IRON IS A TRIGGERING FACTOR FOR DIFFERENTIAL EXPRESSION OF ERWINIA-CHRYSANTHEMI STRAIN-3937 PECTATE LYASES IN PATHOGENESIS OF AFRICAN VIOLETS

This work was aimed at investigating possible links between pectinolys is and chrysobactin-dependent iron assimilation, two major determinant s of the pathogenicity of Erwinia chrysanthemi 3937 on African violets , Transcriptional expression of the relevant pectinase-encoding genes (pelA to pelE and pem) was analyzed during the early stages of pathoge nesis by means of GUS fusions, The pelD::uidA fusion was induced 6 h a fter inoculation, earlier and in higher levels than pelB::uidA, pelC:: uidA, pelE::uidA, and pem::uidA; pelA::uidA was not induced, PelD::uid A expression was compared with that of pelE:: uidA in different mutant s, In a cbr mutant, derepressed for chrysobactin, pelD::uidA was weakl y expressed, pelE::: uidA expression did not change significantly, and production of the cognate PelD and Pelf isoenzymes was reduced, A chr ysobactin-deficient pelD::uidA mutant grew poorly; no pelD::uidA expre ssion was recovered, In a kdgR mutant derepressed for pectinolytic fun ctions, pelD::uidA expression was the same until 15 h postinoculation, after which a substantial increase was apparent. The expression of pe lD::uidA was stimulated by the presence of iron chelators in the growt h medium and the absence of functional chrysobactin-mediated iron upta ke. The data provide evidence supporting the central role of iron in p lant pathogenesis.