Plant sterol-C24-methyl transferases: different profiles of tobacco transformed with SMT1 or SMT2

Phytosterol Profiles, Genomes and Enzymes - An Overview

The remarkable diversity of sterol biosynthetic capacities described in living organisms is enriched at a fast pace by a growing number of sequenced genomes. Whereas analytical chemistry has produced a wealth of sterol profiles of species in diverse taxonomic groups including seed and non-seed plants, algae, phytoplanktonic species and other unicellular eukaryotes, functional assays and validation of candidate genes unveils new enzymes and new pathways besides canonical biosynthetic schemes. An overview of the current landscape of sterol pathways in the tree of life is tentatively assembled in a series of sterolotypes that encompass major groups and provides also peculiar features of sterol profiles in bacteria, fungi, plants, and algae.

Highlights to phytosterols accumulation and equilibrium in plants: Biosynthetic pathway and feedback regulation

Phytosterols are a group of sterols exclusive to plants and fungi, but are indispensable to humans because of their medicinal and nutritional values. However, current raw materials used for phytosterols extraction add to the cost and waste in the process. For higher sterols production, major attention is drawn to plant materials abundant in phytosterols and genetic modification. To provide an insight into phytosterols metabolism, the research progress on key enzymes involved in phytosterols biosynthesis and conversions were summarized. CAS, SSR2, SMT, DWF1 and CYP710A, the enzymes participating in the biosynthetic pathway, and PSAT, ASAT and SGT, the enzymes involved in the conversion of free sterols to conjugated ones, were reviewed. Specifically, SMT and CYP710A were emphasized for their function on modulating the percentage composition of different kinds of phytosterols. The thresholds of sterol equilibrium and the resultant phytosterols accumulation, which vary in plant species and contribute to plasma membrane remodeling under stresses, were also discussed. By retrospective analysis of the previous researches, we proposed a feedback mechanism regulating sterol equilibrium underlying sterols metabolism. From a strategic perspective, we regard salt tolerant plant as an alternative to present raw materials, which will attain higher phytosterols production in combination with gene-modification.

Regulation of sterol content and biosynthetic gene expression during flower opening and early fruit development in olive

Phytosterols are lipophilic membrane components essential not only for diverse cellular functions but also are biosynthetic precursors of the plant hormone, brassinosteroid (BR). However, the interaction between phytosterol and BR during early fleshy-fruit growth remains largely uncharacterized. In olive, phytosterols are important lipids because they affect oil quality, but phytosterol composition during flowering and early fruit development has not been explored. Here, we first investigated the temporal changes in phytosterol composition, and biosynthetic gene expression that occurred during olive flower opening and early fruit growth. Next, we analyzed the interrelationship between phytosterol and BR, whose levels we manipulated through the application of exogenous BRs (24-epibrassinolide, EBR) or a BR biosynthesis inhibitor (brassinazole, Brz). In this report, the profiling of phytosterol measurement revealed that β-sitosterol is the most abundant in olive reproductive organs. Our data demonstrate that both OeCYP51 and OeSMT2 genes are upregulated during floral anthesis in good agreement with the rise in cholesterol and β-sitosterol contents in olive flower. By contrast, the OeCYP51 and OeSMT2 genes displayed different expression patterns during early olive-fruit development. Furthermore, our data show that exogenous EBR enhanced the early olive-fruit growth, as well as the OeSMT2 transcript and β-sitosterol levels, but decreased the OeCYP51 transcript, squalene, campesterol and cholesterol levels, whereas the Brz treatment exerted the opposite effect. Overall, our findings indicate an up-regulation of β-sitosterol biosynthesis by BR at the transcriptional level during early olive-fruit growth, providing a valuable tool to unravel the physiological function of SMT2 in future studies.

