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

Background and objectives
Erysiphe graminis (Blumeria graminis) DC. f.sp. hordei Em. Marchal is the fungal pathogen which causes powdery mildew of barley. Erysiphe graminis is an obligate biotroph and has evolved an intricate dependency upon its host plant. Prior to establishment of infection, the conidia produce an initial, or primary, germ tube (PGT). The PGT interacts with the host and is thought to perceive host-derived signals which promote the differentiation of a second formed germ tube into an appressorium; a specialized infection structure for penetration of the host tissue. Host recognition is an essential trigger for these early stages of conidial development and is believed to occur via an integrated set of physical and chemical signals [1].

Cyclic-AMP (cAMP) has been established as an intracellular signalling intermediate in fungi with a role in pathogenicity during fungal-plant interactions [2]. This study aims to investigate the potential role of cAMP as a mediator of host recognition during the early development of Erysiphe graminis conidia.

Results and conclusions
Erysiphe graminis conidia differentiate to varying degrees on artificial surfaces [1]. The ability of a surface to induce conidial development has been shown to correlate with hydrophobicity or the presence of potential leaf-derived factors: thus, glass is virtually non-inductive to appressorium formation; hydrated cellulose membrane is semi-inductive; whereas only the leaf itself induces high levels (80-90%) of appressoria. This study investigated the responses of Erysiphe graminis conidia to addition of either exogenous cAMP, cAMP analogues or pharmacological agents which influence cAMP metabolism, on a range of artificial and host surfaces. In addition, we looked at the effects of metabolites of adenine nucleotide metabolism; AMP, adenosine and adenine. The results suggest that elevated levels of CAMP do not induce conidial germination or differentiation and, indeed, that elevation of cAMP is inhibitory to development. This contrasts to the stimulatory role of cAMP in other phytopathogenic fungi [2]. Rapid metabolism of AMP (the degradation product of cAMP) appears also to be important for appressorial differentiation in Erysiphe graminis.

A competitive ELISA assay was used to determine the levels of cAMP in Erysiphe graminis conidia which had either just been inoculated on to a surface (leaf or artificial), or were collected 4 or 12 h post inoculation (h.p.i.). This assay demonstrated that cAMP levels are relatively high upon first contact with the leaf surface, but decrease rapidly by 4 h.p.i. and remain low by 12 h.p.i. The decrease in cAMP in conidia inoculated onto glass was less marked and indeed following an initial decrease, an increase in cAMP levels was observed in spores inoculated onto cellulose membrane.

Taken together, the results suggest that a decrease in cAMP levels correlates with spore germination, and that both efficient metabolism of AMP and a continued low pool of cAMP are required for appressorium formation. Moreover, the changes in cAMP levels may be responsive to the surface upon which the spore lands.

1. Carver TLW et al., 1995. Canadian Journal of Botany 73 (Suppl. 1), S632-S639.
2. Kronstad JW, 1997. Trends in Plant Science 2, 193-199.