Long-Distance Calcium Signalling from the Root Cap to the Elongation Zone Is Essential for Water Tracking

Ever since Darwin postulated that the tip of the root is sensitive to moisture differences across the root, and that it “transmits an influence to the upper adjoining part, which bends towards the source of moisture”, two important questions remained to be answered in order to explain the response of roots to uneven distribution of water in their microenvironment; which longitudinal signal transmits hydro-perception from the root tip to the elongation zone, where bending occurs, and which cross-root asymmetric lateral signal confers differential cell elongation on different sides of the root, resulting in root bending toward the water source. Here we show that a water potential gradient across the root tip generates a slow long-distance asymmetric cytosolic Ca²⁺ signal in the phloem, which peaks at the elongation zone, where it is dispersed laterally and asymmetrically to peripheral cells, where cell elongation occurs. Moreover, we demonstrate that the MIZ1 protein, which is indispensable for root curvature towards water, directly inhibits the activity of ECA1, an endoplasmic reticulum Ca²⁺ transporter (Ca²⁺-ATPase). This inhibition is required for generating the long-distance cytosolic Ca²⁺ signal upon hydrostimulation.