Methyl Jasmonate Mediates ALA-Induced Salt Tolerance in Tomato Seedlings

5-Aminolevulinic acid (ALA) is a natural and environmentally friendly plant growth regulator that plays an important role in enhancing plant tolerance to a wide range of environmental stresses. Exogenous application of ALA enables rapid and efficient physiological regulation. Additionally, methyl jasmonate (MeJA) enhances salt tolerance in tomato seedlings by regulating ALA to promote jasmonic acid (JA) accumulation and strengthening the antioxidant defense system. To investigate how exogenous ALA alleviates salt stress physiologically, this study used ‘Condine Red’ tomato as the experimental material and examined the effects of MeJA-mediated ALA on the growth characteristics and stress tolerance mechanisms of tomato seedlings under salt stress. The results indicated that salt stress significantly inhibited tomato seedling growth, leading to marked reductions in biomass, chlorophyll content, and the enzymatic activities of POD, CAT, and APX. In contrast, SOD activity, MDA content, NPQ, soluble protein content, proline content, endogenous JA levels, and the expression of related genes were significantly increased. Under salt stress, exogenous application of ALA and MeJA alleviated the inhibitory effects on tomato seedlings. However, SHAM (salicylhydroxamic acid) aggravated salt stress damage to plants. The addition of ALA significantly mitigated these salt stress-induced injuries. These findings suggest that ALA may enhance salt tolerance in tomato seedlings by promoting JA accumulation and bolstering the antioxidant defense system.