Apoplastic Antioxidant Research Articles

Changes in apoplastic antioxidants induced by powdery mildew attack in oat genotypes with race non-specific resistance

Three oat (Avena sativa L.) lines which show differential responses to attack by the biotrophic fungal pathogen Blumeria graminis DC f. sp. avenae Marchal, which causes powdery mildew, were studied: Maldwyn shows the strongest resistance in adult plants; Selma shows greater susceptibility; while a Selma × Maldwyn hybrid, OM1387, has a similar degree of resistance to Maldwyn. Host responses to pathogen attack were complete 48 h after inoculation but largely accomplished within the first 24 h, the point when material was taken for enzyme and metabolic assays. In Maldwyn and OM1387 about 80% of attacked cells showed localized autofluorescent host-cell responses but this fell to less than 20% in Selma. A cytoplasmic marker enzyme, glucose 6-phosphate dehydrogenase, was used to determine contamination of the apoplastic extracts by cellular components. After correction for cytoplasmic contamination, up to 4% of the total foliar activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase activities were detected in the apoplast. The apoplast contained about 2% of the total foliar glutathione pool and dehydroascorbate, but not ascorbate, at values amounting to 10% of the total foliar ascorbate plus dehydroascorbate pool. Twenty-four hours after inoculation the foliar or apoplastic ascorbate pools were similar in inoculated and control leaves. Foliar catalase activity increased in both susceptible and resistant responses. Resistance correlated with increased total foliar glutathione, an increase in the ratio of reduced to oxidized glutathione and with decreased total activities of foliar ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase.

Pathogen-induced changes in the antioxidant status of the apoplast in barley leaves

Leaves of two barley (Hordeum vulgare L.) isolines, Alg-R, which has the dominant Mla1 allele conferring hypersensitive race-specific resistance to avirulent races of Blumeria graminis, and Alg-S, which has the recessive mla1 allele for susceptibility to attack, were inoculated with B. graminis f. sp. hordei. Total leaf and apoplastic antioxidants were measured 24 h after inoculation when maximum numbers of attacked cells showed hypersensitive death in Alg-R. Cytoplasmic contamination of the apoplastic extracts, judged by the marker enzyme glucose-6-phosphate dehydrogenase, was very low (less than 2%) even in inoculated plants. Dehydroascorbate, glutathione, superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase, and dehydroascorbate reductase were present in the apoplast. Inoculation had no effect on the total foliar ascorbate pool size or the redox state. The glutathione content of Alg-S leaves and apoplast decreased, whereas that of Alg-R leaves and apoplast increased after pathogen attack, but the redox state was unchanged in both cases. Large increases in foliar catalase activity were observed in Alg-S but not in Alg-R leaves. Pathogen-induced increases in the apoplastic antioxidant enzyme activities were observed. We conclude that sustained oxidation does not occur and that differential strategies of antioxidant response in Alg-S and Alg-R may contribute to pathogen sensitivity.

Ozone stimulates apoplastic antioxidant systems in pumpkin leaves

The phytotoxiticky of ozone is due to its high oxidant capacity and to its ability to generate toxic molecular species. It is well known that intracellular peroxidases play an important role in eliminating toxic forms of oxygen but little evidence has been reported on the role of peroxidases in the apoplastic compartment. The detoxification systems located in the foliar extracellular matrix and in the intracellular fluid of sensitive pumpkin plants (Cucurbita pepo L. cv. Ambassador) exposed to ozone (150 ppb. 5 days. 5 h day‐1) in a fumigation chamber, were analyzed. The analyses were carried out on both young and mature leaves. Ascorbate peroxidase (EC 1.11.1.11) was found in the extracellular matrix of the pumpkin tissues. Its activity increased in both young and mature leaves as a consequence of the treatment, while at intraeellular levels its effect was most prominent in mature leaves. Analysis of the ascorbie‐dehydroascorbic acid system revealed an enhancement of the pool content in the extracellular matrix of both kinds of leaves as a consequence of fumigation, while at the intracelluiar level small variations were found. Very little variation was observed in the glutathione pool as a consequence of fumigation. The analysis of a lipid peroxidation marker, malondi‐aldehyde. showed the significant effect of ozone on membrane lipids. Following fumigation, the free phenols in the extracellular matrix decreased in both young and mature leaves, while the free and glycoside‐bound phenols of the intracellular fluid showed little increase. The results support the hypothesis that ozone stimulates the an‐tioxidant systems mainly in the apoplast and that ascorbic peroxidase activity, ascorbic acid levels and cell wall stiffening are the most influenced parameters.

Apoplastic antioxidants as decisive elimination factors within the uptake process of nitrogen dioxide into leaf tissues.

Uptake of NO2 via stomata and its elimination in the mesophyll are investigated by use of mathematical models. Systems of differential equations describe simultaneous diffusion and reaction of the chemical species taken into account. The common hypothesis, postulating the disproportionation reaction of NO2 being the predominant path for the transformation of NO2 into the cellular nitrate and nitrite pools, could be false because of its failure to reproduce the measured sorption characteristics. If however the reduction of the pro-oxidative NO2 by apoplastic ascorbate is taken into account the calculated uptake rates and the effects of changes in immission concentration and vegetational characteristics fit well within corresponding experimental results. The onset of acute injury in case of a fall in apoplastic ascorbate concentration below a critical level is postulated.