Genetic study and further biochemical characterization of a tobacco mutant that overproduces sterols

Effects of impaired steryl ester biosynthesis on tomato growth and developmental processes

Steryl esters (SE) are stored in cytoplasmic lipid droplets and serve as a reservoir of sterols that helps to maintain free sterols (FS) homeostasis in cell membranes throughout plant growth and development, and provides the FS needed to meet the high demand of these key plasma membrane components during rapid plant organ growth and expansion. SE are also involved in the recycling of sterols and fatty acids released from membranes during plant tissues senescence. SE are synthesized by sterol acyltransferases, which catalyze the transfer of long-chain fatty acid groups to the hydroxyl group at C3 position of FS. Depending on the donor substrate, these enzymes are called acyl-CoA:sterol acyltransferases (ASAT), when the substrate is a long-chain acyl-CoA, and phospholipid:sterol acyltransferases (PSAT), which use a phospholipid as a donor substrate. We have recently identified and preliminary characterized the tomato (Solanum lycopersicum cv. Micro-Tom) SlASAT1 and SlPSAT1 enzymes. To gain further insight into the biological role of these enzymes and SE biosynthesis in tomato, we generated and characterized CRISPR/Cas9 single knock-out mutants lacking SlPSAT1 (slpsat1) and SlASAT1 (slasat1), as well as the double mutant slpsat1 x slasat1. Analysis of FS and SE profiles in seeds and leaves of the single and double mutants revealed a strong depletion of SE in slpsat1, that was even more pronounced in the slpsat1 x slasat1 mutant, while an increase of SE levels was observed in slasat1. Moreover, SlPSAT1 and SlASAT1 inactivation affected in different ways several important cellular and physiological processes, like leaf lipid bo1dies formation, seed germination speed, leaf senescence, and the plant size. Altogether, our results indicate that SlPSAT1 has a predominant role in tomato SE biosynthesis while SlASAT1 would mainly regulate the flux of the sterol pathway. It is also worth to mention that some of the metabolic and physiological responses in the tomato mutants lacking functional SlPSAT1 or SlASAT1 are different from those previously reported in Arabidopsis, being remarkable the synergistic effect of SlASAT1 inactivation in the absence of a functional SlPSAT1 on the early germination and premature senescence phenotypes.

HIGH STEROL ESTER 1 is a key factor in plant sterol homeostasis

Plants strictly regulate the levels of sterol in their cells, as high sterol levels are toxic. However, how plants achieve sterol homeostasis is not fully understood. We isolated an Arabidopsis thaliana mutant that abundantly accumulated sterol esters in structures of about 1 µm in diameter in leaf cells. We designated the mutant high sterol ester 1 (hise1) and called the structures sterol ester bodies. Here, we show that HISE1, the gene product that is altered in this mutant, functions as a key factor in plant sterol homeostasis on the endoplasmic reticulum (ER) and participates in a fail-safe regulatory system comprising two processes. First, HISE1 downregulates the protein levels of the β-hydroxy β-methylglutaryl-CoA reductases HMGR1 and HMGR2, which are rate-limiting enzymes in the sterol synthesis pathway, resulting in suppression of sterol overproduction. Second, if the first process is not successful, excess sterols are converted to sterol esters by phospholipid sterol acyltransferase1 (PSAT1) on ER microdomains and then segregated in SE bodies.

Identification and Characterization of Sterol Acyltransferases Responsible for Steryl Ester Biosynthesis in Tomato

