Functional competition between antioxidants in transformed tomato fruit synthesising resveratrol.

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, have a great antioxidant activity, important both for plant physiology and human nutrition. 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 and 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, both depending 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), in both transgenic and wild type fruits tissues at different stages of maturity, were compared. 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 analyse whether its presence in tomato fruit affected the level and/or the metabolism of other antioxidants naturally present in 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 seem to be proportional to the amount of the resveratrol accumulated in the different tissues. Our results support the view-point that cross-talk between endogenous and introduced pathways is much more complex that the simple competition of enzymes for the same substrates.