Effect of Increased β-Glucosidase Activity on Virulence of Erwinia amylovora

Abstract

Plant tissues often contain β-glucosides that can be enzymatically hydrolyzed to produce toxic aglycones. It has been suggested that the low β-glucosidase activity found in Erwinia amylovora contributes to bacterial virulence by allowing the bacteria to infect plants that contain β-glucosides without inducing the formation of toxic aglycones. To test this suggestion, we created strains of E. amylovora which had high β-glucosidase activities and studied the ability of these strains to cause fire blight disease in pears ( Pyrus communis ). We isolated spontaneous mutants that were able to utilize β-glucosides as the sole carbon source and showed that one class had about 10 times as much β-glucosidase activity as the wild-type strain. In addition, we constructed several plasmids that carry the Escherichia coli bgl operon under the control of a transposon Tn 5 promoter that is expressed in E. amylovora . These plasmids were introduced in E. amylovora by transformation. Pathogenesis studies in immature Bartlett pear fruits, etiolated sprouts, and young shoots showed that a 100-fold increase in β-glucosidase activity does not interfere with normal development of fire blight disease in these model systems.