Abstract
In higher plants, blue light (BL) phototropism is primarily controlled by the phototropins, which are also involved in stomatal movement and chloroplast relocation. These photoresponses are mediated by two phototropins, phot1 and phot2. Phot1 mediates responses with higher sensitivity than phot2, and phot2 specifically mediates chloroplast avoidance and dark positioning responses. Here, we report the isolation and characterization of a Nonphototropic seedling1 (Nps1) mutant of tomato (Solanum lycopersicum). The mutant is impaired in low-fluence BL responses, including chloroplast accumulation and stomatal opening. Genetic analyses show that the mutant locus is dominant negative in nature. In dark-grown seedlings of the Nps1 mutant, phot1 protein accumulates at a highly reduced level relative to the wild type and lacks BL-induced autophosphorylation. The mutant harbors a single glycine-1484-to-alanine transition in the Hinge1 region of a phot1 homolog, resulting in an arginine-to-histidine substitution (R495H) in a highly conserved A'α helix proximal to the light-oxygen and voltage2 domain of the translated gene product. Significantly, the R495H substitution occurring in the Hinge1 region of PHOT1 abolishes its regulatory activity in Nps1 seedlings, thereby highlighting the functional significance of the A'α helix region in phototropic signaling of tomato.
Highlights
In higher plants, blue light (BL) phototropism is primarily controlled by the phototropins, which are involved in stomatal movement and chloroplast relocation
These responses include chloroplast movements (Sakai et al, 2001), nuclear positioning (Iwabuchi et al, 2007), stomatal opening (Kinoshita et al, 2001), sun tracking (Inoue et al, 2008b), leaf expansion (Ohgishi et al, 2004), leaf movements (Inoue et al, 2005), leaf photomorphogenesis (Kozuka et al, 2011), leaf flattening (Sakamoto and Briggs, 2002), and the rapid inhibition of the growth of etiolated hypocotyls (Folta and Spalding, 2001). While both phot1 and phot2 overlap in function in regulating phototropism, chloroplast accumulation, leaf expansion, and stomatal opening, they exhibit differential photosensitivity to BL, where phot1 is more sensitive to low-fluence BL than phot2. Both phot1 and phot2 redundantly regulate the chloroplast accumulation toward low-fluence BL, and phot2 exclusively regulates the chloroplast avoidance from high-fluence BL (Jarillo et al, 2001; Kagawa et al, 2001), while phot1 solely mediates the rapid inhibition of the elongation of etiolated hypocotyls (Folta and Spalding, 2001)
The Nonphototropic seedling1 (Nps1) mutant was isolated in a screen for nonphototropic tomato seedlings exposed to prolonged (12-h) unilateral low-fluence BL (Fig. 1A)
Summary
Blue light (BL) phototropism is primarily controlled by the phototropins, which are involved in stomatal movement and chloroplast relocation. Physiological analyses with Arabidopsis mutants lacking phot and phot have revealed that, in addition to regulating the hypocotyl curvature of seedlings toward BL (Huala et al, 1997; Christie et al, 1998), phototropins regulate a diverse range of responses in flowering plants (Christie and Murphy, 2013; Hohm et al, 2013) These responses include chloroplast movements (Sakai et al, 2001), nuclear positioning (Iwabuchi et al, 2007), stomatal opening (Kinoshita et al, 2001), sun tracking (Inoue et al, 2008b), leaf expansion (Ohgishi et al, 2004), leaf movements (Inoue et al, 2005), leaf photomorphogenesis (Kozuka et al, 2011), leaf flattening (Sakamoto and Briggs, 2002), and the rapid inhibition of the growth of etiolated hypocotyls (Folta and Spalding, 2001). Mutational studies revealed that the photosensory N-terminal domain of phototropin acts as a kinase inhibitor, where the LOV2 domain inhibits the activity of kinase domain by binding to it, and BL exposure is required for the dissociation of the LOV2 domain, enabling phosphorylation of the kinase domain (Matsuoka and Tokutomi, 2005; Jones et al, 2007)
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