From the seeds and siliques of Arabidopsis thaliana, six brassinosteroids, brassinolide, castasterone, typhasterol, 6-deoxocastasterone, 6-deoxotyphasterol and 6-deoxoteasterone, were identified by GC-mass spectrometry or GC-selected ion monitoring. As the occurrence of castasterone, typhasterol, 6-deoxocastasterone and 6-deoxotyphasterol in the shoots of A. thaliana has already been reported, this study provides evidence for the occurrence of the above four brassinosteroids in different organs, seeds and siliques, and the first evidence for the occurrence of brassinolide and 6-deoxoteasterone in A. thaliana. All brassinosteroids identified in this study belong to important components of both the early and late C-6 oxidation pathways, which were established in the cultured cells of Catharanthus roseus. This suggests that both pathways are operating in A. thaliana to produce the most biologically active brassinosteroid, brassinolide, which is responsible for growth and development of the plant.

Nucleolar dominance is an epigenetic phenomenon in which one parental set of ribosomal RNA (rRNA) genes is silenced in an interspecific hybrid. In natural Arabidopsis suecica, an allotetraploid (amphidiploid) hybrid of Arabidopsis thaliana and Cardaminopsis arenosa, the A. thaliana rRNA genes are repressed. Interestingly, A. thaliana rRNA gene silencing is variable in synthetic Arabidopsis suecica F1 hybrids. Two generations are needed for A. thaliana rRNA genes to be silenced in all lines, revealing a species-biased direction but stochastic onset to nucleolar dominance. Backcrossing synthetic A. suecica to tetraploid A. thaliana yielded progeny with active A. thaliana rRNA genes and, in some cases, silenced C. arenosa rRNA genes, showing that the direction of dominance can be switched. The hypothesis that naturally dominant rRNA genes have a superior binding affinity for a limiting transcription factor is inconsistent with dominance switching. Inactivation of a species-specific transcription factor is argued against by showing that A. thaliana and C. arenosa rRNA genes can be expressed transiently in the other species. Transfected A. thaliana genes are also active in A. suecica protoplasts in which chromosomal A. thaliana genes are repressed. Collectively, these data suggest that nucleolar dominance is a chromosomal phenomenon that results in coordinate or cooperative silencing of rRNA genes.

Sulfate uptake of Arabidopsis thaliana seedlings were characterized. Sulfate uptake obeyed single Michaelis-Menten's kinetics. When plants were grown in the medium containing 1,500 mM sulfate (control), Km was 20.0 +/- 3.60 mM and the Vmax ranged from 21 to 54 nmol g-1 fresh weight hr-1. While Km of plants grown in the sulfate-deficient (-S) medium containing 10 mM sulfate was not significantly changed, Vmax of plants grown in the -S medium increased by two fold. In the presence of selenate, an analogue to sulfate, Vmax remained unchanged, indicating that selenate is a competitive inhibitor for sulfate uptake. Metabolic inhibitors and high pH decreased sulfate uptake, suggesting that sulfate uptake system requires a H+ gradient generated by a H+-ATPase. Transfer of seedlings from the control to the -S medium increased the rate of sulfate uptake and vice versa, suggesting that sulfate uptake in A. thaliana was regulated by external sulfate concentrations. These characteristics of sulfate uptake in A. thaliana are similar to those reported for other plant species.

Plastid transformation is reported in Arabidopsis thaliana following biolistic delivery of transforming DNA into leaf cells. Transforming plasmid pGS31A carries a spectinomycin resistance (aadA) gene flanked by plastid DNA sequences to target its insertion between trnV and the rps12/7 operon. Integration of aadA by two homologous recombination events via the flanking ptDNA sequences and selective amplification of the transplastomes on spectinomycin medium yielded resistant cell lines and regenerated plants in which the plastid genome copies have been uniformly altered. The efficiency of plastid transformation was low: 2 in 201 bombarded leaf samples. None of the 98 plants regenerated from the two lines were fertile.

Plastid transformation is reported in Arabidopsis thaliana following biolistic delivery of transforming DNA into leaf cells. Transforming plasmid pGS31A carries a spectinomycin resistance (aadA) gene flanked by plastid DNA sequences to target its insertion between trnV and the rps12/7 operon. Integration of aadA by two homologous recombination events via the flanking ptDNA sequences and selective amplification of the transplastomes on spectinomycin medium yielded resistant cell lines and regenerated plants in which the plastid genome copies have been uniformly altered. The efficiency of plastid transformation was low: 2 in 201 bombarded leaf samples. None of the 98 plants regenerated from the two lines were fertile.

A mutant of Saccharomyces cerevisiae deleted for the COQ3 gene was constructed. COQ3 encodes a 3,4‐dihydroxy‐5‐hexaprenylbenzoate (DHHB) methyltransferase that catalyses the fourth step in the biosynthesis of ubiquinone from p‐hydroxybenzoic acid. A full length cDNA encoding a homologue of DHHB‐methyltransferase was cloned from an Arabidopsis thaliana cDNA library by functional complementation of a yeast coq3 deletion mutant. The Arabidopsis thaliana cDNA (AtCOQ3) was able to restore the respiration ability and ubiquinone synthesis of the mutant. The product of the 1372 bp cDNA contained 322 amino acids and had a molecular mass of 35 360 Da. The predicted amino acid sequence contained all consensus regions for S‐adenosyl methionine methyltransferases and presented 26% identity with Saccharomyces cerevisiae DHHB‐methyltransferase and 38% identity with the rat protein, as well as with a bacterial (Escherichia coli and Salmonella typhimurium) methyltransferase encoded by the UBIG gene. Southern analysis showed that the Arabidopsis thaliana enzyme was encoded by a single nuclear gene. The NH2‐terminal part of the cDNA product contained features consistent with a putative mitochondrial transit sequence. The cDNA in Escherichia coli was overexpressed and antibodies were raised against the recombinant protein. Western blot analysis of Arabidopsis thaliana and pea protein extracts indicated that the AtCOQ3 gene product is localized within mitochondrial membranes. This result suggests that at least this step of ubiquinone synthesis takes place in mitochondria.

In order to characterize new GC-rich minisatellites present in the Arabidopsis thaliana genome, a genomic library was screened at low stringency with a probe containing nine repeated-units of a minisatellite (CMs1) previously identified. Both minisatellites and minisatellite-like elements were identified. The minisatellites, with a tandemly-repeated structure, all contain the Arabidopsis thaliana-core sequence previously defined (Tourmente et al., 1994). Both minisatellite and minisatellite-like sequences occur in the Arabidopsis genome in low copy and are weakly polymorphic between ecotypes. The genetic mapping of these markers has shown that they are dispersed on the genome. YACs clones of the CIC library carrying these minisatellites and minisatellite-like sequences were identified.

Transient expression of the beta-glucuronidase (GUS) gene introduced into Arabidopsis thaliana intact plants by T-DNA after vacuum infiltration of Agrobacterium tumefaciens was followed. The first incidence of GUS activity was found 2-3 d after treatment and a peak of activity one week after treatment in both A. thaliana races, Columbia and C24. GUS activity was sharply increased by cultivation of Arabidopsis plants at elevated temperature (29 deg C) compared to cultivation at 25 deg C. The density of inocula also influenced the GUS activity.