Physiological and Biochemical Mechanisms in Host Resistance and Susceptibility to Wilt Pathogens

Abstract

The literature on plant host-pathogen interactions which contribute to symptom development in vascular wilt diseases is extensive, controversial and comprises some elegant and some notso elegant studies. Certain controversies revolve around the possible roles of wilt toxins, the importance of mechanical plugging of xylem elements, the involvement of cell wall degrading enzymes, the effects and origin of plant growth regulating substances that appear to influence the wilting of plants, and the mechanisms of susceptibility and resistance to pathogens and disease development. Another poorly understood process in wilt diseases involves recognition phenomena between host and pathogen. The more we know about the various processes that contribute to a compatible or incompatible association of a plant host-pathogen relationship, the more we can appreciate the many subtle transitions and the sequential interdependence of physiological interactions between host and parasite that lead to disease development. A reading of the many excellent monographs, reviews, and symposium articles that have been written on various topics concerning wilt diseases of plants, hardly leaves much more to be said (Beckman 1964 and 1967, Duniway 1973, Mace et al 1981); yet there are concepts and some new information to be reviewed that may contribute to a better understanding of wilt diseases. The object of this essay is to focus on certain phenomena which may involve recognition between host and pathogen and the apparent cause and effect relationships among ethylene, induction of polygalacturonase activity, vascular gelation and the resulting changes in plant-water relationships that contribute to vascular wilt symptoms.