A possible mechanism of action of plant growth-promoting rhizobacteria (PGPR) strain Bacillus pumilus WP8 via regulation of soil bacterial community structure.

According to the traditional view, establishment and maintenance of critical population densities in the rhizosphere was the premise of PGPR to exert growth-promoting effects. In light of the facts that soil bacterial community structures can be changed by some PGPR strains including Bacillus pumilus WP8, we hypothesize that regulation of soil bacterial community structure is one of the plant growth-promoting mechanisms of B. pumilus WP8, rather than depending on high-density cells in soil. In this study, denaturing gradient gel electrophoresis (PCR-DGGE) was performed to evaluate the relationship between changes in soil bacterial community structure and growth-promoting effect on the seedling growth of fava beans (Vicia faba L.) during three successive cultivations. We found that B. pumilus WP8 lacks capacity to reproduce in large enough numbers to survive in bulk soil more than 40 days, yet the bacterial community structures were gradually influenced by inoculation of WP8, especially on dominant populations. Despite WP8 being short-lived, it confers the ability of steadily promoting fava bean seedling growth on soil during the whole growing period for at least 90 days. Pseudomonas chlororaphis RA6, another tested PGPR strain, exists in large numbers for at least 60 days but less than 90 days, whilst giving rise to slight influence on bacterial community structure. In addition, along with the extinction of RA6 cells in bulk soils, the effect of growth promotion disappeared simultaneously. Furthermore, the increment of soil catalase activity from WP8 treatment implied the ability to stimulate soil microbial activity, which may be the reason why the dominant population changed and increased as time passed. Our study suggests that regulation of treated soil bacterial community structure may be another possible action mechanism.