The biostimulant role of microbial exopolysaccharides: mechanisms and agricultural applications.

Microbial exopolysaccharides (EPS) are high-molecular-weight carbohydrate polymers secreted by bacteria (including cyanobacteria) and fungi that have attracted increasing interest as biostimulants for sustainable crop production. Despite a growing body of literature, an integrated analysis connecting EPS structural and physicochemical properties to downstream plant molecular responses has been lacking. This review addresses that gap by adopting a structure-function-omics framework, tracing a sequence from EPS chemical composition, including charge, molecular weight, hydrophilicity, and rheological behavior, through plant perception mechanisms, to the transcriptomic and metabolic changes that follow. In the rhizosphere, EPS contribute to soil aggregate stabilisation, water retention, and heavy metal chelation, improving root-zone conditions under drought, salinity, and metal toxicity. At the plant surface, LysM-domain receptor-like kinases recognize structurally defined EPS and initiate signaling cascades. The outcome, such as symbiosis, immunity, or growth promotion, depends on the EPS structural identity. Transcriptomic and metabolomic studies across multiple crop systems indicate that EPS exposure is associated with modulation of photosynthesis, carbohydrate metabolism, antioxidant defense, and secondary metabolite biosynthesis, including phenylpropanoids, flavonoids, and terpenoids. Phytohormone networks involving salicylic acid, jasmonic acid, abscisic acid, and auxin are also influenced, though evidence for intact high-molecular-weight EPS as direct hormonal regulators remains limited. Dedicated coverage is provided for cyanobacterial EPS, an underexplored source with distinctive structural properties. The review concludes by identifying priority knowledge gaps, notably the complete absence of studies on EPS-mediated epigenetic effects in plants, and outlines directions for translating EPS research into field-applicable biostimulant technologies.