Steryl esters (SEs) serve as a storage pool of sterols that helps to maintain proper levels of free sterols (FSs) in cell membranes throughout plant growth and development, and participates in the recycling of FSs and fatty acids released from cell membranes in aging tissues. SEs are synthesized by sterol acyltransferases, a family of enzymes that catalyze the transfer of fatty acil groups to the hydroxyl group at C-3 position of the sterol backbone. Sterol acyltransferases are categorized into acyl-CoA:sterol acyltransferases (ASAT) and phospholipid:sterol acyltransferases (PSAT) depending on whether the fatty acyl donor substrate is a long-chain acyl-CoA or a phospolipid. Until now, only Arabidopsis ASAT and PSAT enzymes (AtASAT1 and AtPSAT1) have been cloned and characterized in plants. Here we report the identification, cloning, and functional characterization of the tomato (Solanum lycopersicum cv. Micro-Tom) orthologs. SlPSAT1 and SlASAT1 were able to restore SE to wild type levels in the Arabidopsis psat1-2 and asat1-1 knock-out mutants, respectively. Expression of SlPSAT1 in the psat1-2 background also prevented the toxicity caused by an external supply of mevalonate and the early senescence phenotype observed in detached leaves of this mutant, whereas expression of SlASAT1 in the asat1-1 mutant revealed a clear substrate preference of the tomato enzyme for the sterol precursors cycloartenol and 24-methylene cycloartanol. Subcellular localization studies using fluorescently tagged SlPSAT1 and SlASAT1 proteins revealed that SlPSAT1 localize in cytoplasmic lipid droplets (LDs) while, in contrast to the endoplasmic reticulum (ER) localization of AtASAT1, SlASAT1 resides in the plasma membrane (PM). The possibility that PM-localized SlASAT1 may act catalytically in trans on their sterol substrates, which are presumably embedded in the ER membrane, is discussed. The widespread expression of SlPSAT1 and SlASAT1 genes in different tomato organs together with their moderate transcriptional response to several stresses suggests a dual role of SlPSAT1 and SlASAT1 in tomato plant and fruit development and the adaptive responses to stress. Overall, this study contributes to enlarge the current knowledge on plant sterol acyltransferases and set the basis for further studies aimed at understanding the role of SE metabolism in tomato plant growth and development.

Functional analysis of 3-hydroxy-3-methylglutaryl coenzyme a reductase encoding genes in triterpene saponin-producing ginseng

Ginsenosides are glycosylated triterpenes that are considered to be important pharmaceutically active components of the ginseng (Panax ginseng 'Meyer') plant, which is known as an adaptogenic herb. However, the regulatory mechanism underlying the biosynthesis of triterpene saponin through the mevalonate pathway in ginseng remains unclear. In this study, we characterized the role of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) concerning ginsenoside biosynthesis. Through analysis of full-length complementary DNA, two forms of ginseng HMGR (PgHMGR1 and PgHMGR2) were identified as showing high sequence identity. The steady-state mRNA expression patterns of PgHMGR1 and PgHMGR2 are relatively low in seed, leaf, stem, and flower, but stronger in the petiole of seedling and root. The transcripts of PgHMGR1 were relatively constant in 3- and 6-year-old ginseng roots. However, PgHMGR2 was increased five times in the 6-year-old ginseng roots compared with the 3-year-old ginseng roots, which indicates that HMGRs have constant and specific roles in the accumulation of ginsenosides in roots. Competitive inhibition of HMGR by mevinolin caused a significant reduction of total ginsenoside in ginseng adventitious roots. Moreover, continuous dark exposure for 2 to 3 d increased the total ginsenosides content in 3-year-old ginseng after the dark-induced activity of PgHMGR1. These results suggest that PgHMGR1 is associated with the dark-dependent promotion of ginsenoside biosynthesis. We also observed that the PgHMGR1 can complement Arabidopsis (Arabidopsis thaliana) hmgr1-1 and that the overexpression of PgHMGR1 enhanced the production of sterols and triterpenes in Arabidopsis and ginseng. Overall, this finding suggests that ginseng HMGRs play a regulatory role in triterpene ginsenoside biosynthesis.

Impaired sterol ester synthesis alters the response of Arabidopsis thaliana to Phytophthora infestans

Non-host resistance of Arabidopsis thaliana against Phytophthora infestans, the causal agent of late blight disease of potato, depends on efficient extracellular pre- and post-invasive resistance responses. Pre-invasive resistance against P. infestans requires the myrosinase PEN2. To identify additional genes involved in non-host resistance to P. infestans, a genetic screen was performed by re-mutagenesis of pen2 plants. Fourteen independent mutants were isolated that displayed an enhanced response to Phytophthora (erp) phenotype. Upon inoculation with P. infestans, two mutants, pen2-1 erp1-3 and pen2-1 erp1-4, showed an enhanced rate of mesophyll cell death and produced excessive callose deposits in the mesophyll cell layer. ERP1 encodes a phospholipid:sterol acyltransferase (PSAT1) that catalyzes the formation of sterol esters. Consistent with this, the tested T-DNA insertion lines of PSAT1 are phenocopies of erp1 plants. Sterol ester levels are highly reduced in all erp1/psat1 mutants, whereas sterol glycoside levels are increased twofold. Excessive callose deposition occurred independently of PMR4/GSL5 activity, a known pathogen-inducible callose synthase. A similar formation of aberrant callose deposits was triggered by the inoculation of erp1 psat1 plants with powdery mildew. These results suggest a role for sterol conjugates in cell non-autonomous defense responses against invasive filamentous pathogens.