Membrane sterols and genes of sterol biosynthesis are involved in the response of Triticum aestivum seedlings to cold stress

Cold stress can significantly alter the composition and functioning of the major membrane lipids in plants. However, the roles of the sterol component of plant membranes in stress tolerance remain unclear. In the work presented here we investigated the role of sterols in the response of wheat to cold stress. Initial experiments demonstrated that the roots and leaves of wheat seedlings are differentially sensitive to low positive temperatures. In the roots, cold stress induced disturbance of membrane integrity and accumulation of ROS followed by the induction of autophagy. The absence of such changes in leaves suggests that in wheat, the roots are more sensitive to cold than the leaves. The roots display a time-dependent parabolic pattern of cold stress response, characterized by raised levels of sterols and markers of oxidative stress during short-term treatment, and a decline of these parameters after prolonged treatment. MβCD-induced sterol depletion aggravated the negative effects of cold on the roots. In the leaves the changes also displayed parabolic patterns, with significant changes occurring in 24-ethyl sterols and major PLs. Constitutively high levels of sterols, glycolipids and PLs, and up-regulation of TaSMTs in the leaves may provide membrane stability and cold tolerance. Taken together, results suggest that sterols play important roles in the response of wheat seedlings to cold stress.

RNAi of Sterol Methyl Transferase1 Reveals its Direct Role in Diverting Intermediates Towards Withanolide/Phytosterol Biosynthesis in Withania somnifera

The medicinal properties of Ashwagandha (Withania somnifera) are accredited to a group of compounds called withanolides. 24-Methylene cholesterol is the intermediate for sterol biosynthesis and a proposed precursor of withanolide biogenesis. However, conversion of 24-methylene cholesterol to withaferin A and other withanolides has not yet been biochemically dissected. Hence, in an effort to fill this gap, an important gene, encoding S-adenosyl l-methionine-dependent sterol-C24-methyltransferase type 1 (SMT1), involved in the first committed step of sterol biosynthesis, from W. somnifera was targeted in the present study. Though SMT1 has been characterized in model plants such as Nicotiana tabacum and Arabidopsis thaliana, its functional role in phytosterol and withanolide biosynthesis was demonstrated for the first time in W. somnifera. Since SMT1 acts at many steps preceding the withanolide precursor, the impact of this gene in channeling of metabolites for withanolide biosynthesis and its regulatory nature was illustrated by suppressing the gene in W. somnifera via the RNA interference (RNAi) approach. Interestingly, down-regulation of SMT1 in W. somnifera led to reduced levels of campesterol, sitosterol and stigmasterol, with an increase of cholesterol content in the transgenic RNAi lines. In contrast, SMT1 overexpression in transgenic N. tabacum enhanced the level of all phytosterols except cholesterol, which was not affected. The results established that SMT1 plays a crucial role in W. somnifera withanolide biosynthesis predominantly through the campesterol and stigmasterol routes.

De novo transcriptome analysis deciphered polyoxypregnane glycoside biosynthesis pathway in Gymnema sylvestre

Gymnema sylvestre is an important medicinal plant containing antidiabetic activity. Through de novo transcriptomic study, the pathways of polyoxypregnane glycosides were explored and candidate genes of these pathways were identified in G. sylvestre. High-quality raw reads were assembled into transcripts which resulted in 193,615 unigenes. These unigenes further decoded 58,274 coding DNA sequences (CDSs). Functional annotation of predicted CDSs was carried out using the protein databases, i.e., NCBI's non-redundant, Uniprot and Pfam. Eukaryotic orthologous group (KOG) classification and transcription factor analysis has revealed most CDS-enriched categories as "Signal transduction mechanism" and "Basic Helix loop helix" (bHLH) transcription factor family, respectively. A total of 16,569 CDSs were assigned minimum one Gene Ontology (GO) term. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis disclosed 235 CDSs which represented total 27 genes of pregnane glycoside pathways and 19 CDSs represented 10 important enzymes of polyoxypregnane glycoside biosynthesis, i.e., sterol 24-C-methyltransferase, cycloeucalenol cycloisomerase, Δ¹⁴-sterol reductase, C-8,7 sterol isomerase, sterol methyltransferase 2, C-5 sterol desaturase, sterol Δ⁷ reductase, Δ²⁴ sterol reductase, 3β-hydroxysteroid dehydrogenase and progesterone 5β reductase (5βPOR). This transcriptome analysis provided an important resource for future functional genomic studies in G. sylvestre.

