Sustainable Horticulture: Microbial Inoculants and Stress Interaction

By Ajay Kumar

Sustainable Horticulture: Microbial Inoculants and Stress Interaction gives insights into the applications and formulations of microbial inoculants. In recent years, the optimum yields of horticultural plants largely influenced by rising global temperature, biotic stress (attack of pathogens) and abiotic stresses has created extra pressure for the horticulturalist to meet the need of optimum yield production for the burgeoning global population. However, the challenges of biotic and abiotic stress factors mitigated by traditional physical or chemicals methods include high application cost and adverse impact on quality limit the frequent use, hence the solutions in this book create new avenues for progress.

This book covers those challenges and how microbial based bio inoculants are broadly used in horticulture to mitigate the challenges of biotic and abiotic stresses. It provides an important contribution on how to apply efficient beneficial microbes (microbial inoculants) for a sustainable society.

• Provides quality chapters from the leading academician and researchers from the different parts of the world
• Gives insights on the applications and formulations of microbial inoculants
• Covers the challenges of biotic and abiotic stress factors mitigated by traditional physical or chemicals methods that are costly

• Language: English
• Publisher: Academic Press
• Release date: Apr 21, 2022
• ISBN: 9780323916769

Book preview

Front Cover for Sustainable Horticulture - Microbial Inoculants and Stress Interaction - 1st edition - by Musa Seymen, Ertan S Kurtar, Ceknas Erdinc, Ajay Kumar

Sustainable Horticulture

Microbial Inoculants and Stress Interaction

Edited by

Musa Seymen

Horticulture Department of Agriculture, Selcuk University, Konya, Turkey

Ertan S Kurtar

Horticulture Department of Agriculture, Selcuk University, Konya, Turkey

Ceknas Erdinc

Agricultural Biotechnology Department of Agriculture, Van Yuzuncu Yil University, Turkey

Ajay Kumar

Agricultural Research Organization, The Volcani Center, Rishon LeZion, Israel

Table of Contents

Cover image

Title page

Copyright

List of contributors

About the editors

Preface

Chapter 1. Effects of microbial inoculants on growth, yield, and fruit quality under stress conditions

Abstract

Chapter Outline

1.1 Introduction

1.2 Biotic stresses

1.3 Abiotic stresses

1.4 Postharvest fruit storage

1.5 Future perspectives

1.6 Conclusion

Acknowledgments

References

Chapter 2. Nutrient availability in temperate fruit species: new approaches in bacteria and mycorrhizae

Abstract

Chapter Outline

2.1 Introduction

2.2 Microbial microorganisms

2.3 The role of bacteria in nutrient availability

2.4 The role of mycorrhizae in nutrient availability

2.5 Future perspectives and conclusion

References

Chapter 3. The effects of microbial inoculants on secondary metabolite production

Abstract

Chapter Outline

3.1 Introduction

3.2 Bacteria

3.3 Fungi

3.4 Nematodes

3.5 Viruses

3.6 Protozoa

3.7 Conclusion

References

Chapter 4. Sustainable stress mitigation with microorganisms in viticulture

Abstract

Chapter Outline

4.1 Introduction

4.2 Viticulture under environmental stress

4.3 Interactions between grapevine and beneficial microorganisms

4.4 Microorganism employment for precision viticulture

4.5 Arbuscular mycorrhiza symbiosis in viticulture

4.6 Plant growth–promoting rhizobacteria in viticulture

4.7 Concluding remarks and future perspectives

References

Further reading

Chapter 5. Mitigation of heavy metal toxicity by plant growth–promoting rhizobacteria

Abstract

Chapter Outline

5.1 Introduction

5.2 Effects of heavy metals on plants

5.3 Plant growth–promoting rhizobacteria

5.4 Plant growth–promoting rhizobacteria and heavy metal stress

5.5 Conclusion

References

Chapter 6. Regulatory role of microbial inoculants to induce salt stress tolerance in horticulture crops

Abstract

Chapter Outline

6.1 Introduction

6.2 Soil microbes and their abundance in soil

6.3 Origin of salinity and its impact on crops

6.4 Salinity effects on crops

6.5 Benefits and effects of microbial inoculants/plant growth–promoting bacteria to plants’ attributes

6.6 Impact of salinity on soil

6.7 Microbial functional genes that help to alleviate stress tolerance in plants

6.8 Impact of soil salinity on crops

6.9 Regulation of plant response to soil salinity

6.10 Role of microbial phytohormone signaling in conferring salt stress tolerance in plants

6.11 Plants with plant growth–promoting rhizobacteria-associated salinity stress tolerance

6.12 Plant growth–promoting bacteria alleviating plant stress due to soil salinity

6.13 Plant growth–promoting rhizobacteria modulation of salinity stress response genes to induce plant tolerance

6.14 Conclusion and future prospects

References

Chapter 7. Arbuscular mycorrhizal fungi in biotic and abiotic stress conditions: function and management in horticulture

Abstract

Chapter Outline

7.1 Introduction

7.2 Principles of arbuscular mycorrhizal fungi symbiosis

7.3 Functions of arbuscular mycorrhizal fungi in abiotic stress conditions

7.4 Arbuscular mycorrhizal fungi as a biocontrol agent

7.5 Arbuscular mycorrhizal fungi technology

7.6 Conclusions and future directions

References

Chapter 8. Enhancing the physiological and molecular responses of horticultural plants to drought stress through plant growth–promoting rhizobacterias

Abstract

Chapter Outline

8.1 Introduction

8.2 Effects of drought stress on plants

8.3 Mechanism of the drought tolerance

8.4 Plant growth–promoting rhizobacteria under drought stress

8.5 Future perspectives and conclusion

References

Chapter 9. Nanotechnologies for microbial inoculants as biofertilizers in the horticulture

Abstract

Chapter Outline

9.1 Introduction

9.2 Characteristics of nanomaterials

9.3 Impact of nanomaterials on plant systems

9.4 Nanotechnology in agriculture

9.5 Nanoformulations for the crops

9.6 Nanotechnology in horticultural systems

9.7 Green nanotechnology

9.8 Conclusion and future perspective

Acknowledgments

References

Chapter 10. Use of microbial inoculants against biotic stress in vegetable crops: physiological and molecular aspect

Abstract

Chapter Outline

10.1 Why do we need methods as alternatives to the usage of pesticides in agriculture?

10.2 Pathogen biocontrol

10.3 Physiological effects of microbial agents on plants

10.4 Use of microbial agents on solanaceae

10.5 Use of microbial agents on cucurbitaceae

10.6 Use of microbial agents on Brassicaceae

10.7 Other vegetables

10.8 Conclusion

References

Chapter 11. Seed application with microbial inoculants for enhanced plant growth

Abstract

Chapter Outline

11.1 Introduction

11.2 Methods to inoculate microbial applications

11.3 Plant beneficial microorganisms

11.4 Microbial seed applications in agriculture

11.5 Cost-efficient microbial biomass preparations for seed treatments

11.6 Comparison of microbial seed applications with other inoculating methods

11.7 Limitations of microbial seed applications

11.8 Conclusion and future prospective

References

Chapter 12. Organic waste separation with microbial inoculants as an effective tool for horticulture

Abstract

Chapter Outline

12.1 Introduction

12.2 Sorption of polyaromatic hydrocarbons

12.3 Half-lives of polyaromatic hydrocarbons in soils

12.4 Presence of microbial genera/strains in organic waste

12.5 Taxonomical distribution of bacteria in organic waste

12.6 Thermophilic bacteria significance

12.7 Molecular technique to isolate thermophilic bacteria

12.8 Recent advances in characterization of novel metagenome

12.9 Micorbial consortium, an effective tool to degrade polyaromatic hydrocarbons in organic waste via composting

12.10 Microbial consortium (thermophilic or mesophilic), the best option for horticulture crop

12.11 Conclusion

References

Chapter 13. Preharvest and postharvest application of microbial inoculants influencing postharvest storage technology in horticultural crops

