Cyanobacteria in sustainable agriculture: from bioactive metabolites toward mechanistically validated biostimulants.

Cyanobacteria are emerging as multifunctional components of sustainable agroecosystems, with the potential to influence nutrient cycling, soil structure, plant-associated microbiomes, and crop responses to environmental stress. This review integrates current knowledge of the molecular and physiological mechanisms underlying these effects, emphasizing nitrogen fixation, nutrient mobilization, phytohormone biosynthesis, extracellular polymeric substances, biocontrol-related metabolites, and stress mitigation. Despite the growing number of cyanobacterial metabolites and traits associated with plant growth promotion and stress tolerance, direct functional and causal evidence linking specific compounds or pathways to plant responses remains limited. The work therefore critically evaluates the evidence supporting cyanobacterial biostimulant functions, distinguishing associative observations from functional and causal validation, while considering translational evidence from controlled experiments to field conditions. This review examines whole-cell inoculants, metabolite-based formulations, hybrid strategies, synthetic consortia, and engineered cyanobacteria in terms of mechanistic support, formulation, reproducibility, scalability, and agricultural applicability. Biosafety, particularly strain-level cyanotoxin risk and the need for appropriate toxicological screening, is also considered an important component of biostimulant development. Because performance depends on interactions among cyanobacterial strain, host plant, formulation, soil properties, microbiomes, and environmental conditions, future progress will require integrating mechanistic biology, metabolomics, formulation development, safety assessment, and field-scale validation. Bridging mechanistic and translational evidence will be essential to convert cyanobacterial functional diversity into reliable, targeted, and scalable technologies for sustainable agriculture.