Dissecting the sterol C-4 demethylation process in higher plants. From structures and genes to catalytic mechanism

Sterols become functional only after removal of the two methyl groups at C-4. This review focuses on the sterol C-4 demethylation process in higher plants. An intriguing aspect in the removal of the two C-4 methyl groups of sterol precursors in plants is that it does not occur consecutively as it does in yeast and animals, but is interrupted by several enzymatic steps. Each C-4 demethylation step involves the sequential participation of three individual enzymatic reactions including a sterol methyl oxidase (SMO), a 3β-hydroxysteroid-dehydrogenase/C4-decarboxylase (3βHSD/D) and a 3-ketosteroid reductase (SR). The distant location of the two C-4 demethylations in the sterol pathway requires distinct SMOs with respective substrate specificity. Combination of genetic and molecular enzymological approaches allowed a thorough identification and functional characterization of two distinct families of SMOs genes and two 3βHSD/D genes. For the latter, these studies provided the first molecularly and functionally characterized HSDs from a short chain dehydrogenase/reductase family in plants, and the first data on 3-D molecular interactions of an enzyme of the postoxidosqualene cyclase sterol biosynthetic pathway with its substrate in animals, yeast and higher plants. Characterization of these three new components involved in C-4 demethylation participates to the completion of the molecular inventory of sterol synthesis in higher plants.

Involvement of the phospholipid sterol acyltransferase1 in plant sterol homeostasis and leaf senescence

Genes encoding sterol ester-forming enzymes were recently identified in the Arabidopsis (Arabidopsis thaliana) genome. One belongs to a family of six members presenting homologies with the mammalian Lecithin Cholesterol Acyltransferases. The other one belongs to the superfamily of Membrane-Bound O-Acyltransferases. The physiological functions of these genes, Phospholipid Sterol Acyltransferase1 (PSAT1) and Acyl-CoA Sterol Acyltransferase1 (ASAT1), respectively, were investigated using Arabidopsis mutants. Sterol ester content decreased in leaves of all mutants and was strongly reduced in seeds from plants carrying a PSAT1-deficient mutation. The amount of sterol esters in flowers was very close to that of the wild type for all lines studied. This indicated further functional redundancy of sterol acylation in Arabidopsis. We performed feeding experiments in which we supplied sterol precursors to psat1-1, psat1-2, and asat1-1 mutants. This triggered the accumulation of sterol esters (stored in cytosolic lipid droplets) in the wild type and the asat1-1 lines but not in the psat1-1 and psat1-2 lines, indicating a major contribution of the PSAT1 in maintaining free sterol homeostasis in plant cell membranes. A clear biological effect associated with the lack of sterol ester formation in the psat1-1 and psat1-2 mutants was an early leaf senescence phenotype. Double mutants lacking PSAT1 and ASAT1 had identical phenotypes to psat1 mutants. The results presented here suggest that PSAT1 plays a role in lipid catabolism as part of the intracellular processes at play in the maintenance of leaf viability during developmental aging.

Homology modeling and site-directed mutagenesis reveal catalytic key amino acids of 3beta-hydroxysteroid-dehydrogenase/C4-decarboxylase from Arabidopsis

Sterols become functional only after removal of the two methyl groups at C4 by a membrane-bound multienzyme complex including a 3beta-hydroxysteroid-dehydrogenase/C4-decarboxylase (3betaHSD/D). We recently identified Arabidopsis (Arabidopsis thaliana) 3betaHSD/D as a bifunctional short-chain dehydrogenase/reductase protein. We made use of three-dimensional homology modeling to identify key amino acids involved in 4alpha-carboxy-sterol and NAD binding and catalysis. Key amino acids were subjected to site-directed mutagenesis, and the mutated enzymes were expressed and assayed both in vivo and in vitro in an erg26 yeast strain defective in 3betaHSD/D. We show that tyrosine-159 and lysine-163, which are oriented near the 3beta-hydroxyl group of the substrate in the model, are essential for the 3betaHSD/D activity, consistent with their involvement in the initial dehydrogenation step of the reaction. The essential arginine-326 residue is predicted to form a salt bridge with the 4alpha-carboxyl group of the substrate, suggesting its involvement both in substrate binding and in the decarboxylation step. The essential aspartic acid-39 residue is in close contact with the hydroxyl groups of the adenosine-ribose ring of NAD+, in good agreement with the strong preference of 3betaHSD/D for NAD+. Data obtained with serine-133 mutants suggest close proximity between the serine-133 residue and the C4beta domain of the bound sterol. Based on these data, we propose a tentative mechanism for 3betaHSD/D activity. This study provides, to our knowledge, the first data on the three-dimensional molecular interactions of an enzyme of the postoxidosqualene cyclase sterol biosynthesis pathway with its substrate. The implications of our findings for studying the roles of C4-alkylated sterol precursors in plant development are discussed.

