PATHOGEN-INDUCED CHANGES IN THE ANTIOXIDANT STATUS OF THE APOPLAST INBARLEY LEAVES

Leaves of two barley (Hordeum vulgare L.) isolines, Alg-R, which has t he dominant Mla1 allele conferring hypersensitive race-specific resist ance to avirulent races of Blumeria graminis, and Alg-S, which has the recessive mla1 allele for susceptibility to attack, were inoculated w ith 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-phos phate dehydrogenase, was very low (less than 2%) even in inoculated pl ants. Dehydroascorbate, glutathione, superoxide dismutase, catalase, a scorbate peroxidase, glutathione reductase, monodehydroascorbate reduc tase, and dehydroascorbate reductase were present in the apoplast. Ino culation had no effect on the total foliar ascorbate pool size or the redox state. The glutathione content of Alg-S leaves and apoplast decr eased, whereas that of Alg-R leaves and apoplast increased after patho gen attack, but the redox state was unchanged in both cases. Large inc reases in foliar catalase activity were observed in Alg-S but not in A lg-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 respons e in Alg-S and Alg-R may contribute to pathogen sensitivity.