ARG1 (Altered Response to Gravity) encodes a DnaJ-like protein that potentially interacts with the cytoskeleton

Gravitropism allows plant organs to direct their growth at a specific angle from the gravity vector, promoting upward growth for shoots and downward g rowth for roots. Little is known about the mechanisms underlying gravitropi c signal transduction. We found that mutations in the ARG1 locus of Arabido psis thaliana alter root and hypocotyl gravitropism without affecting photo tropism, root growth responses to phytohormones or inhibitors of auxin tran sport, or starch accumulation. The positional cloning of ARG1 revealed a Dn aJ-like protein containing a coiled-coil region homologous to coiled coils found in cytoskeleton-interacting proteins. These data suggest that ARG1 pa rticipates in a gravity-signaling process involving the cytoskeleton. A com bination of Northern blot studies and analysis of ARG1-GUS fusion-reporter expression in transgenic plants demonstrated that ARG1 is expressed in all organs. Ubiquitous ARG1 expression in Arabidopsis and the identification of an ortholog in Caenorhabditis elegans suggest that ARG1 is involved in oth er essential processes.