Overexpression of CYP710A1 and CYP710A4 in transgenic Arabidopsis plants increases the level of stigmasterol at the expense of sitosterol

Sitosterol and stigmasterol are major sterols in vascular plants. An altered stigmasterol:sitosterol ratio has been proposed to influence the properties of cell membranes, particularly in relation to various stresses, but biosynthesis of stigmasterol is poorly understood. Recently, however, Morikawa et al. (Plant Cell 18:1008-1022, 2006) showed in Arabidopsis thaliana that synthesis of stigmasterol and brassicasterol is catalyzed by two separate sterol C-22 desaturases, encoded by the genes CYP710A1 and CYP710A2, respectively. The proteins belong to a small cytochrome P450 subfamily having four members, denoted by CYP710A1-A4, and are related to the yeast sterol C-22 desaturase Erg5p acting in ergosterol synthesis. Here, we report on our parallel investigation of the Arabidopsis CYP710A family. To elucidate the function of CYP710A proteins, transgenic Arabidopsis plants were generated overexpressing CYP710A1 and CYP710A4. Compared to wild-type plants, both types of transformant displayed a normal phenotype, but contained increased levels of free stigmasterol and a concomitant decrease in the level of free sitosterol. CYP710A1 transformants also displayed higher levels of esterified forms of stigmasterol, cholesterol, 24-methylcholesterol and isofucosterol. The results confirm the findings of Morikawa et al. (Plant Cell 18:1008-1022, 2006) regarding the function of CYP710A1 in stigmasterol synthesis, and show that CYP710A4 also has this capacity. Furthermore, our results suggest that an increased stigmasterol level alone is sufficient to stimulate esterification of other major sterols.

Biogenesis, molecular regulation and function of plant isoprenoids

Isoprenoids represent the oldest class of known low molecular-mass natural products synthesized by plants. Their biogenesis in plastids, mitochondria and the endoplasmic reticulum-cytosol proceed invariably from the C5 building blocks, isopentenyl diphosphate and/or dimethylallyl diphosphate according to complex and reiterated mechanisms. Compounds derived from the pathway exhibit a diverse spectrum of biological functions. This review centers on advances obtained in the field based on combined use of biochemical, molecular biology and genetic approaches. The function and evolutionary implications of this metabolism are discussed in relation with seminal informations gathered from distantly but related organisms.

Cellular sterol ester synthesis in plants is performed by an enzyme (phospholipid:sterol acyltransferase) different from the yeast and mammalian acyl-CoA:sterol acyltransferases

A gene encoding a sterol ester-synthesizing enzyme was identified in Arabidopsis. The cDNA of the Arabidopsis gene At1g04010 (AtPSAT) was overexpressed in Arabidopsis behind the cauliflower mosaic virus 35S promoter. Microsomal membranes from the leaves of overexpresser lines catalyzed the transacylation of acyl groups from phosphatidylethanolamine to sterols. This activity correlated with the expression level of the AtPSAT gene, thus demonstrating that this gene encodes a phospholipid:sterol acyltransferase (PSAT). Properties of the AtPSAT were examined in microsomal fractions from the tissues of an overexpresser. The enzyme did not utilize neutral lipids, had the highest activity with phosphatidylethanolamine, had a 5-fold preference for the sn-2 position, and utilized both saturated and unsaturated fatty acids. Various sterols and sterol intermediates, including triterpenic precursors, were acylated by the PSAT, whereas other triterpenes were not. Sterol selectivity studies showed that the enzyme is activated by end product sterols and that sterol intermediates are preferentially acylated by the activated enzyme. This indicates that PSAT both regulates the pool of free sterols as well as limits the amount of free sterol intermediates in the membranes. Two T-DNA insertion mutants in the AtPSAT gene, with strongly reduced (but still measurable) levels of sterol esters in their tissues, had no detectable PSAT activity in the microsomal fractions, suggesting that Arabidopsis possess other enzyme(s) capable of acylating sterols. The AtPSAT is the only intracellular enzyme found so far that catalyzes an acyl-CoA-independent sterol ester formation. Thus, PSAT has a similar physiological function in plant cells as the unrelated acyl-CoA:sterol acyltransferase has in animal cells.

