Effect of canavanine from alfalfa seeds on the population biology of Bacillus cereus

Bacillus cereus UW85 suppresses diseases of alfalfa seedlings, although alf alfa seed exudate inhibits the growth of UW85 in culture (J. L. Milner, S. J. Raffel, B. J. Lethbridge, and J. Handelsman, Appl. Microbiol. Biotechnol , 43:685-691, 1995), In this study, we determined the chemical basis for an d biological role of the inhibitory activity. All of the alfalfa germ plasm tested included seeds that released inhibitory material. We purified the i nhibitory material from one alfalfa cultivar and identified it as canavanin e, which was present in the cultivar Iroquois seed exudate at a concentrati on of 2 mg/g of seeds. Multiple lines of evidence suggested that canavanine activity accounted for all of the inhibitory activity. Both canavanine and seed exudate inhibited the growth of UW85 on minimal medium; growth inhibi tion by either canavanine or seed exudate was prevented by arginine, histid ine, or lysine; and canavanine and crude seed exudate had the same spectrum of activity against B, cereus, Bacillus thuringiensis, and Vibrio cholerae , The B. cereus UW85 populations surrounding canavanine-exuding seeds were up to 100-fold smaller than the populations surrounding non-canavanine-exud ing seeds, but canavanine did not affect the growth of UW85 on seed surface s. The spermosphere populations of canavanine-resistant mutants of UW85 wer e larger than the spermosphere populations of UW85, but the mutants and UW8 5 were similar in spermoplane colonization. These results indicate that can avanine exuded from alfalfa seeds affects the population biology of B. cere us.