RBOHC-Generated ROS Tune GNOM-Dependent Root Halotropism in Arabidopsis

Halotropism--the directional growth of roots away from saline environments--requires coordinated integration of tropic cues. We show that halotropic bending in Arabidopsis thaliana roots is fine-tuned by a spatially confined, symmetric reactive oxygen species (ROS) domain generated by the NADPH oxidase RBOHC in elongation-zone epidermal cells. This domain, visualized by dihydrorhodamine-123 staining and confocal microscopy, emerges during the first hours of halostimulation and limits excessive curvature. Reducing ROS, either chemically with ascorbate or diphenyleneiodonium, or genetically in rbohC mutants, enhances halotropic bending, whereas miz2, defective in the ARF-GEF GNOM, exhibits negative halotropism due to an expanded and mislocalized ROS domain that disrupts spatial restriction. The miz2 rbohC double mutant shows a much weaker halotropic response than rbohC alone and similarly lacks the halotropic ROS signals in the elongation zone, indicating that GNOM acts upstream of RBOHC-mediated ROS production. Comparisons with hydrotropism--a moisture-seeking response also involving defined ROS distribution--suggest that GNOM-dependent regulation of RBOHC constitutes a shared module for adjusting root orientation to environmental gradients. Understanding these molecular mechanisms is essential for enhancing crop resilience to soil salinity, particularly in the context of increasing soil salinization driven by climate change.