Plant growth-promoting rhizobacteria (PGPR) interact with host plants through chemical signals. However, the specific signals in citrus–PGPR interactions remain unclear. Here, we show that a predominant and growth-promoting Burkholderia strain (Burk_2H3) isolated from citrus rhizosphere promotes plant growth by secreting N-(3-oxo-octanoyl)-L-homoserine lactone (PGPHL). Metabolomic analysis revealed that PGPHL abundance in Burk_2H3 secretions was 9.7- to 17.2-fold higher than that in three non-promoting Burkholderia strains. Exogenous application of PGPHL, but not other secretory metabolites, increased citrus seedling dry weight by 43.12%. Transcriptomic analysis showed that Burk_2H3, its cell-free supernatant, or PGPHL consistently upregulated key nutrient transporter genes in roots. Consistently, ionomic analysis confirmed higher root concentrations of nitrogen, phosphorus, and potassium. Field trials further demonstrated that PGPHL increased biomass by 21% in pepper, 15% in celery, and 18% in mustard. Together, these findings identify PGPHL as a candidate for developing plant growth stimulants and biofertilizers.

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