Cloning and functional analysis of two sterol-C24-methyltransferase 1 (SMT1) genes from Paris polyphylla

The biosynthetic pathways of phytosterols and steroidal saponins are located in two adjacent branches which share cycloartenol as substrate. The rate-limiting enzyme S-adenosyl-L-methionine-sterol-C24-methyltransferase 1 (SMT1) facilitates the metabolic flux toward phytosterols. It catalyzes the methylation of the cycloartenol in the side chain of the C24-alkyl group, to generate 24(28)-methylene cycloartenol. In this study, we obtained two full-length sequences of SMT1 genes from Pari polyphylla, designated PpSMT1-1 and PpSMT1-2. The full-length cDNA of PpSMT1-1 was 1369 bp long with an open reading frame (ORF) of 1038 bp, while the PpSMT1-2 had a length of 1222 bp, with a 1005 bp ORF. Bioinformatics analysis confirmed that the two cloned SMTs belong to the SMT1 family. The predicted function was further validated by performing in vitro enzymatic reactions, and the results showed that PpSMT1-1 encodes a cycloartenol-C24-methyltransferase, which catalyzes the conversion of cycloartenol to 24-methylene cycloartenol, whereas PpSMT1-2 lacked this catalytic activity. The tissue expression patterns of the two SMTs revealed differential expression in different organs of Paris polyphylla plants of different developmental stage and age. These results lay the foundation for detailed genetic studies of the biosynthetic pathways of steroid compounds, which constitute the main class of active substances found in P. polyphylla.

Molecular cloning and functional identification of sterol C24-methyltransferase gene from Tripterygium wilfordii

Sterol C24-methyltransferase (SMT) plays multiple important roles in plant growth and development. SMT1, which belongs to the family of transferases and transforms cycloartenol into 24-methylene cycloartenol, is involved in the biosynthesis of 24-methyl sterols. Here, we report the cloning and characterization of a cDNA encoding a sterol C24-methyltransferase from Tripterygium wilfordii (TwSMT1). TwSMT1 (GenBank access number KU885950) is a 1530 bp cDNA with a 1041 bp open reading frame predicted to encode a 346-amino acid, 38.62 kDa protein. The polypeptide encoded by the SMT1 cDNA was expressed and purified as a recombinant protein from Escherichia coli (E. coli) and showed SMT activity. The expression of TwSMT1 was highly up-regulated in T. wilfordii cell suspension cultures treated with methyl jasmonate (MeJA). Tissue expression pattern analysis showed higher expression in the phellem layer compared to the other four organs (leaf, stem, xylem and phloem), which is about ten times that of the lowest expression in leaf. The results are meaningful for the study of sterol biosynthesis of T. wilfordii and will further lay the foundations for the research in regulating both the content of other main compounds and growth and development of T. wilfordii.

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.

MdMyb93 is a regulator of suberin deposition in russeted apple fruit skins

A comparison of the transcriptomes of russeted vs nonrusseted apple skins previously highlighted a tight relationship between a gene encoding an MYB-type transcription factor, MdMYB93, and some key suberin biosynthetic genes. The present work assesses the role of this transcription factor in the suberization process. A phylogenetic analysis of MdMYB93 and Arabidopsis thaliana MYBs was performed and the function of MdMYB93 was further investigated using Agrobacterium-mediated transient overexpression in Nicotiana benthamiana leaves. An RNA-Seq analysis was performed to highlight the MdMYB93-regulated genes. Ultraperformance liquid chromatography-triple time-of-flight (UPLC-TripleTOF) and GC-MS were used to investigate alterations in phenylpropanoid, soluble-free lipid and lipid polyester contents. A massive accumulation of suberin and its biosynthetic precursors in MdMYB93 agroinfiltrated leaves was accompanied by a remobilization of phenylpropanoids and an increased amount of lignin precursors. Gene expression profiling displayed a concomitant alteration of lipid and phenylpropanoid metabolism, cell wall development, and extracellular transport, with a large number of induced transcripts predicted to be involved in suberin deposition. The present work supports a major role of MdMYB93 in the regulation of suberin deposition in russeted apple skins, from the synthesis of monomeric precursors, their transport, polymerization, and final deposition as suberin in primary cell wall.

