Abstract
The anthocyanin biosynthetic pathway is well characterized in plants. However, in tomato (Solanum lycopersicum L.) an exhaustive knowledge of its regulation is still lacking. Tomato mutants showing higher levels of anthocyanins in fruits or vegetative tissues, such as Anthocyanin fruit (Aft) or atroviolacea (atv), have been extensively exploited in the attempt to clarify the process. Nevertheless, only candidate genes have been proposed as responsible for such phenotypes. The recessive atv mutation likely represents an allelic variant of a gene introgressed in tomato from wild Solanum species. We performed genome sequencing of atv/atv plants followed by candidate gene analysis, and identified a mutated gene encoding an R3-MYB protein. When overexpressed, this protein abolished anthocyanin production in tomato seedlings and plants, by silencing key regulators and biosynthetic genes of the pathway. The functional analysis of the protein clearly showed that it can negatively interfere with the activation of the anthocyanin biosynthetic pathway mediated by the endogenous MYB-bHLH-WDR (MBW) complexes. In particular, this R3-MYB protein can directly bind the bHLH factors which are part of the MBW complexes, therefore acting as a competitive inhibitor. The R3-MYB protein here described is therefore involved in a feedback mechanism that dampens the production of anthocyanins once activated by endogenous or exogenous stimuli. The atv mutation causes the production of a truncated version of the R3-MYB factor that cannot retain the full potential to inhibit the MBW complexes, thus leading to a constitutively higher production of anthocyanins.
Highlights
Anthocyanins are water soluble pigments mainly synthesized in epidermal and sub-epidermal cells of shoots, roots, flowers, and fruits
We found a strong inhibition exerted by SlMYB-ATV on the MBW complex composed by SlAN2 and SlJAF13 (SlJAF13 is equivalent to bHLH1 in the model of Albert et al, 2014a; Figure 8), which, in accordance to such model, should act upstream to induce the transcription of SlAN1 (SlAN1 is equivalent to bHLH2), in a reinforcement mechanism, and the formation of the core complex (Albert et al, 2014a)
In this paper we experimentally demonstrated that the atv phenotype of tomato mutants is genetically associated with a mutation of the gene Solyc07g052490 encoding the R3-MYB protein recently named SlMYB-ATV (Cao et al, 2017)
Summary
Anthocyanins are water soluble pigments mainly synthesized in epidermal and sub-epidermal cells of shoots, roots, flowers, and fruits. They represent the glycosylated forms of the anthocyanidins, secondary metabolites belonging to the class of flavonoids and synthesized through the phenylpropanoid pathway (Winkel-Shirley, 2001). Cyanidin, one of the most common anthocyanidin, has been shown to alleviate inflammation in vivo thanks to its specific capacity to inhibit signaling by the proinflammatory cytokine interleukin-17A (Liu et al, 2017) In this sense anthocyanins can be considered as nutraceuticals and the edible vegetables and fruits containing them are functional foods
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