Genetic Dissection of Photoprotection and Characterization of NPQ Mutants

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During mid-day, plants are absorbing much more light energy than they can use to fuel photosynthesis. This excess absorbed light energy photosensitizes pigments in plants and can cause extreme damage to cells because of the oxidation of various cellular building blocks such as proteins and lipids. To alleviate this situation plants and algae have developed efficient mechanisms for eliminating excess absorbed light energy as heat. One of these mechanisms can be measured as a change in the chlorophyll fluorescence of the cell, or nonphotochemical quenching of chlorophyll fluorescence. Using measurements of chlorophyll fluorescence, mutant strains of the alga Chlamydomonas reinhardtii were isolated that could not dissipate excess light energy as efficiently as wild-type or normal cells. A number of these strains appeared to be more sensitive than wild-type cells to high light; they bleach and die upon exposure to high light. Analyses of these mutants have led to the identification of both pigment and protein molecules that are critical for the elimination of excess absorbed light energy in photosynthetic organisms. Developing a greater understanding of how plants respond to high light and the mechanisms used to eliminate excess light energy offers the possibility of engineering specific, agronomically important plants to survive longer periods of time under harsh environmental conditions, where plants are often absorbing excess light energy. This may extend the range of environments in which certain crop plants can grow, which in turn could increase the yields of specific crops.