Characterization, mutagenesis and mechanistic analysis of an ancient algal sterol C24-methyltransferase: Implications for understanding sterol evolution in the green lineage

Sterol C24-methyltransferases (SMTs) constitute a group of sequence-related proteins that catalyze the pattern of sterol diversity across eukaryotic kingdoms. The only gene for sterol alkylation in green algae was identified and the corresponding catalyst from Chlamydomonas reinhardtii (Cr) was characterized kinetically and for product distributions. The properties of CrSMT were similar to those predicted for an ancient SMT expected to possess broad C3-anchoring requirements for substrate binding and formation of 24β-methyl/ethyl Δ(25(27))-olefin products typical of primitive organisms. Unnatural Δ(24(25))-sterol substrates, missing a C4β-angular methyl group involved with binding orientation, convert to product ratios in favor of Δ(24(28))-products. Remodeling the active site to alter the electronics of Try110 (to Leu) results in delayed timing of the hydride migration from methyl attack of the Δ(24)-bond, that thereby produces metabolic switching of product ratios in favor of Δ(25(27))-olefins or impairs the second C1-transfer activity. Incubation of [27-(13)C]lanosterol or [methyl-(2)H3]SAM as co-substrates established the CrSMT catalyzes a sterol methylation pathway by the "algal" Δ(25(27))-olefin route, where methylation proceeds by a conserved SN2 reaction and de-protonation proceeds from the pro-Z methyl group on lanosterol corresponding to C27. This previously unrecognized catalytic competence for an enzyme of sterol biosynthesis, together with phylogenomic analyses, suggest that mutational divergence of a promiscuous SMT produced substrate- and phyla-specific SMT1 (catalyzes first biomethylation) and SMT2 (catalyzes second biomethylation) isoforms in red and green algae, respectively, and in the case of SMT2 selection afforded modification in reaction channeling necessary for the switch in ergosterol (24β-methyl) biosynthesis to stigmasterol (24α-ethyl) biosynthesis during the course of land plant evolution.

Plant Sterol Diversity in Pollen from Angiosperms

Here we have examined the composition of free sterols and steryl esters of pollen from selected angiosperm species, as a first step towards a comprehensive analysis of sterol biogenesis in the male gametophyte. We detected four major sterol structural groups: cycloartenol derivatives bearing a 9β,19-cyclopropyl group, sterols with a double bond at C-7(8), sterols with a double bond at C-5(6), and stanols. All these groups were unequally distributed among species. However, the distribution of sterols as free sterols or as steryl esters in pollen grains indicated that free sterols were mostly Δ(5)-sterols and that steryl esters were predominantly 9β,19-cyclopropyl sterols. In order to link the sterol composition of a pollen grain at anthesis with the requirement for membrane lipid constituents of the pollen tube, we germinated pollen grains from Nicotiana tabacum, a model plant in reproductive biology. In the presence of radiolabelled mevalonic acid and in a time course series of measurements, we showed that cycloeucalenol was identified as the major neosynthesized sterol. Furthermore, the inhibition of cycloeucalenol neosynthesis by squalestatin was in full agreement with a de novo biogenesis and an apparent truncated pathway in the pollen tube.

Sterol side chain reductase 2 is a key enzyme in the biosynthesis of cholesterol, the common precursor of toxic steroidal glycoalkaloids in potato

Potatoes (Solanum tuberosum) contain α-solanine and α-chaconine, two well-known toxic steroidal glycoalkaloids (SGAs). Sprouts and green tubers accumulate especially high levels of SGAs. Although SGAs were proposed to be biosynthesized from cholesterol, the biosynthetic pathway for plant cholesterol is poorly understood. Here, we identify sterol side chain reductase 2 (SSR2) from potato as a key enzyme in the biosynthesis of cholesterol and related SGAs. Using in vitro enzyme activity assays, we determined that potato SSR2 (St SSR2) reduces desmosterol and cycloartenol to cholesterol and cycloartanol, respectively. These reduction steps are branch points in the biosynthetic pathways between C-24 alkylsterols and cholesterol in potato. Similar enzymatic results were also obtained from tomato SSR2. St SSR2-silenced potatoes or St SSR2-disrupted potato generated by targeted genome editing had significantly lower levels of cholesterol and SGAs without affecting plant growth. Our results suggest that St SSR2 is a promising target gene for breeding potatoes with low SGA levels.

