Ogawa, S. T.; Zhang, W.; Staiger, C. J.; Kessler, S. A.

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 bypasses FER signaling in root hairs, complementing not only the previously reported fer-4 root hair bursting phenotype, but also the complete loss of polarity in spherical root hairs. This complementation correlates with the restoration of tip-focused [Ca2+]cyt oscillations that are disrupted in fer mutants, suggesting that MLOs are involved in regulating [Ca2+]cyt dynamics in growing root hairs. 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 and that ROS levels are decreased in mlo15-4 root hairs. We propose that MLOs act downstream of FER to mediate Ca2+ influx and promote ROS production in order to regulate root hair tip-growth.