Fungal morphogenesis and enzyme secretion during pathogenesis

Fungi represent a highly diverse evolutionary group of eukaryotic, heterotrophic organisms which share a common cellular organisation. The fungill thallus or mycelium consists of hyph<le, tubular slructures which may be divided into cellular units by sepIa. Growth of fungal hyph;lC occurs al thc lip and is medialed by continuous synthesis aIIII scnction of cell w;i11 material to the apex. Hyphal lip growth allows the myceliulll to rapidly spread over, ,lIld feed on, a variety of substrata. While many fungi are feeding saprophytically on dead organic matter , several fungal groups are able to exploit otl1er organisms as nutrient sources during sym-biotic or parasitic stages. Plant parasitic fungi have developed a variety or rcl'ined mechanisms to invade their hosts and to live at their .expcnse. So-called nccrotrophic (pertholrophic) fungi rilpidly kill the infected host by secretion of loxins (Chapter 4) and wall-Illazerating enzymes, In contrast, biotrophic fungi are able to absorb nutrients from living host cells while maintaining host integr'ily and viability for an extended period of time. Intermediate relationships are called hemibiotrophic, because an initiill biotrophic phase is followed by tissue destruction and colonization of the dead host tissue. In fact, it has become evident that many of the necrotrophic rungi are going through a short initial infection phase without arrecting hosl cell viability (Rodriguez-Galvez & Mendgen 1995). Successful infection depends on correct recognition of, and adaplalion to, the host ;1nd is orten accompanied by a high degree of cytological and molecular dirrerentiation of the I'ungus. Fungal pathogenicity <lnd plant susceptibililY or resistance are meehanistically inseparable from each other, and only <l combined understanding of both aspects will give a comprehensive perspective of the parasite-host relationship. This ;1pplies in particular to those fungal-plant interact ions determined by gene-for-gene relal ionshi ps, which arc now beginning to he unoerstood on a molecul<lr Jevel (see Cl1<lpters H;1nd 10). Wc expect that this knowledge wjll expand our potential to develop novel strategies for plant rJro[ection, e.g. by inlerrering with specil'ic slers of pathogen development. In this Chapter. we will describe the various stages of fungal infection ,IIH1 discuss propenies ano fUllctions of specialiLeo hyphae (inl"cuion structures) ill this process. The morphogenetic differentiation of the invading fungus is accompanied by a regulated expression of genes, such as thosc encoding cuticJe-and wall-degrading enzymes. Our present knowledge of their properties and their particular role during pathogenesis will also be discussed.