Abstract

Chapter Outline

13.1 Introduction

13.2 Some relevant preharvest and postharvest factors influencing horticultural crop quality

13.3 Preharvest microbial inoculants, the allies of postharvest management technologies

13.4 Potential of bioinoculants in postharvest horticultural crops protection and preservation

13.5 Postharvest preservation technologies incorporating microbial inoculants or their metabolites

13.6 Conclusion and future prospective

Acknowledgments

References

Chapter 14. Nano-based biofertilizers for horticulture

Abstract

Chapter Outline

14.1 Introduction

14.2 Fertilizers

14.3 Microbial inoculants as fertilizers

14.4 Types of biofertilizers

14.5 Nanotechnology—strategic potential in sustainable horticulture

14.6 Nanofertilizers—role in improving crop productivity and crop protection

14.7 Nanobiofertilizers—an emerging eco-friendly approach for a smart nutrient delivery system for horticulture

14.8 Advantage of nanobiofertilizers over chemical fertilizers

14.9 Conclusion and future perspective

Acknowledgments

References

Chapter 15. Biochemical and molecular effectiveness of Bacillus spp. in disease suppression of horticultural crops

Abstract

Chapter Outline

15.1 Introduction

15.2 Plant growth promotion by Bacillus spp

15.3 Antagonistic effects of Bacillus species in management of the plant pathogens

15.4 Plant–pathogen–Bacillus interactions

15.5 Future perspectives

References

Index

Copyright

Academic Press is an imprint of Elsevier

125 London Wall, London EC2Y 5AS, United Kingdom

525 B Street, Suite 1650, San Diego, CA 92101, United States

50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States

The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom

Copyright © 2022 Elsevier Inc. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

ISBN: 978-0-323-91861-9

For Information on all Academic Press publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Stacy Masucci

Acquisitions Editor: Linda Versteeg-Buschman

Editorial Project Manager: Czarina Mae S. Osuyos

Production Project Manager: Selvaraj Raviraj

Cover Designer: Matthew Limbert

Typeset by MPS Limited, Chennai, India

List of contributors

Muhammad Asif Ali,     Department of Life Sciences, Abasyn University, Islamabad, Pakistan

Şeyma Arıkan,     Department of Horticulture, Selçuk University, Konya, Turkey

Tayyaba Asif,     Department of Biosciences, COMSATS University Islamabad, Islamabad, Pakistan

Angelika Astaikina,     Faculty of Soil Science, Lomonosov Moscow State University, Moscow, Russia

Kubilay Kurtulus Bastas,     Department of Plant Protection, Faculty of Agriculture, Selcuk University, Konya, Turkey

Gökhan Boyno,     Department of Plant Protection, Faculty of Agriculture, Van YYU, Van, Turkey

Maryam Bozorg-Amirkalaee,     Department of Plant Protection, Faculty of Agricultural Sciences, University of Mohaghegh Ardabili, Ardabil, Iran

Haris Butt,     Department of Plant Protection, Faculty of Agriculture, Selcuk University, Konya, Turkey

Hasan Can,     Eregli Faculty of Agriculture, Necmettin Erbakan University, Konya, Turkey

Paul A. Correa,     Department of Biosciences, COMSATS University, Islamabad, Pakistan

Yesim Dal,     Department of Horticulture, Faculty of Agriculture, Selcuk University, Konya, Turkey

Younes Rezaee Danesh

Soil, Fertilizer and Water Resources Central Research Institute, Ankara, Turkey

Department of Plant Protection, Faculty of Agriculture, Urmia University, Urmia, Iran

Semra Demir,     Department of Plant Protection, Faculty of Agriculture, Van YYU, Van, Turkey

Melek Ekinci,     Department of Horticulture, Faculty of Agriculture, Atatürk University, Erzurum, Turkey

Hassan Etesami,     Department of Soil Science, University of Tehran, Tehran, Iran

Muzaffer İpek,     Horticulture Department, Faculty of Agriculture, University of Selçuk, Konya, Turkey

Unal Kal,     Department of Horticulture, Faculty of Agriculture, Selcuk University, Konya, Turkey

Merve Karakoyun,     Department of Horticulture, Faculty of Agriculture, Bilecik Seyh Edabali University, Bilecik, Turkey

Manpreet Kaur,     Department of Physics, Chandigarh Group of Colleges, Mohali, Punjab, India

Necibe Kayak,     Department of Horticulture, Faculty of Agriculture, Selcuk University, Konya, Turkey

Rabiya Tabbassum Khan,     Shri Mata Vaishno Devi University, Katra, Jammu and Kashmir, India

Sofia Sharief Khan,     Shri Mata Vaishno Devi University, Katra, Jammu and Kashmir, India

Sehar Khushhal,     Department of Biosciences, COMSATS University Islamabad, Islamabad, Pakistan

Divjot Kour,     Department of Biotechnology, Dr. Khem Singh Gill Akal College of Agriculture, Eternal University, Sirmaur, Himachal Pradesh, India

Harpreet Kour,     Department of Botany, University of Jammu, Jammu and Kashmir, India

Shilpa Kumari,     Department of Physics, Akal College of Basic Sciences, Eternal University, Sirmaur, Himachal Pradesh, India

Jyothis Mathew,     School of Biosciences, Mahatma Gandhi University, Kottayam, Kerala, India

Sara Mubeen,     Department of Biosciences, COMSATS University Islamabad, Islamabad, Pakistan

Emre Mutluay,     Institute of Science, University of Selçuk, Konya, Turkey

Solmaz Najafi,     Department of Field Crops, Faculty of Agriculture, Van YYU, Van, Turkey

Rabia Naz,     Department of Biosciences, COMSATS University Islamabad, Islamabad, Pakistan

Asia Nosheen,     Department of Biosciences, COMSATS University, Islamabad, Pakistan

Maryam Pahlavan Yali,     Department of Plant Protection, Faculty of Agriculture, Shahid Bahonar University, Kerman, Iran

María Valentina Angoa Pérez,     Department of Research, CIIDIR IPN Michoacan Unit, National Polytechnic Institute, Jiquilpan, Mexico

Abdullah Kaviani Rad,     Department of Soil Science, School of Agriculture, Shiraz University, Shiraz, Iran

E.K. Radhakrishnan,     School of Biosciences, Mahatma Gandhi University, Kottayam, Kerala, India

Ali Sabir,     Department of Horticulture, Faculty of Agriculture, Selçuk University, Konya, Turkey

Samina Siddiqui,     National Centre of Excellence in Geology, University of Peshawar, Peshawar, Pakistan

Shaveta Singh,     Department of Microbiology, Shoolini University, Solan, Himachal Pradesh, India

Rostislav Streletskii,     Faculty of Soil Science, Lomonosov Moscow State University, Moscow, Russia

Metin Turan,     Department of Genetic and Bioengineering, Faculty of Engineering and Architecture, Yeditepe University, Istanbul, Turkey

Onder Turkmen

Eregli Faculty of Agriculture, Necmettin Erbakan University, Konya, Turkey

Department of Horticulture, Faculty of Agriculture, Selcuk University, Konya, Turkey

Hortencia Gabriela Mena Violante,     Department of Research, CIIDIR IPN Michoacan Unit, National Polytechnic Institute, Jiquilpan, Mexico

T.N. Vipina Vinod,     School of Biosciences, Mahatma Gandhi University, Kottayam, Kerala, India

Ajar Nath Yadav,     Department of Biotechnology, Dr. Khem Singh Gill Akal College of Agriculture, Eternal University, Sirmaur, Himachal Pradesh, India

Humaira Yasmin,     Department of Biosciences, COMSATS University, Islamabad, Pakistan

