13 The Biotrophy–Necrotrophy Switch in Fungal Pathogenesis

Abstract

In recent years, increasing numbers of plant pathogenic fungi have been identified as hemibiotrophs, i.e., pathogens sequentially differentiating biotrophic and necrotrophic hyphae in the host tissue. Interestingly, these pathogens make it possible to analyze biotrophic and necrotrophic lifestyles at the molecular level in the same genetic background. Biotrophic development strictly depends on prevention of initiation of plant defense responses, either by masking pathogen-associated molecular patterns or by secretion of effectors which efficiently interfere with recognition of the invading pathogen and/or signal transduction. After establishment of biotrophic hyphae, hemibiotrophs switch to necrotrophic development. The signal(s) leading to switching of hyphal morphology and lifestyles is/are so far unknown, but fundamental physiological changes are associated with these changes. For example, the carbon sources available during biotrophic growth in the interfacial matrix are probably strikingly different from those available during growth on killed host tissue, and, accordingly, different sets of sugar transporters differing in their substrate specificity are activated. Furthermore, associated with changing lifestyle, dramatic changes in fungal secondary metabolism occur, and these changes probably pave the necrotrophic pathway.