Humic Acid Regulates Root Growth through ROS-Dependent Pathway and Hormone Signaling in Rice

Humic acid (HA) enhances plant development, but the integration between reactive oxygen species (ROS) and hormone signaling in HA-induced root growth remains unclear. This study investigated the mechanisms of HA extracted from vermicompost on rice root development using ROS scavengers, hormone pathway inhibitors, and gene expression analyses. HA application increased root dry weight by 27% and lateral root length by 20%. ROS staining revealed higher O2•- accumulation in root tips and H2O2 in the elongation zone. Treatments with SB, TIRON, DMTU, DPI, PCIB, and FLD suppressed HA-induced growth, indicating redox and hormonal modulation. HA also upregulated genes related to ROS signaling (OsGPX3, CuZnSOD1, OsPRX112, BAS1) and hormone signaling (OsTOR, OsIAA11). These findings support the idea that HA triggers a controlled oxidative signal that interacts with hormone pathways to regulate gene expression and root development. This ROS-hormone crosstalk reveals a key mechanism underlying HA bioactivity and reinforces its potential as a biostimulant for sustainable crop production.