Objectives This study was conducted to explore the promoting effect of plant growth promoting rhizobacteria and their metabolites on crop growth, and deeply understand the mechanism of their promoting crop growth, so as to provide excellent strain resources and technical support for the application of efficient microbial fertilizer.

Methods A pot experiment was carried out to clarify the growth-promoting effect of the fermentation broth components of Bacillus subtilis subsp. subtilis NRCB002. Five treatments were set up, namely fermentation medium (P), fermentation broth (F), fermentation broth supernatant (S), resuspended bacteria (G) and resuspended bacteria + acetoin (A) respectively, and the control treatment was irrigated with the same amount of deionized water (CK). Another pot experiment on the growth-promoting effect and application methods of acetoin was conducted. Two treatments of root irrigation (RI) and foliar spray (FS) with 0.1 g/L acetoin solution were set up, with the same amount of deionized water as the control (CK). Two weeks after treatment imposition in both studies, lettuce seedlings were characterized for plant height, leaf area, fresh and dry weights of shoot and root, and root index. NRCB002 was cultured under K medium with 0, 0.075, 0.375, and 1.5 g/L acetoin addition. The amount of NRCB002 biofilm formation was determined by crystal violet staining method.

Results 1) Compared with CK, fresh and dry weight of shoot and root were significantly increased by the other four treatments except for the P treatment. Treatments of S, F, and A also significantly increased the plant height, leaf area, total root length, total root surface area, total root volume and number of lateral roots of lettuce seedlings. Except for average root diameter and number of lateral roots, the growth characteristics of lettuce seedlings treated with S, F, and A were similar, S, F and A treatments significantly enhanced shoot dry weight, fresh and dry weight of root compared with G treatment. 2) Compared with the control, plant height, total root length, total root surface area, total root volume, number of lateral roots, root-shoot ratio, fresh and dry weight of shoot and root of lettuce seedlings treated with root application (RI) and leaf application (FS) of acetoin were significantly increased. Except for root dry weight and root-shoot ratio, there was no significant difference between the two treatments. 3) Acetoin significantly increased the biofilm formation of NRCB002. Under three application concentrations, the biomass of NRCB002 were similar, but the biofilm formation of NRCB002 with acetion of 0.375 g/L was significantly lower than that with acetion of 0.075 and 1.5 g/L.

Conclusions Acetoin is a key promoting component of NRCB002 fermentation broth. Root irrigation and foliar spray, especially foliar spray of trace amounts of acetoin directly promotes the growth of lettuce seedlings. Acetoin significantly enhances the biofilm formation of NRCB002, which would improve the stability of NRCB002, therefore acetoin and NRCB002 have synergistical effects.