Subcellular distribution of steryl ester biosynthesis in spinach leaves

Plant Sterols: Diversity, Biosynthesis, and Physiological Functions

Sterols, which are isoprenoid derivatives, are structural components of biological membranes. Special attention is now being given not only to their structure and function, but also to their regulatory roles in plants. Plant sterols have diverse composition; they exist as free sterols, sterol esters with higher fatty acids, sterol glycosides, and acylsterol glycosides, which are absent in animal cells. This diversity of types of phytosterols determines a wide spectrum of functions they play in plant life. Sterols are precursors of a group of plant hormones, the brassinosteroids, which regulate plant growth and development. Furthermore, sterols participate in transmembrane signal transduction by forming lipid microdomains. The predominant sterols in plants are β-sitosterol, campesterol, and stigmasterol. These sterols differ in the presence of a methyl or an ethyl group in the side chain at the 24th carbon atom and are named methylsterols or ethylsterols, respectively. The balance between 24-methylsterols and 24-ethylsterols is specific for individual plant species. The present review focuses on the key stages of plant sterol biosynthesis that determine the ratios between the different types of sterols, and the crosstalk between the sterol and sphingolipid pathways. The main enzymes involved in plant sterol biosynthesis are 3-hydroxy-3-methylglutaryl-CoA reductase, C24-sterol methyltransferase, and C22-sterol desaturase. These enzymes are responsible for maintaining the optimal balance between sterols. Regulation of the ratios between the different types of sterols and sterols/sphingolipids can be of crucial importance in the responses of plants to stresses.

New aspects of sterol biosynthesis in growth and development of higher plants

The characterization of the enzymatic components of plant sterol biosynthesis, the phenotypic description of a set of Arabidopsis thaliana sterol mutants, and consequently, the identification of aspects of growth and development influenced by sterols have been in recent years a very fruitful area of research. The overall data obtained in the field have shown an essential role of sterols at the cellular level in hormone signaling, organized divisions and embryo patterning. Indeed, current research efforts strongly suggest that membrane bound proteins implicated in polarized auxin transport or ethylene signaling have altered activity or functionality in a modified sterolic environment.

Biosynthesis and accumulation of sterols

In recent years, the impressive development of molecular genetics tools, the sequencing of the Arabidopsis thaliana genome, the availability of DNA or transposon tagged mutants, and the multiple possibilities offered by stable transformation with DNA in sense and antisense orientation have enabled the application of a strategy of gain or loss of function to study the sterol biosynthesis pathway. Here we describe the results obtained with these techniques. The results essentially confirm data obtained previously with sterol biosynthesis inhibitors (SBIs) and enable the precise dissection of biosynthetic pathways. We discuss the advantages and disadvantages of molecular genetics techniques as applied to sterol metabolism. The greater selectivity of these techniques constitutes an invaluable advantage and has led to the discovery of a role for sterols in plant development.

Three series of high molecular weight alkanoates found in Amazonian plants

Electron impact mass spectra were measured by high temperature high resolution gas chromatography-mass spectrometry (HT-HRGC-MS) for three homologous series of high molecular weight compounds present in the Amazonian plants Marupá (Simaruba amara) and Brazil nut (Bertholettia excelsa). Based on their mass spectra, the compounds were identified as three wax ester series of alpha-tocopherol (vitamin E), beta-tocopherol and phytol (2,6,10,14-tetramethylhexadec-14-en-16-ol). The interpretations are supported by high resolution mass spectrometry and GC retention indices of authentic standards.

Partial characterization of steryl ester biosynthesis in spinach leaves

Acetone powders of a 20,000g pellet fraction from spinach leaves (Spinacia oleracea L.) synthesized [4-(14)C]cholesteryl esters when incubated with [4-(14)C]cholesterol. The reaction was inhibited by digitonin. There was a reciprocal relationship between the decline of label in cholesterol and its incorporation into cholesteryl ester, indicating that free cholesterol was the direct precursor for cholesteryl ester biosynthesis. The hydrolysis of cholesteryl [1-(14)C]palmitate into free cholesterol and [1-(14)C]palmitate was not detected in these acetone powder preparations. Exogenous cholesteryl palmitate had no effect on the esterification of [4-(14)C]cholesterol. The data indicate that an esterase-type mechanism was not involved in the biosynthesis of these steryl esters. Label from [1-(14)C]palmitoyl-CoA was incorporated into steryl esters when incubated with spinach leaf acetone powder preparations. The optimal buffer for steryl ester biosynthesis was 2-(N-morpholino)ethanesulfonate and the optimal pH was 6. Iodoacetamide, N-ethylmaleimide, and dithiothreitol had no effect on the esterification reaction. Ethylenediaminetetraacetate, MgCl(2), CaCl(2), MnCl(2), and ZnSO(4) inhibited at concentrations of 10 to 30 mm.