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PKS1 is involved in red-light-based positive phototropism in Arabidopsis roots.   M.L. Molas, J.Z. Kiss Dept. of Botany, Miami University, Oxford, Ohio, 45056 

   Plants direct their growth in response to unidirectional light, while aerial parts curve toward a source of light (i.e. positive phototropism), roots typically grow away from the source of light (i.e. negative phototropism). Recently we have shown a positive response to red light (RL) in Arabidopsis roots that is mediated by phytochrome A and B (PHYA and PHYB). Upon light stimulation, PHYA and PHYB phosphorylate a phytochrome kinase substrate (PKS1) in the cytoplasm. At the same time, the active forms of PHYA and PHYB migrate to the nucleus where they interact with transcription factors to regulate the transcription of light-dependent genes. In this study, we investigated the interaction of PHYA and PHYB with PKS1 in the positive phototropic response to RL in roots. Using a high-resolution computer-based feedback system, we studied the phenotypic response of phyA, phyB, pks1, phyA pks1 and phyB pks1 null mutants and PKS1-overexpressing line after exposure to unidirectional RL. PHYA was essential for a positive response to RL, and PKS1 attenuated PHYA action. Our results suggest that the PHYA mode of action involved at least two pathways: a PKS1-independent and PKS1-dependent. The PKS1-dependent pathway elicits a signaling cascade in the cytosol that counteracts the PKS1-independent pathway. Moreover, PKS1 appears to promote a negative response to RL. In addition, PHYB is also necessary for a positive curvature, but PKS1 seems not to be directly involved in PHYB signaling in root phototropism. Thus, PKS1 plays a role in downstream signal transduction of PHYA in red-light-based root phototropism. As a working model, we propose that PKS1 counteracts PHYA action by stimulating a negative response to RL and that PHYB regulates phototropic response independent of PKS1.