PHOTOSYNTHETIC ELECTRON-TRANSPORT REGULATES THE EXPRESSION OF CYTOSOLIC ASCORBATE PEROXIDASE GENES IN ARABIDOPSIS DURING EXCESS LIGHT STRESS

Exposure of Arabidopsis plants that were maintained under low tight (2 00 mu mol of photons m(-2) sec(-1)) to excess light (2000 mu mol of ph otons m(-2) sec(-1)) for 1 hr caused reversible photoinhibition of pho tosynthesis. Measurements of photosynthetic parameters and the use of electron transport inhibitors indicated that a novel signal transducti on pathway was initiated at plastoquinone and regulated, at least in p art, by the redox status of the plastoquinone pool. This signal, which preceded the photooxidative burst of hydrogen peroxide (H2O2) associa ted with photoinhibition of photosynthesis, resulted in a rapid increa se (within 15 min) in mRNA levels of two cytosolic ascorbate peroxidas e genes (APX1 and APX2). Treatment of leaves with exogenous reduced gl utathione abolished this signal, suggesting that glutathione or the re dox status of the glutathione pool has a regulatory impact on this sig naling pathway. During recovery from photooxidative stress, transcript s for cytosolic glutathione reductase (GOR2) increased, emphasizing th e role of glutathione in this stress.