Colonization of soybean roots by Bacillus megaterium B 153-2-2

Abstract

Colonization of soybean seedling roots and hypocotyls by Bacillus megaterium strain B 153-2-2 was studied in the greenhouse using two autoclaved growth media, vermiculite or a mixture of sand, soil, pea gravel and vermiculite (1:1:1:1, v/v), and in the field with natural soil. In the greenhouse B153-2-2 was recovered from hypocotyls of colonized seedlings at 5 cm above the medium line and the entire length of roots at 1 wk after coating the seed with the bacterium. No differences were recorded between the two media in the recovery of B153-2-2 from roots or hypocotyl with time. Populations of B153-2-2 on roots declined toward the root tip and approached a linear relationship with root depth. At 4 wk, populations of B153-2-2 were significantly greater than those at 1 wk on deeper roots. Bacterial cells were found to attach on the root surface of 1-wk-old seedlings. In the field, populations of B153-2-2 were recovered from hypocotyls and from roots at least 20cm deep with the highest populations occurring on the upper 10cm of root. Distributions of populations of B153-2-2 on hypocotyl and roots approached a cubic relationship. In the presence of an introduced fungal pathogen, Rhizoctonia solani isolate 65L-2, populations of B153-2-2 varied, but root colonization by B153-2-2 was not affected. Once introduced either by seed coating or soil treatment, soybean seedling, roots and hypocotyls were colonized by B153-2-2 for 4wk, whether grown in sterile media in the greenhouse or in natural soil in the field.