Physiological and Biochemical Investigations on the Phototropins: Plant Blue Light Receptors

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

The phototropins (phot1 and phot2) are a unique family of plant photoreceptors that bind two molecules of flavin mononucleotide (FMN) as their chromophores. Blue and UV-A light cause rapid autophosphorylation of these photoreceptors. In all plans thus far investigated, phot1 is associated with the plasma membrane and preliminary evidence indicates that phot2 is as well. The phototropins participate in phototropism (growth toward a blue or UV-A light source), light perception for chloroplast movement (avoidance by mutual shading in high light, moving to maximize light interception in low light), stomatal opening, rapid inhibition of growth of dark-grown seedlings, and likely solar tracking in those species that have that response. The FMN chromophores are bound tightly in specialized domains designated LOV1 and LOV2 (domains found in signaling proteins that respond to Light, Oxygen, or Voltage). The phototropin LOV domains undergo a unique photochemistry involving light-activated formation of a flavin triplet state that decays within microseconds to a metastable signaling state, a cysteinyl adduct at the C94a) carbon of the FMN. This signaling state decays to the dark state in a few seconds or minutes depending on which LOV domain is involved. The project will explore how this identical initial photochemistry (adduct formation) and biochemistry (autophosphorylation) can activate such different processes, some requiring cell-cell communication and other entirely cell autonomous. It will also explore the roles of the individual LOV domains in these various processes. We already know that only LOV2 is required for full autophosphorylation in phot1, but that either LOV1 or LOV2 can mediate phosphorylation of phot2. We also know that LOV2 is sufficient to restore the phototropic response in a null phot1 mutant but do not know for any of the other responses. Sorting out the roles of the two phototropins and of the individual LOV domains in each of the responses is the first major goal of the project. A second major goal is to determine the cellular and subcellular distribution of phot2 (we already have this information for phot1), information basic to an understanding of its function. Finally we hope to identify downstream reacting partners in one or more of the responses and determine where the signaling pathways diverge for the different responses. The phototropins are vital to normal plant growth and development for several reasons: first, phototropism is a mechanism to insure maximal harvest of light energy for photosynthesis, clearly of importance in maximizing plant growth for whatever purpose-food, structural materials, fiber, production of medicinal drugs, etc. Second, for plants like cotton that have leaves that track the sun and remain at right angles to the direction of the incident sunlight the sun's angle changes, solar tracking of mature leaves serves the same function: maximizing photosynthesis and hence plant growth. Third, leaf expansion is an absolute requisite for photosynthesis. In the absence of the phototropins, leaves are dwarfed and curled, and severely impaired in their function. Fourth, the capacity to regulate the position of the chloroplasts in the leaf cells is essential a) to maximize photosynthesis by minimizing mutual shading in dim light, and b) to prevent photodamage to the chloroplasts by maximizing mutual shading in very bright light. Finally, light regulation of stomatal opening controls the rate at which carbon dioxide is taken in for photosynthesis balanced against loss of water. Without this regulation, normal photosynthetic function would be severely impaired. Thus along with the other known plant photoreceptors, the phytochromes and cryptochromes, the phototropins play a vital role in plant development and homeostasis. Their role in human welfare is clearly a major one.