Different responses of sugarcane (Saccharum spp. hybrid) and rhizosphere microorganisms to application of single or mixed plant growth-promoting bacterial stains

BACKGROUND: Plant growth-promoting bacteria (PGPB) benefit plant growth and development via different direct and indirect mechanisms. However, our knowledge of rhizosphere responses at different plant growth stages to diverse PGPB applications in sugarcane (Saccharum spp. hybrid) is limited. In this study, four strains of bacterial genera, Gluconacetobacter diazotrophicus PAL5 (PAL5), Streptomyces chartreusis WZS021 (WZS021), Bacillus spp. CA1 (CA1), and Pseudomonas mosselii CN11 (CN11) were inoculated into two sugarcane varieties (B8, ROC22) as single and mixed cultures in a pot experiment. The effects of single and combined application of PGPB on nitrogen metabolism and agronomic traits of sugarcane, as well as on chemical and biological properties and the diversity of microbial communities in the rhizosphere, were observed. RESULTS: The inoculation of PGPB resulted in changes in microbial communities, which improved soil N and P cycling and increased the activities of GOT, GPT, NR, and GS, chlorophyll content (SPAD), plant height, and biomass in sugarcane. High-throughput sequencing (HTS) analysis revealed that Proteobacteria, Firmicutes, Chloroflexi, and Actinobacteria were the main microbial communities at the phylum level. Spearman's correlation analysis showed that Bacillus (a genus of Firmicutes) was positively correlated with the physiological properties of sugarcane and the physicochemical properties of soil. Using MicroPITA analysis, this study found that PAL5 treatment was the most representative at the seedling stage, and the maximum diversity of soil bacteria occurred at the elongation stage of sugarcane. At the elongation stage, the species diversity in the PAL5 treatment was abundant, and the species composition was the least similar to that in other treatments. In the WZS021 treatment, there were multiple PGPB and the highest diversity of soil bacteria, which could be beneficial throughout the entire growth duration of sugarcane. CONCLUSIONS: Inoculation of the strains of bacterial genera Gluconacetobacter diazotrophicus PAL5 (PAL5), Streptomyces chartreusis WZS021 (WZS021), Bacillus spp. CA1 (CA1), and Pseudomonas mosselii CN11 (CN11) could enrich the beneficial bacteria in sugarcane. Bacillus and Firmicutes were the key strains that improved soil physicochemical properties and sugarcane growth indexes. Therefore, to develop a new environmental protection biological agent, we should focus on its effects on the enrichment of soil Bacillus and Firmicutes, as well as on different sugarcane varieties.