MIZ1 regulates ECA1 to generate a slow, long-distance phloem-transmitted Ca²⁺ signal essential for root water tracking in Arabidopsis

Ever since Darwin postulated that the tip of the root is sensitive to moisture differences and that it “transmits an influence to the upper adjoining part, which bends towards the source of moisture” [Darwin C, Darwin F (1880) The Power of Movement in Plants, pp 572–574], the signal underlying this tropic response has remained elusive. Using the FRET-based Cameleon Ca²⁺ sensor in planta, we show that a water potential gradient applied 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. In addition, the MIZ1 protein, whose biochemical function is unknown but is required for root curvature toward water, is indispensable for generating the slow, long-distance Ca²⁺ signal. Furthermore, biochemical and genetic manipulations that elevate cytosolic Ca²⁺ levels, including mutants of the endoplasmic reticulum (ER) Ca²⁺-ATPase isoform ECA1, enhance root curvature toward water. Finally, coimmunoprecipitation of plant proteins and functional complementation assays in yeast cells revealed that MIZ1 directly binds to ECA1 and inhibits its activity. We suggest that the inhibition of ECA1 by MIZ1 changes the balance between cytosolic Ca²⁺ influx and efflux and generates the cytosolic Ca²⁺ signal required for water tracking.