Endophytes as Next-Generation Plant Biostimulants: From Root-Associated Microbial Colonisation and Molecular Signalling to Stress Resilience and Field Application

Climate change, soil degradation, water scarcity and increasing food demand are intensifying the need for sustainable strategies that improve crop productivity and resilience while reducing chemical inputs. Beneficial bacterial and fungal endophytes associated with plant roots are promising next-generation plant biostimulants because they colonize internal plant tissues and modulate nutrient acquisition, phytohormone signalling, metabolism and stress responses. This review focuses specifically on beneficial bacterial and fungal endophytes, particularly root-associated endophytes, and examines their roles in plant growth promotion, nutrient mobilisation, and tolerance to abiotic and biotic stresses. Emphasis is placed on microbial colonisation, molecular signalling and emerging multi-omics approaches for elucidating endophyte–plant interactions. Pathogenic endophytes and seed-specific endophyte systems are outside the primary scope of this review. Despite promising mechanistic evidence, translation to field applications remains constrained by host specificity, environmental variability, microbial compatibility, formulation and delivery, persistence, biosafety and regulatory requirements. Future development should therefore move beyond the selection of individual strains towards predictive, mechanism-based and microbiome-informed strategies supported by multi-omics and rigorous field validation. Such approaches could enable reliable, context-adapted endophyte-based biostimulants for sustainable and climate-resilient agriculture.