PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Low molecular weight antioxidants and photosynthesis. Christine Foyer^*,1, Graham Noctor², ¹ Crop Performance and Improvement Division, Harpenden, Hertfordshire, U.K.² Institut de Biotechnologie de Plantes, Paris, France

ABSTRACT- Antioxidants are crucial to the defence and function of both chloroplasts and mitochondria, which use oxygen in essential energy exchange reactions. Ascorbate, glutathione and tocopherol are three major low molecular weight antioxidants of plant cells [Foyer et al. (2004) in: Advances in Photosynthesis and Respiration, B. Demmig-Adams, WW Adams, eds (in press)]. While tocopherol is hydrophobic and is found only in the lipid membranes, ascorbate and glutathione are hydrophilic, accumulating to high concentrations in the chloroplast stroma and other compartments of the plant cell. It is generally accepted that there is a high degree of redox coupling between these antioxidants that has implications for regulation, function and signalling associated with photosynthesis and respiration. In terms of photosynthesis, tocopherol has an essential protective function in counteracting the harmful effects of singlet oxygen production at photosystem II (PSII). While it has long been recognised that ascorbate reduces and thus regenerates oxidised tocopherol, the effectiveness of tocopherol in protecting PSII and its interactions with ascorbate have only recently been demonstrated. Moreover, while reduced glutathione will always reduce oxidised ascorbate (dehydroascorbate), the degree of coupling between the ascorbate and glutathione redox couples varies in different cellular compartments. The tightness of coupling between these metabolites has important implications for photosynthetic function. Accumulating evidence suggests that low molecular weight antioxidants not only limit the extent of accumulation of reactive forms of oxygen, but also act independently as signal-transducing molecules. For example, the regulation of a number of key genes associated with photosynthesis is modified in response to ascorbate abundance. Thus, in addition to its role in chemical and enzyme-catalysed detoxification of reactive oxygen, ascorbate may provide information on tissue oxidative load and redox-buffering capacity.

KEY WORDS: antioxidant coupling , Stress Tolerance, redox signalling, Ascorbate