Plant Growth-Promoting Microorganisms

Plant Growth Promoting Microorganisms (PGPM) are a diverse group of bacteria and fungi that enhance plant growth and productivity through a combination of direct and indirect mechanisms. These microbes, which include well characterized genera such as Bacillus, Pseudomonas, Rhizobium, Azospirillum, Trichoderma, Streptomyces and arbuscular mycorrhizal fungi (AMF), establish themselves in the rhizosphere, phyllosphere, or endosphere of plants. PGPM contribute to plant vigor by improving nutrient availability, synthesizing growth-stimulating hormones, and modulating root development. Their ability to colonize plant roots and interact with host physiology makes them essential components of sustainable crop management systems.

Figure X. Network of plant growth-promoting rhizobacteria (PGPR) interactions supporting plants under abiotic stress, adapted from Vocciante et al. (2022), Applied Sciences, 12(3), 1231. CC BY 4.0. https://doi.org/10.3390/app12031231

Figure X. Simplified scheme of the main PGPR activities and root interactions, adapted from Vocciante et al. (2022), Applied Sciences, 12(3), 1231. CC BY 4.0. https://doi.org/10.3390/app12031231

Direct mechanisms by which PGPM promote growth include biological nitrogen fixation, phosphate solubilization, siderophore production, and phytohormone synthesis. Diazotrophic bacteria such as Rhizobium and Azotobacter convert atmospheric nitrogen (N₂) into ammonium (NH₄⁺), while phosphate solubilizing microbes release organic acids and enzymes that mobilize inorganic and organic phosphorus pools. Many PGPM also produce indole-3-acetic acid (IAA), gibberellins, and cytokinins, which influence root elongation, cell division, and lateral root formation traits directly tied to increased water and nutrient uptake. In addition, siderophore-producing bacteria enhance iron acquisition under iron limiting conditions, further supporting chlorophyll biosynthesis and metabolic activity.

Indirectly, PGPM protect plants against biotic and abiotic stresses through multiple modes of action. They suppress phytopathogens via antibiosis, competition, and mycoparasitism, and many can trigger Induced Systemic Resistance (ISR), priming the plant’s defense machinery. Furthermore, PGPM enhance tolerance to drought, salinity, and heavy metal stress by improving antioxidant enzyme activities and modulating stress-responsive gene expression. When integrated into cropping systems as biofertilizers, seed inoculants, or soil amendments, PGPM reduce reliance on chemical inputs while boosting yield and crop quality. Their multifunctionality and adaptability make them powerful tools for advancing agroecological intensification and achieving climate-resilient, low-input farming systems.