The R2R3-MYB TT2b and the bHLH TT8 genes are the major regulators of proanthocyanidin biosynthesis in the leaves of Lotus species

Abstract

By exploiting interspecific hybrids and their progeny, we identified key regulatory and transporter genes intimately related to proanthocyanidin biosynthesis in leaves of Lotus spp. Proanthocyanidins (PAs), known as condensed tannins, are polymeric flavonoids enriching forage legumes of key nutritional value to prevent bloating in ruminant animals. Unfortunately, major forage legumes such as alfalfa and clovers lack PAs in edible tissues. Therefore, engineering the PA trait in herbage of forage legumes is paramount to improve both ecological and economical sustainability of cattle production system. Progresses on the understanding of genetic determinants controlling PA biosynthesis and accumulation have been mainly made studying mutants of Arabidopsis, Medicago truncatula and Lotus japonicus, model species unable to synthesize PAs in the leaves. Here, we exploited interspecific hybrids between Lotus corniculatus, with high levels of PAs in the leaves, and Lotus tenuis, with no PAs in these organs, and relative F progeny, to identify among candidate PA regulators and transporters the genes mainly affecting this trait. We found that the levels of leaf PAs significantly correlate with the expression of MATE1, the putative transporter of glycosylated PA monomers, and, among the candidate regulatory genes, with the expression of the MYB genes TT2a, TT2b and MYB14 and the bHLH gene TT8. The expression levels of TT2b and TT8 also correlated with those of all key structural genes of the PA pathways investigated, MATE1 included. Our study unveils a different involvement of the three Lotus TT2 paralogs to the PA trait and highlights differences in the regulation of this trait in our Lotus genotypes with respect to model species. This information opens new avenues for breeding bloat safe forage legumes.

Targeted mutagenesis of BnTT8 homologs controls yellow seed coat development for effective oil production in Brassica napus L.

“…Accumulating evidence demonstrates that TT8 is a central component of the well‐conserved complex that controls flavonoid accumulation in various crops (Escaray et al , ; Li et al , ; Li et al , ; Lim et al , ; Nemesio‐Gorriz et al , ; Schaart et al , ). Taking into account the close phylogenetic relationship between Arabidopsis and Brassica , TT8 gene homologs could play comparable roles in Brassica species.…”

“…In A. thaliana, a ternary complex (known as the MBW complex) comprising three TF regulators, namely AtMYB123/TT2 (R2R3-MYB), TT8 (basic helix-loop-helix, bHLH) and TTG1 (WD40 protein), plays a key role in activating PA-specific genes in seed coat development (Xu et al, 2014(Xu et al, , 2015. Accumulating evidence demonstrates that TT8 is a central component of the well-conserved complex that controls flavonoid accumulation in various crops (Escaray et al, 2017;Li et al, 2016;Lim et al, 2017;Nemesio-Gorriz et al, 2017;Schaart et al, 2013). Taking into account the close phylogenetic relationship between Arabidopsis and Brassica, TT8 gene homologs could play comparable roles in Brassica species.…”

CRISPR-Cas9 Mediated Knockout of NtAn1 to Enhance the Lipid Accumulation in Tobacco Seed for Biodiesel Production

“…The formation of seed color in most plant species is due to the deposition of PAs within the endothelial layer of the inner integument of the seed coat [13]. Previous studies demonstrated that TT8 played a key role in regulating PAs accumulation in various plants [18,19]. In this paper, mutation of the NtAn1, the TT8 homolog genes in tobacco, generated yellow-seeded phenotype (Fig.…”

“…Most recently, targeted mutation of the TT8 homologs through CRISPR-Cas9 system in Brassica napus also generated yellow-seeded phenotype [12]. Homologs of TT8 gene were also reported to be involved in PAs biosynthesis in diverse plant species, including Medicago truncatula, Lotus corniculatus, and Raphanus sativus [18][19][20].…”

Anthocyanin Synthesis and the Expression Patterns of bHLH Transcription Factor Family during Development of the Chinese Jujube Fruit (Ziziphus jujuba Mill.)

“…Its core conserved domain contains approximately 60 amino acid residues [6]. The bHLH has been shown to be involved in numerous biological processes, including stress resistance [7][8][9], growth and development [10,11], signal transduction [12,13] and synthesis [14][15][16]. The first bHLH transcription factor was discovered in maize, where its function is involved in the synthesis of anthocyanins [17].…”