Fulvic acids act as chemical eustressors driving redox and hormonal reprogramming in rice roots

Chemical eustress is a beneficial form of stress that induces adaptive responses in plants without causing physiological damage. Owing to their conformational flexibility and structural heterogeneity, fulvic acids (FA) can act as chemical eustressors at the soil-root interface. However, the molecular mechanisms underlying their action remain poorly understood. In this study, FA from vermicompost were structurally characterized by ¹H NMR, HSQC, SEM-EDS, and DLS. On the basis of the biologically active dose, the transcriptomic response was investigated through a time-series RNA-seq experiment. Early responses (2 h) revealed rapid modulation of ion transporter genes, redox homeostasis regulators, and hormone signaling pathways involving jasmonates, abscisic acid, and ethylene, indicating the perception of a transient stress. At 48 h, a phase of metabolic reprogramming was observed, marked by the repression of classical stress-responsive genes (class III peroxidases, catalases), suggesting adaptation. The sustained expression of redox and hormone-regulating genes (e.g., OsGSTU, Os1-CysPrxB, OsUGT98B1, and OsJAZ2) indicates the establishment of a homeostatic readiness state. After 48 h, at 72 h, plants exhibited significant increases in root growth, reflecting the transition from molecular signaling to functional adaptation. We propose a stepwise mode of action for FA in rice plants: (i) physicochemical interactions at the root interface; (ii) functional disturbance of tissues; (iii) activation of redox and hormonal signaling pathways; and (iv) enhanced resilience and growth. The data demonstrate that FA act as chemical eustressors by transiently activating defense and adaptation pathways that promote root development and physiological preparedness in plants.