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CELL BIOLOGY AND SIGNAL TRANSDUCTION

PHYTOCHROME KINASE SUBSTRATE1 Regulates Root Phototropism and Gravitropism^{1,[C],[W],[OA]}

IFEVA, Facultad de Agronomía, Universidad de Buenos Aires and Consejo Nacional de Investigaciones Científicas y Técnicas, 1417-Buenos Aires, Argentina (H.E.B., S.N.D.S., J.J.C.); Center for Integrative Genomics, University of Lausanne, CH–1015 Lausanne, Switzerland (I.S., C.F.); and Department of Molecular Biology, University of Geneva, CH–1211 Geneva 4, Switzerland (A.B.-H.)

Light promotes the expression of PHYTOCHROME KINASE SUBSTRATE1 (PKS1) in the root of Arabidopsis thaliana, but the function of PKS1 in this organ is unknown. Unilateral blue light induced a negative root phototropic response mediated by phototropin 1 in wild-type seedlings. This response was absent in pks1 mutants. In the wild type, unilateral blue light enhanced PKS1 expression in the subapical region of the root several hours before bending was detectable. The negative phototropism and the enhanced PKS1 expression in response to blue light required phytochrome A (phyA). In addition, the pks1 mutation enhanced the root gravitropic response when vertically oriented seedlings were placed horizontally. The negative regulation of gravitropism by PKS1 occurred even in dark-grown seedlings and did not require phyA. Blue light also failed to induce negative phototropism in pks1 under reduced gravitational stimulation, indicating that the effect of pks1 on phototropism is not simply the consequence of the counteracting effect of enhanced gravitropism. We propose a model where the background level of PKS1 reduces gravitropism. After a phyA-dependent increase in its expression, PKS1 positively affects root phototropism and both effects contribute to negative curvature in response to unilateral blue light.

² These authors contributed equally to the article.

³ Present address: Cátedra de Química Orgánica y Biológica, Facultad de Ciencias Agrarias, Universidad Nacional de Cuyo, Almirante Brown 500, 5505 Chacras de Coria, Mendoza, Argentina.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Jorge J. Casal (casal{at}ifeva.edu.ar).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.107.106468

^* Corresponding author; e-mail casal{at}ifeva.edu.ar.

Received August 7, 2007; accepted October 31, 2007; published November 16, 2007.

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