Objective To investigate the effects of Burkholderia ubonensis P5 and Pseudomonas grimontii RP22 on the growth of Chinese fir seedlings for revealing the mechanism of the two types of PSBs in promoting growth of Chinese fir.

Method Based on the six-month-old live Chinese fir seedling, the formulated P5, RP22 and P5 + RP22 fungicides were applied by root irrigation once a month for 3 months. Seedlings and soil were sampled after 3 months to determine the growth properties, physiological properties of seedlings and soil properties; transcriptome analysis of seedlings was performed based on Illumina HiSeq 4000 sequencing platform and screening of Chinese fir seedling growth-promoting genes.

Result Compared with the control group (CK), microbial agent addition of all three treatments promoted the growth of Chinese fir seedlings. The physiological mechanisms of the three treatments on the growth of Chinese fir seedlings included: PSB increased the physiological indexes such as chlorophyll, soluble sugars, soluble proteins, and the phosphorus content of Chinese fir seedlings; and at the same time, it increased the content of active phosphorus, active potassium, and total phosphorus of soil. Transcriptome results indicated that differentially expressed genes (DEGs) in Chinese fir seedlings after three treatments were significantly enriched in metabolite synthesis, phenylpropane biosynthesis, linoleic acid metabolism, α-linolenic acid metabolism, and photosynthesis pathways. The pathways analysis revealed that PAL, COMT, 4CL, LOX1_5, AOS and AOC genes were significantly up-regulated in the P5 treatment; PAL, TOGT1, CYP73A, LOX1_5, psbS and psaF genes were significantly up-regulated in the RP22 treatment; and LOX1_5, psbO, petH and psaF genes were significantly up-regulated in the P5 + RP22 treatment. It indicated that the growth-promoting mechanism of the three treatments on Chinese fir seedlings was the result of multiple metabolic pathways coordinating with each other.

Conclusion The three fungicides treatments can promote the growth of Chinese fir seedlings by regulating the physiology, increasing soil nutrient content and up-regulating the expression of genes in the pathways of phenylpropane biosynthesis, linoleic acid metabolism and photosynthesis.