Phototropins, Other Photoreceptors, and Associated Signaling: The Lead and Supporting Cast in the Control of Plant Movement Responses

Abstract

Publisher Summary This chapter focuses on light-driven plant movements that occur in response to the perception of changes in light direction, intensity, and/or duration within the near-ultraviolet (UV), wavelengths between 320–390 nm and blue (390 and 500 nm) regions of the electromagnetic spectrum. The most obvious plant movement induced by this portion of the light spectrum is phototropism, directional growth response to directional light cues. Plants also respond to changes in the intensity of incident near-UV or blue light by adjusting the position of chloroplasts (organelles that contain light-harvesting complexes required for photosynthesis) within cells. All of the plant movements reviewed in the chapter require light perception by at least one of two phototropin light sensors. The first phototropin, PHOT1, was identified genetically in a screen for Arabidopsis mutants with altered phototropic responses in low-intensity, directional blue light. The second phototropin, PHOT2, was identified initially by its sequence homology to PHOT1. Phototropins are members of a larger family of proteins known as the “LOV” domain family. The ability of the phototropins to function as near-UV–blue-light receptors is derived from the association (noncovalent in a dark state) of a single flavin mononucleotide (FMN) molecule with each LOV domain, and the unique photocycle that occurs within this chromopeptide. Phototropins are the ‘‘on’’ and ‘‘off’’ switch for phototropism, but other photoreceptors are the ‘‘dimmer’’ switch, controlling the degree of phototropic response. When light is optimal, chloroplasts are scattered within the cell; however, if the light intensity changes, the chloroplasts are relocated to adjust to that change. The isolation of mutants in all phototropins, cryptochromes, and phytochromes have led to a cascade of discoveries and provided powerful tools for elucidating the molecular processes involved in blue light-driven plant movement responses.