Promotive effects of exogenous 5-aminolevulinic acid (ALA) on the biosynthetic pathway of volatile aromatic compounds in tomato fruits

Tomato fruit aroma, determined by diverse volatile organic compounds, is crucial for consumer preference yet often neglected in breeding programs. This study investigates the effects of 5-aminolevulinic acid (ALA) on the biosynthesis of volatile compounds in tomato fruits (Solanum lycopersicum cv. ‘184’). Treatment with 200 ml/L ALA was applied at the fruit setting stage (fruit diameter of approximately 1 cm), repeated at 10-d intervals, and the volatile profiles were subsequently analyzed. The results showed that ALA significantly enhanced fruit quality indexes, including soluble solids, soluble protein, and vitamin C. Combined headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME/GC-MS) and electronic nose analyses revealed that ALA increased the diversity and abundance of volatiles, particularly fatty acid-derived and carotenoid-derived volatiles. Partial least squares regression (PLSR) analysis identified 2-(2-methylpropyl)-thiazole and (E)-3-hexen-1-al as key green aroma contributors. ALA also modulated the activities and gene expression of key enzymes (including lipoxygenase (LOX), hydroperoxide lyase (HPL), carotenoid cleavage dioxygenases (CCDs), and branched-chain amino acid transaminase (BCAT)) involved in volatile biosynthesis. These findings demonstrate that exogenous ALA enhances tomato fruit aroma by regulating volatile biosynthesis, providing a theoretical basis for improving fruit quality.