Phototropins are autophosphorylating protein kinases of plantspecific blue light receptors. They regulate various blue light responses, including phototropism, chloroplast movements, hypocotyl growth inhibition, leaf flattening, and stomatal opening. However, the physiological role of autophosphorylation remains unknown. Here, we identified phosphorylation sites of Ser or Thr in the N terminus, Hinge1 region, kinase domain, and C terminus in Arabidopsis phototropin1 (phot1) by liquid chromatographytandem mass spectrometry in vivo. We substituted these Ser or Thr residues with Ala in phot1 and analyzed their functions by inspecting the phot1-mediated responses of stomatal opening, phototropism, chloroplast accumulation, and leaf flattening after the transformation of the phot1 phot2 double mutant. Among these sites, we found that autophosphorylation of Ser-851 in the activation loop of the kinase domain was required for the responses mentioned above, whereas the phosphorylation of the other Ser and Thr, except those in the activation loop, was not. Ser-849 in the loop may have an additional role in the responses. Immunological analysis revealed that Ser-851 was phosphorylated rapidly by blue light in a fluence-dependent manner and dephosphorylated gradually upon darkness. We conclude that autophosphorylation of Ser-851 is a primary step that mediates signaling between photochemical reaction and physiological events.protein kinase ͉ stomata ͉ photoreceptor ͉ light signaling P hototropins (phot1 and phot2) mediate multiple blue light responses in Arabidopsis, including phototropism, chloroplast movements, leaf flattening, leaf positioning, stomatal opening, and rapid inhibition of hypocotyl growth (1-5). These responses enhance photosynthesis and optimize plant growth, particularly under weak light (6). Under strong light, phot2 induces a chloroplast-avoidance response to prevent photodamage to photosynthetic machinery (7). Phototropins, therefore, are essential proteins for the survival of higher plants and the extension of their living areas, particularly under ever-changing environments of light.Phototropins are blue light receptor protein kinases with two light, oxygen, voltage (LOV) domains in the N terminus and a Ser/Thr protein kinase in the C terminus (8). The LOV domains possess noncovalent binding sites for the chromophore flavin mononucleotide (FMN) and cysteine residues (2, 9-11) and produce a covalent cysteinyl adduct with FMN when LOVs are illuminated with blue light (12, 13). The cysteinyl adduct formation is the primary photochemical process; the adduct formation induces conformational changes in the LOV2 domain (14, 15) and J␣-helix (16) and leads to phototropin phosphorylation and subsequent physiological processes (17, 18). Phototropin phosphorylation by blue light is demonstrated to be autophosphorylation through the use of recombinant proteins of PHOT1 and PHOT2 expressed in insect cells (2, 9, 17).Phototropin was initially found as a plasma membraneassociated phosphorylated protein in etiolate...

Phototropins (phot1 and phot2) are plant-specific blue light receptors for phototropism, chloroplast movement, leaf expansion, and stomatal opening. All these responses are thought to optimize photosynthesis by helping to capture light energy efficiently, reduce photodamage, and acquire CO 2 . However, experimental evidence for the promotion of plant growth through phototropins is lacking. Here, we report dramatic phototropin-dependent effects on plant growth. When plants of Arabidopsis thaliana wild type, the phot1 and phot2 mutants, and the phot1 phot2 double mutant were grown under red light, no significant growth differences were observed. However, if a very low intensity of blue light (0.1 mmol m ÿ2 s ÿ1 ) was superimposed on red light, large increases in fresh weight up to threefold were found in those plants that carried functional PHOT1 genes. When the intensity of blue light was increased to 1 mmol m ÿ2 s ÿ1 , the growth enhancement was also found in the phot1 single mutant, but not in the double mutant, indicating that phot2 mediated similar responses as phot1 with a lower sensitivity. The effects occurred under low photosynthetically active radiation in particular. The wellknown physiological phototropin-mediated responses, including chloroplast movement, stomatal opening, and leaf expansion, in the different lines tested indicated an involvement of these responses in the blue light-induced growth enhancement. We conclude that phototropins promote plant growth by controlling and integrating a variety of responses that optimize photosynthetic performance under low photosynthetically active radiation in the natural environment.

Appropriate leaf positioning is essential for optimizing photosynthesis and plant growth. However, it has not been elucidated how green leaves reach and maintain their position for capturing light. We show here the regulation of leaf positioning under blue light stimuli. When 1-week-old Arabidopsis seedlings grown under white light were transferred to red light (25 micromol m(-2) s(-1)) for 5 d, new petioles that appeared were almost horizontal and their leaves were curled and slanted downward. However, when a weak blue light from above (0.1 micromol m(-2) s(-1)) was superimposed on red light, the new petioles grew obliquely upward and the leaves were flat and horizontal. The leaf positioning required both phototropin1 (phot1) and nonphototropic hypocotyl 3 (NPH3), and resulted in enhanced plant growth. In an nph3 mutant, neither optimal leaf positioning nor leaf flattening by blue light was found, and blue light-induced growth enhancement was drastically reduced. When blue light was increased from 0.1 to 5 micromol m(-2) s(-1), normal leaf positioning and leaf flattening were induced in both phot1 and nph3 mutants, suggesting that phot2 signaling became functional and that the signaling was independent of phot1 and NPH3 in these responses. When plants were irradiated with blue light (0.1 micromol m(-2) s(-1)) from the side and red light from above, the new leaves became oriented toward the source of blue light. When we transferred these plants to both blue light and red light from above, the leaf surface changed its orientation to the new blue light source within a few hours, whereas the petioles initially were unchanged but then gradually rotated, suggesting the plasticity of leaf positioning in response to blue light. We showed the tissue expression of NPH3 and its plasma membrane localization via the coiled-coil domain and the C-terminal region. We conclude that NPH3-mediated phototropin signaling optimizes the efficiency of light perception by inducing both optimal leaf positioning and leaf flattening, and enhances plant growth.