Exopolysaccharides in the Interaction of the Fire-Blight Pathogen Erwinia Amylovora with its Host Cells

Abstract

The fireblight pathogen Erwinia amylovora depends on synthesis of exopolysaccharides in order to prevent defense reactions of the host plants. Mutants in genes for capsular polysaccharides, for levan surcrase and in the galactose metabolism were created by mutagenesis with transposon Tn5. A deficiency for levan sucrase did not affect virulence on pears. Mutants in synthesis of acidic exopolysaccharides (EPS) were apathogenic. The gal-mutant induced phytoalexins and callose production in plant cells. Both defense reactions were suppressed by dihydrophenylalanine, a phenylalanine analog secreted by some E. amylovora strains. This compound is synthesized in the shikimic acid pathway of the bacteria and is inhibitory in plants for synthesis of aromatic amino acids. The other mutants without EPS-synthesis were genetically and physically characterized. The insertions were mapped in a gene cluster, which is controlled by regulatory genes like the cloned and sequenced rcsA-gene.