The TRANSPARENT TESTA GLABRA1 Locus, Which Regulates Trichome Differentiation and Anthocyanin Biosynthesis in Arabidopsis, Encodes a WD40 Repeat Protein

Abstract

The TRANSPARENT TESTA GLABRA1 ( TTG1 ) locus regulates several developmental and biochemical pathways in Arabidopsis, including the formation of hairs on leaves, stems, and roots, and the production of seed mucilage and anthocyanin pigments. The TTG1 locus has been isolated by positional cloning, and its identity was confirmed by complementation of a ttg1 mutant. The locus encodes a protein of 341 amino acid residues with four WD40 repeats. The protein is similar to AN11, a regulator of anthocyanin biosynthesis in petunia, and more distantly related to those of the ␤ subunits of heterotrimeric G proteins, which suggests a role for TTG1 in signal transduction to downstream transcription factors. The 1.5-kb TTG1 transcript is present in all major organs of Arabidopsis. Sequence analysis of six mutant alleles has identified base changes producing truncations or single amino acid changes in the TTG1 protein. INTRODUCTIONThe TRANSPARENT TESTA GLABRA1 ( TTG1 ) locus controls many apparently unrelated characters of Arabidopsis (catalogued by Koornneef, 1981), several of which appear to be confined to the epidermal cell layer of different tissues. ttg1 mutants have a glabrous phenotype, possessing none of the leaf or stem hairs (trichomes) that normally are derived from the meristematic L1 cell layer. Purple anthocyanin pigments are absent from the ttg1 seed coat, causing the transparent testa phenotype in which the yellow cotyledons are visible through the testa. In wild-type plants, anthocyanins are present in the hypocotyl of seedlings and in the stem and leaves of plants as they age, and they are inducible by many forms of stress, including high light, poor nutrients, or water stress. ttg1 mutants completely lack anthocyanins in the epidermis and in subepidermal layers of leaves and stems. Mucilage normally found in the cell wall of the seed coat is absent in ttg1 mutants. Seeds of ttg1 plants do not require drying and cold treatments to germinate and therefore exhibit an altered seed dormancy when compared with ecotypes, such as Landsberg erecta (L er ;Koornneef, 1981;Léon-Kloosterziel et al., 1994). This characteristic of ttg1 mutants may be linked to an altered seed coat structure. The TTG1 gene appears to have the opposite effect on root hair formation when compared with its effect on leaf hair initiation. In Arabidopsis, root hairs extend from root epidermal cells only in files of cells that contact two underlying cortical cells, whereas in ttg1 mutants, extra root hairs occur in the atrichoblast cell files (Galway et al., 1994). Under laboratory growth conditions, mutations at the ttg1 locus do not greatly affect the viability of the plants.In ttg1 mutants, the anthocyanin biosynthetic pathway is blocked at the dihydroflavonol-4-reductase (DFR) step, because DFR-encoding transcripts have not been detected in these mutants (Shirley et al., 1995). By contrast, transcripts of the chalcone synthase and chalcone isomerase genes are unaffected. The point of regulation of the pathway by TTG1 was confirmed by the clon...

Mapping quantitative trait loci for yield, yield components and morphological traits in an advanced backcross population between Oryza rufipogon and the Oryza sativa cultivar Jefferson

“…Although these traits do not seem to be related, it is still possible that a single gene has pleiotropic effects on all three traits. One precedent for this is the ttg1 mutant in Arabidopsis that affects three seemingly unrelated traits: trichome cell-fate determination, root hair spacing, and anthocyanin secondary metabolism (Walker et al 1999;Payne et al 2000). Fine mapping of these clusters would be needed to differentiate between gene linkage and pleiotropy.…”

Transcriptome analysis and anthocyanin-related genes in red leaf lettuce

“…EGL1 and GL3 are known to regulate anthocyanin biosynthesis in seedlings and mature plants of Arabidopsis (Payne et al, 2000;Feyissa et al, 2009). In the same species, two unigenes from the WD40 family showed similarity to TTG1 (Walker et al, 1999). The predicted regulatory mechanism of anthocyanin in red leaf lettuce is shown in Figure 4.…”

Candidate gene analysis of anthocyanin pigmentation loci in the Solanaceae

“…In the current study, chi also mapped to this region, 0.7 cM south of CT93. The af phenotype is associated with the loss of both anthocyanin pigments and of trichomes and thus mirrors the well-known pleiotropic transparent testa glabra 1 (ttg1) locus in Arabidopsis (Walker et al 1999). The ttg1 gene product is a WD40 repeat protein, very similar to the Petunia an11 gene product.…”