Co-expression of onion chalcone isomerase in Del/Ros1-expressing tomato enhances anthocyanin and flavonol production

Anthocyanins are colorful pigments known for contributing antioxidant effects to the human diet which provide health benefits that protect against several forms of cancer and vascular disease. Unfortunately, tomatoes found in nature have very low anthocyanin content. Anthocyanin-rich purple tomatoes by the ectopic co-expression of two transcription factors Delila (Del) and Rosea1 (Ros1) from the snapdragon Antirrhinum majus have been generated. However, the Del/Ros1 (DR)-expressing tomatoes cannot sufficiently upregulate all necessary key endogenous genes, particularly chalcone isomerase (CHI), for full utilization of the anthocyanin production pathway. In this study, CHI from onion Allium cepa was introduced into DR-expressing tomatoes for a further increase of anthocyanin levels in both the peel and the flesh of tomatoes. We achieved up to 400- and 260-fold increases in the levels of anthocyanins in tomato peel and flesh, respectively, in CHI/DR transgenics compared with 100- and 80-fold increases in tomato peel and flesh in the DR-only-expressing lines. Furthermore, CHI/DR-expressing tomatoes increased up to 200-fold more total flavonol content in flesh compared with the wild-type tomatoes. In summary, stacking CHI with DR could significantly increase the levels of anthocyanins and flavonols in tomato fruit.