Fungal attachment and penetration

Many plant pathogenic fungi infect their hosts without using wounds or natural openings, such as stomata. They can penetrate the plant epider!Jjlis directly, although its structure is composed in such a way as to deter microorganisms Ouniper, 1991). The outermost layer of the epidermis, the cuticle, is a mixture of the polyester cutin and wa.,'{ (Holloway, 1993; see Chapter 2) anchored to the wall layers below which consist mainly of carbohydrates and proteins (Carpita and Gibeaut, 1993). The way fungi manage to infect the epidermis has been a matter of interest since the earliest studies On plant diseases. DeBary (1884) and Chester (1946) were the first to review the literature on fungal penetration. More recently, Martin (1964) and, some years later, Van den Ende (1974) concentrated on the degradation of the cuticle and its role in disease resistance. Kolattukudy (1985) was the first to summarize the exciting new facts on the molecular qualities of the enzyme cutinase and the mechanism of cutin degradation. In most recent years, reviews have concentrated on the different tasks of cutinase isozymes (Kbller, 1991; KblIer et al., 1995) and their importance as a pathogenicity factor during the early stages of infection (Schafer, 1994). Since cutin is only one hurdle in the obstacle race of the fungus to gain access to the epidermal protoplast, more general reviews may also be of interest to the reader. Elad and Evensen (1995) discuss the mode of infection by Bot"rytis and other reviews deal with the different types of fungal infection structures produced by biotrophic or necrotrophic pathogens (Hardham, 1992; Mendgen and Deising, 1993; Mendgen et al., 1996). This chapter concentrates on recent results explaining the factors that contribute to the initiation of the penetration process. In addition, observations demonstrating modifications of the plant cell wall during fungal penetration are included ..