Steroid profiles of transgenic tobacco expressing an Actinomyces 3-hydroxysteroid oxidase gene

Previously, we have shown that the expression of a 3-hydroxysteroid-oxidase gene in transgenic tobacco initiated a series of biochemical events leading to the conversion of sterol to stanol. As a result, the plants maintained a diminished sterol pool and a modified relative sterol ratio but demonstrated no observable morphological abnormalities. The maintenance of normal higher plant physiology in the absence of particular sterols or in the presence of modified sterol ratios is controversial. In this report, we present additional biochemical and physiological characteristics of transgenic tobacco expressing an Actinomyces 3-hydroxysteroid-oxidase gene. The total steroid accumulated in the transgenic plants is 6-fold higher than in control plants and consists of sterol, 3-ketosteroid and stanol. The relative abundance of sterols within whole plant and individual organs is grossly altered as ethylated side chain sterols account for 99% of the total sterol pool in the transgenic tobacco. Stigmasterol is readily apparent in all tissues and cholesterol is found at measurable levels in specific organs, while campesterol and sitosterol are detected at trace levels in the transgenic plants. Stanols and 3-ketosteroids accumulate in all tissues and represent 77% of the measurable steroid pool in the transgenic plants. The sum of sterol, the respective 3-ketosteroid plus stanol provide a relative abundance of steroid, which is similar to the abundance of sterol accumulated in control tissue. In vitro photosynthetic electron transport measurements demonstrate altered activity of chloroplasts under a variety of reaction conditions, indicating a link between the modified steroid pool and a modulation of chloroplast membrane function.

Expression in yeast of a novel phospholipase A1 cDNA from Arabidopsis thaliana

During a search for cDNAs encoding plant sterol acyltransferases, we isolated four full-length cDNAs from Arabidopsis thaliana that encode proteins with substantial identity with animal lecithin : cholesterol acyltransferases (LCATs). The expression of one of these cDNAs, AtLCAT3 (At3g03310), in various yeast strains resulted in the doubling of the triacylglycerol content. Furthermore, a complete lipid analysis of the transformed wild-type yeast showed that its phospholipid content was lower than that of the control (void plasmid-transformed) yeast whereas lysophospholipids and free fatty acids increased. When microsomes from the AtLCAT3-transformed yeast were incubated with di-[1-14C]oleyl phosphatidylcholine, both the lysophospholipid and free fatty acid fractions were highly and similarly labelled, whereas the same incubation with microsomes from the control yeast produced a negligible labelling of these fractions. Moreover when microsomes from AtLCAT3-transformed yeast were incubated with either sn-1- or sn-2-[1-14C]acyl phosphatidylcholine, the distribution of the labelling between the free fatty acid and the lysophosphatidylcholine fractions strongly suggested a phospholipase A1 activity for AtLCAT3. The sn-1 specificity of this phospholipase was confirmed by gas chromatography analysis of the hydrolysis of 1-myristoyl, 2-oleyl phosphatidylcholine. Phosphatidylethanolamine and phosphatidic acid were shown to be also hydrolysed by AtLCAT3, although less efficiently than phosphatidylcholine. Lysophospatidylcholine was a weak substrate whereas tripalmitoylglycerol and cholesteryl oleate were not hydrolysed at all. This novel A. thaliana phospholipase A1 shows optimal activity at pH 6-6.5 and 60-65 degrees C and appears to be unaffected by Ca2+. Its sequence is unrelated to all other known phospholipases. Further studies are in progress to elucidate its physiological role.

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.

