Allelic mutant series reveal distinct functions for Arabidopsis cycloartenol synthase 1 in cell viability and plastid biogenesis

Differential regulation of key triterpene synthase gene under abiotic stress in Withania somnifera L. Dunal and its co-relation to sterols and withanolides

Withania somnifera (Ashwagandha), is one of the most reputed Indian medicinal plants, having immense pharmacological activities due to the occurrence of withanolides. The withanolides are biosynthesized through triterpenoid biosynthetic pathway with the involvement of WsCAS leading to cyclization of 2, 3 oxidosqualene, which is a key metabolite to further diversify to a myriad of phytochemicals. In contrast to the available reports on the studies of WsCAS in withanolide biosynthesis, its involvement in phytosterol biosynthesis needs investigation. Present work deals with the understanding of role of WsCAS triterpenoid synthase gene in the regulation of biosynthesis of phytosterols & withanolides. Docking studies of WsCAS protein revealed Conserved amino acids, DCATE motif, and QW motif which are involved in efficient substrate binding, structure stabilization, and catalytic activity. Overexpression/silencing of WsCAS leading to increment/decline of phytosterols confers its stringent regulation in phytosterols biosynthesis. Differential regulation of WsCAS on the metabolic flux towards phytosterols and withanolide biosynthesis was observed under abiotic stress conditions. The preferential channelization of 2, 3 oxidosqualene towards withanolides and/or phytosterols occurred under heat/salt stress and cold/water stress, respectively. Stigmasterol and β-sitosterol showed major contribution in high/low temperature and salt stress, and campesterol in water stress management. Overexpression of WsCAS in Arabidopsis thaliana led to the increment in phytosterols in general. Thus, the WsCAS plays important regulatory role in the biosynthetic pathway of phytosterols and withanolides under abiotic stress conditions.

Cuticular wax biosynthesis in blueberries (Vaccinium corymbosum L.): Transcript and metabolite changes during ripening and storage affect key fruit quality traits

In fruits, cuticular waxes affect fruit quality traits such as surface color at harvest and water loss during postharvest storage. This study investigated the transcriptional regulation of cuticular wax deposition in northern highbush blueberries (Vaccinium corymbosum L.) in relation to fruit water loss and surface color during ripening and postharvest storage, as well as the effects of abscisic acid (ABA)-mediated changes in cuticular wax deposition on these fruit quality traits. Total cuticular wax content (μg∙cm-2) decreased during fruit ripening and increased during postharvest storage. Transcriptome analysis revealed a transcript network for cuticular wax deposition in blueberries. Particularly, five OSC-Likes were identified as putative genes for triterpene alcohol production, with OSC-Like1 and OSC-Like2 encoding mixed amyrin synthases, OSC-Like3 encoding a lupeol synthase, and OSC-Like4 and OSC-Like5 encoding cycloartenol synthases. The expression of three CYP716A-like genes correlated to the accumulation of two triterpene acids oleanolic acid and ursolic acid, the major wax compounds in blueberries. Exogenous ABA application induced the expression of triterpenoid biosynthetic genes and the accumulation of β-amyrin and oleanolic acid, as well as increased the ratio of oleanolic acid to ursolic acid. These changes were associated with reduced fruit water loss. The content of β-diketones was also increased by ABA application, and this increase was associated with increased fruit lightness (measured as L* using CIELAB Color Space by a colorimeter). This study provided key insights on the molecular basis of cuticular wax deposition and its implications on fruit quality traits in blueberries.

Arabidopsis 3β-Hydroxysteroid Dehydrogenases/C4-Decarboxylases Are Essential for the Pollen and Embryonic Development

The biosynthesis of C27-29 sterols from their C30 precursor squalene involves C24-alkylation and the removal of three methyl groups, including two at the C4 position. The two C4 demethylation reactions require a bifunctional enzyme known as 3β-hydroxysteroid dehydrogenase/C4-decarboxylase (3βHSD/D), which removes an oxidized methyl (carboxylic) group at C4 while simultaneously catalyzing the 3β-hydroxyl→3-keto oxidation. Its loss-of-function mutations cause ergosterol-dependent growth in yeast and congenital hemidysplasia with ichthyosiform erythroderma and limb defect (CHILD) syndrome in humans. Although plant 3βHSD/D enzymes were well studied enzymatically, their developmental functions remain unknown. Here we employed a CRISPR/Cas9-based genome-editing approach to generate knockout mutants for two Arabidopsis 3βHSD/D genes, HSD1 and HSD2, and discovered the male gametophytic lethality for the hsd1 hsd2 double mutation. Pollen-specific expression of HSD2 in the heterozygous hsd1 hsd2/+ mutant not only rescued the pollen lethality but also revealed the critical roles of the two HSD genes in embryogenesis. Our study thus demonstrated the essential functions of the two Arabidopsis 3βHSD/D genes in male gametogenesis and embryogenesis.

