Fungal pathogenicity—establishing infection
The first obstacle encountered by any plant pathogen is the outer surface of a plant, whether in the form of a tough leaf cuticle, a fibrous stem, or the root surface submerged within soil. In each case, pathogenic fungi have evolved mechanisms to breach these surfaces, by using natural openings, such as stornata, or direct rupture of the surface layers. This chapter describes the early events in plant infection and the specific morphogenetic pathways that have evolved in fungi for this purpose. It concentrates mainly on the developmental biology of the rice blast fungus, Magnaporthe grisea, an appressorium-forming cereal pathogen. In recent years, rapid progress has occurred in identifying genes from this fungus that are involved in the plant infection process and in determining their function. A view of the specific signal transduction pathways and gene expression patterns required for plant colonisation is, therefore, emerging and can act as a framework for understanding the biology of fungal pathogens. Based on this information, experiments can be designed to determine the conservation of pathogenic processes among diverse species. This chapter follows the chronology of plant infection by M. grisea and describes the key genetic components so far identified and their known, or predicted, functions. Comparisons to other fungal pathogens are made throughout the text.