Ertan Yildirim,     Department of Horticulture, Faculty of Agriculture, Atatürk University, Erzurum, Turkey

Mehdi Zarei

Department of Soil Science, School of Agriculture, Shiraz University, Shiraz, Iran

Department of Agriculture and Natural Resources, Higher Education Center of Eghlid, Eghlid, Iran

About the editors

Assoc Dr. Musa Seymen: Faculty of Agriculture, Department of Horticulture, Selcuk University, Konya, Turkey

Assoc Dr. Ceknas Erdinc: Faculty of Agriculture, Department of Plant Biotechnology, Van Yuzuncu Yıl University, Van, Turkey

Prof. Dr. Ertan S Kurtar: Faculty of Agriculture, Department of Horticulture, Selcuk University, Konya, Turkey

Dr. Ajay Kumar: Visiting Scientist, Agriculture Research Organization, Volcani Center, Ministry of Agriculture and Rural Development, Rishon Leziyon, Israel

Preface

In the current global climate change scenario and rising human population, sustainable practices for horticultural crop production are the immediate need. Nevertheless, conventional practices for horticultural crop production mainly rely on chemical pesticides to speed up and enhance crop production. However, the undistributed and regular use of chemical pesticides affects the soil and crop quality and negatively impacts human and environmental health. However, in the last few years, microbes or microbial products have frequently been practiced as an alternative to chemical fertilizers or chemical pesticides to enhance and protect crop plants. The utilization of beneficial microbes, including bacteria, fungi, yeast, and actinomycetes, has been the focus of many agricultural studies. This book focuses on the effects of microbial inoculants and the agronomic, physiological, and molecular mechanisms of these effects to make plant production more efficient in a sustainable framework. PGPR and AMFs play a supportive role in biotic (disease and pest management) and abiotic (drought, salinity, extreme heat and cold, heavy metal pollution) stress conditions, and under these conditions, they positively affect plant growth, yield, fruit quality, nutrient content, and shelf life. In addition, microbial inoculant applications enhance the production of some secondary metabolites (terpenes, phenolics, nitrogenated compounds) by programming plant metabolism. The use of nanoparticles as nanofertilizers (NF) and nanopesticides (NP)-based PGPR and AMFs has tremendous advantages in eco-friendly and cost-effective agricultural production by minimizing synthetic chemical inputs. Microinoculants also have an essential role in postharvest technology and preserve the quality and extend the shelf life of agricultural products. Some PGPR and AMF strains promote the plant immune system against viral, bacterial, and fungal diseases. We hope that this work will be helpful not only for agricultural fields but also for other branches of science. We want to thank all the subject specialists, our research and compilation team members, who contributed to the production of this book, for their cooperation and patience.

Editors

Chapter 1

Effects of microbial inoculants on growth, yield, and fruit quality under stress conditions

Abdullah Kaviani Rad¹, Mehdi Zarei¹,², Angelika Astaikina³, Rostislav Streletskii³ and Hassan Etesami⁴,    ¹Department of Soil Science, School of Agriculture, Shiraz University, Shiraz, Iran,    ²Department of Agriculture and Natural Resources, Higher Education Center of Eghlid, Eghlid, Iran,    ³Faculty of Soil Science, Lomonosov Moscow State University, Moscow, Russia,    ⁴Department of Soil Science, University of Tehran, Tehran, Iran

Abstract

Fruits as nutritious meals are enriched with bioactive compounds, antioxidants, vitamins, and micronutrients, constituting a significant portion of the human diet and agriculture production. Nevertheless, the horticultural trade faces many challenges, such as biotic and abiotic stresses, including plant diseases, pests, drought, heat, and salinity. Ecological stresses directly lead to physiological and biochemical alterations such as osmotic and oxidative stress, insufficient nutrients uptake, and decreased photosynthesis in plants; therefore growers utilize chemical pesticides and fertilizers to prevent fruit loss, leading to adverse impacts on environmental health. On the other hand, microbial inoculants and biofertilizers are practical solutions that have been shown to enhance crop yield during stressful conditions. Plant growth–promoting rhizobacteria and endophytic microbes can overcome the stresses by employing mechanisms such as changes in root system architecture, induction of systematic resistance, and regulation of expression of stress-responsive genes. Microbial strains were also advantageous agents in shielding postharvest fruits against various pathogens. The present chapter aims to review studies about the performance of microbial inoculants in fruits during various biotic and abiotic stresses. Future studies should be focused on enhancing the efficiency of microbial strains by addressing the crucial role of microbial inoculants in improving crop yield and combating the negative consequences of environmental stresses.

Keywords

Biotic and abiotic stress; fruit; drought; heat; salinity; plant disease; pests; PGPR; AMF; biofertilizer

Chapter Outline

Outline

1.1 Introduction 1

1.2 Biotic stresses 7

1.2.1 Plant diseases 7

1.2.2 Plant pests 10

1.3 Abiotic stresses 12

1.3.1 Drought stress 12

1.3.2 Heat stress 14

1.3.3 Salinity stress 15

1.4 Postharvest fruit storage 17

1.5 Future perspectives 19

1.6 Conclusion 19

Acknowledgments 20

References 20

1.1 Introduction

Growing urbanization and lifestyle transformations have changed consumer demand and food production (Philippe et al., 2021), and increasing consumption of poor and hyperenergy foods has led to an unhealthy diet that causes numerous diseases such as obesity, overweight, stroke, hypertension, and cancer, especially in young persons. Hence, a healthy diet is a vital agent for physical and mental wellness. Consumption of fruits and vegetables (400 g/day) is recommended to adults due to their short and long-term advantages (Dai et al., 2021; Li et al., 2021; Swallah et al., 2020). Because fruits as a herbal snack are enriched with macro-and micronutrients, bioactive compounds, antioxidants, carbohydrates, lipids, organic acids, vitamins, carotenoids, phenolic, anthocyanin, fibers, and other metabolites that can improve the body immune system and mitigate the risk of chronic and heart diseases (Alasalvar et al., 2020; Jideani et al., 2021; Meena et al., 2021; Wallace et al., 2020). Carotenoids are pigments of fruits, and Marhuenda-Muñoz et al. (2021) detected that the carotenoid was higher in the cell plasma of persons who consumed more fruits and vegetables. dos Santos et al. (2021) concluded that constipation prevalence decreased along with fruit consumption enhancement. Pentacyclic triterpenes and phenols are bioactive molecules in olive trees, and triterpenes such as oleanolic acid, malonic acid, erythrodiol, and uvaol have antitumor activity, and phenols such as oleuropein, tyrosol, and hydroxytyrosol are natural antioxidants (Jiménez-Herrera et al., 2019).

Date palm fruit is an excellent food that contains a wide range of nutrients such as carbohydrates, fibers, proteins, and minerals. Fructose and glucose carbohydrates consist of 70% of date palm fruit, and its minerals consist of calcium, iron, selenium, copper, phosphorous, potassium, zinc, sulfur, cobalt, fluorine, and manganese. Therefore date palm is an essential economic product that endures food security, specifically in the arid regions of the world (Aljaloud et al., 2020; Hazzouri et al., 2020; Olakunle-Moses & Aderonke, 2019). Ceri Terengganu also has a high phenolic and flavonoid content and antioxidant activity and can be used as a natural antioxidant source (Looi et al., 2020). Investigating the capability of Sageretia theezans for improving the body’s immune system indicated that this fruit enhanced cell survival and phagocytosis by producing immune system modulators (Eo et al., 2021).

