The Arabidopsis TT2 Gene Encodes and R2R3 MYB Domain Protein That Acts as a Key Determinant for Proanthocyanidin Accumulation in Developing Seed

Abstract

In Arabidopsis, proanthocyanidins specifically accumulate in the endothelium during early seed development. At least three TRANSPARENT TESTA (TT) genes, TT2, TT8, and TTG1, are necessary for the normal expression of several flavonoid structural genes in immature seed, such as DIHYDROFLAVONOL-4-REDUCTASE and BANYULS (BAN). TT8 and TTG1 were characterized recently and found to code for a basic helix-loop-helix domain transcription factor and a WD-repeat-containing protein, respectively. Here the molecular cloning of the TT2 gene was achieved by T-DNA tagging. TT2 encoded an R2R3 MYB domain protein with high similarity to the rice OsMYB3 protein and the maize COLORLESS1 factor. A TT2-green fluorescent protein fusion protein was located mostly in the nucleus, in agreement with the regulatory function of the native TT2 protein. TT2 expression was restricted to the seed during early embryogenesis, consistent with BAN expression and the proanthocyanidin deposition profile. Finally, in gain-of-function experiments, TT2 was able to induce ectopic expression of BAN in young seedlings and roots in the presence of a functional TT8 protein. Therefore, our results strongly suggest that stringent spatial and temporal BAN expression, and thus proanthocyanidin accumulation, are determined at least partially by TT2.

Global Expression Profiling Applied to the Analysis of Arabidopsis Stamen Development

“…The pollen wall is based on the polymer sporopollenin, which is largely composed of acyl lipids and phenylpropanoid precursors, while the pollen coat contains a complex mixture of proteins, lipids, and phenolic compounds such as flavonoids (Piffanelli et al, 1998;Hsieh and Huang, 2007). The regulation of phenylpropanoid metabolism, which contributes to the formation of both structures, is relatively well understood and appears to be dependent to a large extent on the activity of R2R3 MYB transcription factors (Borevitz et al, 2000;Nesi et al, 2001;Preston et al, 2004). To identify genes that might be involved in the regulation of spororopollenin and pollen coat formation, we compared, using cluster analysis, the expression profiles of R2R3 MYB transcription factor and MYB-like coding genes identified in the ms1 developmental series to those of genes known or presumed to be involved in phenylpropanoid metabolism.…”

MYB transcription factor genes as regulators for plant responses: an overview

“…MYB340 (Moyano et al 1996), MYB308 and MYB330 (Tamagnone et al 1998) were reported to be other MYB genes involved in phenylpropanoid metabolism. The Arabidopsis TRANSPARENT TESTA (TT2) gene encoded an R2R3 MYB domain protein that acted as a key determinant for proanthocyanidin accumulation in developing seed (Nesi et al 2001). Huang et al (2013) isolated and characterized 13 full-length cDNA clones of R2R3-MYB TFs from E. sagittatum (EsMYB) and found to regulate the flavonoid biosynthetic pathway.…”

Arabidopsis TRANSPARENT TESTA GLABRA2Is Directly Regulated by R2R3 MYB Transcription Factors and Is Involved in Regulation ofGLABRA2Transcription in Epidermal Differentiation

“…GL1 is also a binding candidate, because it is an early regulator of trichome development and because it is functionally equivalent to WER (Lee and Schiefelbein, 2001). TT2 may also be a candidate, because both TT2 and TTG2 are involved in proanthocyanidin accumulation in seed coats (Nesi et al, 2001;Johnson et al, 2002).…”

“…Anthocyanins accumulate in vegetative tissues and are regulated by two R2R3 MYB genes, PRODUCTION OF ANTHOCYANIN PIGMENT1 (PAP1) and PAP2, and three bHLH genes, GL3, EGL3, and TRANSPARENT TESTA8 (TT8) (Borevitz et al, 2000;Zhang et al, 2003). Proanthocyanidin accumulation in seed coats is regulated by an R2R3 MYB gene, TRANSPARENT TESTA2 (TT2), and a bHLH gene, TT8 (Nesi et al, 2000(Nesi et al, , 2001. Seed coat mucilage production is regulated by an R2R3 MYB gene, MYB61, and two bHLH genes, EGL3 and TT8, although the MYB61-EGL3/TT8 interaction has not been experimentally confirmed (Penfield et al, 2001;Zhang et al, 2003).…”