Poster: Tropisms

Abs # 408: Regulation of the auxin efflux component Arabidopsis thaliana AGRAVITROPIC 1 (AGR1) by protein phosphatases

Presenter:	Chen, Rujin , rchen@noble.org
Authors	Chen, Rujin  (A)   Shin, Heungsop  (A)   Shin, Hwa-Soo  (A)   Guo, Zibiao  (A)   Blancaflro, Elison B. (A)   Masson, Patrick H. (B)
Affiliations:	(A): The Samuel Roberts Noble Foundation (B): University of Wisconsin-Madison

Polar auxin transport has been linked to many auxin-regulated processes including the gravitropic response in roots. We has previously shown that loss-of-function mutations in the Arabidopsis thaliana AGRAVITROPC 1 (AGR1) gene impair the root gravitropic response and the root basipetal auxin transport in the mutants. AGR1 gene is specifically expressed in the root meristem, and distal and central elongation zones (DEZ and CEZ), but not in columella cells. AGR1 protein is localized to the basal plasma membrane of epidermal, cortical and lateral cap cells. To elucidate regulatory mechanisms that underlie the AGR1 function, we compared the expression pattern of an auxin sensor/reporter DR5::GUS and the root basipetal auxin transport between wild type and agr1-5 mutant plants pre-treated with various chemicals. Our studies indicated that a pre-treatment with exogenous-applied auxins readily induced the DR5::GUS gene expression in the CEZ cells of wild type plants, but not of the mutant, suggesting a role of AGR1 in this response. Furthermore, the auxin-induced DR5::GUS expression in CEZ cells was blocked by cantharidin or okadaic acid, inhibitors of protein phosphatases 1 and 2A, and by 2,3,5-triiodobenzoic acid (TIBA), a polar auxin transport inhibitor. Cantharidin or okadaic acid also reduced the root basipetal auxin transport and gravitropic response in a dosage-dependent manner, mimicking the phenotypes of the agr1-5 mutant. Taken together, our data suggest that protein phosphatases 1 and 2A, targeted by cantharidin and okadaic acid are positive regulators of the auxin efflux component AGR1.