Gu, He, et al. (2021) reported that reduction of risk of Chinese adults’ mortality had a positive correlation with a higher level of fruit consumption. Moreover, Głąbska et al. (2020) showed that consumption of citrus and berry decreased psychological distress and depression symptoms. Therefore it is necessary to increase the consumption of various fruits and reduce eating poor nutritious foods (Davison et al., 2021). Fruit advantages are not limited to supplying food demand, and many food industries are using the present pectin of fruits as a stabilizer and thickener agent (Wu et al., 2020). Also, fruits can be used as bioactive compounds in order to produce dyeing chemicals for the food and beverage industries (Di Gioia et al., 2020). Hence, horticulture and fruit trees cultivation are essential in agricultural and industrial production sectors that consist of technologies and commercial aspects (Huang et al., 2020; Li, Wang, et al., 2019).

By considering world population enhancement in 2050 (United Nations, 2017) along with climate change and increasing global warming, crop productivity improvement and sustainable food security attract agricultural scientists’ attention (Bhattarai et al., 2021; Lee et al., 2019). On the other hand, farmers have to apply chemical inputs such as pesticides and fertilizers to maintain crop yield and face increasing food demand (Philippe et al., 2021). Pesticides are organic compounds that increase crop production and help to control the pests and disease transmitters such as mosquitoes, ticks, and mice (de O Gomes et al., 2020; Rojas et al., 2021). Although pesticides have many benefits for controlling plant pests, their residues and metabolites can be transferred to the human body through water and food (Omwenga et al., 2021). Approximately 60,000 chemical compounds are used in the food production process; 90% of these chemical agents are dangerous to the human body (Bursić et al., 2021; Crépet et al., 2021).

According to a study by Olisah et al. (2020), organochlorine pesticides such as Dichlorodiphenyltrichloroethane (DDT) and endosulfans have the highest environmental residues in Africa. Al-Nasir et al. (2020) reported a high level of chlorothalonil and daminozide in irrigation water and garden soil of a garden in Jordan. In a study by Bhandari et al. (2019), pesticide residues of organochlorines, organophosphates, and acaricides in samples of eggplant (4%), tomato (44%), and pepper (19%) were higher than European maximum residue limit standards. In another study, in order to investigate the diet of French infants and children under three ages in 2011–12, it was indicated that 78 types of pesticide residues were presented in 67% of the food samples. Most pesticides include 2-phenylphenol, azoxystrobin, captan, carbendazim, difenoconazole, dodine, and imazalil, and the insecticides were acetamiprid, pirimiphos-methyl, and thiacloprid, which ranged from 0.02 to 594 μg/kg in the food samples (Nougadère et al., 2020).

Despite the fact that horticulture is the primary source of livelihood in many rural regions, the consequences of the unchecked use of chemical pesticides can be more severe in these regions (Khan, Yaqub, et al., 2020). Uncontrolled application of antimicrobial pesticides also increases bacterial tolerance to antimicrobial pesticides so that controlling bacterial pathogens will be complicated by using conventional commercial pesticides (Campos & Ariel, 2021). Table 1.1 summarizes the studies on pesticide residues in fruits in some regions of the world.

Table 1.1

In addition to the protection of horticultural products, fertilizers are a vital factor to fruit yield improvement, and nutrients have a key role in plant growth and metabolism. For instance, potassium has a critical role in physiological and metabolic processes (Sattar et al., 2019), such as water setting and drought tolerance improvement; calcium can increase salinity tolerance levels by removing sodium, and silicon forms a physical obstacle that controls the entrance of disease agents through sediment in epidemic cells of the plant leaves (Fernández-Escobar, 2019). In a study by Boaretto et al. (2020), nutrient supply enhancement increased the plant tolerance to environmental stresses and led to a high photosynthesis and transpiration rate and low electron transferring/photosynthetic carbon ratio. Magnesium supply enhancement also raised the activity of the antioxidant enzyme system and reduced the oxidative stress of plants. In a field experiment in China, magnesium increased pepper yield by 25.6% (Lu et al., 2021). High chlorophyll content and light-energy absorbability were observed in trees treated with extra nitrogen (Boaretto et al., 2020). Therefore fertilizing is a vital tool for raising soil fertility and crop yield improvement, and farmers have depended on fertilizers application for plant nutrition. However, this dependence has caused ecological concerns such as greenhouse emissions, soil degradation, and air and water pollution (Giri & Varma, 2019; Qaswar et al., 2020).

Uncontrolled and long-term application of chemical inputs in food productive ecosystems has led to the accumulation of toxic compounds like heavy metals in soil that reduces crop quality and transfer to plants and the human body in the long term (Amouei et al., 2020; Mirzaei et al., 2020; Vishwakarma et al., 2020; Zhang et al., 2020). Heavy metals and pesticides are ranking at the first position of the list of toxic environmental compounds (Alengebawy et al., 2021), and according to a study by Liao et al. (2021), the share of anthropogenic cultivation activities in polluting soil by heavy metals is 15.85%, which is a significant portion (Du et al., 2020). Chen et al. (2020) found that the presence of Cd, As, Cu, and Ni in farmland soil samples was related to chemical fertilizer application, and the use of sewage sludge was considered as the main reason for the high concentration of heavy metals in farmland soil (Taghipour et al., 2013). Moreover, urban sewage sludge use in farmlands has led to a reduction of pH (due to organic matter decomposition) and raised the extractable concentration of Cr, Cu, Pb, and Zn (Protano et al., 2020). Wei, Yu, et al. (2020) concluded that the application of chemical fertilizers increased the availability and accumulation of Cu, Ni, Pb, and Zn; however, manure application also significantly increased the availability of Cd and Zn in soil.

Cd is a trace element that is famous for its toxicity for humans, and in this regard, Rostami et al. (2021) demonstrated that Cd is responsible for the highest pollution level of the soil (Park et al., 2021). Ugulu et al. (2021) reported that the health risk index for Zn, Co, Fe, Cd, Pb, Cu, and Cr was higher in wheat grown with chemical fertilizers. Overall the abovementioned studies demonstrated that fertilizers’ overuse could threat human and animals health by accumulating heavy metals (Jalali & Karimi Mojahed, 2020). Table 1.2 summarizes some studies about heavy metals distribution and accumulation in farmlands of different regions, and general impacts of chemical inputs overuse (Fig. 1.1).

Table 1.2

Figure 1.1 General adverse consequences of chemical inputs on agriculture and human health.

It is known that the reduction of fruit trees’ yield can be solved only with chemical inputs if the trees are not exposed to biotic and abiotic stresses. However, plants constantly face the challenges imposed by environmental conditions and must cope with various stresses (Etesami & Maheshwari, 2018; Salvatierra et al., 2020; Teklić et al., 2021). Despite the increasing product demand, plants are under high pressure to maintain their yield in the prevalence of environmental stresses such as cold, heat, drought, waterlogging, salinity, nutrient deficiency, and high and low light intensity (Francini & Sebastiani, 2019; Malhi et al., 2021). The listed abiotic stresses have significant effects on plant physiological and biochemical processes and can reduce the fruit yield by 50%–70% (Francini & Sebastiani, 2019). Hence, abiotic stress-related economic losses are expected to reach 0.3-0.8% of the global gross domestic product in 2100 (Stevanović et al., 2016).

Fruit trees primarily are perennials, and they are more exposed to nonliving stresses; hence abiotic stresses have significant effects on the growth and development of these plants (Elsheery et al., 2020; Li, Wang, et al., 2019). In this regard, Bhusal et al. (2019) stated that abiotic stresses are the most important limiting agent of plant growth and food production in many regions of the world, and it is forecasted to be exacerbated along with global warming. Therefore fruit production industries are continuously following the novel technologies which can endure food system sustainability besides the high quality and safety of products. Recently, plant bio-stimulants have been considered a valuable tool for increasing agrochemical inputs efficiency, which can improve the stress tolerance of fruit trees and fruit quality (Basile et al., 2020). These bio-stimulants include microbial stimulators such as arbuscular mycorrhizal fungi (AMF) and plant growth–promoting rhizobacteria (PGPR), humic and fulvic acid, compounds containing nitrogen, silicon, animal and plant hydrolysate proteins that their ability in biofertilizer formulation, nutrients supply for growth of horticultural plants, control of biotic and abiotic stresses, and phytopathogens suppression have been proved (Drobek et al., 2019; Fasusi et al., 2021; Rouphael & Colla, 2020).

