Molecular Soil Biology 2024, Vol.15, No.1, 8-16 http://bioscipublisher.com/index.php/msb 10 Azospirillum brasilense, many of which are involved in hormone signaling pathways that regulate root development. PGPR influence root architecture through the production and modulation of plant hormones such as auxins, cytokinins, and gibberellins. Auxins, in particular, play a crucial role in root elongation and branching. The study by (Thomas et al., 2019) highlights the involvement of hormone signaling pathways in the interaction between rice roots and Azospirillum brasilense, indicating that PGPR can alter the hormonal balance within the plant to promote root growth. PGPR also interact with plant stress hormones such as ethylene and abscisic acid (ABA) to enhance plant stress tolerance. For instance (Pineda et al., 2010), reviews the role of beneficial soil-borne microbes in helping plants cope with biotic and abiotic stresses, including the modulation of stress hormone levels. By influencing the levels of ethylene and ABA, PGPR can help plants better manage stress conditions, thereby improving overall plant health and productivity. 2 Functional Benefits of PGPR in Crop Growth 2.1 Nutrient acquisition Plant Growth-Promoting Rhizobacteria (PGPR) play a crucial role in enhancing nutrient availability to crops through various mechanisms such as nitrogen fixation, phosphate solubilization, and potassium solubilization. For instance, PGPR like Azospirillum, Azotobacter, and Klebsiella are known for their nitrogen-fixing capabilities, which convert atmospheric nitrogen into a form that plants can readily absorb and utilize (Hayat et al., 2012; Kuan et al., 2016; Mohanty et al., 2021). Additionally, phosphate-solubilizing bacteria such as Bacillus megaterium and Pseudomonas species release organic acids that convert insoluble phosphates into soluble forms, making phosphorus more accessible to plants (Wu et al., 2012; Tang et al., 2020; Mohanty et al., 2021). PGPR facilitate nutrient uptake by altering plant hormone levels, which enhances root surface area and morphology, thereby increasing nutrient absorption. For example, the production of indole acetic acid (IAA) by PGPR stimulates root elongation and branching, leading to a larger root surface area for nutrient uptake (Bhattacharyya and Jha, 2011; Ankati and Podile, 2019; Mohanty et al., 2021). Furthermore, PGPR can produce siderophores that chelate iron from the soil, making it more available to plants (Bhattacharyya and Jha, 2011; Hayat et al., 2012). The synergistic interaction between earthworms and PGPR has also been shown to significantly increase the availability of nitrogen, phosphorus, and potassium in the soil, further enhancing nutrient uptake (Wu et al., 2012). 2.2 Stress tolerance PGPR contribute to plant tolerance against various abiotic stresses such as drought, salinity, and heavy metals. For instance, PGPR like Bacillus and Pseudomonas species produce exopolysaccharides that help in soil aggregation and water retention, thereby aiding plants in drought conditions (Hayat et al., 2012; Mohanty et al., 2021). Additionally, PGPR can produce ACC deaminase, which lowers ethylene levels in plants under stress, thus promoting root growth and stress tolerance (Bhattacharyya and Jha, 2011; Hayat et al., 2012). The molecular pathways involved in PGPR-mediated stress mitigation include the production of antioxidants, osmolytes, and stress-related proteins. For example, PGPR can induce the production of osmoprotectants like proline and trehalose in plants, which help in maintaining cellular osmotic balance under stress conditions (Hayat et al., 2012; Vejan et al., 2019). Moreover, PGPR can activate plant defense pathways by inducing the expression of stress-responsive genes, thereby enhancing the plant's ability to withstand adverse environmental conditions (Bhattacharyya and Jha, 2011; Hayat et al., 2012). 2.3 Disease suppression PGPR exhibit biocontrol mechanisms that suppress phytopathogens through the production of antimicrobial compounds, competition for nutrients and niches, and induction of systemic resistance in plants. For instance,
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