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...

Point mutations result from errors made during DNA replication or repair, so they are usually expected to be homogeneous across all regions of a genome. However, we have found a region of chloroplast DNA in plants related to sweetpea (Lathyrus) whose local point mutation rate is at least 20 times higher than elsewhere in the same molecule. There are very few precedents for such heterogeneity in any genome, and we suspect that the hypermutable region may be subject to an unusual process such as repeated DNA breakage and repair. The region is 1.5 kb long and coincides with a gene, ycf4, whose rate of evolution has increased dramatically. The product of ycf4, a photosystem I assembly protein, is more divergent within the single genus Lathyrus than between cyanobacteria and other angiosperms. Moreover, ycf4 has been lost from the chloroplast genome in Lathyrus odoratus and separately in three other groups of legumes. Each of the four consecutive genes ycf4-psaI-accD-rps16 has been lost in at least one member of the legume ''inverted repeat loss'' clade, despite the rarity of chloroplast gene losses in angiosperms. We established that accD has relocated to the nucleus in Trifolium species, but were unable to find nuclear copies of ycf4 or psaI in Lathyrus. Our results suggest that, as well as accelerating sequence evolution, localized hypermutation has contributed to the phenomenon of gene loss or relocation to the nucleus.

We used DNA sequencing and gel blot surveys to assess the integrity of the chloroplast gene infA , which codes for translation initiation factor 1, in Ͼ 300 diverse angiosperms. Whereas most angiosperms appear to contain an intact chloroplast infA gene, the gene has repeatedly become defunct in ‫ف‬ 24 separate lineages of angiosperms, including almost all rosid species. In four species in which chloroplast infA is defunct, transferred and expressed copies of the gene were found in the nucleus, complete with putative chloroplast transit peptide sequences. The transit peptide sequences of the nuclear infA genes from soybean and Arabidopsis were shown to be functional by their ability to target green fluorescent protein to chloroplasts in vivo. Phylogenetic analysis of infA sequences and assessment of transit peptide homology indicate that the four nuclear infA genes are probably derived from four independent gene transfers from chloroplast to nuclear DNA during angiosperm evolution. Considering this and the many separate losses of infA from chloroplast DNA, the gene has probably been transferred many more times, making infA by far the most mobile chloroplast gene known in plants. INTRODUCTIONMany genes have been lost from the chloroplast genome during plant and algal evolution. Most of these losses occurred in the murky interval between the original endosymbiosis of a cyanobacterium (with perhaps 2000 proteincoding genes) and the last common ancestor of all existing chloroplast genomes (with ‫ف‬ 210 protein-coding genes; . Many other genes were lost during the early evolution of photosynthetic eukaryotes, often in parallel in different algal lineages, and some of these losses were the result of gene transfers to the nuclear genome . During the evolution of land plants, relatively few changes occurred to the set of genes found in chloroplast DNA (cpDNA) Palmer and Delwiche, 1998). Nonetheless, the most recent changes are likely to provide the most information about the evolutionary mechanisms involved.Among the six completely sequenced chloroplast genomes from angiosperms (excluding the nonphotosynthetic plant Epifagus virginiana ; Wolfe et al., 1992a), 74 proteincoding genes are held in common and an additional five are present in only some species. These five genes are accD , ycf1 , and ycf2 (pseudogenes in rice and maize; Hiratsuka et al., 1989;Maier et al., 1995), rpl23 (pseudogene in spinach; Thomas et al., 1988), and infA (pseudogene in tobacco, Arabidopsis, and Oenothera elata ; Shinozaki et al., 1986;Wolfe et al., 1992b;Sato et al., 1999; Hupfer et al., 2000). Other chloroplast gene losses in angiosperms that have been confirmed by sequencing include rpl22 , rps16 , and ycf4 (open reading frame 184), all of which have been lost in 1 To whom correspondence should be addressed. E-mail (in Dublin) khwolfe@tcd.ie; fax 353-1-6798558. 646The Plant Cell some or all legumes (Gantt et al., 1991;Nagano et al., 1991; Doyle et al., 1995; K.H. Wolfe, unpublished data), and ycf2 and ndhF , both of which have been lost ...

SUMMARYFew regulators of phenylpropanoids have been identified in monocots having potential as biofuel crops. Here we demonstrate the role of the maize (Zea mays) R2R3-MYB factor ZmMYB31 in the control of the phenylpropanoid pathway. We determined its in vitro consensus DNA-binding sequence as ACC T / A ACC, and chromatin immunoprecipitation (ChIP) established that it interacts with two lignin gene promoters in vivo. To explore the potential of ZmMYB31 as a regulator of phenylpropanoids in other plants, its role in the regulation of the phenylpropanoid pathway was further investigated in Arabidopsis thaliana. ZmMYB31 downregulates several genes involved in the synthesis of monolignols and transgenic plants are dwarf and show a significantly reduced lignin content with unaltered polymer composition. We demonstrate that these changes increase cell wall degradability of the transgenic plants. In addition, ZmMYB31 represses the synthesis of sinapoylmalate, resulting in plants that are more sensitive to UV irradiation, and induces several stress-related proteins. Our results suggest that, as an indirect effect of repression of lignin biosynthesis, transgenic plants redirect carbon flux towards the biosynthesis of anthocyanins. Thus, ZmMYB31 can be considered a good candidate for the manipulation of lignin biosynthesis in biotechnological applications.

SummaryTransgene expression from the plant's plastid genome represents a promising strategy in molecular farming because of the plastid's potential to accumulate foreign proteins to high levels and the increased biosafety provided by the maternal mode of organelle inheritance.In this article, we explore the potential of transplastomic plants to produce human