AN ELICITOR FROM PYRICULARIA-ORYZAE INDUCES RESISTANCE RESPONSES IN RICE - ISOLATION, CHARACTERIZATION AND PHYSIOLOGICAL-PROPERTIES

Resistance of rice to the blast fungus, Pyricularia oryzae, has been correlated with a range of physiological changes in the infected host tissue. Some of these resistance responses can be induced artificially by elicitors, including chitin and crude extracts from the fungus. Here, we report on the isolation of a single elicitor species from P. oryzae and on its purification to apparent homogeneity. The elicitor is a glycoprotein with a moi. wt of 15.6 kDa as indicated after SDS-polyacrylamide gel electrophoresis and carbohydrate specific labelling. Activity of the water-soluble elicitor is not affected by enzymatic digestion of the protein core, but is completely abolished after periodate treatment. The carbohydrate moiety which has no affinity to Con A consists mainly of mannose with some glucose and galactose. After application to rice leaves, enhanced activities of enzymes involved in lignification were observed. Injection into intercellular spaces of the leaves resulted in increased autofluorescence of cell wails, presumably due to the deposition of lignin or lignin-like substances. When injected into the intercellular spaces of genetically susceptible rice leaves prior to inoculation with a virulent race of the fungus, the elicitor enabled the plant to successfully defend itself against the pathogen. In these leaves fungal growth was restricted by active host responses similar to those observed in generically resistant plants. The results suggest that the isolated elicitor may be involved in the recognition process during the natural infection of rice by P, oryzae. It appears that protection is achieved by enhanced sensitivity of recognition or by promoting a faster and/or more intense host response.