Studies on cellular differentiation of Magnaporthe grisea. Physicochemical aspects of substratum surfaces in relation to appressorium formation

Abstract

Pathogencsis by Magnaporthe grisea , the causal fungus of rice blast disease, involves formation of a special infection structure termed an appressorium. Formation of the appressorium occurs apart from the host, but the regulation mechanism is not completely understood. Studies were performed on various liquids and solids to elucidate the physicochemical aspects involved in appressorium formation. Conidia started to germinate when they contacted either a solid or a liquid surface; nutrient was not required. Elongated germ-tubes differentiated to form appressoria on solid surfaces but not on liquid or agar gel surfaces. Both hydrophobic and hydrophilic solid surfaces were highly inductive for appressorium formation. Topographic characteristics did not appear to be important because appressoria were formed on both rough and apparently smooth surfaces. When germinated on freshly prepared agar gel surfaces, elongated germ-tubes penetrated directly into the soft surfaces without forming appressorium. On partially dried agar gel surfaces, however, they differentiated to form appressoria and then penetrated the surface with penetration hyphae elaborated from the appressoria. From these results, we inferred that a hard solid surface triggered appressorium differentiation.