Plants can acclimate by using tropisms to link the direction of growth to environmental conditions. Hydrotropism allows roots to forage for water, a process known to depend on abscisic acid (ABA) but whose molecular and cellular basis remains unclear. Here we show that hydrotropism still occurs in roots after laser ablation removed the meristem and root cap. Additionally, targeted expression studies reveal that hydrotropism depends on the ABA signalling kinase SnRK2.2 and the hydrotropism-specific MIZ1, both acting specifically in elongation zone cortical cells. Conversely, hydrotropism, but not gravitropism, is inhibited by preventing differential cell-length increases in the cortex, but not in other cell types. We conclude that root tropic responses to gravity and water are driven by distinct tissue-based mechanisms. In addition, unlike its role in root gravitropism, the elongation zone performs a dual function during a hydrotropic response, both sensing a water potential gradient and subsequently undergoing differential growth.
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The authors thank C. Howells, K. Swarup and M. Whitworth for technical assistance, J.-K. Zhu for providing snrk2.2 snrk2.3 seeds, W. Grunewald for pDONR-L1-GAL4-VP16-R2 and S. Tsukinoki for generating WER:MIZ1-GFP(HSPter) and PIN2:MIZ1-GFP(HSPter) transgenic plants and acknowledge the following funding agencies for financial support: D.D., J.F., R.A., T.N., D.W., S.T., C.S., S.M., M.R.O., L.R.B., R.D., O.J., J.K., J.R., T.B. and M.J.B. thank the Biological and Biotechnology Science Research Council (BBSRC) for responsive mode and CISB awards to the Centre for Plant Integrative Biology; D.W., C.S., S.M., M.R.O., J.K., T.P. and M.J.B. thank the European Research Council (ERC) for FUTUREROOTS project funding; L.R.B. thanks the Leverhulme Trust for an Early Career Fellowship; V.B., R.B. and L.D.V. are supported by grants of the Research Foundation Flanders (G.002911N). R.B. and M.J.B. thank the Royal Society for Newton and Wolfson Research Fellowship awards; R.A., T.I.B. and M.J.B. thank the FP7 Marie Curie Fellowship Scheme; R.D. thanks the Engineering and Physical Sciences Research Council, J.D. and M.J.B. thank the GII scheme; and V.B., R.B., L.D.V. and M.J.B. thank the Interuniversity Attraction Poles Programme (IUAP P7/29 “MARS”), initiated by the Belgian Science Policy Office. R.B.P. was funded by grants from the Knut and Alice Wallenberg Foundation. This work was also supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 22120004) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan to H.T., a Grant-in-Aid for Young Scientists (B) (No. 26870057) from the Japan Society for the Promotion of Science (JSPS) to A.K., a Grant-in-Aid for Scientific Research on Innovative Areas (No. 22120002) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan to A.N., a Grant-in-Aid for Scientific Research on Innovative Areas (No. 22120010) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan to Y.H. and the Funding Program for Next-Generation World-Leading Researchers (GS002) to Y.M. L.P. was financially supported by a scholarship from the Japanese government. T.-W.B. was financially supported by the Funding Program for Next-Generation World-Leading Researchers (GS002) and the Grant-in-Aid for Scientific Research on Innovative Areas (No. 22120004).
Supplementary Figures 1–8, Supplementary Methods, Supplementary References, Supplementary Table 1, Supplementary Notes 1 and 2.