Induced accessibility and enhanced inaccessibility at the cellular level in barley coleoptiles. XIII: Significance of haustorium formation by the pathogen Erysiphe graminis for induced accessibility to the non-pathogen E. pisi as assessed by nutritional manipulations

Erysiphe pisi , a non-pathogen of barley, never formed haustoria in fresh, healthy, barley coleoptile epidermal cells, even if cells were in constant contact with E. graminis germling structures, unless the attacked cell contained an haustorium of E. graminis . However, as reported in previous papers, where an haustorium of E. graminis was present in a cell, about 10–50% of E. pisi appressoria formed an haustorium in that cell depending on the timing of invasion by the latter fungus. To investigate the possible role of nutrient depletion in this induced accessibility to E. pisi , the nutritional status of host epidermal cells was manipulated in three ways: (a) tissues were inoculated with E. graminis at high infection densities so that almost all cells contained a haustorium; (b) all host cells adjacent to a cell containing a haustorium of E. graminis were killed by puncturing them with a microneedle; and (c) coleoptile tissues were starved for 6 days. In all three cases, development of secondary hyphae of E. graminis was fully or partially inhibited, unless exogenous glucose or sucrose was added, indicating that infected cells had insufficient sugar to support hyphal growth. In addition, treatments b and c induced accessibility to infection by E. pisi . In both cases, this accessibility was reversed by addition of 0·1 m glucose. These results indicate that accessibility of barley cells to E. pisi is induced by depletion of cell nutrients and that this occurs by prior infection of the cell and haustorium formation by E. graminis .