Induction of cellular accessibility and inaccessibility and suppression and potentiation of cell death in oat attacked by ¤Blumeria graminis¤ f.sp. ¤avenae¤

First-formed (seedling) and later-formed leaves of oat cvs Selma (susceptible) and Maldwyn (adult plant resistance under complex genetic control) were subjected to a double inoculation procedure ('inducer' followed by 'challenger') with conidia of Blumeria graminis (DC.) Speer (Syn. Erysiphe graminis DC.). Successful penetration and haustorium formation by the inducer rendered living epidermal cells highly accessible to later challenge attack as judged by increased frequency of challenge penetration success compared to controls. Conversely, where failure of inducer attack on living epidermal cells was associated with papilla deposition, cells were rendered highly inaccessible to challenge attack. These effects were localized, being expressed most strongly in the single cell subject to direct inducer attack (D0 cell), to a lesser degree in immediately adjacent cells, and relatively weakly, if at all, at two cells distance. The degree of induced (in)accessibility in D0 cells appeared correlated to localized autofluorescent host cell responses to challenge appressoria, but independent of the inherent resistance of leaf tissues. These results agree with earlier observations from barley, suggesting that induced changes in (in)accessibility may be a common consequence of B. graminis attack in cereals. As expected, in Maldwyn, cell death was a consistent but infrequent response to attack (5-20%, of attacks caused cell death in controls). Here, the successful formation of an inducer haustorium, or of a papilla due to failed attack, totally suppressed the cell death response to later challenge attack on D0 cells, but had less effect on neighbouring cells. Conversely, death of a Maldwyn epidermal cell due to inducer attack potentiated cell death in adjacent cells where up to 80% of challenge attacks caused death. This effect was transmitted to some extent to two cells distance. (C) 1999 Academic Press.