1Department of Plant Sciences, South Parks Road, University of Oxford, OX1 3RB, UK; 2Scottish Crop Research Institute, lnvergowrie, Dundee, DD2 5DA, Scotland, UK

Background and objectives
Recessive alleles (mlo) of the Mlo locus in barley mediate a broad, non-race-specific, papilla-based resistance reaction to the powdery mildew fungus (Erysiphe graminis (Blumeria graminis) f.sp. hordei). Over the past few years mlo resistance has been used increasingly in European spring barley, reaching over 70% of UK spring barley seed production in 1993. However, reports of heavily infected mlo-resistant barley crops which characteristically regain their resistance in subsequent growth led to the discovery that temporary breakdown of mlo resistance occurs upon relief of water stress [1]. The aim of this project was to investigate the genotypic specificity, cellular characteristics and molecular mechanism of this temporary partial breakdown.

Results and conclusions
Field trials have shown that breakdown occurs in particular resistant and susceptible genotypes and that differences in the degree of breakdown of resistance are attributable to genetic background rather than to the specific mlo allele [2]. A small-scale glasshouse stress-tube experiment, comparing a susceptible to an mlo-resistant genotype, has confirmed these results and demonstrated the temporal nature of the breakdown in the mlo-resistant genotype as resistance is regained some time after relief of water stress.

A comprehensive microscopic examination has determined the cellular characteristics of breakdown. The optimal time-point for resistance breakdown, upon relief of water stress, was found to be 7 h post-inoculation. The degree of breakdown was shown to vary with cell type; notably colony formation on short cells was shown to increase from less than 1 in 100 to more than 1 in 10. Principal components analysis (PCA) has been used to demonstrate that host reactions in resistant and susceptible cultivars can be distinguished at 7 h post-inoculation.

Peroxidases play an important role in both papilla-based resistance and plant responses to water stress where they are involved in protein cross-linking and protection from oxidative damage, respectively. mlo-Resistant plants show more extensive protein cross-linking around reaction sites, in terms of halo size, than mlo-susceptible plants; this phenomenon is reduced under water stress. Furthermore, at the critical time-point, 7 h post-inoculation, protein cross-linking reaction sites were found to be more frequent in non-stressed plants than in water-stressed plants, suggesting that the speed of response is reduced in water-stressed plants. An investigation to assess the effect of relief of water stress on peroxidase transcript levels during pathogenesis and wounding is being carried out.

SJB holds a BBSRC CASE Studentship with SCRI.

1. Newton AC, Young IM, 1996. Plant Pathology 45, 973-977.
2. Baker SJ, Newton AC, Crabb D et al., 1998. Plant Pathology (in press).