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Published on April 4, 2008; 10.1104/pp.108.118166

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Received February 21, 2008
Accepted March 25, 2008

PHYTOCHROME KINASE SUBSTRATE 4 modulates phytochrome-mediated control of hypocotyl growth orientation

Isabelle Schepens , Hernan E. Boccalandro , Chitose Kami , Jorge J. Casal , and Christian Fankhauser *

Center for Integrative Genomics, University of Lausanne, Genopode Building, 1015 Lausanne, Switzerland; IFEVA, Facultad de Agronomia, Universidad de Buenos Aires, and Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), Av. San Martin 4453, 1417-Buenos Aires, Argentina

Gravity and light are major factors shaping plant growth. Light perceived by phytochromes leads to seedling de-etiolation, which includes the deviation from vertical hypocotyl growth and promotes hypocotyl phototropism. These light responses enhance survival of young seedlings during their emergence from the soil. The Phytochrome Kinase Substrate (PKS) family is composed of four members in Arabidopsis: PKS1-PKS4. Here we show that PKS4 is a negative regulator of both phytochrome A and B-mediated inhibition of hypocotyl growth and promotion of cotyledon unfolding. Most prominently, pks4 mutants show abnormal phytochrome-modulated hypocotyl growth orientation. In dark-grown seedlings hypocotyls change from the original orientation defined by seed position to the upright orientation defined by gravity and light reduces the magnitude of this shift. In older seedlings with the hypocotyls already oriented by gravity, light promotes the deviation from vertical orientation. Based on the characterization of pks4 mutants we propose that PKS4 inhibits changes in growth orientation under red or far-red light. Our data suggest that in these light conditions PKS4 acts as an inhibitor of asymmetric growth. This hypothesis is supported by the phenotype of PKS4 over-expressers. Together with previous findings, these results indicate that the PKS family plays important functions during light-regulated tropic growth responses.






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