Chemotactic response of Bacillus megateriumstrain B153-2-2 to soybean root and seed exudates

Abstract

Bacillus megateriumstrain B153-2-2, a root colonizer and potential biocontrol agent for rhizoctonia root rot of soybeans, showed a significant ( P=0.05) positive chemotactic response to soybean root and seed exudates at 0.03mgml -1with a maximum response at 4mgml -1in a capillary assay. B153-2-2 cells at the exponential growth stage in tryptone broth had a significantly higher chemotactic response than those at the stationary growth stage. Chemotactic responses occurred from 5 to 40°C and from pH5.5 to 9.0, with a maximum response at 27±1°C and pH7. Eighteen amino acids were detected in the exudates using HPLC analysis. B153-2-2 cells showed a chemotactic response to alanine, asparagine, glutamine, serine, and threonine at concentrations equal to that in the exudates. B153-2-2 cells had a chemotactic response to three of seven organic acids; malate at concentrations higher than 10 -5 m, malonate at 10 -3 m, and pyruvate and succinate at 10 -2 m. None of seven sugars induced a chemotactic response at the concentrations tested. B153-2-2 cells showed a significantly greater chemotactic response to seed than to root exudates at the optimum concentrations under the same conditions. The chemotactic response of B153-2-2 cells to a mixture of the five chemoattractant amino acids was significantly lower than that to the mixture plus 10 -3 mmalate, which in turn, was lower than that to seed but not root exudates. These results suggested that alanine, asparagine, glutamine, malate, serine, and threonine in soybean root and seed exudates may serve as chemoattractants to B153-2-2 cells. The highly sensitive chemotactic response of B153-2-2 to soybean root and seed exudates over a range of temperatures and pH may be a trait which contributes to its successful colonization of soybean roots and seeds.