Other Large Molecules in Relation to Gene-for-Gene Disease

Abstract

We are in this chapter still concerned with how variation in gene-for-gene diseases is stored chemically and recognized in host-pathogen associations. The general problem was stated in Chapter 2, using wheat stem rust for illustration. There are potentially more than a million phenotypes of wheat that differ in their reactions to Puccinia graminis tritici, and potentially more than a million phenotypes of P. graminis tritici that differ in their reactions on wheat. Axiomatically, all the relevant information is stored in the genes, i.e., in the DNA. However, the expression of the information and its manifestation as potentially more than a million races of stem rust is almost certainly not at DNA level. Molecular recognition and chemical interaction between constituents of the host and those of the pathogen occur between the products of the genes. But which products? In Chapters 2, 3 and, obliquely, 4 proteins were named as the products, susceptibility being seen as host protein-pathogen protein copolymerization. In this chapter we consider candidates other than proteins. This is done, not to challenge the protein copolymerization hypothesis, but to put the chemical storage and recognition of variation in proper perspective. This has not been done adequately before; and the very large literature of phytoalexins testifies to a widespread misconception about the storage of variation in molecules and its part in host-pathogen interaction.