Root gravitropism: a complex response to a simple stimulus?

Roots avoid depleting their immediate environment of essential nutrients by continuous growth. Root growth is directed by environmental cues, includin g gravity. Gravity sensing occurs mainly in the columella cells of the root cap. Upon reorientation within the gravity field, the root-cap amyloplasts sediment, generating a physiological signal that promotes the development of a curvature at the root elongation zones. Recent molecular genetic studi es in Arabidopsis have allowed the identification of genes that play import ant roles in root gravitropism. Among them, the ARG1 gene encodes a DnaJ-li ke protein involved in gravity signal transduction, whereas the AUX1 and AG RI genes encode proteins involved in polar auxin transport. These studies h ave important implications for understanding the intra- and inter-cellular signaling processes that underlie root gravitropism.