MIZU-KUSSEI1 (MIZ1) and GNOM/MIZ2 not only control positive hydrotropism but also phototropism in Arabidopsis roots.

In response to unilateral blue light illumination, roots of some plant species such as Arabidopsis thaliana exhibit negative phototropism (bending away from light), which is important for light avoidance in nature. MIZU-KUSSEI1 (MIZ1) and GNOM/MIZ2 are essential for positive hydrotropism (i.e., in the presence of a moisture gradient, root bending towards greater water availability). Intriguingly, mutations in these genes also cause a substantial reduction in phototropism. Here, we examined whether Arabidopsis root tissue expression domains required for MIZ1- and GNOM/MIZ2-regulated hydrotropism are also required for phototropism. The attenuated phototropic response of miz1 roots was completely restored when a functional MIZ1-GFP fusion was expressed in the cortex of the root elongation zone but not in other tissues such as root cap, meristem, epidermis, or endodermis. The hydrotropic defect and reduced phototropism of miz2 roots were restored by GNOM/MIZ2 expression in either the epidermis, cortex, or stele, but not in the root cap or endodermis. Thus, root tissues that regulate MIZ1- and GNOM/MIZ2-dependent hydrotropism also regulate phototropism. These results suggest that MIZ1- and GNOM/MIZ2-mediated pathways are, at least in part, shared by hydrotropic and phototropic responses in Arabidopsis roots.