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Published on November 3, 2006; 10.1104/pp.106.089839

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Received September 14, 2006
Accepted October 30, 2006

Physiological Roles of the LOV Domains of Phototropin 1 and Phototropin 2 in Arabidopsis

Hae-Young Cho , Tong-Seung Tseng , Eirini Kaiserli , Stuart Sullivan , John M. Christie , and Winslow R. Briggs *

Department of Plant Biology, Carnegie Institution of Washington, Stanford, California
Department of Plant Biology, Carnegie Institution of Washington, Stanford, California; Plant Science Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, University Avenue, Glasgow G12 8QQ, Scotland UK
Plant Science Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, University Avenue, Glasgow G12 8QQ, Scotland UK

* Corresponding author; email: briggs{at}stanford.edu.

Phototropins (phot1 and phot2) are plant blue-light receptors that mediate phototropism, chloroplast movement, stomatal opening, rapid inhibition of growth of etiolated seedlings, and leaf expansion in Arabidopsis thaliana. Their N-terminal region contains two LOV (Light, Oxygen, or Voltage) domains, which bind flavin mononucleotide (FMN) and form a covalent adduct between a conserved cysteine and the FMN chromophore upon photoexcitation. The C-terminal region contains a serine/threonine kinase domain which catalyses blue light-activated autophosphorylation. Here, we have transformed the phot1 phot2 (phot1-5 phot2-1) double mutant with PHOT expression constructs driven by the CaMV35S promoter. These constructs encode either wild-type phototropin or phototropin with one or both LOV-domain cysteines mutated to block their photochemistry. We selected multiple lines in each of the eight resulting categories of transformants for further physiological analyses. Specifically, we investigated whether LOV1 and LOV2 serve the same or different functions for phototropism and leaf expansion. Our results show that the LOV2 domain of phot1 plays a major role in phototropism and leaf expansion, as does the LOV2 domain of phot2. No complementation of phototropism or leaf expansion was observed for the LOV1 domain of phot1. However phot2 LOV1 was unexpectedly found to complement phototropism to a considerable level. Similarly, transformants carrying a PHOT transgene with both LOV domains inactivated developed strong curvatures toward high fluence-rate blue light. However, we found that the phot2-1 mutant is leaky and produces a small level of full-length phot2 protein. In vitro experiments indicate that cross-phosphorylation can occur between functional phot2 and inactivated phot1 molecules. Such a mechanism may occur in vivo and therefore account for the functional activities observed in the PHOT transgenics with both lov domains inactivated. The implications of this mechanism with respect to phototropin function are discussed.




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