Are diverse signalling pathways integrated in the regulation of Arabidopsis antioxidant defence gene expression in response to excess excitation energy?

When low-light-grown Arabidopsis rosettes are partially exposed to excess l ight (EL), the unexposed leaves become acclimated to excess excitation ener gy (EEE) and consequent photo-oxidative stress. This phenomenon, termed sys temic acquired acclimation (SAA), is associated with redox changes in the p roximity of photosystem II, changes in foliar H2O2 content and induction of antioxidant defences. The induction of extra-plastidial antioxidant system s is important in the protection of the chloroplast under EL conditions. A larger range of transcripts encoding different antioxidant defence enzymes may be induced in the systemically acclimated leaves and these include thos e encoded by the glutathione peroxidase (GPX2) and glutathione-S-transferas e (GST) genes, which are also highly induced in the hypersensitive response and associated systemic acquired resistance (SAR) in incompatible plant-pa thogen interactions. Furthermore, the expression of the SAR-inducible patho genesis-related protein gene, PR2, is enhanced in SAA leaves. Wounded leaf tissue also shows enhanced systemic induction of a cytosolic ascorbate pero xidase gene (APX2) under EL conditions. These and other considerations, sug gest H2O2 and other reactive oxygen species (ROS) could be the common facto r in signalling pathways for diverse environmental stresses. These effects may be mediated by changes in the level and redox state of the cellular glu tathione pool. Mutants with constitutive expression of a normally EL-induci ble APX2 gene have much reduced levels of foliar glutathione. The expression of APX1 and APX3 encoding cytosolic and peroxisome-associate d isoforms, respectively, are also under phytochrome-A-mediated control. Th e expression of these genes is tightly linked to the greening of plastids i n etiolated seedlings. These data suggest that part of the developmental pr ocesses that bring about the acclimation of leaves to high light includes t he configuration of antioxidant defences. Therefore, the linkage between im mediate responses of leaves to EL, acclimation of chloroplasts to EEE and t he subsequent changes to leaf form and function in high light could be medi ated by the activity of foliar antioxidant defences and changes in the conc entration of ROS.