The Role of Plant-Associated Microorganisms in Improving Crop Growth and Stress Tolerance

Abstract

Plant-associated microorganisms influence crop growth through interactions with roots, leaves, seeds, and surrounding soil. Certain bacteria and fungi support nutrient acquisition, modify plant signalling, and contribute to protection against drought, salinity, and pathogens. Their effects, however, depend on microbial strain, crop genotype, soil conditions, and management. This narrative review examines plant growth-promoting rhizobacteria, nitrogen-fixing symbioses, carbuncular mycorrhizal fungi, and entophytes, with attention to their ecological roles and proposed mechanisms. It discusses nutrient mobilization, phytohormone regulation, competition with pathogens, and physiological responses to abiotic stress, as well as methods for characterizing microbial communities and evaluating inoculants. A graph illustrates published mean responses to rhizobacteria inoculation under well-watered conditions from a meta-analysis; these values are not guaranteed yield gains and do not represent drought results. Evidence from field studies must be interpreted in relation to experimental design, environmental variability, and economic costs. The review highlights formulation, colonization, biosafety, and quality control as key challenges for translation into agriculture. It concludes that microbial technologies are most useful when evaluated as part of locally adapted crop and soil management. No new field experiment or dataset is reported.