The Regulation of Leaf Cell Expansion by Light and Ion Flux

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The interaction between light and the plasma membrane of growing leaf cells is the focus of this proposal. Leaf expansion requires light. Most dicotyledonous leaves remain very small in the dark, expand slightly in response to (red) light of low fluence rates, but they require continuous exposure to (white) light of high fluence rates to reach full size. this photoresponse is a "high- irradiance" response of phytochrome occurring in green tissue, and it may involve additional activity of a blue-light receptor. The mechanism(s) transducing light absorption to cell growth will be pursued by testing the following hypotheses: 1) red and blue light act through two photoreceptors to stimulate cell growth in leaves, 2) blue light stimulates proton efflux driven by an electron transport system in the plasma membrane, 3) red light acts through phytochrome to modulate potassium flux through ion channels. These possibilities will be tested in primary leaves of Phaseolus vulgaris L. which provide a well documented model system for light stimulated cell expansion. The generality of their responses will be tested in leaves of other species (Populus, Vicia, Impatiens) which grow differently in response to light. %%% The goal of the research is to determine how leaf cell enlargement in plants is controlled and what is the nature of the molecules involved in their growth response to light.