The strawberry transcription factor FaMYB1 inhibits the biosynthesis of proanthocyanidins in Lotus corniculatus leaves

Abstract

Proanthocyanidins (PAs) are agronomically important biopolymers in higher plants composed primarily of catechin and epicatechin units. The biosynthesis of these natural products is regulated by transcription factors including proteins of the R2R3MYB class. To gain insight into the genetic control of the catechin and epicatechin branches of the PA pathway in forage legumes, here the effects of the expression of FaMYB1, a flavonoid R2R3MYB repressor from strawberry, in Lotus corniculatus (birdsfoot trefoil), were tested. It was found that in leaves of T(0) transgenic lines the degree of PA inhibition correlated with the level of FaMYB1 expression. These effects were heritable in the transgene-positive plant T(1) generation and were tissue specific as the suppression of proanthocyanidin biosynthesis was most pronounced in mesophyll cells within the leaf, whereas other flavonoid and phenolic compounds were substantially unaltered. The data suggest that FaMYB1 may counter-balance the activity of the endogenous transcriptional MYB-bHLH-WD40 (MBW) complex promoting proanthocyanidin biosynthesis via the catechin and epicatechin branches and that FaMYB1 does not interfere with the expression levels of a resident R2R3MYB activator of PAs. It is proposed that in forage legumes leaf cell commitment to synthesize proanthocyanidins relies on the balance between the activity of activator and repressor MYBs operating within the MBW complex.

A Conserved Network of Transcriptional Activators and Repressors Regulates Anthocyanin Pigmentation in Eudicots

“…The action on common regulatory targets is not just through the reduction of the bHLH (AN1) levels, as there is also the direct activity on the DFR-A promoter ( Figure 6B). The MYB27 data are in agreement with that for Fa-MYB1; based on timing of gene expression and Y2H studies, Fa-MYB1 is thought to be specifically associated with anthocyanin production in strawberry fruit and not PA biosynthesis (Schaart et al, 2013), yet when overexpressed in Lotus corniculatus, it repressed the PA biosynthetic pathway genes normally activated by the endogenous MBW complex (Paolocci et al, 2011). These observations in both petunia and strawberry suggest the R2R3-MYB repressors bind MBW complexes and that the targeting to promoters is likely determined by the R2R3-MYB activator.…”

The Phenylpropanoid Pathway Is Controlled at Different Branches by a Set of R2R3-MYB C2 Repressors in Grapevine

“…For instance, D1 may be exclusive to strawberry and closely related species. The role of C1 motif has not been characterized yet, but it may not be related to any repression role, because FaMYB1, which lacks this motif, strongly inhibits flavonoid synthesis (Aharoni et al, 2001;Paolocci et al, 2011). The D3 subclade members share the TLLLFR repression motif that is also found in AtMYBL2, a single-repeat MYB with a strong repression activity over flavonoid synthesis (Dubos et al, 2008;Matsui et al, 2008).…”

The Transcriptional Repressor MYB2 Regulates Both Spatial and Temporal Patterns of Proanthocyandin and Anthocyanin Pigmentation inMedicago truncatula

“…In Arabidopsis, MYBL2 promoter activity is expressed in the endothelium of the seed coat, and expression of MYBL2 driven by the TT8 promoter produced seeds with reduced PA content (Dubos et al, 2008). Overexpression of Fa MYB1, grapevine (Vitis vinifera) Vv MYBC2, or poplar (Populus trichocarpa) Ptr MYB182 resulted in reductions of PA or both PA and anthocyanin biosynthesis (Paolocci et al, 2011;Huang et al, 2014;Yoshida et al, 2015;Cavallini et al, 2015). However, information on how MYB repressors may be involved in fine spatio-temporal regulation of pigmentation is still limited, especially for PA deposition, and requires analysis of lossof-function phenotypes.…”