Formation of infection structures as a recognition response in fungi

Abstract

Fungal pathogens develop specialized infection structures by which they penetrate and colonize a host plant. These structures consist of a simple appressorium in some fungi that are little different in appearance from the germ tube, whereas in others the appressorium is thick walled and separated from the germ tube by a septum. For example, in the rust fungi, the appressorium is morphologically prominent and with a septum that aids identification. It is one of five parts of the complex infection structure which includes the peg, substomatal vesicle, infection hyphae, and haustorium mother cell. Over the past few years, a number of researchers have published studies on germinating conidia of fungal plant pathogens which suggest that germ tubes recognize and respond to specific topography of the host. Infection either fails or is erratic unless the correct information is present to induce the infection structures. The most definitive of these studies has been generated from work with fungi that produce appressoria delimited by a septum—the anthracnoses, the powdery mildews, and the rusts. The purpose of this review is to bring together information on induction of appressoria because the work elucidates in more general ways how fungi can perceive and respond to their environment. Our review is not intended to be exhaustive, rather it is to make the point that there are three different stimuli to which fungi must respond in sequence for the infection structures to develop—cellular junction lines of the epidermis, stomatal guard cells, and the mesophyll cell wall. Some of the work on rusts has been summarized by Littlefield and Heath (1979). Other reviewers, e.g., Emmett and Parbery (1975) and Allen (1976), have dealt with infection structure formation in other fungi as well. Although a great deal of new information has been added recently to our understanding of the control of early phases of spore germination when the germ tube begins to elongate (Macko et al., 1976 ; Littlefield and Heath, 1979), the subject of our review covers the period from germ tube elongation to post elongation development.