Functional Differentiation of Lotus japonicus TT2s, R2R3-MYB Transcription Factors Comprising a Multigene Family

Abstract

Leguminous plants have many paralogous genes encoding enzymes involved in the flavonoid biosynthetic pathway. Duplicate genes are predicted to contribute to the production of various flavonoid compounds and to have resulted in a diversity of legume species. We identified gene duplication in the transcription factors regulating flavonoid biosynthesis in the model legume Lotus japonicus. Three copies of a homolog of Arabidopsis thaliana TRANSPARENT TESTA2 (TT2), which is a MYB transcription factor that regulates proanthocyanidin biosynthesis, were present in the L. japonicus genome. The organ specificity and stress responsiveness differed among the three LjTT2s, and correlations between proanthocyanidin accumulation and the expression levels of LjTT2s were observed during seedling development. Moreover, three LjTT2s functionally complemented TT2 in transient expression experiments in A. thaliana leaf cells. The different reporter activity caused by LjTT2a was consistent with the affinity of physical interactions with TT8 and TTG1 in yeast two-hybrid experiments as well as the branching pattern of the phylogenetic tree. These results suggest that LjTT2 factors have diverse functions in the tissues in which they are expressed; in particular, LjTT2a is predicted to have evolved flexibility in interaction with other transcription regulators to resist environmental stresses.

Transcription Factors of Lotus: Regulation of Isoflavonoid Biosynthesis Requires Coordinated Changes in Transcription Factor Activity

“…The abundance of LjMYB136, LjMYB137, and LjMYB139 (subgroup 5) transcripts was very low in young leaves prior to elicitation, consistent with their proposed roles in positively regulating condensed tannin biosynthesis (Yoshida et al, 2008), and were undetectable in elicited samples after 40 PCR cycles. Yoshida et al (2008) reported the same trend for LjMYB136 and LjMYB139 gene expression after GSH treatment.…”

Expression of the R2R3-MYB Transcription Factor TaMYB14 fromTrifolium arvenseActivates Proanthocyanidin Biosynthesis in the LegumesTrifolium repensandMedicago sativa

“…R2R3-MYB TFs have been described to regulate PA biosynthesis, often in a complex with bHLH and WD40 proteins (Baudry et al, 2004;Yoshida et al, 2008Yoshida et al, , 2010Hichri et al, 2011), in a wide range of plants. In our study, ectopic expression of TaMYB14 under the control of the CaMV 35S promoter resulted in PA accumulation in leaves of N. tabacum, M. sativa, and T. repens without the need for coexpression of additional factors.…”

“…Other species in which MYB TFs have been reported to be crucial for PA biosynthesis include Brassica napus, Diospyrios kaki (persimmon), and Populus tremuloides (Wei et al, 2007;Akagi et al, 2009;Mellway et al, 2009), indicating that MYB TFs are common regulators of PA biosynthesis. The first legume R2R3-MYB TFs reported to regulate PA biosynthetic genes are three Lotus japonicus TT2 orthologs, which, if coexpressed with AtTT8, AtTTG1, and LjANR in transient Arabidopsis leaf cell assays, activate the promoter of LjANR (Yoshida et al, 2008). Constitutive expression of LjTT2a results in the production and accumulation of PAs and anthocyanins in Arabidopsis seedlings and induces the expression of CHS, DFR, ANS, ANR, and a glutathione S-transferase predicted to be involved in PA monomer transport (Yoshida et al, 2010).…”

Legume Transcription Factor Genes: What Makes Legumes So Special?

“…Legume TF genes discovered in this way include those involved in the control of floral meristems, such as LjFLO, PsFLO/LFY, PsPEAM4, and MtPIM (Hofer et al, 1997;Berbel et al, 2001;Taylor et al, 2002;Dong et al, 2005;Benlloch et al, 2006). In the same way, three L. japonicus MYB TFs with homology to Arabidopsis TRANSPARENT TESTA2 (AtTT2) were shown to regulate proanthocyanidin biosynthesis after their transient expression in Arabidopsis leaves (Yoshida et al, 2008). More recently, the role of legume TF genes was investigated using reversegenetic approaches.…”