Effects of Exogenous SA/GABA Combined with ZnSO4 Treatment on the Physiological Metabolism and Flavonoid Biosynthesis in Finger Millet (Eleusine coracana L.) Sprouts

Finger millet (Eleusine coracana L.) is rich in bioactive compounds, including flavonoids. Following exogenous substance regulation, its sprouts can achieve efficient flavonoid enrichment. This study investigates the regulatory effects of exogenous salicylic acid (SA) and γ-aminobutyric acid (GABA) on the physiological metabolism, oxidative stress response, and flavonoid biosynthesis of finger millet sprouts subjected to 5 mM zinc sulfate (ZnSO4) stress. Compared to treatment solely with ZnSO4, the application of both 50 μM salicylic acid (SA) and 1 mM gamma-aminobutyric acid (GABA) markedly enhanced flavonoid biosynthesis, with respective yields of 8.53 μg/sprout and 8.85 μg/sprout observed by 6 days post-germination. Concurrently, SA and GABA attenuated ZnSO4-induced oxidative damage. During days 4 and 6 post-germination, malondialdehyde and hydrogen peroxide levels in sprouts were significantly reduced, with levels at 6 days showing a particularly notable decrease. Moreover, the catalytic activities of catalase, peroxidase, superoxide dismutase, and ascorbate peroxidase were significantly upregulated. Further analysis revealed that both treatments significantly activated the phenylpropanoid biosynthesis pathway. The activities of key rate-limiting enzymes, phenylalanine ammonia-lyase, cinnamate-4-hydroxylase, and 4-coumarate-CoA ligase, along with the expression levels of their corresponding genes, were markedly upregulated. Concurrently, the expression of genes and transcription factors, specifically myeloblastosis and NAC transcription factors, involved in regulating reactive oxygen species homeostasis also increased. These findings suggest that exogenous SA, GABA, and ZnSO4 cotreatment can effectively enhance the accumulation of flavonoids and the nutritional quality of finger millet sprouts by bolstering antioxidant capacity and modulating the flavonoid biosynthesis pathway. This investigation establishes a theoretical framework for the production of superior, bioactive finger millet sprout ingredients.