Arabidopsis MLO4 Functions as a Ca2+ Channel Essential for Mechanosensing in Root Tips

Mechanical stimuli guide root growth to avoid obstacles in the soil. Although mechanisms underlying root mechanosensing remain largely unknown, calcium signaling is proposed to be an early event that proceeds directional root growth. In response to mechanical perturbations, root cells produce transient spikes in the cytosolic free Ca2+ levels. Loss-of-function mutations of MLOs cause root curling upon contact with hard surface of obstacles, the mechanism for MLOs action in root thigmomorphogenesis has been enigmatic. WT and mlo4 seedlings were grown on MS media containing gradient agar concentrations mimicking different levels of soil hardness and compared their root growth into the agar gradient mlo4 had much less roots penetrated the harder agar barrier as compared to WT roots. The channel activity of MLO4 was examined in Xenopus oocytes using the two-electrode voltage clamp (TEVC) method. The TEVC recording showed that the oocytes injected with MLO cRNA displayed large inward currents in the presence of Ca2+. MLO4 rescued the cch1/mid1 from growth arrest by the pheromone, these results indicate that MLO4 constitutes a novel Ca2+-permeable channel. Ca2+ spike was hardly detectable in mlo4/GCaMP6s seedings in response to a hard surface during root elongation. In summary, MLO4 functions as a typical Ca2+ channel that links touch stimulation to Ca2+ elevation in root tip cells.