PGPR are famous for their ability to help plants absorb nutrients, synthesize compatible solutes, production of phytohormones, and pathogens biocontrol (Sayyed et al., 2019). These microorganisms are essential biological groups of soil involved in biological, biochemical, and biogeochemical processes and are responsible for increasing soil quality and fertility (Santos et al., 2020; Vadkertiová et al., 2019). Comprehensive combinations of rhizospheric microbes such as Bacillus edaphicus, Bacillus mucilaginosus, Acidothiobacillus ferrooxidans, Bacillus circulans, Paenibacillus sp., and fungal strains of Aspergillus terreus are involved in the dissolution of potassium mineral sources. These microbes improve the availability of nutrients to plants by various mechanisms. For example, potassium dissolution mechanisms by the microbes include organic acid production, soil pH reduction, acidolysis, and chelation (Sattar et al., 2019). In a study by Wei, Liu, et al. (2020) addition of biochar with Pseudomonas putida to soil increased grape weight by 7.6% compared to control samples. Also, the physicochemical properties of treated soil, such as electrical conductivity, available phosphorous and potassium, organic matter, and urease activity, improved by 87.3%, 73.2%, 78.7%, 71.6%, and 17.5%, respectively. Abd et al. (2021) found that Bacillus megaterium and Azotobacter chroococcum can produce the phytohormone, indole-3-acetic acid (IAA). Inoculation of tomato seedlings with Streptomyces thermocarboxydus had a notable positive impression on photosynthesis, and chlorophyll fluorescence parameters improved due to enhanced electron transport rate in the thylakoid membrane (Passari et al., 2019). It is concluded that the inoculation of Pisolithus tinctorius into the soil can alleviate stressful environmental conditions (Maltz et al., 2019).

Bizos et al. (2020) reported that AMF stimulate root growth and increase olive resistance to ecological stresses. Since biofertilizers have a crucial role in nitrogen fixation and dissolution of phosphorus and potassium, Penicillium pinophilum inoculation to soil increased available K and P, leading to pomegranate yield improvement by 35% (Maity et al., 2019). In a study by Javanmardi et al. (2014), inoculation of Pepino (Solanum muricatum Ait.) with Glomus versiforme enhanced dry matter and vitamin C percentage of fruits in comparison to control samples. Also, the treatment of seedlings with Glomus etunicatum significantly increased fruit weight. Gamez et al. (2019) demonstrated that Bacillus amyloliquefaciens and Pseudomonas fluorescens raised the growth of banana plants and could partially replace chemical fertilizers. In a study by Rho et al. (2020), endophyte colonization in apple trees lingered leaf aging and enhanced lateral root biomass and soluble sugar content. Preinoculation of pepper seedlings with Bacillus velezensis led to the formation of a new microbiota in the rhizosphere that significantly correlated with pepper yield (Zhang et al., 2019).

Hu, Li, et al. (2020) indicated that the genera of Flavobacterium and Sorangium in the seedling stage, Klebsiella in flowering, Collimonas in early fruit setting, Pontibacter, Micrococcaceae, and Adhaeribacter have involved in the late tomato setting. Inoculation of Piriformospora indica and Funneliformis mosseae with trifoliate orange improved growth parameters such as plant height, leaf number, and stem and root biomass in comparison to noninoculated samples (Yang et al., 2021). A study by Abdelrahman and Darwesh (2020) demonstrated that the application of Enterobacter ludwigii in apple trees increased the growth, yield, and quality of apples; hence, this strain can be utilized for numerous fruit crops. Many species of Trichoderma as microbial stimulants have direct antagonistic activity and biocontrol ability that can stimulate plant growth (Lombardi et al., 2020). They can also stimulate plant defense mechanisms against pests (Dini et al., 2021). Trichoderma harzianum and Bacillus thuringiensis have raised fruit trees by producing growth-promoting substances, inducing plant resistance to pathogens, and improving soil biological and physical properties (Abdelmoaty et al., 2021). The obtained results by Imran et al. (2020) showed that Trichoderma inoculation to soil led to soybean phenology and yield enhancement. Moreover, the utilization of Trichoderma has caused plant growth stimulation, anthocyanin, and other antioxidants accumulation in strawberries. Proteomic analysis of treated strawberries demonstrated that microbial inoculation significantly impacted stress-responsive proteins, nutrients absorption, and carbon/energy metabolism (Lombardi et al., 2020).

AMF also improve plant tolerance to ecological stresses by altering plant physiological, functional, and biochemical processes (Malhi et al., 2021). Overall soil bio-stimulants can develop beneficial soil microorganisms that provide substrates for the plant (Drobek et al., 2019). Hence, fruit production by utilizing beneficial microorganisms is a sustainable, affordable, and promising approach to face biotic and abiotic stresses (Fasusi et al., 2021; Manja & Aoun, 2019). This chapter aims to review studies on the benefits and impacts of microbial inoculants on the growth, yield, and quality of various fruits and investigates the different biotic and abiotic challenges toward fruit trees and the role of microbes in managing the stressful ecological conditions.

1.2 Biotic stresses

1.2.1 Plant diseases

Although the demand for organic fruits is increasing, products yield under organic cultivation is low compared to conventional farming, and phytopathogens are a severe threat to organic fruits (Pylak et al., 2019). Fruit crops are primarily perennials and are more exposed to biotic stresses than annuals (Li, Wang, et al., 2019; Pineda et al., 2021). Rhizobacteria and fungi as biological control agents (BCAs) can stimulate plant growth and protect trees’ health (Pirttilä et al., 2021), and biocontrol methods such as microbial inoculation (Rabiey et al., 2019) and the use of beneficial nutrients such as silicon (Etesami & Jeong, 2018) and calcium are suitable alternatives of agrochemical pesticides for plant disease management. For instance, silicon foliar sprays in olive trees reduced the leaf spots (Fernández-Escobar, 2019). El-Hady et al. (2020) also found that olive trees that received 0.5% calcium chelate as a spray had higher flowering, fruit setting, yield, and quality.

Microbial communities have complex networks and relationships with plants that help plant productivity and health (Zhimo et al., 2021). Endophytes are fungi and bacteria that live in host plants’ intercellular spaces or vascular tissues without causing apparent disease. Bacteria induce defense mechanisms in plants (Migunova & Sasanelli, 2021). Crown gall disease originated by Agrobacterium tumefaciens severely affects the production of peaches. It was observed that the high abundance and diversity of endophytic and antagonist bacteria could help protect the peach tree against A. tumefaciens (Li, Guo, et al., 2019). Soil replant disease can also affect the growth of peach trees in soils with poor traits. Inoculation of Acaulospora scrobiculata to peach seedlings significantly increased shoot and root biomass, phosphorus, potassium, calcium, phosphatase, and urease activity in infected soil samples (Lü et al., 2019). Solanki et al. (2019) demonstrated that the inoculation of B. velezensis, Trichoderma lixii, and Streptomyces atrovirens to tomatoes increased plant defense enzymes chitinase, β-1,3-glucanase, peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase. S. atrovirens and T. lixii also activated the plant defense system against Rhizoctonia solani. In another study by Zheng et al. (2019), the performance of the microbial community was correlated with resistance to grape disease. P. fluorescens was reported as an effective BCA of Verticillium wilt in olive trees (Bizos et al., 2020). In antagonistic activity tests by López-González et al. (2021), 10 Bacillus strains inhibited the mycelial growth of Penicillium expansum (Blue mold agent) from 30.1% to 60.1%. The treatment of apple seedlings with the endophytic fungus strain, Crinipellis tabim, controlled the Dematophora necatrix (white root rot) by 93.5% (Pal et al., 2020). Also, the inoculation of Pseudomonas chlororaphis into the rhizospheric soil of the avocado tree significantly decreased the presence of the phytopathogen, Rosellinia necatrix, in the rhizosphere and its impacts on other microbial communities (Tienda et al., 2020).

