Unraveling the roles of PGPR and endophytic bacteria as antagonists against fungal pathogens and mechanisms of disease resistance in crop plants

Plant growth-promoting rhizobacteria (PGPR) and bacterial endophytes improve crop performance through nutrient acquisition, phytohormone modulation, stress alleviation and biological control. Their antagonistic effects against fungal pathogens arise from several complementary mechanisms, including competition for nutrients and colonization sites, siderophore-mediated iron sequestration, antibiosis, lipopeptide and volatile production, hydrolytic enzymes, detoxification of virulence factors and activation of induced systemic resistance (ISR). Effective ISR begins with plant perception of microbial signals at plasma-membrane pattern-recognition receptors and proceeds through calcium, reactive oxygen species and kinase signaling to jasmonic acid-, ethylene- and salicylic acid-associated defense networks. Culture conditions, host genotype and environmental variables such as temperature, moisture, pH, salinity and nutrient availability strongly influence endophyte recovery, establishment and persistence. This review integrates these mechanisms with culture-dependent and culture-independent approaches, measurable plant-growth and disease outcomes, and practical pre- and post-harvest application methods. It also identifies the need for standardized inoculum quality, strain–host matching, multi-environment validation and molecular monitoring to translate promising laboratory antagonists into reliable microbial products for sustainable crop protection.