Metabolic engineering of soybean affords improved phytosterol seed traits

Different combinations of three rate-limiting enzymes in phytosterol biosynthesis, the Arabidopsis thaliana hydroxyl methylglutaryl CoA1 (HMGR1) catalytic subunit linked to either constitutive or seed-specific β-conglycinin promoter, and the Glycine max sterol methyltransferase1 (SMT1) and sterol methyltransferase2-2 (SMT2-2) genes, under the control of seed-specific Glycinin-1 and Beta-phaseolin promoters, respectively, were engineered in soybean plants. Mature seeds of transgenic plants displayed modest increases in total sterol content, which points towards a tight control of phytosterol biosynthesis. However, in contrast to wild-type seeds that accumulated about 35% of the total sterol in the form of intermediates, in the engineered seeds driven by a seed-specific promoter, metabolic flux was directed to Δ(5) -24-alkyl sterol formation (99% of total sterol). The engineered effect of end-product sterol (sitosterol, campesterol, and stigmasterol) over-production in soybean seeds resulted in an approximately 30% increase in overall sitosterol synthesis, a desirable trait for oilseeds and human health. In contradistinction, increased accumulation of cycloartenol and 24(28)-methylencylartanol (55% of the total sterol) was detected in plants harbouring the constitutive t-HMGR1 gene, consistent with the previous studies. Our results support the possibility that metabolic flux of the phytosterol family pathway is differentially regulated in leaves and seeds.

Molecular characterization and functional analysis of Glycine max sterol methyl transferase 2 genes involved in plant membrane sterol biosynthesis

Sterol C24 methyltransferase (SMT2) genes governing the pattern of phytosterols synthesized in higher plants have been studied in Glycine seedlings and wild-type and engineered Arabidopsis thaliana plants. The SMT2 genes of soybean (SMT2-1 and SMT2-2) previously cloned and characterized (Neelakandan et al. 2009) were shown to complement the SMT deficient cvp1 mutant Arabidopsis plants, consistent with their role in regulation of 24-alkyl sterol-controlled plant physiology. Further analysis of these genes showed that environmental cues, including dehydration, cold, and abscisic acid induced differential changes in transcript levels of the SMT2 during soybean seedling growth. Sterol analyses of transgenic Arabidopsis seeds originating in variant constructs of AtHMGR1, GmSMT1, and GmSMT2 engineered in seeds showed relevant modifications in the ratio of 24-methyl to 24-ethyl sterol in the direction of sitosterol formation. To provide insight into the structural features of the sterol gene that affects transcript regulation, the upstream promoter sequences of soybean SMT2 genes were cloned and characterized. Sequence analysis revealed several important cis-elements and transcription factor binding sites. The analysis of promoter-GUS fusions in transgenic Arabidopsis plants revealed shared and distinct expression features in different developmental stages and tissues. The data are interpreted to imply that SMT2 is an important contributor to normal plant growth and development.

Alteration of fatty acid and sterol metabolism in miltefosine-resistant Leishmania donovani promastigotes and consequences for drug-membrane interactions

Miltefosine (hexadecylphosphocholine [HePC]) is the first orally active drug approved for the treatment of visceral leishmaniasis. In order to investigate the biochemical modifications occurring in HePC-resistant (HePC-R) Leishmania donovani promastigotes, taking into account the lipid nature of HePC, we investigated their fatty acid and sterol metabolisms. We found that the content of unsaturated phospholipid alkyl chains was lower in HePC-R parasite plasma membranes than in those of the wild type, suggesting a lower fluidity of HePC-R parasite membranes. We also demonstrated that HePC insertion within an external monolayer was more difficult when the proportion of unsaturated phospholipids decreased, rendering the HePC interaction with the external monolayer of HePC-R parasites more difficult. Furthermore, HePC-R parasite membranes displayed a higher content of short alkyl chain fatty acids, suggesting a partial inactivation of the fatty acid elongation enzyme system in HePC-R parasites. Sterol biosynthesis was found to be modified in HePC-R parasites, since the 24-alkylated sterol content was halved in HePC-R parasites; however, this modification was not related to HePC sensitivity. In conclusion, HePC resistance affects three lipid biochemical pathways: fatty acid elongation, the desaturase system responsible for fatty acid alkyl chain unsaturation, and the C-24-alkylation of sterols.

