| Gravitropism is a process that allows plant organs to use gravity as a growth guide. The ARG1 DnaJ-like protein is required for normal root and hypocotyl gravitropism. I have shown that expression of ARG1 in gravity-perceiving cells is sufficient to rescue the gravitropic defects of arg1-2. I also demonstrated that arg1-2 columella cells maintain a constant cytosolic pH in response to gravistimulation, unlike the wild type. Also, mutant roots appear to accumulate a larger amount of auxin in an expanded domain of the cap, and lack any detectable gravity-induced lateral auxin gradient across the cap. These observations indicate that ARG1 functions in an early step of gravity signal transduction, potentially by regulating specific transport processes at the plasma membrane.; Using GFP fusion and immunolocalization experiments, I showed that ARG1 is a peripheral membrane protein that localizes in cellular components that contribute to vesicular trafficking between endosomal compartments and the plasma membrane. This pathway may be shared with the PIN auxin efflux carriers. Thus, it appears that ARG1 may mediate gravity signaling by affecting activity and/or localization of these proteins.; Two other genes, ARG1-LIKE 1 (ARL1) and ARL2, encode proteins that share similarity with ARG1 throughout their length. Mutations at ARL2 but not ARL1 result in gravitropic defects in the root and the hypocotyl that resemble those of arg1-2. Double and triple mutant analyses reveal the lack of genetic interactions between ARG1 and its paralogs. However, genetic interactions were observed between ARL2 and PHOSPHOGLUCOMUTASE (PGM), which is involved in the synthesis of amyloplastidial starch. These results indicate that ARG1 and ARL2 participate in the same gravity-signaling pathway, which may be different from the PGM pathway. |
