Functional characteristics and culture conditions of five plant growth-promoting rhizobacteria strains

Studying the functional characteristics and culture conditions is essential for successfully exploiting and industrializing plant growth-promoting rhizobacteria (PGPR). To further develop and utilize the strains, five strains obtained from the rhizosphere of Polygonum viviparum, Koeleria glauca, Trifolium pratense, and Poa pratensis were selected as materials in a previous study. The functional characteristics of the strains included the ability to dissolve phosphorus, fix nitrogen, and secrete indoleacetic acid (IAA). The taxonomic status of the strains was confirmed by 16S rDNA molecular biotechnology. The growth characteristics, temperature tolerance, pH tolerance, gas producing characteristics, and acid producing characteristics of the strains were determined, as well as their most appropriate culture conditions. The results showed that M1 had the highest amount of soluble organic phosphorus, 170.37 µg·mL⁻¹, and Y3 had the highest amount of soluble inorganic phosphorus, 308.50 µg·mL⁻¹. The strains had the ability to secrete IAA, and the amount secreted was between 24.23 and 104.74 µg·mL⁻¹. Furthermore, the Y1 and Y3 strains had the ability to fix nitrogen, and produced 175.30 and 255.55 nmol·(h·mL)⁻¹(C₂H₄) from nitrogenase activities, respectively. From the determination of the growth characteristics of the strains, it was found that the five strains had entered the stable growth phase at 40 h, the suitable growth temperature was between 25 and 31 ℃, the suitable growth pH range was between 6.5 and 7.5, and there was no gas production when they were grown in LB medium. The pH was stable between 6.23 and 7.52 during the growth process, indicating their potential as industrial fermentation strains. The 16S rDNA identification confirmed that Y5 and Y3 were Bacillus subtilis, Y1 was Bacillus mycoides, M1 was Pseudomonas synxantha, and M6 was Pseudomonas simiae. This study lays the foundation for the industrial production of the strains.