Land plants are sessile and have developed sophisticated mechanisms that allow for both immediate and acclimatory responses to changing environments. Partial exposure of low light-adapted Arabidopsis plants to excess light results in a systemic acclimation to excess excitation energy and consequent photooxidative stress in unexposed leaves. Thus, plants possess a mechanism to communicate excess excitation energy systemically, allowing them to mount a defense against further episodes of such stress. Systemic redox changes in the proximity of photosystem II, hydrogen peroxide, and the induction of antioxidant defenses are key determinants of this mechanism of systemic acquired acclimation.

Using two cultivars of Pisum sativum L. with different sensitivity to NaCl, the effect of long-term (15 d) NaCl (70 mM) treatments on the activity and expression of the foliar ascorbate-glutathione cycle enzymes, superoxide dismutase isozymes and their mRNAs was evaluated and related to their ascorbate and glutathione contents. High-speed supernatant (soluble) fractions, enriched for cytosolic components of the antioxidant system, were used. In this fraction from the NaCl-tolerant variety (cv Granada), the activities of ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), Mn-superoxide dismutase (Mn-SOD) and dehydroascorbate reductase (DHAR) increased, while CuZn-SOD activity remained constant. In the NaClsensitive plants (cv Challis), salinity did not produce significant changes in APX, MDHAR and GR activities. Only DHAR activity was induced in cv Challis, whereas soluble CuZn-SOD activity decreased by about 35%. Total ascorbate and glutathione contents decreased in both cultivars, but the decline was greater in NaCl-sensitive plants. This difference between the two cultivars was more pronounced when the transcript levels of some these enzymes were examined. Transcript levels for mitochondrial Mn-SOD, chloroplastic CuZn-SOD and phospholipid hydroperoxide glutathione peroxidase (PHGPX), cytosolic GR and APX were strongly induced in the NaCl-tolerant variety but not in the NaCl-sensitive variety. These data strongly suggest that induction of antioxidant defences is at least one component of the tolerance mechanism of peas to long-term salt-stress.

Exposure of Arabidopsis plants that were maintained under low light (200 pmol of photons m-2 sec-I) to excess light (2000 pmol of photons m-2 sec-I) for 1 hr caused reversible photoinhibition of photosynthesis. Measurements of photosynthetic parameters and the use of electron transport inhibitors indicated that a nove1 signal transduction pathway was initiated at plastoquinone and regulated, at least in part, by the redox status of the plastoquinone pool. This signal, which preceded the photooxidative burst of hydrogen peroxide (H202) associated with photoinhibition of photosynthesis, resulted in a rapid increase (within 15 min) in mRNA levels of two cytosolic ascorbate peroxidase genes ( A f X 1 and APX2). Treatment of leaves with exogenous reduced glutathione abolished this signal, suggesting that glutathione or the redox status of the glutathione pool has a regulatory impact on this signaling pathway. During recovety from photooxidative stress, transcripts for cytosolic glutathione reductase (GOR2) increased, emphasizing the role of glutathione in this stress.