Institute of Grassland and Environmental Research, Aberystwyth, SY23 3EB, UK

Background and objectives
Hydrogen peroxide (H2O2) production by the plasmalemma appears to be a common feature of plant-pathogen interactions. This oxidant is considered to be involved in signal transduction processes leading to resistance [1]. However, uncontrolled H2O2 accumulation would lead to extensive oxidation, lipid peroxidation and tissue damage. A fine balance must be achieved between allowing pathogen-induced H2O2 signalling etc. to occur, and preventing destruction of surrounding cells by oxidative damage. Plant cells contain antioxidants which destroy H2O2 [2] but the role of these in plant-pathogen interactions is largely unexplored.

Materials and methods
The anti-oxidant content and composition of whole leaves and the leaf apoplast were measured in cereals expressing race-specific and race-non-specific resistance to attack by Blumeria graminis which causes cereal powdery mildew. Whole-leaf and apoplastic extracts from barley and oat were examined 24 h after inoculation, when maximal induction of certain pathogen-related proteins is known to occur in oat [3] and in barley [4]. The oats used were Maldwyn and OM1387 (with genetically complex race-non-specific resistance) and Selma (susceptible). The barley lines were Alg-R (with specific resistance conditioned by the Mla1 allele) and its susceptible sister isoline Alg-S (mla1 allele). Extracts from uninoculated leaves and leaves inoculated with appropriate formae speciales of B. graminis were compared for anti-oxidant content and activity.

Results and conclusions
Significant activities of anti-oxidant enzymes as well as ascorbate and glutathione were measured in the apoplast. Consistent features in the responses of the two species to pathogen attack were observed. There was no prolonged oxidation of the total foliar pools of ascorbate or glutathione 24 h after inoculation, despite the presence of substantial cell death in some varieties. The ascorbate pool was largely unchanged following B. graminis attack, while substantial enhancement of the glutathione pool was observed. In resistant lines of both cereal species, foliar glutathione consistently increased following inoculation. This accords with the possible role of glutathione as a signal-transducing molecule involved in elicitation of defence responses.

1. Levine A, Tenhaken R, Dixon R, Lamb C, 1994. Cell 79, 583-593.
2. Foyer CH, Descourvieres P, Kunert K-J, 1994. Plant Cell and Environment 17, 507-523.
3. Zhang L, Robbins MP, Carver TLW, Zeyen RJ, 1997. Physiological and Molecular Plant Pathology 51, 15-33.
4. Boyd LA, Smith PH, Green RM, Brown JKM, 1994. Molecular Plant-Microbe Interactions 7, 401-410.