Plant Growth-Promoting Traits of Bacillus and Related Genera

Rhizosphere, the thin layer of soil under the direct influence of plant root, is a nutrient-rich microhabitat. The numbers of bacteria colonizing this niche are 100–1000 times more than the surrounding non-rhizosphere soil. More than 25 bacterial genera have been characterized as plant growth-promoting rhizobacteria, among which Bacillus sp. play a predominant role. Because of the non-fastidious nature and quick colonization of rhizosphere, these gram-positive rods are relatively abundant in the rhizosphere and can exert its plant growth-promoting benefits on the plant involved. Brevibacillus, Lysinibacillus, Bacillus subtilis, Bacillus cereus, and Bacillus amyloliquefaciens are some of the species that can act as plant growth-promoting rhizobacteria (PGPR). Bacillus sp. readily qualify as a PGPR owing to its phytohormone production, nitrogen fixation, siderophore production, hydrogen cyanide production, antagonism against plant pathogens, and production of certain allelochemicals. Some strains of Bacillus sp. show extreme tolerance to heavy metals and can be coupled with phytoremediating plants to remove heavy metal pollutants from contaminated soils. Bacillus sp. isolated from degraded mine soils also show plant growth-promoting effects and can be potentially used as a bioinoculant during the revegetation process of reclamation of mine ecosystem.