We have used in situ hybridization to determine the number of sites of rDNA in species in the genus Arabidopsis. A. wallichii (2n = 16) has one major pair of sites and one minor pair of sites, while A. pumila and A. griffithiana (both 2n = 32) have six major and two minor rDNA sites. A. thaliana (2n = 10) is known to have two pairs of rDNA sites. A highly repeated para-centromeric sequence from A. thaliana, pAL1, is absent in the other three species. Hence the A. thaliana genome is not present (or the centromeric DNA has evolved substantially) in the polyploid species A. pumila and A. griffithiana. Analysis of Arabidopsis species is a valuable complement to the large programmes for genetic analysis of A. thaliana.

Sitosterol was the principal sterol in Arabidopsis thaliana with campesterol being second most abundant. Small amounts of cholesterol, stigmasterol, campestanol, brassicasterol, stigmastanol, fucosterol and isofucosterol were identified by GC-MS. Free sterol made up 88% of the total sterol, but both esters and glycosides were detected. No significant changes in sterol composition were noted in Arabidopsis thaliana or in mutant PM-11 in response to chilling temperatures. Phytyl esters accumulated in mutant PM-11 after two or more days of chilling temperature, but did not accumulate in the wild type under chilling conditions. The fatty acids in these phytyl esters are atypical of the total fatty acids of Arabidopsis and are dominated by short chain, saturated fatty acids and hexadecatrienoic acid (16:3). The accumulation of these phytyl esters would be consistent with chlorophyll degradation, inhibition of late stages in fatty acid biosynthesis and degradation of the chloroplast.

Plant roots accumulate K+ from micromolar external concentrations. However, the absence of a firm determination of the trans-plasma-membrane electrochemical gradient for K+ in these conditions has precluded an assessment of whether K+-accumulation requires energization in addition to the driving force provided by the inside-negative membrane electrical potential (Em). To address this question unequivocally, we measured Em, and the cytosolic and external K+-activities in root cells of Arabidopsis thaliana (L.) Heynh. cv. Columbia in conditions in which net K+-accumulation occurs at low external K+ (10 micromolar). In these conditions, net K+- uptake was about 0.1 micromole.(g FW)-1.h-1, Em varied between -153 and -129 mV and the cytosolic K+-activity, determined with K+-selective electrodes, was 83 +/- 4 mM. These values yield an outwardly-directed driving force on K+ of at least 6.5 kJ.mol-1. Only if external potassium is raised to the region of 1 mM does Em become sufficient to drive net K+-accumulation. It is therefore concluded that at micromolar external K+-activities which prevail in most soils, K+-uptake cannot be solely energized by Em--as exemplified by a channel-mediated mechanism. The nature of the energization mechanism is discussed in relation to processes operating in fungal and algal cells.

We examined the effects of brassinosteroids on Arabidopsis thaliana (L.) Henyh. ecotype Columbia in order to develop a model system for studying gene regulation by plant steroids. Submicromolar concentrations of two brassinosteroids, brassinolide and 24-epibrassinolide, stimulated elongation of Arabidopsis peduncles and inhibited root elongation, respectively. Furthermore, brassinolide altered the abundance of specific in vitro translatable mRNAs from peduncles and whole plants of Arabidopsis. Root elongation in the auxin-insensitive Arabidopsis mutant axr1 was inhibited by 24-epibrassinolide but not by 2,4-D, indicating an independent mode of action for these growth regulators in this physiological response.

Arabidopsis thaliana has emerged as a model species for the analysis of genes controlling plant development. However, its small size has impaired biochemical analyses, and the absence of a transient expression system has hampered promoter analysis. Here, we report a method for rapidly establishing A. thaliana suspension cultures that yield protoplasts that can be readily transfected. We have optimized transient expression conditions using a modified polyethylene glycol/calcium nitrate transformation protocol and a Cauliflower Mosaic Virus 35S promoter-luciferase reporter gene construct. Our methods permit isolation of large quantities of rapidly growing cells and analysis of Arabidopsis promoters in vivo in a homologous system.

We examined the effects of brassinosteroids on Arabidopsis thaliana (L.) Henyh. ecotype Columbia in order to develop a model system for studying gene regulation by plant steroids. Submicromolar concentrations of two brassinosteroids, brassinolide and 24-epibrassinolide, stimulated elongation of Arabidopsis peduncles and inhibited root elongation, respectively. Furthermore, brassinolide altered the abundance of specific in vitro translatable mRNAs from peduncles and whole plants of Arabidopsis. Root elongation in the auxin-insensitive Arabidopsis mutant axr1 was inhibited by 24-epibrassinolide but not by 2,4-D, indicating an independent mode of action for these growth regulators in this physiological response.

We have isolated cDNA and genomic clones for Arabidopsis thaliana cytosolic ribosomal protein S15 and determined their sequences. Like animal S15 genes, this plant S15 gene is composed of four exons and the first intron is located immediately following the ATG translational start codon. The 5' end of the S15 mRNA was mapped by RNase protection experiments which showed that this mRNA contains a 5' untranslated region of approx. 83 nucleotides. Southern blot analyses suggest that Arabidopsis S15 is encoded by a small family of genes. The sequences of the predicted exons in the cloned S15 gene arc identical to that of the S15 cDNA, demonstrating that this gene is transcriptionally active. Sequence analysis of the cloned A. thaliana S15 gene shows that it is tightly linked (approx. 500 nucleotides distant) to a gene of unknown function. The Arabidopsis S15 protein described here is about 75% identical to vertebrate S15, 70% identical to the homologous yeast protein (S21), 50% identical to archaebacterial S19, 30% identical to eubacterial S19, and about 30% identical to plant mitochondrial and plastid S19.