Induced accessibility and enhanced inaccessibility at the cellular level in barley coleoptiles. V. Duration of stimulus by a non-pathogen in relation to enhanced inaccessibility

Abstract

The influence of the duration of a direct stimulus by a non-pathogen to the enhancement of inaccessibility was examined using barley coleoptile cells and Erysiphe pisi. The degree of enhanced inaccessibility was evaluated with the rate of haustorium formation (penetration efficiency, PE) of E. graminis which attempted penetration of the same coleoptile cells as E. pisi. Partially dissected coleoptiles (coleoptiles A) were inoculated with E. pisi conidia. By observing the specimens every 15 min, the time of initiation of cytoplasmic aggregation in coleoptile cells below an individual E. pisi appressorium was recorded. At 0·5, 1·0 or 3·0 h after the initiation of cytoplasmic aggregation, germlings of E. pisi were removed by a micromanipulator. Germinating conidia of E. graminis which had been inoculated onto other coleoptiles were then transferred onto coleoptile A by micromanipulation so that they could attempt to penetrate the same cells in which E. pisi had induced a cytoplasmic aggregate. When E. pisi was removed 0·5 h after the initiation of cytoplasmic aggregation, the PE of E. graminis was reduced by 35·4%, suggesting that the inaccessibility was enhanced by the presence of E. pisi. When E. pisi was removed at 1·0 and 3·0 h, the subsequent PEs of E. graminis were 24·8% and 5·8%, respectively, indicating that the prolonged presence of E. pisi further enhanced the inaccessibility. The degree of enhanced inaccessibility was influenced by the size of coleoptile cells but not by the distances between penetration sites of the two fungi within the same cell and the time intervals of the initiation of cytoplasmic aggregation induced by both fungi. It is suggested that unknown mechanism(s) unrelated to papilla formation may be involved in the enhancement of inaccessibility.