<b>Lipids in the root tips were largely altered by loss-function of three RLKs in </b><b><i>Arabidopsis thaliana</i></b>

Root tips can sense moisture gradients and grow into environments with higher water potential. This process is called root hydrotropism. Here, we report three closely related receptor-like kinases (RLKs) that play critical roles in root hydrotropism: ALTERED ROOT HYDROTROPIC RESPONSE 1 (ARH1), FEI1, and FEI2. Overexpression of these RLKs strongly reduced root hydrotropism, but corresponding loss-of-function mutants exhibited an increased hydrotropic response in their roots. All these RLKs showed polar localization at the plasma membrane regions in the root tips. The biosynthesis of the cell wall, cutin, and wax (CCW) was significantly impaired in the root tips of arh1-2 fei1-C fei2-C. A series of known CCW mutants also exhibited increased root hydrotropism and reduced osmotic tolerance, similar to the characteristics of the triple mutant. Our results demonstrated that the integrity of the cell wall, cutin, and previously unreported root cap wax mediate the trade-off between root hydrotropism and osmotic tolerance.