The role of sterols in plant growth and development

Sterols found in all eukaryotic organisms are membrane components which regulate the fluidity and the permeability of phospholipid bilayers. Certain sterols in minute amounts, such as campesterol in Arabidopsis thaliana, are precursors of oxidized steroids acting as growth hormones collectively named brassinosteroids. The crucial importance of brassinosteroids upon growth and development has been established through the study of a set of dwarf mutants affected in brassinosteroid synthesis or perception. Some of these dwarfs are, in fact, deficient in the final steps of sterol biosynthesis and their developmental phenotypes are primarily caused by a depletion in the sterol precursor for brassinosteroids. Recently, the characterization of genes encoding sterol biosynthetic enzymes and the isolation of novel plant lines affected in the expression of those genes, either by insertional or classical mutagenesis, overexpression or cosuppression, have shed new light on the involvement of sterols in biological processes such as embryonic development, cell and plant growth, and fertility, which will be presented and discussed in this review article.

Reduction of cholesterol and glycoalkaloid levels in transgenic potato plants by overexpression of a type 1 sterol methyltransferase cDNA

Transgenic potato (Solanum tuberosum cv Désirée) plants overexpressing a soybean (Glycine max) type 1 sterol methyltransferase (GmSMT1) cDNA were generated and used to study sterol biosynthesis in relation to the production of toxic glycoalkaloids. Transgenic plants displayed an increased total sterol level in both leaves and tubers, mainly due to increased levels of the 24-ethyl sterols isofucosterol and sitosterol. The higher total sterol level was due to increases in both free and esterified sterols. However, the level of free cholesterol, a nonalkylated sterol, was decreased. Associated with this was a decreased glycoalkaloid level in leaves and tubers, down to 41% and 63% of wild-type levels, respectively. The results show that glycoalkaloid biosynthesis can be down-regulated in transgenic potato plants by reducing the content of free nonalkylated sterols, and they support the view of cholesterol as a precursor in glycoalkaloid biosynthesis.

Enhancement of seed phytosterol levels by expression of an N-terminal truncated Hevea brasiliensis (rubber tree) 3-hydroxy-3-methylglutaryl-CoA reductase

Dietary intake of phytosterols (plant sterols) has been shown to be effective in reducing blood cholesterol levels, thereby reducing the risk of cardiovascular disease. Phytosterols are most commonly sourced from vegetable oils, where they are present as minor components. We report here the generation of transgenic tobacco seeds substantially enhanced in phytosterol content by the expression of a modified form of one of the key sterol biosynthetic enzymes, 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR). The constitutive expression of an N-terminal truncated Hevea brasiliensis HMGR (t-HMGR), lacking the membrane binding domain, enhanced seed HMGR activities by 11-fold, leading to increases in total seed sterol of 2.4-fold. Seed-specific expression of t-HMGR enhanced total seed sterol levels by 3.2-fold, to 1.36% dry weight or 3.25% of oil. 4-desmethylsterols were increased by 2.2-fold, whilst certain sterol biosynthetic intermediates, in particular cycloartenol and 24-ethylidene lophenol, also accumulated. The additional sterol in seed tissue was present in the form of fatty acid esters. Constitutive expression of t-HMGR increased leaf phytosterol sterol levels by 10-fold, representing 1.8% dry weight, and the sterol was sequestered, in acyl ester form, as cytoplasmic 'oil droplets'. These studies establish HMGR as a key enzyme controlling overall flux into the sterol biosynthesis pathway in seed tissue, but the accumulation of certain intermediates suggests additional slow steps in the pathway. The expression of an N-truncated HMGR activity has generated novel phytosterol-enriched raw materials that may provide the basis of new sourcing opportunities for this important class of cholesterol-lowering actives.

Phytosterols, phytostanols, and their conjugates in foods: structural diversity, quantitative analysis, and health-promoting uses

