Bacillus-related Spore Formers: Attractive Agents for Plant Growth Promotion

Aerobic spore-forming bacteria, such as Bacillus spp., Paenibacillus spp., and Brevibacillus spp., are one of the major soil bacteria (1, 2, 15). This finding is based on the culture-dependent methods. The culture-independent methods have also revealed that Bacillus spp. constitute a majority of the bacteria in a Dutch soil (6) and a soil contaminated with chlorinated compounds in Taiwan (3). In addition to soil communities, Bacillus spp. have been isolated from many crops (e.g., 11, 13). Bacillus spp. were a dominant bacterial group in the leaves and roots of rice plants; B. megaterium, B. luciferensis, Brevibacilllus agri were isolated from the roots and B. pumilus, B. subtilis, Paenibacillus amylolyticus from the seeds and leaves (12). In a recent study that compared the bacterial community structure of rice plants between leaf blade and leaf sheath, a variety of Bacillus spp., including B. aquimaris, B. cereus, B. gibsonii, B. megaterium, B. drentensis, B. koreensis, accounted for 6% to 27% of the total bacterial populations in the leaf blades, while those for 0.6% to 3.3% in the leaf sheaths (10). Bacillus spp. and Paenibacillus spp. are listed as major components of the bacterial population in the rhizosphere of potato, while Brevibacillus spp. are as a minor component (5). The bacterial community of a potato phytosphere was examined using both culture-dependent and independent methods and the results showed that Bacillus spp. and Paenibacillus spp. composed of 3% to 25% of 16S rRNA gene libraries of the root and tuber, and these ratios were higher than those obtained with the culture-dependent method (21). Paenibacillus pabuli, P. chitinolyticus, P. humicus, and B. thuringiensis were isolated from the rhizo-sphere of agronomic plants as chitinolytic bacteria (20). Metagenomic analysis of the rhizosphere soil of Lotus japon-icas found that Firmicutes constituted 5% of the total bacterial community, although it is not known whether or not they are affiliated with the genus Bacillus (24). Bacillus-related species have been also isolated from the gut of a soil-feeding termite (16). As described above, Bacillus-related species have abilities to colonize a variety of the soil and plant environment. Their most common characteristic is an ability to form endospores and thereby to persist in the environment. Survivability and stress tolerance are always the issues that many microbiologists have tackled for years, and those are now the topic in the context of ecological impacts (8). While the inoculation of beneficial microbes has attracted attention to …