<scp>MLO</scp> ‐mediated Ca ²⁺ influx regulates root hair tip growth in Arabidopsis

Root hair tip-growth involves coordinated Ca²⁺ and ROS signaling to promote growth while maintaining tip integrity. MILDEW RESISTANCE LOCUS-O (MLO) proteins act downstream of FERONIA (FER) receptor-like kinases in pollen tubes and synergids to regulate calcium dynamics. This study uses a constitutively active MLO (faNTA) to identify a new role for the FER/MLO signaling module in regulating [Ca²⁺]_cyt oscillations in growing root hairs. faNTA was used as a tool to manipulate Ca²⁺ influx in fer mutants. Light sheet fluorescence imaging was used to image the reporter R-GECO1 to observe [Ca²⁺]_cyt dynamics during root hair elongation in various genotypes. We show that faNTA is sufficient to restore normal root hair development, [Ca²⁺]_cyt oscillations, and ROS levels to fer-4. MLO15 was identified as a regulator of root hair tip growth based on disrupted root hair growth and [Ca²⁺]_cyt signatures in mlo15-4. We also link the FER/MLO module to ROS accumulation by showing that faNTA is sufficient to restore ROS levels in fer-4 root hairs, but is unable to complement the burst root hairs of rbohc. We propose that MLOs act downstream of FER to mediate Ca²⁺ influx and promote ROS production to regulate root hair tip growth.