Plant growth promotion by Bacillus amyloliquefaciens FZB45 depends on inoculum rate and P-related soil properties

We have studied the effect of soil P-related properties and inoculum concentration on plant growth promotion by the phytase-producing strain Bacillus amyloliquefaciens FZB45. The response (shoot fresh weight/Pi content) of a mycorrhizal-independent plant, Chinese cabbage, was evaluated in a soil with well-defined P-related properties. Two inoculum concentrations were evaluated under four P regimes: no P addition, inorganic P, and two levels of phytate. Significant interaction between P regime and bacterial inoculation was found. FZB45 only promoted plant growth and P uptake at the higher rate of phytate, confirming that phytase activity is its major mechanism and that is limited by soil phytate availability. The effect caused by the lower inoculum concentration was superior than that by the higher, suggesting the simultaneous involvement of a direct effect. This effect was confirmed by a soilless test, which showed a hormone-like response. FZB45 produced IAA in vitro, but its role is to be determined. These results demonstrate that soil modulates the performance of plant growth-promoting rhizobacteria in a specific manner, consistent to the mechanisms of action involved. Determination of those mechanisms and their modulating factors helps predict conditions where plant growth promotion will result, an important step in increasing the consistency of PGPR.