Engineering Nicotiana tabacum trichomes for triterpenic acid production

In this work, we aimed at implementing the biosynthesis of triterpenic acids in Nicotiana tabacum glandular trichomes. Although endogenous genes coding for enzymes involved in such biosynthetic pathway are found in the Nicotiana tabacum genome, implementing such pathway specifically in glandular trichomes required to boost endogenous enzymatic activities. Five transgenes coding for a farnesyl-diphosphate synthase, a squalene synthase, a squalene epoxidase, a beta-amyrin synthase and a beta-amyrin 28-monooxygenase were introduced in N.tabacum, their expression being driven by pMALD1, a trichome-specific transcriptional promoter. This study aimed at testing whether sinking isoprenoid precursors localized in plastids, by exploiting potential cross-talks allowing the exchange of terpenoid pools from the chloroplast to the cytosol, could be a way to improve overall yield. By analyzing metabolites extracted from entire leaves, a low amount of ursolic acid was detected in plants expressing the five transgenes. Our study shows that the terpene biosynthetic pathway could be, in part, redirected in N.tabacum glandular trichomes with no deleterious phenotype at the whole plant level (chlorosis, dwarfism,…). In light of our results, possible ways to improve the final yield are discussed.

Pearl millet response to drought: A review

The C4 grass pearl millet is one of the most drought tolerant cereals and is primarily grown in marginal areas where annual rainfall is low and intermittent. It was domesticated in sub-Saharan Africa, and several studies have found that it uses a combination of morphological and physiological traits to successfully resist drought. This review explores the short term and long-term responses of pearl millet that enables it to either tolerate, avoid, escape, or recover from drought stress. The response to short term drought reveals fine tuning of osmotic adjustment, stomatal conductance, and ROS scavenging ability, along with ABA and ethylene transduction. Equally important are longer term developmental plasticity in tillering, root development, leaf adaptations and flowering time that can both help avoid the worst water stress and recover some of the yield losses via asynchronous tiller production. We examine genes related to drought resistance that were identified through individual transcriptomic studies and through our combined analysis of previous studies. From the combined analysis, we found 94 genes that were differentially expressed in both vegetative and reproductive stages under drought stress. Among them is a tight cluster of genes that are directly related to biotic and abiotic stress, as well as carbon metabolism, and hormonal pathways. We suggest that knowledge of gene expression patterns in tiller buds, inflorescences and rooting tips will be important for understanding the growth responses of pearl millet and the trade-offs at play in the response of this crop to drought. Much remains to be learnt about how pearl millet's unique combination of genetic and physiological mechanisms allow it to achieve such high drought tolerance, and the answers to be found may well be useful for crops other than just pearl millet.

Gene expression and genetic divergence in oak species highlight adaptive genes to soil water constraints

Drought and waterlogging impede tree growth and may even lead to tree death. Oaks, an emblematic group of tree species, have evolved a range of adaptations to cope with these constraints. The two most widely distributed European species, pedunculate (PO; Quercus robur L.) and sessile oak (SO; Quercus petraea Matt. Lieb), have overlapping ranges, but their respective distribution are highly constrained by local soil conditions. These contrasting ecological preferences between two closely related and frequently hybridizing species constitute a powerful model to explore the functional bases of the adaptive responses in oak. We exposed oak seedlings to waterlogging and drought, conditions typically encountered by the two species in their respective habitats, and studied changes in gene expression in roots using RNA-seq. We identified genes that change in expression between treatments differentially depending on species. These "species × environment"-responsive genes revealed adaptive molecular strategies involving adventitious and lateral root formation, aerenchyma formation in PO, and osmoregulation and ABA regulation in SO. With this experimental design, we also identified genes with different expression between species independently of water conditions imposed. Surprisingly, this category included genes with functions consistent with a role in intrinsic reproductive barriers. Finally, we compared our findings with those for a genome scan of species divergence and found that the expressional candidate genes included numerous highly differentiated genetic markers between the two species. By combining transcriptomic analysis, gene annotation, pathway analyses, as well as genome scan for genetic differentiation among species, we were able to highlight loci likely involved in adaptation of the two species to their respective ecological niches.

