Plants respond to unilateral light by growing toward the light source (in the case of shoots) or away from the light source (in the case of roots). This response, known as phototropism, has an action spectrum with a broad peak in the blue region, and a maximum at approximately 450 nm. A biochemical approach isolated several membrane-bound proteins whose phosphorylation was required for the phototropic response, implicating a common signal transduction component in the plant’s response to unilateral light.
Using a mutant screen in the model plant Arabidopsis, several novel mutants were isolated that showed non-phototropic hypocotyls (the nph mutants). Cloning and characterization of the nph1 mutant revealed it to encode a polypeptide with several intriguing domains. On one end of the protein was a kinase domain, responsible for phosphorylating a target substrate, and on the other end of the protein was a LOV domain. LOV, (for light, oxygen, and voltage sensing), is an evolutionarily conserved domain found in a number of organisms and involved in sensing one of these physical cues. The protein encoded by NPH1, which has been named PHOTOTROPIN1 (phot1), also binds a flavin mononucleotide that acts as a chromophore.
PHOT1 protein has been localized to the plasma membrane in cells of dark-adapted hypocotyl tissue. Upon exposure to blue light, PHOT1 undergoes autophosphorylation and is relocalized to the cytoplasm. One role of PHOT1 appears to involve interacting with the auxin efflux carrier ABCB19 and causing its inhibition through phosphorylation. When ABCB19 is inhibited, auxin accumulates in the tip of the stem where it undergoes lateral transport to the shaded side of the stem through an efflux carrier.