Degradation patterns of pectic substrates related to the localization of bacterial pectate-lyases in the modelErwinia chrysanthemi/Saintpaulia ionantha

Erwinia chrysanthemiis a bacterium which macerates host plant tissues by secreting at least five major pectate-lyases (PLa–e) which depolymerize the homogalacturonans of the cell walls. Immunocytochemical studies were carried out onSaintpaulia ionanthaleaves inoculated withE. chrysanthemito localize both the homogalacturonans of the plant cell walls and the PLs a/d secreted by the bacterium. Different degradation patterns of pectic substrates were observed by following the loss of immunolabelling: several cell walls were highly degraded whereas some areas were still highly labelled, giving the walls a dotted appearance. The first attachment sites of PLa/d were on the side of the wall facing the intercellular spaces. In more degraded walls the enzymes were more regularly distributed throughout the walls. Local domains were evidenced where the substrate was not accessible for the PLs a/d. The involvement of the battery of PLs secreted byE. chrysanthemiin host–plant interactions is discussed.