The Arabidopsis TT2 gene encodes an R2R3 MYB domain protein that acts as akey determinant for proanthocyanidin accumulation in developing seed

In Arabidopsis, proanthocyanidins specifically accumulate in the endotheliu m during early seed development. At least three TRANSPARENT TESTA (TT) gene s, TT2, TT8, and TTG1, are necessary for the normal expression of several f lavonoid structural genes in immature seed, such as DIHYDROFLAVONOL-4-REDUC TASE and BANYULS (BAN). TTS and TTG1 were characterized recently and found to code for a basic helix-loop-helix domain transcription factor and a WD-r epeat-containing protein, respectively. Here the molecular cloning of the T T2 gene was achieved by T-DNA tagging. TT2 encoded an R2R3 MYB domain prote in 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 embryogene sis, consistent with BAN expression and the proanthocyanidin deposition pro file. Finally, in gain-of-function experiments, TT2 was able to induce ecto pic expression of BAN in young seedlings and roots in the presence of a fun ctional TT8 protein. Therefore, our results strongly suggest that stringent spatial and temporal BAN expression, and thus proanthocyanidin accumulatio n, are determined at least partially by TT2.