Abstract
Root hydrotropism is a response to a moisture gradient and is considered to have an important role for drought avoidance. Previous physiological studies have elucidated that hydrostimulation immediately induces degradation of starch. Recently, we succeeded in identifying the genes required for hydrotropism, namely MIZ1 and MIZ2/GNOM. In contrast to GNOM, which encodes an ARF-GEF, the function of MIZ1 is unknown. To gain a new insight to the function of MIZ1, we generated the transgenic lines over-expressing MIZ1, and analyzed genetic and physiological relationship between over-expressed MIZ1 and the above-mentioned factors that regulate root hydrotropism. Upon hydrostimulation, rapid starch degradation was observed in both miz1 and miz2. Furthermore, we found that miz2 mutation was epistatic to MIZ1 over-expression. This observation was also confirmed by the fact that the effect of over-expressed MIZ1 was completely nullified by treating MIZ1 over-expressors with BFA. These results suggest that following the rapid degradation of starch, MIZ1 functions upstream of MIZ2/GNOM-mediated vesicle transport.
