A POSSIBLE MOLECULAR BASIS FOR OBLIGATE HOST‐PATHOGEN INTERACTIONS

Abstract

SUMMARY This article is concerned with the molecular basis of host‐obligate‐parasite interactions that lead to either resistance or susceptibility of the host plants to obligate fungal pathogens. A scheme of molecular events that may occur during the primary stages of host‐pathogen interactions is proposed. It implies derepression of novel genes, intercistronic complementation of oligomeric enzymes in the host and the pathogen and, in some cases, complementation between host‐specific and pathogen‐specific polypeptides leading to the formation of catalytically different enzymes whose functions are essential for the continuation of the host‐parasite relationship. The basic tenets of the scheme are (a) transfer of nucleic acids, (b) synthesis of new RNA and proteins, (c) subunit interactions and formation of hybrid enzymes and (d) enzyme complementation during host‐parasite interactions. These are discussed in the light of some recent discoveries in plant pathology, biochemistry, virology and molecular biology. Transfer of macromolecules from one organism to another at the levels of (i) DNA, (ii) RNA or (iii) polypeptides frequently occurs in nature. Transfer of polypeptides (or amplified DNA segments or mRNA molecules coding for them) from pathogen to the host cells could lead to complementation between host‐specific and pathogen‐specific subunits of oligomeric enzymes. There are readily detectable changes in the transcription pattern at early stages of rust infection. Further analyses have shown that the observed changes in the pattern of gene expression are due to biochemically detectable alterations in the transcriptive specificity of RNA polymerases as well as in the template activity of chromatin. These findings are consistent with the derepression of novel genes in the host elicited by host‐pathogen interactions. Host‐pathogen interactions lead to the appearance of RNases with unique physical and catalytic properties. The appearance of these RNases coincides with (i) a remarkable increase in the rate of synthesis of all major classes of RNA molecules and (ii) an accumulation of RNA in the inoculated host suggesting an essential role of the new RNases in the post‐transcriptional processing of precursor RNA molecules.According to the postulated scheme, changes in the pattern of transcription and post‐transcriptional processing are brought about by intragenic and intergenic complementation of the enzymes involved in these processes including RNA poly‐merase and RNase molecules. The phenomenon of intragenic complementation has been observed in a large number of organisms including plants and fungi. In addition, fully functional hybrids of oligomeric enzymes have been produced in vitro with subunits from different tissues, species, genera, families and even phyla. In procaryotic systems, interactions of host‐specific and phage‐specific subunits leading to the formation of new enzymes are of common occurrence. In all oligomeric enzymes, relatively subtle changes in the subunit structure lead to radical changes in their catalytic and regulatory properties. This phenomenon is illustrated by the two enzyme systems, RNA polymerases and RNases. The mode of operation of the complementation model and its relevance to the host‐specificity of the pathogens and resistance or susceptibility of the host plants are described.