Abstract
In Arabidopsis (Arabidopsis thaliana), the blue light photoreceptor phototropins (phot1 and phot2) fine-tune the photosynthetic status of the plant by controlling several important adaptive processes in response to environmental light variations. These processes include stem and petiole phototropism (leaf positioning), leaf flattening, stomatal opening, and chloroplast movements. The PHYTOCHROME KINASE SUBSTRATE (PKS) protein family comprises four members in Arabidopsis (PKS1-PKS4). PKS1 is a novel phot1 signaling element during phototropism, as it interacts with phot1 and the important signaling element NONPHOTOTROPIC HYPOCOTYL3 (NPH3) and is required for normal phot1-mediated phototropism. In this study, we have analyzed more globally the role of three PKS members (PKS1, PKS2, and PKS4). Systematic analysis of mutants reveals that PKS2 (and to a lesser extent PKS1) act in the same subset of phototropin-controlled responses as NPH3, namely leaf flattening and positioning. PKS1, PKS2, and NPH3 coimmunoprecipitate with both phot1-green fluorescent protein and phot2-green fluorescent protein in leaf extracts. Genetic experiments position PKS2 within phot1 and phot2 pathways controlling leaf positioning and leaf flattening, respectively. NPH3 can act in both phot1 and phot2 pathways, and synergistic interactions observed between pks2 and nph3 mutants suggest complementary roles of PKS2 and NPH3 during phototropin signaling. Finally, several observations further suggest that PKS2 may regulate leaf flattening and positioning by controlling auxin homeostasis. Together with previous findings, our results indicate that the PKS proteins represent an important family of phototropin signaling proteins.
Highlights
In Arabidopsis (Arabidopsis thaliana), the blue light photoreceptor phototropins fine-tune the photosynthetic status of the plant by controlling several important adaptive processes in response to environmental light variations
Since PKS1/2/4 are required for phototropism and PKS1 is associated with phot1 in vivo (Lariguet et al, 2006; Boccalandro et al, 2008), we used a genetic approach and analyzed leaf flattening, leaf positioning, chloroplast movements, and stomatal opening in the pks mutants to determine whether members of the PHYTOCHROME KINASE SUBSTRATE (PKS) gene family are global regulators of phototropin signaling
Analysis of double and triple nph3pks mutants revealed a predominant role for PKS2 and a minor role for PKS1, while PKS4 did not seem to contribute to leaf flattening (Supplemental Fig. S1, A and C)
Summary
In Arabidopsis (Arabidopsis thaliana), the blue light photoreceptor phototropins (phot and phot2) fine-tune the photosynthetic status of the plant by controlling several important adaptive processes in response to environmental light variations. These processes include stem and petiole phototropism (leaf positioning), leaf flattening, stomatal opening, and chloroplast movements. In Arabidopsis (Arabidopsis thaliana), phototropin (phot1) and its paralog phot were discovered as primary photoreceptors for blue light-induced hypocotyl phototropism and for high light-induced chloroplast avoidance movements, respectively (Liscum and Briggs, 1995; Huala et al, 1997; Jarillo et al, 2001; Kagawa et al, 2001). Phot mediates the chloroplast accumulation, leaf positioning, and phototropic responses under very low light (Demarsy and Fankhauser, 2009). Phot and phot act redundantly over a broad range of light intensity (Kinoshita et al, 2001; Doi et al, 2004)
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