Abstract
Phototropins (phot1 and phot2) are autophosphorylating blue-light receptor kinases that mediate blue-light responses such as phototropism, chloroplast accumulation, and stomatal opening in Arabidopsis (Arabidopsis thaliana). Only phot2 induces the chloroplast avoidance response under strong blue light. The serine (Ser) residues of the kinase activation loop in phot1 are autophosphorylated by blue light, and autophosphorylation is essential for the phot1-mediated responses. However, the role of autophosphorylation in phot2 remains to be determined. In this study, we substituted the conserved residues of Ser-761 and Ser-763 with alanine (S761A S763A) in the phot2 activation loop and analyzed their function by investigating the phot2-mediated responses after the transformation of phot1 phot2 double mutant with this mutant phot2 gene. Transgenic plants expressing the mutant phot2 protein exhibited impaired responses in chloroplast movement, stomatal opening, phototropic bending, leaf flattening, and plant growth; and those expressing phot2 with S761D S763D mutations showed the normal responses. Substitution of both Ser-761 and Ser-763 with alanine in phot2 did not significantly affect the kinase activity in planta. From these results, we conclude that phosphorylation of Ser-761 and Ser-763 in the activation loop may be a common primary step for phot2-mediated responses.
Highlights
Phototropins are autophosphorylating blue-light receptor kinases that mediate blue-light responses such as phototropism, chloroplast accumulation, and stomatal opening in Arabidopsis (Arabidopsis thaliana)
In phot1, we previously demonstrated that the autophosphorylation of the Ser-849 and Ser-851 in the kinase activation loop is required for the phot1-mediated responses mentioned above
763 in the activation loop of phot2 (Fig. 2A), and we suspected that phototropin-mediated responses such as phototropism, chloroplast accumulation, stomatal opening, and leaf flattening may require the phosphorylation of the two Ser residues but the responses of phot2-specific chloroplast avoidance and dark positioning may not
Summary
Phototropins (phot and phot2) are autophosphorylating blue-light receptor kinases that mediate blue-light responses such as phototropism, chloroplast accumulation, and stomatal opening in Arabidopsis (Arabidopsis thaliana). Transgenic plants expressing the mutant phot protein exhibited impaired responses in chloroplast movement, stomatal opening, phototropic bending, leaf flattening, and plant growth; and those expressing phot with S761D S763D mutations showed the normal responses Substitution of both Ser-761 and Ser-763 with alanine in phot did not significantly affect the kinase activity in planta. Phototropins (phot and phot2) are plant-specific blue-light receptors that regulate a wide range of bluelight responses in Arabidopsis (Arabidopsis thaliana), these including phototropism (Liscum and Briggs, 1995; Huala et al, 1997), chloroplast accumulation and avoidance responses (Jarillo et al, 2001; Kagawa et al, 2001; Sakai et al, 2001), nuclear positioning (Iwabuchi et al, 2007), stomatal opening (Kinoshita et al, 2001; Doi et al, 2004), rapid inhibition of hypocotyl growth (Folta and Spalding, 2001), leaf flattening (Sakamoto and Briggs, 2002; Takemiya et al, 2005), Phototropins contain two light, oxygen, and voltage (LOV) domains, named LOV1 and LOV2, in the N terminus, and a canonical Ser/Thr kinase domain in the C terminus (Huala et al, 1997). We provided information on in vivo autophosphorylation sites in phot
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