A network of redundant bHLH proteins functions in all TTG1-dependent pathways of Arabidopsis

GLABRA3 (GL3) encodes a bHLH protein that interacts with the WD repeat protein, TTG1. GL3 overexpression suppresses the trichome defect of the pleiotropic ttg1 mutations. However, single gl3 mutations only affect the trichome pathway with a modest trichome number reduction. A novel unlinked bHLH-encoding locus is described here, ENHANCER OF GLABRA3 (EGL3). When mutated, egl3 gives totally glabrous plants only in the gl3 mutant background. The double bHLH mutant, gl3 egl3, has a pleiotropic phenotype like ttg1 having defective anthocyanin production, seed coat mucilage production, and position-dependent root hair spacing. Furthermore, the triple bHLH mutant, gl3 egl3 tt8, phenocopies the ttg1 mutation. Yeast two-hybrid and plant overexpression studies show that EGL3, like GL3, interacts with TTG1, the myb proteins GL1, PAP1 and 2, CPC and TRY, and it will form heterodimers with GL3. These results suggest a combinatorial model for TTG1-dependent pathway regulation by this trio of partially functionally redundant bHLH proteins.     

Arabidopsis TTG2 Regulates TRY Expression through Enhancement of Activator Complex-Triggered Activation

Trichome patterning in Arabidopsis thaliana is regulated by a regulatory feedback loop of the trichome promoting factors TRANSPARENT TESTA GLABRA1 (TTG1), GLABRA3 (GL3)/ENHANCER OF GL3 (EGL3), and GL1 and a group of homologous R3MYB proteins that act as their inhibitors. Together, they regulate the temporal and spatial expression of GL2 and TTG2, which are considered to control trichome cell differentiation. In this work, we show that TTG2 is a specific activator of TRY (but not CPC or GL2). The WRKY protein TTG2 binds to W-boxes in a minimal promoter fragment of TRY, and these W-boxes are essential for rescue of the try mutant phenotype. We further show that TTG2 alone is not able to activate TRY expression, but rather drastically enhances the activation by TTG1 and GL3. As TTG2 physically interacts with TTG1 and because TTG2 can associate with GL3 through its interaction with TTG1, we propose that TTG2 enhances the activity of TTG1 and GL3 by forming a protein complex.     

GL3 encodes a bHLH protein that regulates trichome development in arabidopsis through interaction with GL1 and TTG1

Arabidopsis trichome development and differentiation is a well-studied model for plant cell-fate determination and morphogenesis. Mutations in TRANSPARENT TESTA GLABRA1 (TTG1) result in several pleiotropic defects including an almost complete lack of trichomes. The complex phenotype caused by ttg1 mutations is suppressed by ectopic expression of the maize anthocyanin regulator R. Here it is demonstrated that the Arabidopsis trichome development locus GLABRA3 (GL3) encodes an R homolog. GL3 and GLABRA1 (GL1) interact when overexpressed together in plants. Yeast two-hybrid assays indicate that GL3 participates in physical interactions with GL1, TTG1, and itself, but that GL1 and TTG1 do not interact. These data suggest a reiterated combinatorial model for the differential regulation of such diverse developmental pathways as trichome cell-fate determination, root hair spacing, and anthocyanin secondary metabolism.