Phytotoxic material from associations betweenErwinia amylovora and pear tissue culture: Possible role in necrotic symptomatology of fireblight disease

Abstract

Pear cell suspension cultures (PCSC) inoculated with virulent strains of the fireblight bacterium,Erwinia amylovora, exhibited massive necrosis within 7 days, whereas avirulentE. amylovora strains and other enterobacteria generally elicited very slight or no necrotic reactions. These results were generally repeated when pear seedlings (1–2 months old) were inoculated with these same bacterial strains. Fractions derived from massively necrotized PCSC were tested for biological activity against healthy PCSC and pear seedling cuttings. Of these fractions, the dialyzable, 70%-ethanol-soluble material—remaining after precipitation by ethanol of high-molecular-weight polysaccharides (“amylovorin”)—exclusively inhibited growth of pear callus, plasmolyzed and disintegrated pear cells in PCSC, and caused some browning of pear callus. Although cuttings of young pear seedlings placed in solutions containing this dialyzable, ethanol-soluble material generally became massively necrotized within 3 days, some residual methodological problems with this bioassay procedure must be solved. These observations suggest that some relatively small molecule(s) formed in PCSC inoculated with virulentE. amylovora can exert antagonistic biological activity against pear tissue and may play a role in the symptomatology characteristic of the fireblight disease.