SENSING ENVIRONMENTAL-TEMPERATURE CHANGE THROUGH IMBALANCES BETWEEN ENERGY SUPPLY AND ENERGY-CONSUMPTION - REDOX STATE OF PHOTOSYSTEM-II

A basic requirement of all photosynthetic organisms is a balance betwe en overall energy supply through temperature-independent photochemical reactions and energy consumption through the temperature-dependent bi ochemical reactions of photosynthetic electron transport and contiguou s metabolic pathways. Since the turnover of photosystem II (PSII) reac tion centers is a limiting step in the conversion of light energy into ATP and NADPH, any energy imbalance may be sensed through modulation of the redox state of PSII. This can be estimated in vivo by chlorophy ll a fluorescence as changes in the redox state of PSII, or photosyste m II excitation pressure, which reflects changes in the redox poise of intersystem electron transport carriers. Through comparisons of photo synthetic adjustment, We show that growth at low temperature mimics gr owth at high light. We conclude that terrestrial plants, green algae a nd cyanobacteria do not respond to changes in growth temperature or gr owth irradiance per se, but rather, respond to changes in the redox st ate of intersystem electron transport as reflected by changes in PSII excitation pressure. We suggest that this chloroplastic redox sensing mechanism may be an important component for sensing abiotic stresses i n general. Thus, in addition to its role in energy transduction, the c hloroplast may also be considered a primary sensor of environmental ch ange through a redox sensing/signalling mechanism that acts synergisti cally with other signal transduction pathways to elicit the appropriat e molecular and physiological responses.