Plysiologicnl a’nd Molecular Plmr~ Pafholou (1994) 44, 227-236 227 Studies on cellular differentiation of Magnaporthe grisea. Physicochemical aspects of substratum surfaces in relation to appressorium formation J.-Z. XIAO, T. WATANABE, T. KAMAKURA, A. OHSHIMA* and I. YAMAGUCHI Microbial Toxicology Labornloy, The hslilule of Plysical and Chemical Research (RIKEN), IV&o, Sailomn 351-01, Japaa (hepled for publication Febrtql 1994) Pathogenesis by Magtmporthe g&en, 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 physicochcmical 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 apprcssorium. 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 apprcssorium differentiation. INTRODUCTION Many plant pathogens form a specialized infection structure, the appressorium which facilitates adhesion and penetration of the host [4]. The formation of an appressorium involves cellular differentiation. Considerable research has been devoted to the biochemistry and ultrastructure of appressorium formation [for review see 4, 71. Formation of appressoria by rust fungi, especially by Uromyces spp. has been extensively studied to date. Staples and colleagues reported changes in protein and RNA synthesis during appressorium formation by Uromyces appendiculatus, and they described the substratum surface parameters necessary to trigger the formation of infection structures [7,8, 12, 141. Recently, two genes specifically expressed during infection structure formation were cloned and characterized in this fungus [I, 171. Magnaporthe grisea Bar (imperfect stage Pyricularia oryzae Car.) is a pathogen of rice causing a devastating disease called rice blast. This fungus produces darkly melanized appressoria which are a prerequisite for penetration into host tissues. An essential role of * Present address: Yuki Research Center, Nihon Bayer Agrochcm K.K., Yuki 951 l-4, Yuki, Ibaraki 307, Japan. 0885-5765/94/030227 + IO $08.00/O 0 1994 Academic Press Limited