Overexpression of TwSQS, TwSE, and TwOSC Regulates Celastrol Accumulation in Cambial Meristematic Cells and Dedifferentiated Cells

Squalene synthase (SQS), squalene epoxidase (SE), and oxidosqualene cyclase (OSC) are encoding enzymes in downstream biosynthetic pathway of triterpenoid in plants, but the relationship between three genes and celastrol accumulation in Tripterygium wilfordii still remains unknown. Gene transformation system in plant can be used for studying gene function rapidly. However, there is no report on the application of cambial meristematic cells (CMCs) and dedifferentiated cells (DDCs) in genetic transformation systems. Our aim was to study the effects of individual overexpression of TwSQS, TwSE, and TwOSC on terpenoid accumulation and biosynthetic pathway related gene expression through CMCs and DDCs systems. Overexpression vectors of TwSQS, TwSE, and TwOSC were constructed by Gateway technology and transferred into CMCs and DDCs by gene gun. After overexpression, the content of celastrol was significantly increased in CMCs compared with the control group. However, there was no significant increment of celastrol in DDCs. Meanwhile, the relative expression levels of TwSQS, TwSE, TwOSC, and terpenoid biosynthetic pathway related genes were detected. The relative expression levels of TwSQS, TwSE, and TwOSC were increased compared with the control group in both CMCs and DDCs, while the pathway-related genes displayed different expression trends. Therefore, it was verified in T. wilfordii CMCs that overexpression of TwSQS, TwSE, and TwOSC increased celastrol accumulation and had different effects on the expression of related genes in terpenoid biosynthetic pathway, laying a foundation for further elucidating the downstream biosynthetic pathway of celastrol through T. wilfordii CMCs system.

Immunomodulatory Molecular Mechanisms of Luffa cylindrica for Downy Mildews Resistance Induced by Growth-Promoting Endophytic Fungi

Downy mildew (DM), caused by P. cubensis, is harmful to cucurbits including luffa, with increased shortcomings associated with its control through cultural practices, chemical fungicides, and resistant cultivars; there is a prompt need for an effective, eco-friendly, economical, and safe biocontrol approach. Current research is therefore dealt with the biocontrol of luffa DM1 through the endophytic fungi (EF) consortium. Results revealed that T. harzianum (ThM9) and T. virens (TvA1) showed pathogen-dependent inducible metabolic production of squalene and gliotoxins by higher gene expression induction of SQS1/ERG9 (squalene synthase) and GliP (non-ribosomal peptide synthetase). Gene expression of lytic enzymes of EF was also induced with subsequently higher enzyme activities upon confrontation with P. cubensis. EF-inoculated luffa seeds showed efficient germination with enhanced growth potential and vigor of seedlings. EF-inoculated plants showed an increased level of growth-promoting hormone GA with higher gene expression of GA2OX8. EF-pre-inoculated seedlings were resistant to DM and showed an increased GSH content and antioxidant enzyme activities (SOD, CAT, POD). The level of MDA, H₂O_2, REL, and disease severity was reduced by EF. ACC, JA, ABA, and SA were overproduced along with higher gene expression of LOX, ERF, NCED2, and PAL. Expression of defense-marker genes (PPO, CAT2, SOD, APX, PER5, LOX, NBS-LRR, PSY, CAS, Ubi, MLP43) was also modulated in EF-inoculated infected plants. Current research supported the use of EF inoculation to effectively escalate the systemic immunity against DM corresponding to the significant promotion of induced systemic resistance (ISR) and systemic acquired resistance (SAR) responses through initiating the defense mechanism by SA, ABA, ET, and JA biosynthesis and signaling pathways in luffa.