Phytosterols (plant sterols) are triterpenes that are important structural components of plant membranes, and free phytosterols serve to stabilize phospholipid bilayers in plant cell membranes just as cholesterol does in animal cell membranes. Most phytosterols contain 28 or 29 carbons and one or two carbon-carbon double bonds, typically one in the sterol nucleus and sometimes a second in the alkyl side chain. Phytostanols are a fully-saturated subgroup of phytosterols (contain no double bonds). Phytostanols occur in trace levels in many plant species and they occur in high levels in tissues of only in a few cereal species. Phytosterols can be converted to phytostanols by chemical hydrogenation. More than 200 different types of phytosterols have been reported in plant species. In addition to the free form, phytosterols occur as four types of "conjugates," in which the 3beta-OH group is esterified to a fatty acid or a hydroxycinnamic acid, or glycosylated with a hexose (usually glucose) or a 6-fatty-acyl hexose. The most popular methods for phytosterol analysis involve hydrolysis of the esters (and sometimes the glycosides) and capillary GLC of the total phytosterols, either in the free form or as TMS or acetylated derivatives. Several alternative methods have been reported for analysis of free phytosterols and intact phytosteryl conjugates. Phytosterols and phytostanols have received much attention in the last five years because of their cholesterol-lowering properties. Early phytosterol-enriched products contained free phytosterols and relatively large dosages were required to significantly lower serum cholesterol. In the last several years two spreads, one containing phytostanyl fatty-acid esters and the other phytosteryl fatty-acid esters, have been commercialized and were shown to significantly lower serum cholesterol at dosages of 1-3 g per day. The popularity of these products has caused the medical and biochemical community to focus much attention on phytosterols and consequently research activity on phytosterols has increased dramatically.

Regulation of squalene synthase, a key enzyme of sterol biosynthesis, in tobacco

Squalene synthase (SS) represents a putative branch point in the isoprenoid biosynthetic pathway capable of diverting carbon flow specifically to the biosynthesis of sterols and, hence, is considered a potential regulatory point for sterol metabolism. For example, when plant cells grown in suspension culture are challenged with fungal elicitors, suppression of sterol biosynthesis has been correlated with a reduction in SS enzyme activity. The current study sought to correlate changes in SS enzyme activity with changes in the level of the corresponding protein and mRNA. Using an SS-specific antibody, the initial suppression of SS enzyme activity in elicitor-challenged cells was not reflected by changes in the absolute level of the corresponding polypeptide, implicating a post-translational control mechanism for this enzyme activity. In comparison, the absolute level of the SS mRNA did decrease approximately 5-fold in the elicitor-treated cells, which is suggestive of decreased transcription of the SS gene. Study of SS in intact plants was also initiated by measuring the level of SS enzyme activity, the level of the corresponding protein, and the expression of SS gene promoter-reporter gene constructs in transgenic plants. SS enzyme activity, polypeptide level, and gene expression were all localized predominately to the shoot apical meristem, with much lower levels observed in leaves and roots. These later results suggest that sterol biosynthesis is localized to the apical meristems and that apical meristems may be a source of sterols for other plant tissues.

Expression in yeast and tobacco of plant cDNAs encoding acyl CoA:diacylglycerol acyltransferase

During the course of a search for cDNAs encoding plant sterol acyltransferases, an expressed sequence tag clone presenting substantial identity with yeast and animal acyl CoA:cholesterol acyltransferases was used to screen cDNA libraries from Arabidopsis and tobacco. This resulted in the isolation of two full-length cDNAs encoding proteins of 520 and 532 amino acids, respectively. Attempts to complement the yeast double-mutant are1 are2 defective in acyl CoA:cholesterol acyltransferase were unsuccessful, showing that neither gene encodes acyl CoA:cholesterol acyltransferase. Their deduced amino acid sequences were then shown to have 40 and 38% identity, respectively, with a murine acyl CoA:diacylglycerol acyltransferase and their expression in are1 are2 or wild-type yeast resulted in a strong increase in the incorporation of oleyl CoA into triacylglycerols. Incorporation was 2-3 times higher in microsomes from yeast transformed with these plant cDNAs than in yeast transformed with the void vector, clearly showing that these cDNAs encode acyl CoA:diacylglycerol acyltransferases. Moreover, during the preparation of microsomes from the Arabidopsis DGAT-transformed yeast, a floating layer was observed on top of the 100 000 g supernatant. This fraction was enriched in triacylglycerols and exhibited strong acyl CoA:diacylglycerol acyltransferase activity, whereas almost no activity was detected in the corresponding clear fraction from the control yeast. Thanks to the use of this active fraction and dihexanoylglycerol as a substrate, the de novo synthesis of 1,2-dihexanoyl 3-oleyl glycerol by AtDGAT could be demonstrated. Transformation of tobacco with AtDGAT was also performed. Analysis of 19 primary transformants allowed detection, in several individuals, of a marked increase (up to seven times) of triacylglycerol content which correlated with the AtDGAT mRNA expression. Furthermore, light-microscopy observations of leaf epidermis cells, stained with a lipid-specific dye, showed the presence of lipid droplets in the cells of triacylglycerol-overproducer plants, thus illustrating the potential application of acyl CoA:diacylglycerol acyltransferase-transformed plants.

