Pectate lyase isozymes and the pathogenicity of soft rotting strains of Erwinia for melon and cucumber

Abstract

Two strains of an Erwinia sp. obtained from soft rotting melon (Cucumis melo) plants in Egypt were found to be highly pathogenic on melon but non-pathogenic on the related crop cucumber (C. sativus). Typical isolates of E. carotovora ssp. carotovora (Ecc) were pathogenic on both species while isolates of the related ssp. atroseptica were pathogenic in neither. Diagnostic tests showed that the two strains had characteristics which were not wholly typical of Ecc nor of the related species E. chrysanthemi (Echr) but indicated affinities with both. Thus. both strains produced indole, which is typical of Echr but not Ecc, but fermented lactose and trehalose, which are characteristics of Ecc but not Echr. They also produced basic pectate lyases only on a polypectate medium, a characteristic which is more typical of Ecc than of Echr. One of the strains was tested for its ability to grow in cucumber cotyledons and although the generation time and final population achieved were less than half of those in melon, substantial growth did occur. Thus, although not a soft rotting pathogen of cucumber, it is clearly a parasite of this species. Culture supernates of both isolates were capable of soft rotting cucumber tissue and so the failure to cause a soft rot was not due to the resistance of the tissues to the enzymes produced by the two strains but could be due to the failure of cucumber tissue to induce the production of the appropriate enzymes. An analysis of pectate lyase production by one of the strains showed that it produced basic isozymes only when grown on a polypectate medium or on autoclaved cucumber tissue but it produced an acidic isozyme in addition to basic isozymes when grown on autoclaved melon tissue and when colonising melon seedlings. It is suggested that the failure of the two melon strains to cause a soft rot in cucumber seedlings is due to the failure of the tissue to provide the appropriate environment for the induction of the set of lyase isozymes required for tissue maceration.