γ-Valerolactone Reprograms Tomato Metabolism to Enhance Seedling Growth

Background: γ-Valerolactone (GVL), a green volatile platform compound derived from lignocellulosic biomass, has recently been identified as one volatile organic compound produced by the plant growth-promoting rhizobacterium Stutzerimonas stutzeri NRCB010. Although our previous transcriptomic and phenotypic studies have verified that exogenous GVL can promote the growth of tomato seedlings, the global metabolic reprogramming behind this promoting effect remains unclear. Methods: Tomato seedlings were subjected to exogenous GVL treatments at three concentrations (0, 0.25, 0.5 g/L) for 24 h and 48 h separately. Combined with physiological growth measurements, ultra-high-performance liquid chromatography–tandem mass spectrometry-based widely targeted metabolomics was applied to systematically characterize GVL-mediated metabolic reprogramming. Results: GVL remarkably improved seedling height, biomass and root development, with 0.25 g/L GVL showing the strongest promoting effect. A total of 703 metabolites including lipids, flavonoids and alkaloids were identified. Principal component analysis and orthogonal partial least squares discriminant analysis revealed distinct metabolic separation between control and treated groups, verifying time and concentration dependent metabolic shifts. After 24 h, GVL activated primary pathways (the tricarboxylic acid cycle, amino acid and purine metabolism) to supply growth energy; after 48 h, central carbon and secondary biosynthetic pathways were enriched, and 0.5 g/L GVL specifically triggered defensive isoquinoline and indole alkaloid biosynthesis. Conclusions: GVL coordinately remodels primary and secondary metabolic networks to accelerate tomato seedling growth, which provides sufficient metabolomic evidence for developing GVL as a novel bio-based plant biostimulant.