Plant biostimulants in sustainable agriculture: current understanding, physiological responses, field applications and future perspectives

Global agriculture faces the dual challenge of increasing food production while reducing environmental impacts. Conventional reliance on agrochemicals has improved yields but accelerated soil degradation, biodiversity loss, and pollution. Plant biostimulants (PBs) have emerged as sustainable inputs; however, knowledge remains fragmented regarding biochemical mechanisms, field-scale efficacy, dose–response relationships, and regulatory alignment. This critical review addresses these gaps by integrating microbial and non-microbial biostimulants into a multi-scale synthesis. Evidence is consolidated on how seaweed extracts, humic and fulvic substances, protein hydrolysates, chitosan, silicon, phosphite, arbuscular mycorrhizal fungi (AMF), plant-growth-promoting rhizobacteria, and Trichoderma spp. influence nutrient use efficiency, photosynthetic performance, root development, and stress tolerance. Although significant progress has been made in identifying physiological and biochemical responses induced by plant biostimulants, the precise molecular mechanisms regulating plant–biostimulant interactions remain incompletely understood and require further investigation. Special emphasis is placed on hormetic dose–response dynamics and crop- or environment-specific outcomes, supported by analyses of methodological challenges that often obscure reproducibility. Findings indicate that robust effects are consistently obtained under multi-site factorial field trials that measure nutrient dynamics, soil–plant–microbe interactions, and yield stability. The review also maps global regulatory frameworks and highlights the potential of precision agriculture and smart-farming technologies to optimize placement, rate, and timing of application. A forward-looking roadmap is provided, prioritizing standardized efficacy metrics, omics-based mechanism discovery, and circular feedstocks for product development. Overall, the synthesis demonstrates the central role of plant biostimulants in advancing sustainable agriculture while setting research and policy priorities for credible and scalable adoption. HIGHLIGHTSIntegrated synthesis from molecular mechanisms to field outcomesHormetic dose–response identified as critical determinant of efficacyMulti-site factorial trials essential for reproducible performanceGlobal regulatory frameworks compared for consistency and gapsSmart farming and circular feedstocks outlined as future priorities