ROLE OF TOPOGRAPHY SENSING FOR INFECTION-STRUCTURE DIFFERENTIATION INCEREAL RUST FUNGI

Over 90% of the germ tubes of Puccinia graminis tritici (wheat stem ru st) and Puccinia hordei (barley brown rust) differentiate appressoria on encountering stomata. There has been controversy as to the role of host topographical signals in the highly precise and efficient inducti on of these infection structures over stomata by cereal rusts. In the present study, polystyrene replicas of microfabricated silicon wafers, bearing precise microtopographies of defined dimensions, were used to investigate the influence of ridge spacing and height on infection-st ructure induction by P. graminis tritici and P. hordei. It was found t hat artificial topographical signals alone can induce a reproducibly h igh percentage (83-86%) of germ tubes to differentiate infection struc tures. Multiple, closely spaced (1.5 mu m) ridges which were 2.0 mu m high provided the most inductive topography. Differentiation on flat s urfaces and over single ridges was < 4%. Appressorium induction common ly initiated a cascade of differentiation events involving the formati on of infection pegs, vesicles, infection hyphae, and occasionally hau storial mother cells. It is suggested that the close spacing of cell j unctions associated with the dumbbell-shaped guard cells of cereal sto matal complexes provide inductive signals for infection-structure form ation by cereal rusts in vivo.