SummaryHD2 (histone deacetylase) proteins are plant-speci®c histone deacetylases (HDACs). The Arabidopsis genome contains four HD2 genes, namely HD2A, HD2B, HD2C, and HD2D. We have previously demonstrated that HD2A, HD2B, and HD2C can repress transcription directly by targeting to promoters in planta. Here, we show that the N-terminal conserved motif (EFWG) and histidine 25 (H25), a potential catalytic residue, were important for the gene repression activity of HD2A. In situ hybridization indicated that HD2A, HD2B, and HD2C were expressed in ovules, embryos, shoot apical meristems, and primary leaves. Furthermore, all three genes were strongly induced during the process of somatic embryogenesis. HD2D mRNA was only detected in the stems and¯owers with young siliques and may have adopted different functions. Using green¯uorescent protein (GFP) fusions, we demonstrated that HD2A, HD2B, and HD2C accumulated in the nuclei of Arabidopsis cells. Overexpression of 35S::GFP±HD2A in transgenic Arabidopsis plants generated pleiotropic developmental abnormalities, including abnormal leaves, delayed¯owering, and aborted seed development. The data showed that normal pattern of HD2 expression was essential for normal plant development and that HD2A, HD2B, and HD2C may be needed for embryogenesis and embryo development. Reverse transcriptase (RT)-PCR analysis revealed that a number of genes involved in seed development and maturation were repressed in the 35S::GFP±HD2A plants, supporting a role of HD2A in the regulation of gene expression during seed development.
Members of the AP2 family of transcription factors, such as BABY BOOM (BBM), play important roles in cell proliferation and embryogenesis in Arabidopsis thaliana (AtBBM) and Brassica napus (BnBBM) but how this occurs is not understood. We have isolated three AP2 genes (GmBBM1, GmAIL5, GmPLT2) from somatic embryo cultures of soybean, Glycine max (L.) Merr, and discovered GmBBM1 to be homologous to AtBBM and BnBBM. GmAIL5 and GmPLT2 were homologous to Arabidopsis AINTEGUMENTA-like5 (AIL5) and PLETHORA2 (PLT2), respectively. Constitutive expression of GmBBM1 in Arabidopsis induced somatic embryos on vegetative organs and other pleiotropic effects on post-germinative vegetative organ development. Sequence comparisons of BBM orthologues revealed the presence of ten sequence motifs outside of the AP2 DNA-binding domains. One of the motifs, bbm-1, was specific to the BBM-like genes. Deletion and domain swap analyses revealed that bbm-1 was important for somatic embryogenesis and acted cooperatively with at least one other motif, euANT2, in the regulation of somatic embryogenesis and embryo development in transgenic Arabidopsis. The results provide new insights into the mechanisms by which BBM governs embryogenesis.Electronic supplementary materialThe online version of this article (doi:10.1007/s11103-010-9674-8) contains supplementary material, which is available to authorized users.
Wild-type tobacco (Nicotiana tabacum L.) seed development was characterized with respect to architecture and carbohydrate metabolism. Tobacco seeds accumulate oil and protein in the embryo, cellular endosperm and inner layer of the seed coat. They have high cell wall invertase (INV) and hexoses in early development which is typical of seeds. INV and the ratio of hexose to sucrose decline during development, switching from high hex to high suc, but not until most oil and all protein accumulation has occurred. The oil synthesis which coincides with the switch is mostly within the embryo. INV activity is greater than sucrose synthase activity throughout development, and both activities exceed the demand for carbohydrate for dry matter accumulation. To investigate the role of INV-mediated suc metabolism in oilseeds, genes for yeast INV and/or hexokinase (HK) were expressed under a seed-specific napin promoter, targeting activity to the apoplast and cytosol, respectively. Manipulating the INV pathway in an oilseed could either increase oil accumulation and sink strength, or disrupt carbohydrate metabolism, possibly through sugar-sensing, and decrease the storage function. Neither effect was found: transgenics with INV and/or HK increased 30-fold and 10-fold above wild-type levels had normal seed size and composition. This contrasted with dramatic effects on sugar contents in the INV lines.
Acetohydroxy acid synthase (AHAS) is an essential enzyme for many organisms as it catalyzes the first step in the biosynthesis of the branched-chain amino acids valine, isoleucine, and leucine. The enzyme is under allosteric control by these amino acids. It is also inhibited by several classes of herbicides, such as the sulfonylureas, imidazolinones and triazolopyrimidines, that are believed to bind to a relic quinone-binding site. In this study, a mutant allele of AHAS3 responsible for sulfonylurea resistance in a Brassica napus cell line was isolated. Sequence analyses predicted a single amino acid change (557 Trp-->Leu) within a conserved region of AHAS. Expression in transgenic plants conferred strong resistance to the three classes of herbicides, revealing a single site essential for the binding of all the herbicide classes. The mutation did not appear to affect feedback inhibition by the branched-chain amino acids in plants.
The Arabidopsis HD2 family of histone deacetylases consist of 4 members (HD2A, HD2B, HD2C, HD2D) that play diverse roles in plant development and physiology through chromatin remodelling. Here, we show that the transcripts of HD2 family members selectively accumulate in response to glucose through a HXK1-independent signal transduction pathway during the early stages of seedling growth. Germination was enhanced in hd2a null mutants relative to wild-type seeds. In contrast, hd2c mutants were restrained in germination relative to wild-type seeds. In hd2a/hd2c double mutants, germination was restored to wild-type levels. The data suggests that HD2A and HD2C may have different and opposing functions in germination with the glucose/HD2A pathway acting to restrain germination and the HD2C pathway acting to enhance germination. These pathways may function early in the regulation of seedling germination, independently of the glucose/HXK1/ABA signal transduction pathway, to fine tune the onset of germination.
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