The banana industry suffered severe economic losses due to the Fusarium wilt epidemic in the last century (Bubici et al., 2019). Fusarium wilt is a common soil-borne disease caused by Fusarium oxysporum and is one of the critical global diseases in numerous plants such as tomatoes and bananas (Izquierdo-García et al., 2020; Mohammed et al., 2019; Zhou et al., 2019). Hence, fertilizing is a critical tool to control this pathogen, and antagonistic PGPR can control it through the induction of defensive mechanisms into plants (Nagpal et al., 2020; Widyantoro et al., 2020). According to a study by Mohammed et al. (2019), P. fluorescence and Bacillus subtilis had antagonistic activity against the growth of F. oxysporum. Also, preventive utilization of B. subtilis in potatoes decreased the severity of Fusarium dry rot by 42.8%–63.8% (Ben Khedher et al., 2021). Evaluating the biocontrol activity of the endophytic bacterium, Achromobacter xylosoxidans, revealed that these bacteria reduced the mycelial growth of F. oxysporum and Fusarium solani and the severity of the disease in melons by 80% and 60% compared to the untreated plants (Dhaouadi et al., 2019). Widyantoro et al. (2020) demonstrated that Bacillus spp. and Streptomyces spp. were the most effective biofertilizers for controlling this disease in bananas. Moreover, Zhu et al. (2021) found that the application of Streptomyces morookaensis to banana plantlets significantly reduced the incidence of Fusarium wilt, and Passari et al. (2019) concluded that S. thermocarboxydus could be applied as a biocontrol tool for Fusarium wilt in tomatoes. Therefore antagonistic fungi either directly through mycoparasitism or indirectly by antibiosis induce the defensive mechanisms and combat infection agents (Maitlo et al., 2019). Hewedy et al. (2020) examined the antagonistic activity of four Trichoderma species against F. oxysporum and observed that these species could inhibit the growth of phytopathogenic mycelium between 35.71% and 85.75% in vitro conditions. According to the results of Izquierdo-García et al. (2020), the consortium of Trichoderma virens and B. velezensis is a potential instrument to control this phytopathogen in cape gooseberry. Generally, F. oxysporum has been controlled by Pseudomonas spp. (79%) and Trichoderma spp. (70%) in field conditions (Bubici et al., 2019).

Erwinia amylovora also causes fire blight, a mortifying global disease for apples, pears, and several rosaceous plants (Bittner et al., 2020; Gayder et al., 2019). In a study by Dagher et al. (2020), apple and pear trees treated with Pantoea agglomerans NY60 had lower levels of this phytopathogen than untreated plants. Furthermore, P. chlororaphis, Pseudomonas congelans, and Pseudomonas protegens inhibited the growth of E. amylovora (Mikiciński et al., 2020). Tafifet et al. (2020) also reported that Pseudomonas brassicacearum had the most significant impact on preventing the infection of pear fruits with fire blight and decreased necrosis by 90%. Therefore microbes can be applied as BCAs due to their various mechanisms to combat various plant diseases in fruit trees. Table 1.3 summarizes some studies on the mechanisms of biocontrol agents against fruit phytopathogens.

Table 1.3

1.2.2 Plant pests

Plant pests are one of the serious concerns and substantial biotic stresses of the fruits production process in horticulture. Insects are a wonderfully diverse group of animals, which cause damage to agricultural and forestry sectors, hence called pests (Gurung et al., 2019). Globally, these pests reduce crop yields by 20% (Hiebert, Kessel, et al., 2020), and it is estimated that parasitic nematodes cause approximately $173 billion losses to plant crops worldwide annually (Gamalero & Glick, 2020). For instance, avocado fruit waste caused by Scirtothrips perseae leads to 12%–51% economic loss in many countries (Tzec-Interián et al., 2020). The invasive pest Drosophila suzukii is an East Asian native insect that infects soft-skinned fruit crops and leads to considerable financial loss worldwide (Gabarra et al., 2015; Hiebert, Carrau, et al., 2020; Wang et al., 2021). Duponchelia fovealis is also an important pest in strawberry production that causes significant crop losses (Araujo et al., 2020). Hence, synthetic chemical pesticides have been practiced for many years to control these pests (Keswani et al., 2019). Meanwhile, improving pest management systems along with maintaining environmental health in sustainable agriculture are increasing by multiple BCAs to enhance resistance to pests and diseases (Regaiolo et al., 2020). Numerous evidence proposes that microorganisms are employed to administer nutrients and protect themselves and crops (Shi & Bode, 2018).

Microbial biocontrol agents include some species of PGPR and fungi that directly attack plant-parasitic agents through diverse mechanisms such as competition, formation of protective biofilms, and release of bioactive compounds. Some strains of Bacillus, Paenibacillus, Brevibacillus, Pseudomonas, Serratia, Burkholderia, and Streptomyces spp. have been utilized for biological control (Köhl et al., 2019; Ruiu, 2020). In this regard, Mastore et al. (2021) stated that B. thuringiensis is an alternative option to chemical insecticides owing to the maximum mortality rate (78%) of D. suzukii recorded at a concentration of 0.564 µg/L of this strain. Chanthini et al. (2018) reported that B. subtilis had caused the highest death rate (87%) of tobacco cutworm worms (Spodoptera litura). Also, entomopathogenic fungi such as Metarhizium spp. and Beauveria bassiana are extensively applied in the biological control of different species of pests (Yang et al., 2019). Abdel-Raheem et al. (2020) reported that the inoculation of Metarhizium anisopliae led to the death of eggs, larvae, and adult Rhynchophorus ferrugineus by 90%, 95%, and 77%, respectively. The tomato leaf miner, Tuta absoluta (Meyrick), is a key pest of tomato in several parts of the world, and its control by pesticides makes several problems such as developing resistance. A study by Zamani (2018) indicated that using F. mosseae and Micrococcus yunnanensis improves the plant growth, and suppresses the damage of tomato leaf miner. The highest mortality rate (79.33%) of the Kharap beetle was recorded 21 days after exposure to M. anisopliae and Spinetoram (Anwar et al., 2020). In an experiment on the red pepper guarding by Kang et al. (2018), it was found that B. bassiana and Lecanicillum spp. caused the death of wax moths. Hiebert, Kessel, et al. (2020) also found that Leuconostoc pseudomesenteroides significantly decreased the survival rate of drosophilid insects and aphids. Pinus pinaster trees treated with diazotrophic bacteria and Cunninghamella elegans biofertilizer recorded 36.3 times less Bursaphelenchus xylophilus (pinwilt) nematode than control plants (Nunes da Silva et al., 2019).

Metabolites produced by microorganisms also play an essential role in combating pests and supporting crop health (Keswani et al., 2019). In this regard, Ghazanchyan et al. (2018) reported that Brevibacillus laterosporus could produce protein compounds against leaf beetles. Chitinase is an enzyme used by insects to damage the structural polysaccharides and hydrolyzes chitin of other insects, and Danai-Tambhale (2018) demonstrated that the chitinolytic activities of actinomycetes destroyed the larvae of Spodoptera frugiperda after a few hours. Microbial chitinases are associated with hydrolyzing the gut epithelium of insects and the cell wall of many fungi, and according to a study by Okongo et al. (2019), chitinases produced by Thermomyces lanuginosus and Bacillus licheniformis were responsible for 70% and 80% of Eldana saccharina larval mortality. Kim et al. (2019) also pronounced that chitinase-producing bacterium, Paenibacillus elgii, as a biocontrol agent in plants can reduce damage caused by microbial pathogens, insects, and nematodes; hence, Subbanna et al. (2018) reported that chitinolytic bacteria could be formulated as pest control tools for direct application. Although BCAs are crucial characteristics of sustainable pest management and copious antagonistic microorganisms have been recognized for pest control, their commercialization is still limited due to their weakness and dependence on chemical, biological, and physical conditions compared to synthesized chemical pesticides (Marian & Shimizu, 2019).

