Characterization and content of stilbenes and other polyphenols in tomato plants transformed with the stilbene synthase gene

There is a growing interest in producing food plants with increased amounts of antioxidants because of their potential health benefits. In particular polyphenol secondary metabolites, such as flavonoids and stilbenes, are of particular interest, both for plant physiology and human nutrition because of their great antioxidant activity. With the aim of generating plants and fruits with increased antioxidant capacity and a wider range of potential health benefits, the tomato flavonoid route was modified through the over-expression of the grape stilbene synthase cDNA under constitutive promoter or a tissue specific, TomLoxB, promoter. The expression of the StS gene in tomato tissues caused the synthesis of new compounds, identified as trans-resveratrol and trans-resveratrol-glucopyranoside which have a different tissue specificity, depending both on the promoter used and on the amount of precursors present in different tissues. In order to verify the metabolic impairment, the amounts of soluble phenolics (i.e., chlorogenic acid), flavonoids (i.e., naringenin) and flavonols (i.e., rutin and quercetin) were compared in both transgenic and wild type fruit tissues at different stages of maturity. The results indicate that the stilbenes and the flavonoids found in transgenic plants are present as conjugates. Since the free radical scavenging capabilities of resveratrol are well known, we wanted to analyze whether its presence in tomato fruit affected the level and/or the metabolism of other antioxidants naturally present in the tomato. The synthesis of resveratrol affects the other antioxidants in different ways; in particular ascorbate was increased in the transformed tissues. The alteration of ascorbate seems to be proportional to the amount of the resveratrol accumulated in the different tissues. Our results support the view that cross-talk between endogenous and introduced pathways is much more complex than the simple competition of enzymes for the same substrates.