In vitro differentiation of haustorial mother cells of the wheat stem rust fungus, Puccinia graminis f. sp. tritici, triggered by the synergistic action of chemical and physical signals.

Biotrophic plant pathogenic fungi often develop a sophisticated series of infection structures for non-destructive host tissue penetration. In vitro, early infection structures of rust fungi-germ tube, appressorium, substomatal vesicle, infection hyphae-can easily be induced, but in vitro differentiation rates of late infection structures-haustorial mother cells (hmc), haustoria-are low at best. Under appropriate conditions (humid atmosphere), a combination of physical (mild heat shock) and chemical signals (trans-2-hexen-1-ol) induced the in vitro differentiation of hmc in the wheat stem rust fungus, Puccinia graminis f. sp. tritici. Around two thirds of the in vitro differentiated germlings developed up to three hmc which were cytologically identical to hmc formed in planta. Efficient in vitro differentiation of hmc will allow us to analyse in molecular detail the processes involved in the induction and differentiation of this critically important developmental stage of the economically important plant pathogenic rust fungi.