Appressorium induction by topographical signals in six cereal rusts

Using polystyrene replicas of microfabricated silicon wafers bearing multiple 1.5 mu m spaced, 2.0 mu m wide ridges (= 2.0 mu m spaced, 1.5 mu m wide grooves) of different heights (0.116-2.4 mu m), appressoria were induced in six cereal rusts (Puccinia graminis f. sp. tritici, P. graminis f. sp. avenae;, P. graminis f.: sp, secalis, P. recondita f. sp. tritici, P. coronata f., sp. avenae and P. horden). The pattern of response to different ridge heights was not correlated with either host or rust species. Only P. graminis f. sp. secalis formed significantly large numbers of appressoria (<65.0% differentiation) on singles ridges spaced 50 mu m apart. Formation of appressoria on host leaves was found to be very efficient, with 92-95% of germlings encountering stomata differentiating appressoria over them. On polystyrene replicas of host leaves, however, the efficiency was much less and varied significantly between rusts (4-63% differentiation). Ag ain, the variation was not correlated with host or rust species. Our results indicate that whilst topographical signals are sufficient to induce appressoria in vitro, host topography alone is not sufficient to explain the high efficiency of appressorium induction observed in vivo. (C) 1997 Academic Press Limited.