Arabidopsis cyp51 mutant shows postembryonic seedling lethality associated with lack of membrane integrity

CYP51 exists in all organisms that synthesize sterols de novo. Plant CYP51 encodes an obtusifoliol 14alpha-demethylase involved in the postsqualene sterol biosynthetic pathway. According to the current gene annotation, the Arabidopsis (Arabidopsis thaliana) genome contains two putative CYP51 genes, CYP51A1 and CYP51A2. Our studies revealed that CYP51A1 should be considered an expressed pseudogene. To study the functional importance of the CYP51A2 gene in plant growth and development, we isolated T-DNA knockout alleles for CYP51A2. Loss-of-function mutants for CYP51A2 showed multiple defects, such as stunted hypocotyls, short roots, reduced cell elongation, and seedling lethality. In contrast to other sterol mutants, such as fk/hydra2 and hydra1, the cyp51A2 mutant has only minor defects in early embryogenesis. Measurements of endogenous sterol levels in the cyp51A2 mutant revealed that it accumulates obtusifoliol, the substrate of CYP51, and a high proportion of 14alpha-methyl-delta8-sterols, at the expense of campesterol and sitosterol. The cyp51A2 mutants have defects in membrane integrity and hypocotyl elongation. The defect in hypocotyl elongation was not rescued by the exogenous application of brassinolide, although the brassinosteroid-signaling cascade is apparently not affected in the mutants. Developmental defects in the cyp51A2 mutant were completely rescued by the ectopic expression of CYP51A2. Taken together, our results demonstrate that the Arabidopsis CYP51A2 gene encodes a functional obtusifoliol 14alpha-demethylase enzyme and plays an essential role in controlling plant growth and development by a sterol-specific pathway.

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.

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.

Sterol C-24 methyltransferase type 1 controls the flux of carbon into sterol biosynthesis in tobacco seed

The first committed step in the conversion of cycloartenol into Delta(5) C24-alkyl sterols in plants is catalyzed by an S-adenosyl-methionine-dependent sterol-C24-methyltransferase type 1 (SMT1). We report the consequences of overexpressing SMT1 in tobacco (Nicotiana tabacum), under control of either the constitutive carnation etched ring virus promoter or the seed-specific Brassica napus acyl-carrier protein promoter, on sterol biosynthesis in seed tissue. Overexpression of SMT1 with either promoter increased the amount of total sterols in seed tissue by up to 44%. The sterol composition was also perturbed with levels of sitosterol increased by up to 50% and levels of isofucosterol and campesterol increased by up to 80%, whereas levels of cycloartenol and cholesterol were decreased by up to 53% and 34%, respectively. Concomitant with the enhanced SMT1 activity was an increase in endogenous 3-hydroxy-3-methylglutaryl coenzyme A reductase activity, from which one can speculate that reduced levels of cycloartenol feed back to up-regulate 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and thereby control the carbon flux into sterol biosynthesis. This potential regulatory role of SMT1 in seed sterol biosynthesis is discussed.

The ratio of campesterol to sitosterol that modulates growth in Arabidopsis is controlled by STEROL METHYLTRANSFERASE 2;1

The Arabidopsis genome contains three distinct genes encoding sterol-C24-methyltransferases (SMTs) involved in sterol biosynthesis. The expression of one of them, STEROL METHYLTRANSFERASE 2;1, was modulated in 35S:SMT2;1 Arabidopsis in order to study its physiological function. Plants overexpressing the transgene accumulate sitosterol, a 24-ethylsterol which is thought to be the typical plant membrane reinforcer, at the expense of campesterol. These plants displayed a reduced stature and growth that could be restored by brassinosteroid treatment. Plants showing co-suppression of SMT2;1 were characterized by a predominant 24-methylsterol biosynthetic pathway leading to a high campesterol content and a depletion in sitosterol. Pleiotropic effects on development such as reduced growth, increased branching, and low fertility of high-campesterol plants were not modified by exogenous brassinosteroids, indicating specific sterol requirements to promote normal development. Thus SMT2;1 has a crucial role in balancing the ratio of campesterol to sitosterol in order to fit both growth requirements and membrane integrity.