Symposium II: Adapting to a Dynamic Environment
Abs #
20001: Acclimation to light and reactive oxygen
Plants require light for photosynthesis, but the supply of light energy in natural environments is not constant. The amount of incident light can vary over several orders of magnitude on a time scale of seconds, hours, or over the course of a growing season. To optimize photosynthetic utilization of available light in different environments, plants have evolved various acclimation responses to changes in light intensity. Within seconds to minutes after a shift from low light to high light, downregulation of photosystem (PS) II light harvesting occurs by a feedback de-excitation mechanism (qE) that helps to minimize generation of oxidizing molecules. Characterization of Arabidopsis thaliana mutants showed the importance of specific xanthophylls for qE and revealed an essential role of a xanthophyll-binding PS II protein, PsbS. The expression level of the PsbS protein was shown to be a determinant of qE capacity, but PsbS-dependent enhancement of qE did not depend on the peripheral antenna proteins of PS II, suggesting that PsbS interacts functionally with the core antenna of PS II. The results of site-directed mutagenesis experiments indicated that PsbS binds protons (and zeaxanthin) when light absorption exceeds the capacity for CO2 fixation, leading to de-excitation of chlorophyll and harmless thermal dissipation of excess absorbed light energy. Ultrafast spectroscopy using mutants and PsbS-overexpressing plants provided evidence for a direct role of xanthophylls in the de-excitation of singlet chlorophyll. Despite protective mechanisms like qE, photo-oxidative damage can be caused by a variety of reactive oxygen species that are produced as unwanted byproducts of photosynthesis, especially in high light. For example, singlet oxygen can be generated by the interaction of triplet excited chlorophyll and ground-state oxygen in vivo. We have found that the unicellular green alga Chlamydomonas reinhardtii exhibits a robust acclimation response to singlet oxygen generated in response to either high light or a photosensitizing dye (rose bengal). Acclimation to singlet oxygen was dependent upon transcription and de novo protein synthesis, and microarray analysis of RNA levels uncovered a subset of genes whose expression was strongly induced during exposure to rose bengal. To identify regulatory components involved in acclimation to singlet oxygen, we have isolated mutants that fail to acclimate.
