Minisymposium 2: Signaling

Abs # 12004: Rac GTPases and hormone signaling

Presenter:	Tao, Li-zhen , ltao@biochem.umass.edu
Authors	Tao, Li-zhen  (A) (C)  Nibau, Candida  (B) (C)  Wu, Hen-ming  (A)   Cheung, Alice  (A) (B)
Affiliations:	(A): Dept. of Biochemistry and Molecular Biology, University of Massachusetts, Amherst MA 01003 (B): Plant Biology Graduate Program, University of Massachusetts, Amherst MA 01003 (C): Equal contribution

Plant hormones, acting coordinately, create an intricate network of signaling cues that must be correctly perceived by the plant in order to generate specific responses. How these signals are sensed and integrated is not yet clear. Rac-like small GTPases (Racs) are emerging as important components of these signaling pathways. We have shown previously that over-expressing a wild type and constitutively active (CA) mutant form of Racs activates auxin-responsive gene expression in transfected protoplasts and in transgenic plants. On the other hand, over-expressing dominant negative (DN) forms of Racs and Rac-negative regulators, or reducing the endogenous Rac level by using RNAi-mediated suppression, down-regulates auxin-induced gene expression. Moreover, endogenous auxin-inducible Aux/IAA genes are expressed in transgenic plants over-expressing Rac activity even without auxin induction and their auxin-induced expression is inhibited in plants with suppressed Rac activity. Auxin is known to induce gene derepression via regulated proteolysis of Aux/IAA repressor proteins. Using representative Aux/IAA proteins, we have observed that over-expression of Rac or Rac(CA) promotes 26S proteasome-dependent degradation of these Aux/IAA-LUC fusion proteins in transfected protoplasts. Over-expressing DN Racs prevents the auxin-induced decline in Aux/IAA-LUC. Together with the observations that increasing Rac or suppression of Rac expression in transgenic seedlings induce phenotypes that are similar to auxin-related defects, our results provide evidence that at least a subset of Rac GTPases serves an integral role in auxin-induced pathway of gene derepresion via regulated proteolysis of Aux/IAA proteins. In our studies on the activity of Racs from Arabidopsis on auxin signaling, we have observed that a group II Arac, Arac7, acts as a negative regulator of auxin signaling while having a positive effect in ABA-related processes. Arac7 is expressed in actively dividing areas and throughout lateral root development. Interestingly, auxin stimulates transcription from the Arac7 promoter while ABA represses it. Because ABA and auxin classically have opposing roles in growth and development, we propose that Arac7 acts as an integration point of these two pathways. Moreover, the fact that Arac7 expression is influenced by auxin and ABA provides a mechanism that may coordinate the two signaling pathways.