Overexpression of MIZU-KUSSEI1 enhances the root hydrotropic response by retaining cell viability under hydrostimulated conditions in Arabidopsis thaliana.

Because of their sessile nature, plants evolved several mechanisms to tolerate or avoid conditions where water is scarce. The molecular mechanisms contributing to drought tolerance have been studied extensively, whereas the molecular mechanism underlying drought avoidance is less understood despite its importance. Several lines of evidence showed that the roots sense the moisture gradient and grow toward the wet area: so-called hydrotropism. We previously identified MIZU-KUSSEI (MIZ) 1 and MIZ2/GNOM as genes responsible for this process. To gain new insight into the molecular mechanism of root hydrotropism, we generated overexpressors of MIZ1 (MIZ1OEs) and analyzed their hydrotropic response. MIZ1OEs had a remarkable enhancement of root hydrotropism. Furthermore, a greater number of MIZ1OE root cells remained viable under hydrostimulated conditions than those of the wild type, which might contribute to retaining root growth under hydrostimulated conditions. Although overexpression of MIZ1 also caused a slight decrease in the root gravitropic response, it was not attributable to the enhanced hydrotropic response. In addition, miz2 mutation or the auxin response inhibitor nullified the enhanced hydrotropic response in MIZ1OEs. Furthermore, the expression of MIZ1 did not alter the expression of typical genes involved in drought tolerance. These results suggest that MIZ1 positively regulates hydrotropism at an early stage and its overexpression results in an enhancement of signal transduction unique to root hydrotropism to increase the degree of hydrotropic root bending.