Abstract
Pigment intensity and patterns are important factors that determine the nutritional and market values of tomato fruits. The acropetal manner of light-dependent anthocyanin accumulation with the highest levels at the stem end of the fruit makes Pro35S:BrTT8 tomato plants an ideal system for investigating the effects of light intensity on anthocyanin biosynthesis. Extensive transcript analyses indicate that anthocyanin pigmentation in Pro35S:BrTT8 plants under high light might be coordinately regulated by the exogenous protein BrTT8 and endogenous proteins SlAN2 and SlMYBL2. Furthermore, yeast two-hybrid assays showed that BrTT8 could interact efficiently with SlAN2, SlMYBL2, and SlAN11. Moreover, the physical interaction between BrTT8 and SlAN2 was validated by FRET. Simultaneous overexpression of SlAN2 and BrTT8 activated significant anthocyanin biosynthesis in infiltrated tobacco leaves. In addition, the ability of SlMYBL2 to suppress anthocyanin accumulation was also demonstrated in infiltrated tobacco leaves. Altogether, these results prove that tissue-specific assemblage of the heterogeneous MYB-bHLH-WD40 complex consisting of SlAN2, BrTT8 and SlAN11 triggers nonuniform anthocyanin accumulation in tomato fruit under high light. Additionally, it is proposed that a negative-feedback loop fulfilled by SlMYBL2 also participates in the regulation of anthocyanin production.
Highlights
Anthocyanins are the main water-soluble pigments widespread in many flowering plants
Chalcone synthase (CHS) catalyzes the condensation reaction with 4-coumaroyl CoA and malonyl CoA as substrates to produce naringenin chalcone, the entry molecule of the flavonoid pathway. 4-Coumaroyl CoA can be esterified with quinic acid by hydroxycinnamoyl CoA-quinate transferase (HQT), leading to the production of chlorogenic acid (CGA), a typical phenolic acid widespread in Solanaceae plants
Anthocyanin pigmentation was uneven on the fruit surface, and the nonuniform distribution pattern was attributed to the direct sunlight flux received by epicarp tissue in the unit area of the fruit surface
Summary
Anthocyanins are the main water-soluble pigments widespread in many flowering plants. Chalcone synthase (CHS) catalyzes the condensation reaction with 4-coumaroyl CoA and malonyl CoA as substrates to produce naringenin chalcone, the entry molecule of the flavonoid pathway. 4-Coumaroyl CoA can be esterified with quinic acid by hydroxycinnamoyl CoA-quinate transferase (HQT), leading to the production of chlorogenic acid (CGA), a typical phenolic acid widespread in Solanaceae plants. The basic skeleton of all flavonoids (including anthocyanins, chalcones, flavones, flavonols, flavandiols, and condensed tannins) consists of three aromatic rings generated by CHS and CHI. Cyanidin and delphinidin can be further methylated at the 3′ or 5′ position of the B-ring by flavonoid 3′, 5′-methyltransferase (FAOMT), resulting in the production of peonidin, petunidin and malvidin, respectively. Except for anthocyanins, chalcones and CGA, flavonols and flavones can be produced in the branch pathway of phenylpropanoid metabolism
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