3 - Endophytic bacteria: Role in sustainable agriculture

The biotic and abiotic stresses are key constraints for food quality, crop yield and global food security. It is necessary to take action to cope with global threats caused by abiotic and biotic stresses such as a reduction in precipitation, increasing drought, salinity, improper temperature, heavy metals, nutrient deficiency and invasion of plants pathogenic microbes, which lead to the reduction of agricultural crops productivity. Plants are severely affected by these stresses and have impaired functioning. One of the strategies to cope with abiotic and biotic stresses is taking advantage of the potential of soil microbes, including plant growth-promoting rhizobacteria (PGPR). The solution is completely environmentally compatible and has no negative effects on the environment. Effective PGPR have a substantial role in biotic and abiotic stresses management, reduce chemical fertilizers and increase the yield of plant cultivars by affecting elemental cycling and nutrient management. These beneficial bacteria can stimulate plant growth under abiotic and biotic stress conditions via mechanisms such as the production of phytohormones, ACC (1-aminocyclopropane-1-carboxylate) deaminase, exopolysaccharides, siderophore, organic acids, nitrogen fixation, phosphate solubilization, various osmolytes, systemic resistance induction, etc. In this review, the role of the PGPR as a new strategy was investigated that may be responsible for the increase in crop productivity and eventually by effectively answering biotic and abiotic stresses leads to food security.

Arbuscular mycorrhizal fungi (AMF) are a ubiquitous group of soil microorganisms that form symbiotic relationships with the roots of over 80% of terrestrial plant species. These beneficial fungi are crucial in plant growth, nutrition enhancement, and abiotic and biotic stress resilience. This review explores the AMF synergistic benefits including their capacity to interact with plant roots system to enhance nutrient absorption, improve stress resilience, and confer disease resistance, and their potential applications in sustainable agriculture. The Review integrates recent insights illustrating the molecular processes responsible for improving plant defense mechanisms by AMF, including the modulation of signaling pathways. It highlights the importance of AMF-induced systemic resistance in enhanced abiotic and biotic stress resistance. Moreover, the article provides an integrative perspective on applying AMF toward sustainable plant protection. Within this context, we discussed how these fungi improve plant performance, including enhanced nutrient acquisition, increased tolerance to environmental stressors, and enhanced protection against pathogens by improving plant resistance to biotic stress through the activation of the plant immune system. We also examine the ecological significance of AMF in maintaining soil health and fertility and highlight the importance of incorporating their management into sustainable agricultural practices. Future research directions and innovative applications are also presented. The literature survey demonstrated these fungi's versatility in improving plant tolerance to several biotic and abiotic stresses. At the scientific level, these abilities are supported by several open-field experiments on different plant species. Available commercial formulations and positive ongoing research of AMF, in combination with other sustainable tools, highlight the solid research outline on these beneficial fungi.

Intensive agricultural practices in conjunction with climate change in the recent past have resulted in outbreaks of abiotic and biotic stresses that pose challenges to modern cotton farming systems around the world. Even with improved transgenic cotton varieties, the average lint yield realised in developing (India) and developed countries (Australia) is about 500 and 2500 kg/ha, respectively, compared with theoretical potential yield of 5000 kg/ha. The yield gap is largely associated with many factors being out of balance in the soil and crop management and climate that induce these biotic and abiotic stresses which impacts on the yield. Filling this yield gap requires a joint venture among various agricultural disciplines that include agronomy, soil science, physiology and molecular biology. Several major research projects have aimed to increase yield, and they are related to management of stress and development of stress-tolerant cotton varieties. Bt cotton and herbicide-tolerant cotton are example outcomes from research conducted to alleviate biotic stress. This review briefly describes the major abiotic and biotic stresses in cotton production. Thereafter, the role of soil and agronomic practices in stress management is outlined. This chapter covers drought stress, temperature stress and the major pathogenic stresses and provides appropriate management strategies. This review will be useful broadly to the plant science community, especially physiologists and molecular biologists who will be encouraged to design their research projects based on field realities, considering soil characteristics and agronomic practices.

