Combined phosphate-solubilizing microorganisms jointly promote Pinus massoniana growth by modulating rhizosphere environment and key biological pathways in seedlings

Pinus massoniana is an important afforestation and industrial timber tree species in China; However, its growth and productivity are often suffered from phosphorus deficiency. Phosphate-solubilizing microorganisms (PSMs) can increase phosphorus efficiency and may have the important potential for use in forestry ecosystem. In order to evaluate whether the co-inoculation of PSMs would enhance plant growth than single inoculation, and better understand the synergistic mechanisms of PSM compound strains regulating P. massoniana growth, we used plant physiology, microbiome, and transcriptome technologies to assess the effects of combined PSMs on plant growth, rhizosphere microecology and plant physiological system. The results demonstrated that combined endophytic fungus (Penicillum oxalicum G17) and rhizosphere bacteria (Burkholderia sp. WJ27) significantly increased phosphate-dissolving ratio and promoted P. massoniana growth compared with a single strain or non-inoculation. The combination of G17 and WJ27 strains (G17&WJ27) evidently enhanced soil nutrients, as well as P, N, and K contents in seedlings. Although the diversities of fungi and bacteria were reduced under G17&WJ27 treatment, the relative abundances of ectomycorrhizal fungi (Trichophaea and Thelephora) and plant growth-promoting rhizobacteria (Sphingomonas, Burkholderia, Chitinophaga, Ramlibacter, and Rhodanobacter) increased. Transcriptome profiles showed that compared with non-inoculated plants, there were 7115 differentially expressed genes (DEGs) identified in G17&WJ27 treatment, which were involved in various biological pathways associated with growth regulation, including photosynthesis, sugar metabolism, antioxidant enzyme activity, plant hormones, amino acid biosynthesis, and inorganic phosphate transporters. This study would provide a theoretical basis for the development of compound microbial fertilizers for cultivation and management of Masson pine forests.