Ascorbate is regenerated from MDHA in the chloroplast membrane by fer-redoxin (32) or in the stroma by MDHA reductase at the expense of NADPH. The MDHA can also spontaneously dissociate into ascorbate and dehydroascorbate (DHA), which is rereduced by DHA reductase (DHAR). The DHAR uses reduced glutathione as an electron donor. Oxidized glutathione is then recycled by NADPH-consuming glutathione reductase (GR). The APX is directly involved in the defense response against oxidative stress. In pea and radish, APX activities are induced by Fe excess and salt and drought stresses (33-35).

Winter-acclimated pine needles contain up to 65-fold more APX than summer needles (36), whereas in maize APX (together with SOD) is con-stitutively higher in a chilling-resistant than in a sensitive line (37). After anoxic stress, APX activity rises in wheat roots and rice seedlings (38,39).

Also in wheat, correct temporal expression of APX is an important factor for efficient seed germination. APX activity increases during germination in parallel with the rise of ascorbate levels (40).

Based on the available sequence data, seven different APXs are distinguished in plants: two soluble cytosolic forms, three cytosol membrane-bound types including a glyoxisome-bound form, one chloroplastic stromal, and one thylakoid membrane-bound APX (41). The various isoforms differ in several molecular and enzymatic properties, such as molecular weight, electron donor specificity, lability in the absence of ascorbate, pH optimum, and ascorbate and H202 affinity (42). In general, the chloroplastic isoforms are very specific for ascorbate as electron donor, whereas the cytosolic APX can also oxidize pyrogallol (43).

C. Catalases

Plants, unlike animals, have multiple forms of catalase (H202:H202 oxido-reductase; EC that are mainly found in peroxisomes and glyoxi-somes. Catalase activity was also found in the mitochondria of maize (44,45). Catalases directly consume H202 or oxidize substrates (R), such as methanol, ethanol, formaldehyde, and formic acid.



There are some striking similarities in the organization of the catalase gene family in different species. Our laboratory showed that catalases can be divided into three classes according to their expression (46). The transition from glyoxisomes to leaf peroxisomes during seedling development is associated with the disappearance of class III catalases and the induction of class I catalases. In maize, however, both class I and class III are expressed in seeds, indicating that the class I catalase has a dual function in maize. Class I is most prominent in photosynthetic tissues, where they are involved in the removal of photorespiratory H202. Class II catalases are highly expressed in vascular tissues, where they might play a role in lignification, but their exact biological role remains unknown. As mentioned before, class III is abundant only in seeds and young seedlings and its activity is linked to the removal of excessive H202 that is produced during fatty acid degradation in the glyoxylate cycle in the glyoxisomes. Because catalase isozymes are rapidly induced by ultraviolet B (UV-B), ozone, and also chilling, they may play a direct role in stress protection (47,48).

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