Bacterial diet and weak cadmium stress affect the survivability of Caenorhabditis elegans and its resistance to severe stress

All plants are continuously subjected to various types of biotic and abiotic stress factors from the time they have been planted in the field up to the time of harvesting, transport, storage, and consumption of the plant or plant-based products. These stresses result in the negative and deleterious effects on crop health and also cause enormous losses across the globe. To reduce the intensity of the losses produced by these stress factors, researchers all across the world are involved in inventing new management practices which may include traditional genetics methodology and various techniques of plant breeding. The use of microorganisms to mitigate both abiotic and biotic stress can provide an economical, eco-friendly solution to the problem of losses due to abiotic and biotic stresses. One such category of microorganisms is root-colonizing nonpathogenic bacteria like plant growth-promoting rhizobacteria (PGPR) which can increase the plant’s resistance to biotic and abiotic stress factors. PGPR is the bacteria residing in the rhizosphere region and is involved in promoting plant growth and suppressing stress components. PGPR colonize the rhizosphere for nutrition which they acquire from plant root exudates. The mechanism by which plant growth-promoting rhizobacteria can accomplish the abovementioned task includes increment in plant growth by enrichment of soil nutrients through nitrogen fixation, solubilization of phosphates, production of metal ion chelators, and elevated production of plant growth-promoting hormones. The mechanism also focuses on elevated protection of the plants through influencing the levels of production of cellulases and β-1,3-glucanases which result in the activation of the defense mechanism of plants against pests and pathogens. PGPR also contains useful variation for making plant tolerant to abiotic stress factors like temperature extremes, pH variations, salinity and drought, and heavy metal and pesticide pollution. Enrichment of plant rhizosphere with such potential stress-tolerating PGPR is expected to provide enhanced plant growth and high yield of plant products in stress-affected areas. This chapter summarizes the research related to PGPR and its benefits and also throws light on the involvement of PGPR in abiotic stress management.

An incessant increase in global population along with a continuous augmentation in abiotic stress conditions, such as temperature, pH, salinity, etc., and limitation of natural resources has posed a serious threat to developing nations in terms of food security and enhanced nutritional value of the yield. Substantial crop losses in both qualitative and quantitative aspects due to the several prevalent phytopathogens are adding severity to the existing trouble. Confrontation with this ongoing problem initially led to the application of chemical fertilizers. However, hazardous aftereffects of the chemical fertilizers on the ecosystem have instigated a demand for a promising eco-friendly substitute that deals with both biotic and abiotic stresses. Rhizospheric microorganisms can be utilized as an effective alternative because they reside in soil and have the intrinsic property of upholding balanced ecosystem. These plant growth-promoting rhizobacteria (PGPRs) enhance plant growth even in poor and stressed environmental conditions by the formation of beneficial associations with the host through biological nitrogen fixation, phosphate solubilization, siderophore and hormone production, etc. They can also trigger host defense mechanism through induced systemic resistance (ISR). These PGPRs are also helpful for phytoremediation by various processes such as direct absorption, accumulation, etc. PGPRs are utilized in the fields of phytostimulation, biofertilization, and biocontrol activities. In the current chapter, we would aim to uphold the mechanisms opted by PGPR for effective plant growth promotion and defense under various abiotic as well as biotic stress conditions. In this context, we would also aim to delve in detail about the host-PGPR cross talk during the onset of stress conditions.

