Insights into phosphorus-solubilizing characteristics of Serratia marcescens BP-2: a key role in promoting Bletilla striata (Thunb.) Reichb. f. growth in karst mountain ecosystems

The rhizosphere microbial community is closely tied to soil physicochemical properties. In karst regions, unique geology promotes phosphorus immobilization through reactions with Ca²⁺, forming insoluble calcium phosphate and worsening phosphorus deficiency. This study characterized phosphorus-solubilizing bacteria (PSB) isolated from the rhizosphere of Bletilla striata (Thunb.) Reichb. f. ( B. striata ) in karst mountainous regions and evaluated their phosphorus-solubilizing capacity and plant growth-promoting traits. Inoculation with microbiomes from karst soils significantly improved B. striata growth, including height, biomass, tuber weight, and length, while increasing soil available phosphorus and altering microbial community structure compared to agricultural cultivated soil treatments. 18 PSB strains were isolated—15 from the karst soil microbiome transplant group (KS-MT) and 3 from the cultivated soil microbiome transplant group (CS-MT). A highly efficient phosphorus-solubilizing bacterium, Serratia marcescens BP-2 ( S. marcescens BP-2). It exhibited exceptional phosphate-solubilizing capacity. Metabolomic analysis revealed that organic acid production-predominantly citric acid and other tricarboxylic acid (TCA) cycle intermediates-contributes significantly to its phosphate solubilization activity. Inoculation with S. marcescens BP-2 significantly enhanced plant height, leaf number, leaf length, and root development. Concurrently, the concentration of bioavailable phosphorus in the culture system increased markedly. Transcriptomic analysis further demonstrated significant upregulation of the high-affinity phosphate transporter gene PHT1 in inoculated plants, indicating enhanced molecular capacity for phosphorus acquisition. Results indicate that KS harbor a highly abundant community of phosphorus-solubilizing bacteria. Among these, S. marcescens BP-2 enhances phosphate solubilization primarily involve TCA cycle-mediated secretion of organic acids-particularly citric acid-thereby increasing phosphorus bioavailability in the rhizosphere. This is accompanied by significant upregulation of plant phosphate transporter genes (e.g., PHT1 family members) in B. striata , collectively promoting morphological development and biomass accumulation. This research provides a theoretical basis for understanding the co-adaptive strategies of plants and microorganisms in specialized habitats.