Analysis of molecular mechanism for root hydrotropism that plays a role in plant adaptation to drought conditions

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We attempted to reveal molecular mechanism of hydrotropism in Arabidopsis roots by analyzing the functions of MIZ1 and MIZ2 that play essential roles in hydrotropism. First, we generated the overexpressors of MIZ1 (MIZ1OEs)and analyzed their phenotypes. We found that MIZ1OEs showed an enhanced hydrotropism and a reduced lateral root formation. These phenomena likely resulted form the decreased level of auxin content in MIZ1OEs. Also, it was shown that the miz2 mutation is epistatic to MIZ1 overexpression. MIZ1-GFP introduced into miz1 mutant complemented the mutant phenotype and was detected at high levels in cortical cells and lateral root cap cells. MIZ1-GFP was fractionated into a soluble protein fraction and an endoplasmic reticulum (ER)membrane fraction, where it was bound to the surface of the ER membrane at the cytosolic side. Furthermore, we succeeded in isolating proteins that bind MIZ1 by immunoprecipitation method and LC-MS/MS analysis. In addition, we obtained mutants of suppressors of both miz1 and miz2. This study showed that light and ABA signaling are independently integrated into MIZ1 expression and regulate hydrotropism. Then, we demonstrated that the MIZ1-regulated hydrotropism of lateral roots plays an important role in root system development in soil and contributes to drought avoidance, which results in better survival and yield under the water-limited conditions.