Microbial-derived metabolites coordinate growth regulation and stress tolerance in Phalaenopsis orchids

Microbial metabolites regulate plant growth and stress responses, yet their roles in crassulacean acid metabolism (CAM) plants remain insufficiently characterized. Here, we investigated the effects of metabolites from Bacillus licheniformis strain BlCP6, identified by 16S rDNA sequencing and whole-genome analysis, on growth and stress responses in Phalaenopsis orchids. Genome sequencing and metabolite profiling revealed that siderophores represent major bioactive compounds with direct plant growth-promoting potential, whereas osmoprotectants and extracellular lytic enzymes may contribute indirectly by enhancing stress tolerance. Application of BlCP6-derived metabolites significantly promoted leaf growth within three weeks and increased plant fresh weight after twelve weeks. Treated plants exhibited elevated phenylalanine ammonia-lyase activity, increased lignin deposition, enhanced accumulation of total phenolic compounds, and strengthened antioxidant capacity, as reflected by higher activities of ascorbate peroxidase, guaiacol peroxidase, and catalase. These biochemical changes were accompanied by improved tolerance to drought and chilling stress, as evidenced by higher relative water content and reduced membrane lipid peroxidation and electrolyte leakage. Metabolite pretreatment also moderately improved resistance to Fusarium yellow-leaf disease. Immunoblot analyses showed higher apparent accumulation of proteins associated with growth regulation, including zeaxanthin epoxidase, sedoheptulose-1,7-bisphosphatase, plasma membrane H⁺-ATPase, and nitrate reductase. BlCP6 treatment was also associated with increased accumulation of stress-related proteins, including superoxide dismutase, dehydrin, aquaporin, and autophagy-related proteins. Notably, increased accumulation of chloroplastic lipoxygenase is consistent with activation of oxylipin- and jasmonate-related signaling, while enhanced levels of cellulose synthase and pathogenesis-related protein 2 suggest cell wall reinforcement and defense responses. Collectively, BlCP6-derived metabolites appear to induce coordinated plant-wide responses that enhance growth performance and stress tolerance in Phalaenopsis orchids.