Roots show tropisms in response to environmental cues such as gravity, light, moisture gradient. Each tropism is affected by the other stimulations. For instance, graviresponse interferes root hydrotropism and phototropism. We have identified MIZ1 gene as a regulator of hydrotropism. However, regulation and function of MIZ1 have been unknown. Here, we showed that light and ABA signal affect hydrotropism via modulation of MIZ1 expression. We found that hydrotropism was reduced in dark-grown roots when compared with light-grown ones. In addition, we found that light signal was necessary for MIZ1-GFP expression at the root cap. Moreover, both of hydrotropism and MIZ1 expression are decreased in hy5 mutant. Because HY5 regulates light-dependent transcription, the result suggests that HY5-dependent light signaling is required for expression of MIZ1 and thus for appropriate hydrotropic response. It is also known that HY5 is involved in ABA signaling, and that ABA is an important player for hydrotropism. To assess the possibility that HY5 also regulates ABA-mediated MIZ1 expression, we analyzed MIZ1-GFP level in ABA-treated hy5 roots. The result shows that ABA could recover MIZ1-GFP level similar to that of WT. Moreover, ABA-treated hy5 showed normal hydrotropic response as did ABA-treated WT. These results suggest that HY5 is not involved in ABA-dependent regulation of hydrotropism which acts even in darkness. These machineries may profit for plants to acquire water adequately.