Metabolome and Transcriptome Analyses of Cucurbitacin Biosynthesis in Luffa (Luffa acutangula)

Cucurbitacins are extremely bitter compounds mainly present in Cucurbitaceae, where Luffa belongs. However, there is no comprehensive analysis of cucurbitacin biosynthesis in Luffa fruit. Therefore, this study analyzed bitter (WM709) and non-bitter (S1174) genotypes of Luffa to reveal the underlying mechanism of cucurbitacin biosynthesis by integrating metabolome and transcriptome analyses. A total of 422 metabolites were detected, including vitamins, essential amino acids, antioxidants, and antitumor substances. Of these, 131 metabolites showed significant differences between bitter (WM709) and non-bitter (S1174) Luffa fruits. The levels of isocucurbitacin B, cucurbitacin D, 23,24-dihydro cucurbitacin E, cucurbitacin F were significantly higher in bitter than in non-bitter Luffa. Transcriptome analysis showed that Bi, cytochromes P450s (CYP450s), and acyltransferase (ACT) of the cucurbitacin biosynthesis pathway, were significantly up-regulated. Moreover, drought stress and abscisic acid (ABA) activated genes of the cucurbitacin biosynthesis pathway. Furthermore, dual-luciferase reporter and yeast one-hybrid assays demonstrated that ABA-response element binding factor 1 (AREB1) binds to the Bi promoter to activate Bi expression. Comparative analysis of the Luffa and cucumber genomes showed that Bi, CYP450s, and ACT are located in the conserved syntenic loci, and formed a cucurbitacin biosynthesis cluster. This study provides important insights into major genes and metabolites of the cucurbitacin biosynthetic pathway, deepening the understanding of regulatory mechanisms of cucurbitacin biosynthesis in Luffa.

Different Inhibitory Effects of Erythromycin and Chlortetracycline on Early Growth of Brassica campestris Seedlings

Veterinary antibiotics, including erythromycin (Ery) and chlortetracycline (CTC), are often detected in agricultural land. Although these contaminants affect plant growth and development, their effects on crops remain elusive. In this study, the effects of Ery and CTC on plant growth were investigated and compared by analyzing transcript abundance in Brassica campestris seedlings. Treatment with Ery and/or CTC reduced chlorophyll content in leaves and photosynthetic efficiency. Examination of the chloroplast ultrastructure revealed the presence of abnormally shaped plastids in response to Ery and CTC treatments. The antibiotics produced similar phenotypes of lower accumulation of photosynthetic genes, including RBCL and LHCB1.1. Analysis of the transcript levels revealed that Ery and CTC differentially down-regulated genes involved in the tetrapyrrole biosynthetic pathway and primary root growth. In the presence of Ery and CTC, chloroplasts were undeveloped and photosynthesis efficiency was reduced. These results suggest that both Ery and CTC individually affect gene expression and influence plant physiological activity, independently of one another.

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.

Physiological changes and transcriptome profiling in Saccharum spontaneum L. leaf under water stress and re-watering conditions

As the polyploidy progenitor of modern sugarcane, Saccharum spontaneum is considered to be a valuable resistance source to various biotic and abiotic stresses. However, little has been reported on the mechanism of drought tolerance in S. spontaneum. Herein, the physiological changes of S. spontaneum GXS87-16 at three water-deficit levels (mild, moderate, and severe) and after re-watering during the elongation stage were investigated. RNA sequencing was utilized for global transcriptome profiling of GXS87-16 under severe drought and re-watered conditions. There were significant alterations in the physiological parameters of GXS87-16 in response to drought stress and then recovered differently after re-watering. A total of 1569 differentially expressed genes (DEGs) associated with water stress and re-watering were identified. Notably, the majority of the DEGs were induced by stress. GO functional annotations and KEGG pathway analysis assigned the DEGs to 47 GO categories and 93 pathway categories. The pathway categories were involved in various processes, such as RNA transport, mRNA surveillance, plant hormone signal transduction, and plant-pathogen interaction. The reliability of the RNA-seq results was confirmed by qRT-PCR. This study shed light on the regulatory processes of drought tolerance in S. spontaneum and identifies useful genes for genetic improvement of drought tolerance in sugarcane.