1.3 Abiotic stresses

1.3.1 Drought stress

Droughts and water scarcity are growing worldwide (Prgomet et al., 2020), and by increasing evapotranspiration, climate change reduces water resource accessibility (Boini et al., 2019). It is the principal cause of crop yield reduction in the majority of agricultural regions of the world because it affects almost all plant functions. Plant growth and farming productivity were significantly reduced by drought, which leads to stresses such as oxidative, osmotic shock, insufficient nutrient uptake, and decreased photosynthesis in host plants (Kaushal, 2019; Kour et al., 2020). The analysis by Semida et al. (2021) showed that drought stress reduced membrane stability index, relative water content (RWC), photosynthetic efficiency, and yield in eggplants. In another study, water deficit decreased banana weight by 65% (Panigrahi et al., 2021). Alipour et al. (2020) reported that severe water scarcity decreases RWC and chlorophyll content and expands malondialdehyde and proline content. Apple has a high nutritional value, and water scarcity impressively reduces its growth rate and yield. In this context, Faghih et al. (2021) indicated that water stress boosted the activity of nonenzymatic defense systems of apple trees. Water stress can enhance the content of fructose, glucose, and sorbitol in apples, but the weight will decrease if this stress occurs in the fruit’s development (Wang et al., 2019). Elkelish et al. (2021) demonstrated that plant growth, chlorophyll, RWC, and tomato yield decreased during water stress. Nagaz et al. (2020) reported that the yield of orange trees was reduced by 24% through employing a low irrigation plan.

Nevertheless, plants and microbes have specific mechanisms for drought tolerance which is a complicated characteristic controlled by various genes. Understanding the molecular processes associated with stresses is crucial for tolerating stress and ultimately ensuring crop yield (Shameer et al., 2019). The molecular physiology of trees determines that osmotic regulation, antioxidant defense, and advancing water use efficiency are essential intentions in order to improve drought tolerance at the cellular and tissue levels (Polle et al., 2019). Several antioxidant systems are activated at the biochemical level, and numerous enzymes generate metabolites such as proline, glycine betaine, and amino acids to inhibit cell destruction (Francini & Sebastiani, 2019). Therefore the synthesis of protective bio compounds is regarded as a response to stress, and many of these compounds are produced in the plant’s primary metabolism and act as functional compounds in other organisms (Teklić et al., 2021).

1-Aminocyclopropane-1-carboxylate (ACC) is an essential precursor to ethylene production, which rises in plants during drought stress (Yaseen et al., 2020). Increasing the concentration of ethylene diminishes root development and elongation, and thereby decreases plant growth and yield. Hence, the enzymatic compound, ACC deaminase, converts ACC to ammonia and α-ketobutyrate rather than ethylene and overcomes drought stress in plants by controlling ethylene (Danish & Zafar-ul-Hye, 2019). The response of soil microorganisms to drought stress includes changes in root system architecture, osmoregulation, induction of systematic resistance, and the regulation of transcription of drought-responsive genes in plants (Zia et al., 2021). PGPR can produce ACC deaminase, and Gupta and Pandey (2020) reported that the Aneurinibacillus aneurinilyticus and Paenibacillus consortium could decrease ethylene level by 61% through ACC deaminase activity. Mukhtar et al. (2020) also observed that ACC deaminase–producing bacteria significantly reduced the adverse impressions of ethylene on tomato plants.

Melatonin is a pleiotropic molecule that stimulates growth, protects plants against various stress, and regulates fruit flowering and ripening. The fundamental role of melatonin is generating the primary line of defense against oxidative stress, which occurs in all unfavorable environmental conditions in plants. It adjusts the expression of factors such as enzymes, receptors, transcription agents in signaling auxin, gibberellic acid (GA), ethylene, and abscisic acid (ABA), as well as strigolactones and brassinosteroids (Sun et al., 2021). Jiao et al. (2016) observed that colonization of grape seedlings exposed to drought with B. amyloliquefaciens led to upregulation of melatonin production. Asif et al. (2020) also recorded that the combined application of melatonin and B. licheniformis boosted antioxidant enzymes, superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) activity in spinach.

More so, PGPR amplify plant drought tolerance through a series of mechanisms of physicochemical alterations named rhizobacterial-induced drought endurance and resilience (RIDER). The RIDER mechanism includes alterations in phytohormone levels, metabolic adjustment, production of exopolysaccharides (EPS), biofilm formation, antioxidant resistance, and accumulation of organic solutes such as carbohydrates, amino acids, and polyamines (Fig. 1.2) (Khan, Ali, et al., 2020; Yadav et al., 2020). AMF and PGPR improve the situation of host nutrients and diminish osmotic and oxidative stresses by adjusting the content of osmolytes, antioxidant enzymes, and ion transfer activity. Also, PGPR helps improve hydraulic conductivity by expanding aggregation due to the production of EPS and modifying the root architecture by using IAA and ethylene (Kaushal, 2019). Anli et al. (2020) indicated that AMF and PGPR biofertilizers promoted plant growth, leaf water potential, and electrical conductivity in date palms in water scarcity conditions. Biofertilizers additionally improved physiological parameters in combination or independently by rising stomatal conductance, photosynthetic pigments (chlorophyll content and carotenoids), and efficiency. Ojuederie et al. (2019) showed that inoculation of P. fluorescens and AMF dramatically reduced H2O2 and malondialdehyde in the leaves of Arizona cypress trees. In the study by Zhang et al. (2020), Pseudomonas lini and Serratia plymuthica significantly increased plant height, stem and root dry matter weight, and RWC in jojoba seedlings. Sepahvand et al. (2021) also stated that stem and root dry weight, leaf area, seedling height, root length, and chlorophyll content of Celtis australis coexisted with AMF were more than control samples. According to Shabani-Mazooi (2019), the application of AMF can play a crucial role in improving the resistance of Iranian pot marigolds to water deficit. Zarei et al. (2016) demonstrated that the inoculation of sour orange with Glomus mosseae increased dry weight of shoot, nitrogen and phosphorus uptake, and the activity of antioxidant enzymes SOD, CAT, glutathione peroxidase, and APX during drought stress. In another research, inoculation of Acinetobacter calcoaceticus into seedlings of Sambucus williamsii raised the photosynthetic rate by 12.99% in comparison to control samples during water stress (Liu et al., 2019). Silva et al. (2019) reported that inoculation of soybean plants under drought stress with Bradyrhizobium japonicum and Azospirillum brasilense promoted leaf cell membrane stability (Silva et al., 2019). Moreover, Ilyas et al. (2020) proved that B. subtilis and A. brasilense had a high capacity to produce EPS and osmolytes and remarkably altered the levels of phytohormones IAA, GA, and cytokinin (CK). Regarding agriculture as the highest user of water resources, water use efficiency should be developed by adopting strategies such as microbial inoculants in order to prevent diminished fruit yield due to drought stress (Wu et al., 2021).

Figure 1.2 Mechanisms of plant growth–promoting rhizobacteria (PGPR) to improve drought tolerance in plants. Adapted from Yadav, V. K., Raghav, M., Sharma, S. K., & Bhagat, N. (2020). Rhizobacteriome: Promising candidate for conferring drought tolerance in crops. Journal of Pure and Applied Microbiology, 14(1), 73–92. https://doi.org/10.22207/JPAM.14.1.10.