Silicon (Si): Review and future prospects on the action mechanisms in alleviating biotic and abiotic stresses in plants

Under green revolution practices, the imbalanced use of chemical fertilizers and pesticides causes a negative impact on soil health due to the loss of soil microbial flora and fauna. To overcome this negative impact of the green revolution and to increase sustainable agricultural production without damaging further agricultural lands, the only alternative and effective means is to reduce the use of chemicals in agriculture specifically for plant nutrition and plant protection. Under sustainable agricultural practices, plant growth-promoting rhizobacteria (PGPR) can be effective tools to increase productivity while ensuring sustainability in agriculture. PGPR colonize the rhizosphere zone and help in promoting plant growth and development by regulating nutrient acquisition, modulation of plant hormones, and ameliorating various negative effects of various pathogens. PGPR also help sustain the plant growth productivity and significantly increase soil fertility and health under different biotic and abiotic stresses. As per the literature, many studies prove to increase agriculture productivity due to the use of PGPR as eco-friendly microbial inoculants for promoting plant growth attaributes through various direct and indirect mechanisms. The mechanisms of PGPR include biological nitrogen fixation, phytohormones production, Phosphate, potassium, and zinc solubilization, siderophores production, and secretion of other secondary metabolites (phenolic compounds (phenylpropanoids and flavonoids)) that enhance crop productivity and control phytopathogens. Therefore, this chapter focuses on a detailed description of PGPR keeping in view their functional mechanisms as eco-friendly approaches to increase productivity and enhance soil fertility. PGPR can be used as an eco-friendly, socially acceptable, and cost-effective technology for challenges in the future.KeywordsPGPRProductivitySoil fertility and healthRhizosphereSustainable agriculture

Azospirillum is a well-studied genus of plant growth-promoting rhizobacteria (PGPR) and one of the most extensively researched diazotrophs. This genus can colonize rhizosphere soil and enhance plant growth and productivity by supplying essential nutrients to the host. Azospirillum–plant interactions involve multiple mechanisms, including nitrogen fixation, the production of phytohormones (auxins, cytokinins, indole acetic acid (IAA), and gibberellins), plant growth regulators, siderophore production, phosphate solubilization, and the synthesis of various bioactive molecules, such as flavonoids, hydrogen cyanide (HCN), and catalase. Thus, Azospirillum is involved in plant growth and development. The genus Azospirillum also enhances membrane activity by modifying the composition of membrane phospholipids and fatty acids, thereby ensuring membrane fluidity under water deficiency. It promotes the development of adventitious root systems, increases mineral and water uptake, mitigates environmental stressors (both biotic and abiotic), and exhibits antipathogenic activity. Biological nitrogen fixation (BNF) is the primary mechanism of Azospirillum, which is governed by structural nif genes present in all diazotrophic species. Globally, Azospirillum spp. are widely used as inoculants for commercial crop production. It is considered a non-pathogenic bacterium that can be utilized as a biofertilizer for a variety of crops, particularly cereals and grasses such as rice and wheat, which are economically significant for agriculture. Furthermore, Azospirillum spp. influence gene expression pathways in plants, enhancing their resistance to biotic and abiotic stressors. Advances in genomics and transcriptomics have provided new insights into plant-microbe interactions. This review explored the molecular mechanisms underlying the role of Azospirillum spp. in plant growth. Additionally, BNF phytohormone synthesis, root architecture modification for nutrient uptake and stress tolerance, and immobilization for enhanced crop production are also important. A deeper understanding of the molecular basis of Azospirillum in biofertilizer and biostimulant development, as well as genetically engineered and immobilized strains for improved phosphate solubilization and nitrogen fixation, will contribute to sustainable agricultural practices and help to meet global food security demands.

The tomato (Solanum lycopersicum L.), a widely cultivated and economically important vegetable crop, is subject to a number of biotic and abiotic stresses in nature. Several abiotic and biotic stresses have been demonstrated to elevate the concentration of cytosolic free Ca2+ ([Ca2+]i) in Arabidopsis due to the influx of calcium ions. In this study, recombinant aequorin was introduced into the tomato in order to investigate the change in [Ca2+]i when treated with exogenous Ca2+. This resulted in strong luminescence signals, which were mainly observed in the roots. Luminescence signals were also detected in the whole plant, including the leaves, when a surfactant (Silwet L-77) was added to coelenterazine. The concentration of [Ca2+]i increased with the dosage of NaCl/elf18. The luminescence signals also showed a lower increase in intensity with elf18 treatment compared to NaCl treatment. Furthermore, the [Ca2+]i responses to other abiotic or biotic stresses, such as H2O2 and Pep1, were also evaluated. It was found that this transgenic tomato expressing aequorin can effectively detect changes in [Ca2+]i levels. The transgenic tomato expressing aequorin represents an effective tool for detecting changes in [Ca2+]i and provides a solid basis for investigating the adaptation mechanisms of tomatoes to various abiotic and biotic stresses. Moreover, the aequorin-based system would be a highly valuable tool for studying the specificity and crosstalk of plant signalling networks under abiotic and biotic stresses in tomatoes.

