Infection structures of fungal plant pathogens : a cytological and physiological evaluation

Many fungi differentiate specific infection structures m order to infect the host plant. The spore attaches to the host surface the cuticle, and the germ tube may recognize suitable penetration sites, over which an appressorium ,s formed Additional wall layers in appressoria of many fungi suggest that this structure supports mcreasing pressure during the penetration process. During appressorium formation, synthesis of polymer-degrading enzymes is often mitiated. Cutinases, cellulases and pectm-degrading enzymes can be formed in a developmentally controlled or adaptive, i.e. substrate-dependent, fashion. The penetration hypha develops below the appressorium. This hypha has a new wall structure and exhibits features which serve to breach the plant cell wall. However, at present it is not clear whether penetration hyphae arising from appressoria are more efficient in penetration or induce less damage than hvphae which penetrate without detectable special adaptations. The infection hypha differentiates within the host. During differentiation a characteristic set of enzymes ,s synthesized to enable successful establishment of the host-pathogen relationship. If, as in most cases, multiple forms of cell walldegrading enzymes are formed by the pathogen, mutagenes.s or deletion of a gene encoding one of these enzynies very often has no effect on pathogen.city or even virulence. Proof is missing very often that an enzyme is needed at the right time and at the right site of infection. Events occurring during differentiation of fungal infection structures are reviewed with special emphasis on Magnaporthe grtsea, Colletotrichum spp., and rust fungi, and common features which may be of importance to the success of infection are discussed.