Antioxidative MEtabolism

In order to understand the role of cytosolic antioxidant enzymes in drought stress protection, transgenic tobacco
(Nicotiana tabacum cv. Xanthi) plants overexpressing cytosolic Cu/Zn-superoxide dismutase (cytsod) (EC 1.15.1.1)
or ascorbate peroxidase (cytapx) (EC 1.11.1.1) alone, or in combination, were produced and tested for tolerance
against mild water stress. The results showed that the simultaneous overexpression of Cu/Znsod and apx or at least
apx in the cytosol of transgenic tobacco plants alleviates, to some extent, the damage produced by water
stress conditions. This was correlated with higher water use efficiency and better photosynthetic rates. In general,
oxidative stress parameters, such as lipid peroxidation, electrolyte leakage, and H2O2 levels, were higher in
non-transformed plants than in transgenic lines, suggesting that, at the least, overexpression of cytapx protects
tobacco membranes from water stress. In these conditions, the activity of other antioxidant enzymes was induced in
transgenic lines at the subcellular level. Moreover, an increase in the activity of some antioxidant enzymes was also
observed in the chloroplast of transgenic plants overexpressing cytsod and/or cytapx. These results suggest the
positive influence of cytosolic antioxidant metabolism on the chloroplast and underline the complexity of the
regulation network of plant antioxidant defences during drought stress.
Key words: Cytosolic ascorbate peroxidase, cytosolic superoxide dismutase, tobacco, water stress.

We studied the effect of salicylic acid treatments on the response of pea plants to salinity. The NaCl-induced damage in leaves increased by salicylic acid: this response correlated with a reduction in plant growth.
The contents of reduced ascorbate and glutathione in leaves of salt-treated plants increased by effect of salicylic acid, although an accumulation of the respective oxidised forms occurred. An increase in hydrogen peroxyde also occurred in the leaves of salt-exposed plants which were treated by salicylic acid.
In the absence of NaCl, salicylic acid increased the ascorbate peroxidase (100 µM) and glutathione-S transferase (50 µM) activities and increased catalase activity in a concentration-dependent manner. Salinity decreased glutathione reductase activity, but increased glutathione-S transferase and catalase activities. In salt-stressed plants, salicylic acid also produced changes in antioxidative enzymes: 100 µM decreased ascorbate peroxidase but increased glutathione-S transferase. Finally, a concentration-dependent increase in superoxide dismutase activity was induced by salicylic acid treatments in salt-stressed plants.
An induction of PR-1b was observed in NaCl-stressed plants treated with salicylic acid. The treatment with salicylic acid, as well as the interaction between the salinity and salicylic acid treatments, had a significant effect on PsMAPK3 expression. The expression of PsMAPK3 was not altered by 70 mM NaCl, but it was statistically higher in the absence than in the presence of salicylic acid. Globally, the results show that salicylic acid treatments affected negatively the response of pea plants to NaCl, and this response correlated with an imbalance in antioxidative metabolism. The data also show that salicylic acid treatment could enhance the resistance of salt-stressed plants to a possible opportunistic pathogen attack, as suggested by increased PR-1b gene expression.

In this work we used two different pea cultivars: JI281 is a semidomesticated land race of pea from Ethiopia whereas JI399 is a typical domesticated pea variety. Exposure of pea leaves to excess light (EL) for 1 h caused a reversible photoinhibition of photosynthesis as showed by changes in Fv/Fm. Although little difference existed between the two pea genotypes with respect to photoinhibition, after 60 min of EL the decline in Fv/Fm was higher in JI281 than in JI399 leaves. As a consequence of EL, H2O2 increased in both pea cultivars, whereas lipid peroxidation and protein oxidation slightly increased, althought differenced between cultivars were minimal. The redox state of ascorbate shifted towards its oxidized form under EL stress in both cultivars. Transcript levels of genes coding antioxidant enzymes varied upon EL in both cultivars, but the response was more pronounced in JI399. The induction observed during EL was maintained or increased after the stress period, as occurred for cytGR and chlMDHAR. GR protein accumulation and activity correlated to the transcript accumulation in JI399, but not in JI288. In this work, a possible role for H2O2 and redox status of ascorbate in the photoxidative stress signalling is discussed.

Sharka, a disease caused by plum pox virus (PPV), has a significant economic impact on fruit tree production. In this work, we analysed the effect of (2,1,3)-benzothiadiazole (BTH) and L-2-oxo-4-thiazolidine-carboxylic acid (OTC) on plant growth and virus content. OTC reduced sharka symptom, stimulated plant growth and alleviated PPV-induced oxidative stress, indicated by a lack of changes in some oxidative stress parameters. PPV infection reduced chloroplast electron transport efficiency. However, in the presence of BTH or OTC, no changes in the chlorophyll fluorescence parameters were observed. PPV produced an alteration in chloroplast ultrastructure, giving rise to a decrease in starch contents that was less dramatic in OTC-treated plants. Furthermore, PPV reduced the abundance of proteins associated with photosynthesis, carbohydrate and amino acid metabolism and photorespiration. These changes did not take place in OTC-treated plants, and increases in the expression of proteins related with the aforementioned processes, including ADP-glucose pyro-phosphorylase, were produced, which correlated with the lower decrease in starch contents observed in PPV-infected plants treated with OTC. The results suggested that OTC treatment provides protection to the photosynthetic machinery and/or the chloroplast metabolism in PPV-infected peaches. Thus, OTC could have practical implications in agriculture in improving the vigour of different plant species as well as in immunizing plants against pathogens.