Toxins

Abstract

Anton de Bary (1866) was the first to suggest that one reason why plant parasites damage their hosts could be that they secrete toxins into them. Gäumann (1954) even went as far as to say that ‘microorganisms are pathogenic only if they are toxigenic’. Most plant pathologists today would feel that this is too embracing a concept since there are many parasites which cause severe damage to their hosts by secreting degradative enzymes (section 7.6.2), as well as by altering the concentrations of plant growth regulating substances (Chapter 11). Traditionally, neither class of compound has been regarded by plant pathologists as toxins although animal pathologists recognize some enzymes as toxins (section 7.8). In consequence, it is difficult to formulate an acceptable definition of a toxin. Some authors have attempted to differentiate toxins affecting plants from those affecting animals by calling them phytotoxins, but some compounds affect both plants and animals. The terminology is further confused by the application of the name mycotoxins not, as might be expected, to toxins affecting fungi but to toxins of fungal origin which affect man or animals (Forgacs and Carll, 1962). Some mycotoxins also affect plants. In view of these difficulties the simple term toxin will be used in this book to describe compounds which have deleterious effects on living organisms and we shall be concerned primarily with those compounds of pathogen origin which affect plants. Here we shall not be concerned with enzymes since they have already been discussed in the previous chapter nor plant growth regulators, which will be reserved for Chapter 11.