APPRESSORIUM FORMATION IN RESPONSE TO TOPOGRAPHICAL SIGNALS BY 27 RUST SPECIES

The formation of urediniospore appressoria in response to microfabricated topographies was observed for 27 species of rust fungi representing eight genera. Nine species, including Uromyces appendiculatus, U. vignae, Melampsora medusae, Puccinia antirrhini, P. calcitrapae, P. carduorum, P. melanocephala, P. substriata, and one isolate of P. recondita, developed appressoria on ridges within an optimal height range of approximately 0.4-0.8-mu-m. Appressorium formation was considerably reduced on ridge heights above and below this range. A broader range of ridge heights was observed to be inductive for appressorium formation by P. polysora, P. menthae, P. hieracii, P. sorghi, P. arachidis, and Physopella zeae. These rusts maintained high levels of appressorium formation on ridges up to 2.25-mu-m high. Seven species, including Coleosporium asterum, C. tussilaginis, Phragmidium potentillae, Puccinia coronata, P. graminis f. sp. tritici, P. g. f. sp. avenae, and Tranzschelia discolor, did not form appressoria on microfabricated topographies of defined heights, but they did form very low numbers of appressoria on scratched polystyrene membranes. Phakopsora pachyrhizi formed appressoria both on smooth membranes and in apparent association with ridges. The unique characteristics of the thigmotropic responses demonstrated by some of the rust species will be valuable in studying the mechanisms of thigmotropic sensing and appressorium formation.