THE PHYTOALEXIN RESPONSE: ELICITATION, SIGNALLING AND CONTROL OF HOST GENE EXPRESSION

Abstract

Summary1. Induced resistance of plants to incompatible races of their microbial pathogens is often characterized by rapid cell death (the hypersensitive response) and the accumulation of low‐Mr antimicrobial compounds termed phytoalexins. There is much indirect evidence to support a major role for phytoalexin induction as an event determining host resistance, and genetical analysis of race‐specific pathogen‐host interactions predicts that induced resistance occurs via interaction between a pathogen avirulence gene product and a host resistance gene product. Elicitors (inducing agents) of phytoalexin accumulation are possible candidates for the active products directly or indirectly resulting from the expression of microbial avirulence genes. The present review considers the process of phytoalexin induction, at the molecular level, from the fungal elicitor to the early changes in host gene expression associated with its action.2. A number of microbial polysaccharides, glycoproteins, pectic enzymes, peptides and fatty acids have potent elicitor activity.3. The results of studies on the purification and characterization of microbial elicitors depend on the nature of the bioassay used. Methods for elicitor extraction may lead to artifacts or loss of race specificity.4. Elicitors isolated from plant pathogens may be race‐specific or race‐non‐specific. In cases where only race‐non‐specific elicitors can be shown, race‐specific induction of phytoalexins may result from the action of enhancer or suppressor molecules. Fungal glucans have been proposed as candidates to act in both these roles.5. Molecular genetic approaches to the identification of phytopathogenic bacterial avirulence genes may help to prove or disprove the role of elicitors of the phytoalexin response as agents responsible for the induction of host resistance. Similar analyses, involving genetic transformation, should soon be possible for phytopathogenic fungi.6. Studies on the nature of host receptors for microbial elicitors are still in their infancy. Such receptors are probably localized in the plant plasma membrane, and elicitation results in often striking changes in host membrane properties.7. Cyclic 3′,5′‐adenosine monophosphate and polyamines do not appear to act in plants as intracellular transducers of the phytoalexin response. Interest is now being shown in a possible role for calcium in intracellular signalling.8. Plant cells contain endogenous elicitor molecules whose synthesis or release may play a role in the intercellular transmission of the phytoalexin response. The main candidates for endogenous elicitors are pectic fragments of the host cell wall, although no direct evidence for their involvement in plant‐pathogen interactions is available. Pectic fragments may act as synergists in parallel with, rather than as couplers in series with, microbial elicitors. The role of ethylene as a response coupler for induced resistance phenomena is difficult to assess.9. Some elicitors may themselves move to the host cell nucleus, although whether this is their effective site of action remains unclear.10. Induction of the phytoalexin response is associated with specific changes in host gene expression related to the selective induction of new mRNA species and enzyme activities.11. Progress is now being made in the characterization of enzymes specific for phytoalexin biosynthesis. Enzyme induction and/or infection appears to involve rapid modulation of gene transcription, although some post‐translational events may also be involved in determining induction patterns.12. Work is now commencing on the sequencing of the genes encoding elicitor‐inducible enzymes. Multigene families have been identified for phenylalanine ammonia‐lyase and chalcone synthase.13. Phytoalexin accumulation may be accompanied by the rapid induction of ethylene biosynthesis and the expression of genes encoding activities involved in the synthesis of hydroxyproline‐rich glycoproteins.14. Pectic endogenous elicitors of phytoalexin accumulation are also active as inducers of systemic proteinase inhibitor synthesis in some solanaceous species.15. Results of studies on the mechanisms of signal transmission and differential gene expression in relation to active defence reactions should be of relevance to many other aspects of the plant's response to environmental stimuli at the physiological, biochemical and molecular genetical levels.