An O‐methyltransferase modifies accumulation of methylated anthocyanins in seedlings of tomato

Abstract

SUMMARYAnthocyanins contribute to the appearance of fruit by conferring to them a red, blue or purple colour. In a food context, they have also been suggested to promote consumer health. In purple tomato tissues, such as hypocotyls, stems and purple fruits, various anthocyanins accumulate. These molecules have characteristic patterns of modification, including hydroxylations, methylations, glycosylations and acylations. The genetic basis for many of these modifications has not been fully elucidated, and nor has their role in the functioning of anthocyanins. In this paper, AnthOMT, an O-methyltransferase (OMT) mediating the methylation of anthocyanins, has been identified and functionally characterized using a combined metabolomics and transcriptomics approach. Gene candidates were selected from the draft tomato genome, and their expression was subsequently monitored in a tomato seedling system comprising three tissues and involving several time points. In addition, we also followed gene expression in wild-type red and purple transgenic tomato fruits expressing Rosea1 and Delila transcription factors. Of the 57 candidates identified, only a single OMT gene showed patterns strongly correlating with both accumulation of anthocyanins and expression of anthocyanin biosynthesis genes. This candidate (AnthOMT) was compared to a closely related caffeoyl CoA OMT by recombinant expression in Escherichia coli, and then tested for substrate specificity. AnthOMT showed a strong affinity for glycosylated anthocyanins, while other flavonoid glycosides and aglycones were much less preferred. Gene silencing experiments with AnthOMT resulted in reduced levels of the predominant methylated anthocyanins. This confirms the role of this enzyme in the diversification of tomato anthocyanins.

Expression and functional analysis of the nobiletin biosynthesis-related gene CitOMT in citrus fruit

“…Plant OMTs are a large gene family, which are categorized into two types, COMT and CCoAOMT, according to their molecular weight and bivalent ion dependency. In plants, numerous OMT genes have been identified, and their functions have been extensively investigated in various plant species, such as Arabidopsis 39 , barley 40 , mango 41 , rice 42 , tomato 43 , and sweet basil 44 . In citrus, it was reported that 58 OMT genes existed and were unevenly distributed on the nine chromosomes of Citrus sinensis.…”

Accumulation of Anthocyanins through Overexpression of AtPAP1 in Solanum nigrum Lin. (Black Nightshade)

“…Specifically, the mass spectrum of peak 8 with a retention time at 16 (trans-p-coumaroyl)-5-O-glucoside. The peak 15 with retention time at 32.68 min that had a molecular ion and fragment of 963.2803 and 317 [Pt]+ corresponded to a petunidin aglycone, which was tentatively identified as petunidin-3-(feruloyl)-rutinoside-5-glucoside [21][22][23]. Finally, the peak 17 with a retention time at 36.54 min was identified as malvidin-3-(feruloyl)-rutinoside…”

Characterization of a caffeoyl-CoA O-methyltransferase-like enzyme involved in biosynthesis of polymethoxylated flavones in Citrus reticulata

“…The recombinant enzyme methylated flavones and flavonols in the meta -position, while it methylated dihydroflavones and dihydroflavonols in the para -position, and the role of a single amino acid required for the conversion of para to meta was verified by incubating substrates with the CCoAOMT7 wild type and CCoAOMT7 single mutant, respectively ( Wils et al , 2013 ). In addition, CCoAOMT-like enzymes involved in the methylation of anthocyanins have been functionally characterized in vivo and vitro , including VvAOMT from Vitis vinifera ( Hugueney et al , 2009 ), SlAnthOMT from Solanum lycopersicum ( Gomez Roldan et al , 2014 ), and PsAOMT from Paeonia suffruticosa cv. ‘Gunpohden’ ( Du et al , 2015 ).…”