Poster: Signaling, cell-to-cell

Abs # 431: Plant Rac-like GTPases are activated by auxin and mediate auxin-responsive gene expression

Presenter:	Tao, Li-zhen , tao@nsm.umass.edu
Authors	Tao, Li-zhen  (A)   Cheung, Alice  (A) (B)  Nibau, Candida  (C)   Wu, Hen-ming  (A) (B)
Affiliations:	(A): Department of Biochemistry and Molecular Biology, University of Massachusetts (B): Molecular and Cellular Biology Program (C): Plant Biology Graduate Program

The Rho family of GTPases in plants is emerging as important signaling molecules that mediate a wide variety of external stimuli to intracellular signaling pathways. Using pull-down assays based on PBD from PAK that specifically binds activated Racs, we have obtained evidence that auxin activates plant Rac GTPases. Activated Racs in turn stimulate auxin-responsive gene expression. In particular, we show that overexpressing a wild-type tobacco Rac GTPase, NtRac1, and its constitutively active mutant form activates auxin-responsive gene expression in transfected protoplasts and in transgenic plants. On the other hand, overexpressing dominant-negative NtRac1 and Rac-negative regulators such as AtRac-GDI and AtRac-GAP, or reducing the endogenous NtRac1 level by using NtRac-RNAi, suppresses auxin-induced gene expression. RNA blot analysis shows that endogenous auxin-inducible Aux/IAA genes are also upregulated in NtRac1(CA) transgenic plants and downregulated in NtRac1(DN) and RNAi transgenic plants. Auxin is known to induce gene derepression via regulated proteolysis of Aux/IAA repressor proteins. We have observed that overexpression of NtRac1(CA) stimulates the instability of two representative Aux/IAA-luc fusion proteins in transfected protoplasts. Expressing NtRac1(DN) prevents the auxin-induced decline in Aux/IAA-luc. These observations provide evidence that auxin-induced proteolysis of Aux/IAA proteins requires active Rac GTPases. Overexpression of NtRac1 or suppression of its expression in transgenic seedlings induces phenotypes that are similar to auxin-related defects. Take together, our results show that a subset of plant Rac GTPases functions in mediating the auxin signal to downstream responsive genes via regulated proteolysis of AUX/IAA proteins.