Molecular Microbiology Research 2024, Vol.14, No.5, 218-225 http://microbescipublisher.com/index.php/mmr 221 3.3 Induced systemic resistance and defense responses Induced systemic resistance (ISR) is a plant defense mechanism activated by certain rhizosphere microbes. Bacillus subtilis, for instance, can induce ISR by producing secondary metabolites and cell-wall-degrading enzymes that enhance plant defense against pathogens (Hashem et al., 2019). This biocontrol mechanism not only suppresses disease but also improves plant resilience to environmental stresses. AMF have been shown to alter the rhizosphere microbial community, promoting the enrichment of beneficial microbes that contribute to plant growth and stress tolerance (Shi et al., 2022). 4 Role of Rhizosphere Microbes in Drought Tolerance 4.1 Water retention and soil structure improvement Rhizosphere microbes play a crucial role in enhancing water retention and improving soil structure, which are vital for plant survival under drought conditions. Plant growth-promoting rhizobacteria (PGPR) and mycorrhizal fungi are known to produce extracellular polysaccharides (EPS) that improve soil aggregation and water retention capacity. For instance, the co-inoculation of rhizobia and arbuscular mycorrhizal fungi (AMF) has been shown to significantly enhance soil aggregate stability, thereby improving water retention and plant growth under drought stress (Zhang et al., 2020). The presence of specific bacterial communities, such as those enriched in the rhizosphere of drought-resistant sugarcane varieties, can improve soil structure and water uptake by increasing root length and root tip numbers (Dao et al., 2023). 4.2 Osmotic adjustment and drought signaling Rhizosphere microbes also contribute to osmotic adjustment and drought signaling in plants. These microbes can produce and modulate phytohormones such as abscisic acid (ABA), auxins, and cytokinins, which are crucial for plant drought responses. For example, the production of ABA by rhizosphere bacteria helps in stomatal closure, reducing water loss during drought (Mathur and Roy, 2021). The accumulation of osmolytes like trehalose by rhizobia during symbiosis with legumes protects plant cells from osmotic shock and desiccation, enhancing drought tolerance (Sharma et al., 2020). Microbial interactions also induce systemic tolerance in plants through the production of antioxidant enzymes and stress-responsive genes. The inoculation of drought-tolerant PGPR strains, such as Pseudomonas and Serratia, has been shown to increase the activity of antioxidant enzymes like superoxide dismutase (SOD) and peroxidase (POD), which mitigate oxidative stress in plants under drought conditions 8. Furthermore, the induction of stress-responsive genes by plant-associated microbes, including bacteria, fungi, and viruses, plays a significant role in enhancing plant drought tolerance (Poudel et al., 2021). 5 Role of Rhizosphere Microbes in Salinity Tolerance 5.1 Ion homeostasis and salt exclusion Rhizosphere microbes play a crucial role in maintaining ion homeostasis and facilitating salt exclusion in plants under saline conditions. These beneficial microorganisms, including plant growth-promoting rhizobacteria (PGPR) and endophytic fungi, help plants manage the toxic effects of high salinity by modulating ion transport and accumulation. For instance, the inoculation of maize with the PGPR strain Kocuria rhizophilaY1 has been shown to significantly reduce Na+ levels and electrolyte leakage while enhancing the expression of genes involved in salt tolerance, such as ZmNHX1, ZmNHX2, and ZmNHX3 (Li et al., 2020). Similarly, endophytic fungi can improve nutrient uptake and maintain ionic homeostasis by modulating ion accumulation, thereby restricting the transport ofNa+ to leaves and ensuring a low cytosolic Na+: K+ ratio in plants (Gupta et al., 2020). Moreover, the presence of specific root-associated bacterial consortia can enhance plant adaptability to salt stress. These consortia, recruited by plants under saline conditions, provide enduring resistance against salt stress by maintaining a balanced ion homeostasis (Li et al., 2021). The recruitment of beneficial soil bacteria, such as Bacillus, Ensifer, and Pseudomonas, in the rhizosphere of salt-tolerant alfalfa varieties further underscores the importance of microbial diversity in managing ion homeostasis and enhancing salt tolerance (Fan et al., 2023).
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