Abstract

Various Erwinia amylovora strains from different origins were assayed for phytotoxin production. When cultivated pear cells were embedded in solidified plant medium and inoculated with bacteria, a zone of growth inhibition was observed for some of the strains. A membrane-diffusible toxic compound was identified as L-2,5-dihydrophenylalanine (DHP), an inhibitor of the shikimic acid pathway. In plants DHP caused a reduction of elicitor-induced synthesis of phytoalexins and killed cultivated pear cells within 1 day. Bacterial synthesis of DHP was prevented by aromatic amino acids in the growth medium and was increased when pear juice or malate was present. Pear cells also were killed by strains that did not produce DHP, indicating a second plant cell-damaging pathway.