Melatonin is a tryptophan‐based indole molecule found in primitive photosynthetic bacteria to mammals. It performs different functions in plants like rhizogenesis, enhancing plant growth, seed germination, plant yield, biomass production, photosynthesis, circadian rhythm, and fruit ripening. In addition, one of the most significant attributes of melatonin is its antioxidant activity. Moreover, it works as an anti‐stress agent against different biotic and abiotic stresses like drought, salinity, potentially toxic metals, and pathogens. Melatonin forms antioxidant cascade reaction by scavenging free radicals that enhances its antioxidant capacity. In response to different stress conditions, expression of genes involved in melatonin synthesis is increased. In the same way, plant‐growth‐promoting rhizobacteria colonize plant roots and enhance plant growth by a number of mechanisms like phosphate solubilization, nitrogen fixation, siderophore production, production of phytohormones, phytoremediation, disease suppression, and production of 1‐aminocyclopropane‐1‐carboxylate deaminase. Thus, melatonin and plant‐growth‐promoting rhizobacteria are involved in enhancing plant growth under abiotic and biotic stress but the mechanisms of action of both are different. Therefore, in this study, the data on the impact of melatonin and plant‐growth‐promoting rhizobacteria on plants are combined for the first time and how these could be useful in enhancing the plant growth is examined. In addition, the research gaps are identified in melatonin and plant‐growth‐promoting rhizobacteria research already conducted from the last few decades that will help the scientific community in further research.

Can interaction between silicon and plant growth promoting rhizobacteria benefit in alleviating abiotic and biotic stresses in crop plants?

Does plant—Microbe interaction confer stress tolerance in plants: A review?

22 - Plant growth-promoting potential of endophytic bacteria for sustainable agriculture

Endophytic microbes are plant-associated microorganisms that reside in the interior tissue of plants without causing damage to the host plant. Endophytic microbes can boost the availability of nutrient for plant by using a variety of mechanisms such as fixing nitrogen, solubilizing phosphorus, potassium, and zinc, and producing siderophores, ammonia, hydrogen cyanide, and phytohormones that help plant for growth and protection against various abiotic and biotic stresses. The microbial endophytes have attained the mechanism of producing various hydrolytic enzymes such as cellulase, pectinase, xylanase, amylase, gelatinase, and bioactive compounds for plant growth promotion and protection. The efficient plant growth promoting endophytic microbes could be used as an alternative of chemical fertilizers for agro-environmental sustainability. Endophytic microbes belong to different phyla including Euryarchaeota, Ascomycota, Basidiomycota, Mucoromycota, Firmicutes, Proteobacteria, and Actinobacteria. The most pre-dominant group of bacteria belongs to Proteobacteria including α-, β-, γ-, and δ-Proteobacteria. The least diversity of the endophytic microbes have been revealed fromBacteroidetes, Deinococcus-Thermus, and Acidobacteria. Among reported genera, Achromobacter, Burkholderia, Bacillus, Enterobacter, Herbaspirillum, Pseudomonas, Pantoea, Rhizobium, and Streptomyces were dominant in most host plants. The present review deals with plant endophytic diversity, mechanisms of plant growth promotion, protection, and their role for agro-environmental sustainability. In the future, application of endophytic microbes have potential role in enhancement of crop productivity and maintaining the soil health in sustainable manner.