The Plant Cuticle

Most plant-pathogenic fungi gain access into their host by penetration of unwounded tissue. Some pathogens such as rusts invade the host via stomata (Hoch and Staples, 1987 and Chapter 2), whereas others penetrate the intact leaf surface without the requirement of natural openings (Aist, 1976; Emmett, 1975; Kunoh, 1984). The latter type of direct penetration encounters the plant cuticle, a noncellular hydrophobic structure covering the layer of epidermal cells. The cuticle thus serves as the first surface barrier that the pathogen has to breach. There is little evidence for the mere physical strength of the plant cuticle as a major factor in plant defense against pathogens. In some cases, the thickness of plant cuticles has been correlated with an increased passive resistance against fungal attack. This correlation, however, appears to be an exception rather than the rule (Martin, 1964). Furthermore, the cuticle has not been considered to play a major role in the active defense mechanisms of disease-resistant cultivars. There is good evidence that the cuticle is penetrated by the attacking pathogen before the sequential steps of disease development are halted by the active defense reactions of the challenged plant. Recent examples for this lack of cuticle involvement in cultivar resistance are the host—pathogen interactions of Venturia ivaequalis—apple (Valsangiacomo and Gessler, 1988) or Phytophthora infestans—potato (Gees and Hohl, 1987). The breaching of the cuticle can also be accomplished in many interactions of pathogens with nonhost plants (Heath, 1987).