Some new aspects of isoprenoid biosynthesis in plants--a review

Plants are capable of synthesizing a myriad of isoprenoids and prenyl lipids. Much attention has been focused on 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the enzyme that synthesizes mevalonate and is generally considered responsible for the regulation of substrate flux to isoprenoids. In contrast to vertebrates, where there seems to exist only one HMGR gene, in plants a small family of isogenes appears differentially expressed in regard to location and time. Much less is known in plants about the preceding steps, viz. the conversion of acetyl-CoA to HMG-CoA. An enzyme system has been isolated from radish that can catalyze this transformation, and which shows some unusual properties in vitro. The intracellular localization of the early steps of isoprenoid biosynthesis in plant cells is still a matter of debate. The various observations and hypotheses derived from incorporation and inhibition studies are somewhat contradictory, and an attempt is being made to rationalize various findings that do not at first seem compatible. There are good arguments in favor of an exclusively cytoplasmic formation of isopentenyl pyrophosphate (IPP) via mevalonic acid, but other studies and observations suggest an independent formation in plastids. Other possibilities are being considered, such as the existence of independent (compartmentalized) biosynthetic pathways of IPP formation via the so-called Rohmer pathway. Substrate channeling through the formation of end product-specific multienzyme complexes (metabolons) with no release of substrate intermediates will also be discussed.

Biochemistry and molecular biology of chromoplast development

Plant cells contain a unique class of organelles, designated the plastids, which distinguish them from animal cells. According to the largely accepted endosymbiotic theory of evolution, plastids are descendants of prokaryotes. This process requires several adaptative changes which involve the maintenance and the expression of part of the plastid genome, as well as the integration of the plastid activity to the cellular metabolism. This is illustrated by the diversity of plastids encountered in plant cells. For instance, in tissues undergoing color changes, i.e., flowers and fruits, the chromoplasts produce and accumulate excess carotenoids. In this paper we attempt to review the basic aspects of chromoplast development.

UDP-glucose sterol beta-D-glucosyltransferase, a plasma membrane-bound enzyme of plants: enzymatic properties and lipid dependence

UDP-glucose sterol beta-D-glucosyltransferase (UDPG-SGTase) catalyzes the glucosylation of plant sterols. This enzyme has been shown to be membrane-bound, most of its activity being associated with plasma membrane in etiolated maize coleoptiles. After solubilization with detergents, total delipidation and purification, kinetic studies performed with a purified enzyme preparation in the presence of detergent and soybean phosphatidylcholine (PC) strongly suggest an ordered bi-bi mechanism for the glucosylation of sterols. A reduced sulfhydryl group and an arginyl residue were shown to be essential for activity. Lipid dependence studies have been performed on the delipidated enzyme in two systems: a micellar one composed of a mixture of enzyme, detergent and phospholipids and another one where the enzymatic activity was reconstituted in unilamellar lipid vesicles. In both systems it was shown that the UDPG-SGTase activity was stimulated to a large extent by negatively charged phospholipids. Enzymatic assays were performed with membrane fractions originating from plants whose sterol content was profoundly modified following treatment with a sterol biosynthesis inhibitor. Results showed that the sterol glucosylating activity was strongly inhibited in these fractions in accordance with sterol substrate specificity studies. All these results show that the UDPG-SGTase is exquisitely sensitive to its lipid environment. Physiological implications of these data are discussed in the light of the putative role of sterols in the plant cell.

Regulatory role of microsomal 3-hydroxy-3-methylglutaryl-coenzyme A reductase in a tobacco mutant that overproduces sterols

In a tobacco mutant callus, containing up to tenfold more sterols than the wild-type genotype, HMG-CoA reductase activity is increased by a factor of approximately three, as is the case in mutant seedlings and plants. The rate of HMG-CoA synthesis from acetyl-CoA by the coupled enzyme system acetoacetyl-CoA thiolase/HMG-CoA synthase, as well as its conversion to acetyl-CoA plus acetoacetate by action of HMG-CoA lyase are not affected. These results confirm the key-regulating role of HMG-CoA reductase in sterol biosynthesis, which seems not to be confined only to the animal kingdom, but can also be extended to plants.