Dissecting cholesterol and phytosterol biosynthesis via mutants and inhibitors

Plants stand out among eukaryotes due to the large variety of sterols and sterol derivatives that they can produce. These metabolites not only serve as critical determinants of membrane structures, but also act as signaling molecules, as growth-regulating hormones, or as modulators of enzyme activities. Therefore, it is critical to understand the wiring of the biosynthetic pathways by which plants generate these distinct sterols, to allow their manipulation and to dissect their precise physiological roles. Here, we review the complexity and variation of the biosynthetic routes of the most abundant phytosterols and cholesterol in the green lineage and how different enzymes in these pathways are conserved and diverged from humans, yeast, and even bacteria. Many enzymatic steps show a deep evolutionary conservation, while others are executed by completely different enzymes. This has important implications for the use and specificity of available human and yeast sterol biosynthesis inhibitors in plants, and argues for the development of plant-tailored inhibitors of sterol biosynthesis.

A central role for polyprenol reductase in plant dolichol biosynthesis

Dolichol is an essential polyisoprenoid within the endoplasmic reticulum of all eukaryotes. It serves as a membrane bound anchor onto which N-glycans are assembled prior to being transferred to nascent polypeptides, many of which enter the secretory pathway. Historically, it has been posited that the accumulation of dolichol represents the 'rate-limiting' step in the evolutionary conserved process of N-glycosylation, which ultimately affects the efficacy of approximately one fifth of the entire eukaryotic proteome. Therefore, this study aimed to enhance dolichol accumulation by manipulating the enzymes involved in its biosynthesis using an established Nicotiana benthamiana platform. Co-expression of a Solanum lycopersicum (tomato) cis-prenyltransferase (CPT) and its cognate partner protein, CPT binding protein (CPTBP), that catalyze the antepenultimate step in dolichol biosynthesis led to a 400-fold increase in the levels of long-chain polyprenols but resulted in only modest increases in dolichol accumulation. However, when combined with a newly characterized tomato polyprenol reductase, dolichol biosynthesis was enhanced by approximately 20-fold. We provide further evidence that in the aquatic macrophyte, Lemna gibba, dolichol is derived exclusively from the mevalonic acid (MVA) pathway with little participation from the evolutionary co-adopted non-MVA pathway. Taken together these results indicate that to effectively enhance the in planta accumulation of dolichol, coordinated synthesis and reduction of polyprenol to dolichol, is strictly required.

Membrane Sterol Composition in Arabidopsis thaliana Affects Root Elongation via Auxin Biosynthesis

Plant membrane sterol composition has been reported to affect growth and gravitropism via polar auxin transport and auxin signaling. However, as to whether sterols influence auxin biosynthesis has received little attention. Here, by using the sterol biosynthesis mutant cyclopropylsterol isomerase1-1 (cpi1-1) and sterol application, we reveal that cycloeucalenol, a CPI1 substrate, and sitosterol, an end-product of sterol biosynthesis, antagonistically affect auxin biosynthesis. The short root phenotype of cpi1-1 was associated with a markedly enhanced auxin response in the root tip. Both were neither suppressed by mutations in polar auxin transport (PAT) proteins nor by treatment with a PAT inhibitor and responded to an auxin signaling inhibitor. However, expression of several auxin biosynthesis genes TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 (TAA1) was upregulated in cpi1-1. Functionally, TAA1 mutation reduced the auxin response in cpi1-1 and partially rescued its short root phenotype. In support of this genetic evidence, application of cycloeucalenol upregulated expression of the auxin responsive reporter DR5:GUS (β-glucuronidase) and of several auxin biosynthesis genes, while sitosterol repressed their expression. Hence, our combined genetic, pharmacological, and sterol application studies reveal a hitherto unexplored sterol-dependent modulation of auxin biosynthesis during Arabidopsis root elongation.

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.

Aphid growth and reproduction on plants with altered sterol profiles: Novel insights using Arabidopsis mutant and overexpression lines