1.3.2 Heat stress

Simultaneous occurrence of high temperature and water shortage leads to damage plants (Alhaithloul et al., 2020); accordingly, heat stress caused by global warming is a principal concern for crop production due to the reduction of fruit quality and production sustainability (Morales-Quintana et al., 2020). Previous studies have determined that chloroplasts are highly susceptible to heat stress which influences several photosynthetic processes such as chlorophyll biosynthesis, electron transference, and CO2 assimilation (Hu, Ding, et al., 2020). Unusual high temperatures induce reactive oxygen species (ROS) accumulation, which damages the cell membrane and chloroplasts (Sarkar et al., 2018). Hence, heat stress impairs the phenological, physiological, and biochemical processes of plants at different lifetimes (Hassan et al., 2021). These damages include destruction of the membrane and some proteins, inactivation of critical enzymes, interruption of biomolecule synthesis, and cell division suppression (Mitra et al., 2021). The experiment by Zhang, Gao, et al. (2019) on Tung trees showed that long-term heat stress significantly decreased photosynthesis of seedlings, altered the composition of internal leaf structure, and reduced the performance of chloroplasts. Heat can affect the flowering of citrus trees and lessen fruit yield (Boaretto et al., 2020). Rahaman et al. (2018) reported that loss of canola plants (Brassica napus L.) could arrive 15%–20% due to heat stress.

To perception heat stress, plants have multiple mechanisms such as signaling-hormone pathways and generate ethylene, which is essential for surviving during heat and other abiotic stresses; accordingly, targeting ethylene biosynthesis and signaling pathways can effectively induce heat stress tolerance (Poór et al., 2021). Plants can partly resist heat stress by increasing membrane stability, regulating antioxidants, and inhibiting ROS (Ali et al., 2020). Additionally, PGPR promote heat stress tolerance through lower ROS generation, reduced membrane destruction, chloroplast maintenance, increased chlorophyll content, enhanced redox enzyme expression and accumulation of osmolites (proline and glycine betaine) (Sarkar et al., 2018). Ojuederie et al. (2019) reported that PGPR was capable of producing osmolyte, which dramatically reduces the harmful impressions of ROS. Moreover, PGPR can produce heat shock proteins in plants that induce indirect heat tolerance (Kumar et al., 2019). In a study by Bruno et al. (2020), Providencia rettgeri lowered proline synthesis and enhanced antioxidant gene expression and stress resistance. Khan, Asaf, et al. (2020) determined that Bacillus cereus treatment significantly ameliorated the biomass and chlorophyll of tomato plants during heat stress, as well as the increase of Fe+, P, and K+ uptake.

Molecular mechanisms of endophytes to enhance heat stress tolerance in plants include inducing the expression of stress-responsive genes and producing ROS-inhibiting biomolecules (Lata et al., 2018). The presence of AMF accelerated the reproductive characteristics like the number of flowers and inflorescence, percentage of fruit set, flowering, and fruit ripening in two cultivars of strawberries after heat stress (Shirdel, 2017). Treatment of cucumber plants with Thermomyces endophytic fungus diminished the adverse impacts of extreme temperature through keeping photosystem II performance, photosynthesis rate, water use efficiency, and developing root system in comparison to untreated plants (Ali et al., 2018). Application of bio-stimulants as a seed treatment to cucumber seeds increased germination rate and biomass by 6.54% and 13%, respectively, in heat conditions (Campobenedetto et al., 2020). Therefore PGPR can be utilized as bio-stimulants to enhance horticultural production besides an ecofriendly strategy and neutralize the unfavorable influences of heat stress (Khan, Asaf, et al., 2020).

1.3.3 Salinity stress

Salinity is one of the hugest challenges in global crop production, specifically in arid and semiarid regions. More than 100 countries face salt predicaments in soil and groundwater resources (Giri & Varma, 2019), and approximately 800 million hectares of the global agricultural lands have been affected by salinity, which is expected to increase in the future (Niu et al., 2019). Natural soil salts, drought, evaporation, high heat, and reduction of freshwater resources in these regions cause the accumulation of enormous masses of soluble salts in soils (Hazzouri et al., 2020). Extreme soil salinity, besides natural conditions, is caused by improper irrigation management and the application of saline groundwater and fertilization (Niu et al., 2019). Salt uptake in plants inhibits different physiological and metabolic processes and creates osmotic, oxidative, and ionic stresses, which affect plant survival (Egamberdieva et al., 2019; Kumar Arora et al., 2020), and it was determined that 20%–50% of global crop waste is due to drought and salinity stresses (Ha-Tran et al., 2021). Trabelsi et al. (2019) demonstrated that the leaves of olive trees, which were irrigated with saline water, lost 23% of their photosynthetic activity forever. De Sedas et al. (2020) reported that during rising salinity, the growth parameters of eight species of coastal and inland tropical trees such as leaf area, stem height, and total dry mass decreased. Salinity stress in sweet pepper plants reduced chlorophyll content, RWC, fruit yield, and increased electrolyte leakage, malondialdehyde, proline, ROS, and antioxidant enzyme activities (Abdelaal et al., 2020).

PGPR application is addressed as an adequate strategy to respond to salinity stress owing to a crucial role in enhancing product yield in saline soils (Etesami & Glick, 2020; Shafique et al., 2019). In order to overcome salinity intensity in plants, halo-tolerant rhizobacteria have complex physiological, biochemical, and molecular mechanisms that include the expression of defense proteins, EPS synthesis, activation of the antioxidant system, osmolytes accumulation, change of phytohormone levels, and nutrients uptake for plants, besides the modification of signaling under stress conditions by PGPR which causes systemic resistance in plants to salinity stress (Fig. 1.3) (Etesami & Glick, 2020; Kumar Arora et al., 2020). Treatment of strawberry plants with Kocuria E43, Alcaligenes 637Ca, and Pseudomonas 53/6 in saline and calcareous soils significantly decreased leaf Na+ content, membrane permeability, and H2O2 and malondialdehyde. There was also a notable increase in fruit weight and yield, leaf area, macronutrient level in leaves, stomatal conductance, protein content, proline and SOD, CAT, and APX activity (Arıkan et al., 2020).

Figure 1.3 Mechanisms of plant growth–promoting rhizobacteria (PGPR) to improve salinity tolerance in plants. Adapted from Ilangumaran, G., & Smith, D. L. (2017). Plant growth promoting rhizobacteria in amelioration of salinity stress: A systems biology perspective. Frontiers in Plant Science, 8, 1–14. https://doi.org/10.3389/fpls.2017.01768.

Under salinity stress, treatment of rough lemon (Citrus jambheri) with G. etunicatum and Glomus intraradices increased root dry weight and shoot phosphorus uptake in comparison to control samples (Zarei & Paymaneh, 2014). ALKahtani et al. (2020) indicated that application of B. thuringiensis and chitosan led to a significant improvement in RWC, chlorophyll content, and sweet pepper yield under salinity conditions. This PGPR strain additionally regulated proline accumulation and enzyme activity. Proline is one of the primary compatible solutes of plants and bacteria in order to respond to osmotic imbalance and ion toxicity. Its biosynthesis occurs in the cytosol and mitochondria and modulates the function of these components in various cellular physiological pathways, as well as affects cell proliferation and regulates the expression of specific genes to reduce salinity stress. Rhizobacteria, as an efficient inoculant, can accumulate proline by diverse mechanisms (Sayyed et al., 2019). Abdelaal et al. (2020) also proved that proline played a crucial role in improving plant growth and salinity tolerance through modulating physiological parameters and antioxidants. Tomato plants inoculated with AMF have higher yields and maximum production at