Erwinia amylovora amino acid auxotrophy and epiphytic growth on apple stigmas

The Gram-negative bacterium Erwinia amylovora is the causal agent of fire blight, a devastating disease of apples and pears. The main route for E . amylovora infection occurs on apple flowers initiating with epiphytic growth on the nutrient rich apple stigma surface. This work probes the requirement of amino acids by monitoring epiphytic growth of eleven E . amylovora amino acid auxotroph strains inoculated onto stigmas of detached apple flowers. Since asparagine is a major nitrogen transport molecule in apples, similar experiments using E . amylovora strains deficient in asparagine synthesis and uptake are also investigated. Results of the growth assays show that amino acid biosynthesis deficiencies do not prevent E . amylovora epiphytic growth. However, asparagine uptake is required for required for robust epiphytic growth. These data support the hypothesis that amino acid sources, whether free amino acids, peptides of proteins, are available in the stigma environment.