Department of Plant Sciences, South Parks Road, University of Oxford, OX1 3RB, UK

Background and objectives
Erysiphe graminis f.sp. hordei, the causal agent of powdery mildew of barley (Hordeum vulgare), is an obligate biotrophic fungus, which penetrates susceptible barley leaves through the cuticle. Differentiation progresses through a defined sequence of morphological changes beginning with the appearance of a short primary germ tube some 2 h after inoculation, followed by the appearance of the second elongate appressorial germ tube some 5-8 h post inoculation and progressing onwards to the formation of the haustorium.

Our work aims to clone and characterize barley powdery mildew genes expressed temporally at the host pathogen interface and to pinpoint the location within the host 'surface' of critical factors perceived by the pathogen which drive its development. The knowledge gained should allow identification of weak links in the pathogen's relationship with its host which ultimately may be exploited through plant breeding in order to select for plant phenotypes with novel disease resistance characteristics. In addition, there exists the possibility of using the named genes as either molecular markers for studying the mode of action of fungicidal chemicals or to identify new gene targets for the rational design of novel fungicides by looking at homologies to pathogen sequence motifs.

Results and conclusions
What are the exogenous and endogenous signals which influence spore maturation and infection? Is entry into the epidermal cell effected by enzymic or by mechanical means, or by a combination of the two acting in synergy? That critical host-derived moieties, released sequentially from the host surface, may direct germ tube development can be tested by looking at differentiation on host and artificial surfaces. We have shown that cutin monomers act as a signal to trigger AGT development, that esterase inhibitor ebelactone represses AGT formation [1], and that cellulases also play a pivotal role. We are interested in the role played by 'endogenous' signals and have shown that cAMP-mediated signal transduction plays a part in the morphological differentiation.of the fungal spore (see abstracts of Hall and Gurr). Plasmolysis and cytorrhysis experiments reveal that E. graminis appressoria generate a transient turgor pressure of ca 3 MPa around the time of AGT penetration. Taken collectively, these data suggest that conidial differentiation is triggered, in part, by the hydrophobicity of the host surface [2) and is influenced by the integration of endogenous and host-derived signals. Penetration is thence effected by enzymic means and by mechanical pressure.

Which E. graminis genes are expressed during conidial differentiation? Subtraction differential display yielded 100 bands at different stages of spore differentiation, notably in ungerminated, PGT and AGT-stage conidia. Of 50 sequences, five clones revealed homologies to sequences residing within the database. The paucity of relevant tissue precludes inexorable Northern blot analysis with so many clones and so identical dot blots, carrying the clones, were probed with cDNAs from the stages of spore differentiation and seven clones thence chosen for more rigorous Northern analysis (see abstract of Zhang and Gurr). The temporal and spatial pattern of these clones will be discussed.

1. Francis SA, Dewey FM, Gurr SJ, 1996. Physiological and Molecular Plant Pathology 49, 201-211.
2. Carver TLW et al., 1987. Physiological and Molecular Plant Pathology 30, 351-372.