Characterization of Glabra2 and Transparent Testa Glabra2, targets of the TTG1 complex

Studies on epidermal cell fate determination have been important for gaining insight into the genetic and molecular mechanisms leading to the differentiation and patterning of cells. In Arabidopsis, the organization and development of many epidermal characters including trichomes, root hairs and the seed coat have been found to be controlled by a single combinatorial transcription factor complex consisting of a WD-repeat containing protein, Transparent Testa Glabra 1 (TTG1), and various MYB and bHLH proteins. The work here consists of identification of Glabra2 (GL2) and Transparent Testa Glabra2 (TTG2) as direct transcriptional targets of the TTG1 combinatorial complex, further characterization of GL2 function, and identification of transcriptional targets of GL2 and TTG2. Both GL2 and TTG2 are important in the regulation of trichomes, root hairs and seed coat development. vii GL2 has been identified as an important regulator of epidermal cell fate for over fifteen years yet there is little known about its function and only three transcriptional targets are identified, all involved in root hair patterning. Through the examination of its function a nuclear localization signal was verified and shown that GL2 homodimerizes. Through analysis of available expression databases and differential sequence analysis using SOLiD sequencing technology, several direct targets of GL2 and many more possible transcriptional targets of both GL2 and TTG2 were identified in trichomes. Some of these targets are members of the TTG1 complex, and they are all specialized in the maturation of trichomes, suggesting that GL2 switches the focus of the complex by activating the TTG1 complex members involved in maturation of the trichome through a feedback mechanism. Examination of gl2 mutants shows that they do not produce trichome accessory cells which usually surround the trichome. An additional target of GL2 is At5g65300 which when overexpressed results in the elongation and proliferation of trichome accessory cells into a tall pillar of cells. This suggests that GL2 is involved in the regulation of accessory cell development through At5g65300. The work presented here represents important advances of our knowledge of epidermal cell fate through characterization of the major downstream regulators of epidermal development.