Abstract
Proanthocyanidins (PAs) are oligomeric flavonoids and one group of end products of the phenylpropanoid pathway. PAs have been reported to be beneficial for human and animal health and are particularly important in pastoral agricultural systems for improved animal production and reduced greenhouse gas emissions. However, the main forage legumes grown in these systems, such as Trifolium repens and Medicago sativa, do not contain any substantial amounts of PAs in leaves. We have identified from the foliar PA-accumulating legume Trifolium arvense an R2R3-MYB transcription factor, TaMYB14, and provide evidence that this transcription factor is involved in the regulation of PA biosynthesis in legumes. TaMYB14 expression is necessary and sufficient to up-regulate late steps of the phenylpropanoid pathway and to induce PA biosynthesis. RNA interference silencing of TaMYB14 resulted in almost complete cessation of PA biosynthesis in T. arvense, whereas Nicotiana tabacum, M. sativa, and T. repens plants constitutively expressing TaMYB14 synthesized and accumulated PAs in leaves up to 1.8% dry matter. Targeted liquid chromatography-multistage tandem mass spectrometry analysis identified foliar PAs up to degree of polymerization 6 in leaf extracts. Hence, genetically modified M. sativa and T. repens plants expressing TaMYB14 provide a viable option for improving animal health and mitigating the negative environmental impacts of pastoral animal production systems.
Highlights
Proanthocyanidins (PAs) are oligomeric flavonoids and one group of end products of the phenylpropanoid pathway
Leucoanthocyanidins can be converted by anthocyanidin synthase (ANS; syn. leucoanthocyanidin deoxygenase) to anthocyanidins, which are either glycosylated to the stable, colored anthocyanins or reduced by anthocyanidin reductase (ANR) to form the second set of monomeric PA units, cis-flavan-3-ols
Those amplification products likely to be coding for R2R3-MYB transcription factors (TFs) were translated and analyzed using internal National Center for Biotechnology Information TBLASTX analysis against the Database of Arabidopsis Transcription Factors; more than 50% of the sequences did not return any significant hits to known R2R3-MYB protein sequences
Summary
Proanthocyanidins (PAs) are oligomeric flavonoids and one group of end products of the phenylpropanoid pathway. A further reduction by leucoanthocyanidin reductase (LAR) yields the monomeric PA units, transflavan-3-ols (afzelechin, catechin, and gallocatechin) This step seems not to be involved in PA biosynthesis in Arabidopsis, as its PAs do not contain any transflavan-3-ols but appear to be made up exclusively of the cis-flavan-3-ol epicatechin. Leucoanthocyanidin deoxygenase) to anthocyanidins (pelargonidin, cyanidin, and delphinidin), which are either glycosylated to the stable, colored anthocyanins or reduced by anthocyanidin reductase (ANR) to form the second set of monomeric PA units, cis-flavan-3-ols (epiafzelechin, epicatechin, and epigallocatechin). Most of these genes have been identified in legumes (for review, see Rasmussen, 2009). Epicatechin 3-O-glucoside acts as a substrate for the vacuolar multidrug and toxic compound extrusion (MATE) transporters, which transport glycosylated anthocyanidins (Marinova et al, 2007; Zhao and Dixon, 2009)
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