Faculty of Bioresources, Mie University, Tsu-city, 514-8507 Japan

Background and objectives
Attempted infection of plant tissues by any microbes, whether they are pathogens or non-pathogens, initiates a series of complicated processes of physiological interactions which give rise to characteristic visual symptoms at the tissue and/or organ level. Careful observations of microbe behaviour and plant responses often provide significant information for the analysis of physiological and molecular investigations of disease interactions. Since most powdery mildew fungi are ectoparasites, they are suitable for analysing microbe-plant interactions at the single spore and the single plant cell level. In this paper, cellular interactions between barley and powdery mildew fungi (Erysiphe graminis f.sp. hordei, a pathogen, and E. pisi, a non-pathogen) are described at the cytological and physiological level.

Results and conclusions
Partially dissected coleoptiles of barley seedlings were used. Coleoptiles were inoculated with conidia of E. graminis and of E. pisi, so that both conidia would penetrate the same cell. Accessibility induced by E. graminis and inaccessibility induced by E. pisi were observed at time intervals between the initiation of cytoplasmic aggregation induced by these fungi in the same cells.

Results showed that (i) when E. pisi attempted penetration more than 60 min earlier than E. graminis, none of the E. pisi conidia succeeded in penetration, and (ii) if E. graminis attempted penetration 60 min earlier than E. pisi, about 30% of E. pisi succeeded in penetration. These results showed that conditioning coleoptile cells, whether toward accessibility or inaccessibility, is closely associated with the timing of penetration attempt by both fungi in the same cell. However, the results do not necessarily mean that the cells recognize the presence of both fungi at the time of their penetration. The following results suggest that recognition of a non-pathogen by host cells occurred no later than the time of maturation of its appressorium. Conidia of E. pisi were inoculated onto a barley coleoptile, followed by incubation for several hours. Germlings of this fungus were then removed from coleoptile surfaces with a manipulator and germlings of E. graminis were transferred from another coleoptile to the surface of the same cell. The penetration efficiency of E. graminis was suppressed to about 50% of that of controls when E. pisi had been removed at, or 2 h after, maturation of its appressorium. These results suggest that E. pisi germlings release signal(s) (possibly corresponding to an elicitor) at the time of appressorium maturation, and this is associated with the induction of inaccessibility.

Another experiment suggested the possibility that suppressor(s) of inaccessibility is/are released by E. graminis. Germlings of this fungus which had been inoculated onto another coleoptile were then transferred to the coleoptiles inoculated initially with E. pisi. This was done so that the E. graminis germling would attempt to penetrate the same cell from which the E. pisi germling had been removed. The E. graminis transfer was made before, at, or after the time of maturation of an E. pisi appressorium. The induced inaccessibility of cells that suppresses successful penetration by E. graminis was expressed only when this fungus was transferred to the cells at or after the time of maturation of the E. pisi appressorium.

Another transfer experiment revealed that the continued presence of an E. graminis germling on a host cell suppressed the inaccessibility caused by E. pisi on the same cell. These results indicate that E. graminis releases a suppressor(s) which blocks the effect of the E. pisi elicitor. Water-soluble, extracellular materials from germlings of E. pisi suppressed infection by E. graminis in barley coleoptile cells whose upper surfaces were treated with the materials by a bead method. Water-soluble, extracellular materials from germlings of E. graminis had a similar effect. These results suggest that some component(s) on the surface of germlings of both fungi may work as a signal product to allow coleoptile cells to sense the presence of the fungi and/or to induce inaccessibility. However, the isolation of suppressor(s) from E. graminis has been unsuccessful. Recent analysis revealed that water-soluble extracts of coleoptiles inoculated with E. graminis successfully enhanced infection by E. pisi in coleoptiles. Such an effective component(s) was able to [missing text]...