Microbial fertilizers in sustainable crop production: Types, functions and regulatory mechanisms.

Improving crop productivity while reducing excessive fertilizer inputs is a major goal of sustainable agriculture. Microbial fertilizers can support crop nutrition and stress tolerance through microbial activity in soil and interactions with roots. This review summarizes the types, functions and regulatory mechanisms of microbial fertilizers in crop growth, development and responses to biotic and abiotic stresses. Particular emphasis is placed on plant growth-promoting bacteria, arbuscular mycorrhizal fungi and yeasts in improving nutrient uptake and enhancing stress tolerance. Effects on crop yield and quality are discussed in relation to nutrient acquisition, root development and plant defense. The roles of microbial fertilizers in alleviating drought, salinity and metal toxicity and suppressing pathogens are also examined. Finally, we discuss challenges to practical application and future research on microbe-plant interactions, strain selection, formulation and application strategies, with the aim of matching microbial fertilizers to crop requirements and local growing conditions.