Sterols are essential membrane components and are critical for many physiological processes in all eukaryotes. Insects and other arthropods are sterol auxotrophs that typically rely on a dietary source of sterols. Herbivorous insects generally obtain sterols from plants and then metabolize them into cholesterol, the dominant sterol in most insects. However, there is significant variation in phytosterol structure, and not all phytosterols are equally suitable for insects. In the current study, we used seven Arabidopsis thaliana lines that display altered sterol profiles due to mutations in the sterol biosynthetic pathway or to overexpression of key enzymes of the pathway, and investigated how plant sterol profiles affected green peach aphid (Myzus persicae) growth and reproduction. We also characterized the sterol profile of aphids reared on these Arabidopsis genotypes. Aphids on two mutant lines (14R/fk and ste1-1) that accumulated biosynthetic sterol intermediates (Δ^8,14-sterols, and Δ⁷-sterols, respectively) all showed significantly reduced growth and reproduction. Aphids on SMT2^COSUP plants (which have decreased β-sitosterol but increased campesterol) also displayed significantly reduced growth and reproduction. However, aphids on SMT2^OE plants (which have increased β-sitosterol but decreased campesterol) performed similarly to aphids on wild-type plants. Finally, Arabidopsis plants that had an overproduction of sterols (CD-HMGR^OE) or decreased sterol esters (psat1-2) had no impact on aphid performance. Two noteworthy results come from the aphid sterol profile study. First, β-sitosterol, cholesterol and stigmasterol were recovered in all aphids. Second, we did not detect Δ^8,14-sterols in aphids reared on 14R/fk plants. We discuss the implications of our findings, including how aphid sterol content does not appear to reflect plant leaf sterol profiles. We also discuss the potential of modifying plant sterol profiles to control insect herbivore pests, including aphids.

Massive postglacial gene flow between European white oaks uncovered genes underlying species barriers

Oaks are dominant forest tree species widely distributed across the Northern Hemisphere, where they constitute natural resources of economic, ecological, social and historical value. Hybridisation and adaptive introgression have long been thought to be major drivers of their ecological success. Therefore, the maintenance of species barriers remains a key question, given the extent of interspecific gene flow. In this study, we made use of the tremendous genetic variation among four European white oak species (31 million single nucleotide polymorphisms (SNPs)) to infer the evolutionary history of these species, study patterns of genetic differentiation and identify reproductive barriers. We first analysed the ecological and historical relationships among these species and inferred a long-term strict isolation followed by a recent and extensive postglacial contact using approximate Bayesian computation. Assuming this demographic scenario, we then performed backward simulations to generate the expected distributions of differentiation under neutrality to scan their genomes for reproductive barriers. We finally identified important intrinsic and ecological functions driving the reproductive isolation. We discussed the importance of identifying the genetic basis for the ecological preferences between these oak species and its implications for the renewal of European forests under global warming.

Distinct gene expression and secondary metabolite profiles in suppressor of prosystemin-mediated responses2 (spr2) tomato mutants having impaired mycorrhizal colonization

Arbuscular mycorrhizal fungi (AMF) colonization, sampled at 32-50 days post-inoculation (dpi), was significantly reduced in suppressor of prosystemin-mediated responses2 (spr2) mutant tomato plants impaired in the ω-3 FATTY ACID DESATURASE7 (FAD7) gene that limits the generation of linolenic acid and, consequently, the wound-responsive jasmonic acid (JA) burst. Contrary to wild-type (WT) plants, JA levels in root and leaves of spr2 mutants remained unchanged in response to AMF colonization, further supporting its regulatory role in the AM symbiosis. Decreased AMF colonization in spr2 plants was also linked to alterations associated with a disrupted FAD7 function, such as enhanced salicylic acid (SA) levels and SA-related defense gene expression and a reduction in fatty acid content in both mycorrhizal spr2 roots and leaves. Transcriptomic data revealed that lower mycorrhizal colonization efficiency in spr2 mutants coincided with the modified expression of key genes controlling gibberellin and ethylene signaling, brassinosteroid, ethylene, apocarotenoid and phenylpropanoid synthesis, and the wound response. Targeted metabolomic analysis, performed at 45 dpi, revealed augmented contents of L-threonic acid and DL-malic acid in colonized spr2 roots which suggested unfavorable conditions for AMF colonization. Additionally, time- and genotype-dependent changes in root steroid glycoalkaloid levels, including tomatine, suggested that these metabolites might positively regulate the AM symbiosis in tomato. Untargeted metabolomic analysis demonstrated that the tomato root metabolomes were distinctly affected by genotype, mycorrhizal colonization and colonization time. In conclusion, reduced AMF colonization efficiency in spr2 mutants is probably caused by multiple and interconnected JA-dependent and independent gene expression and metabolomic alterations.

Inhibition of Phytosterol Biosynthesis by Azasterols

Inhibitors of enzymes in essential cellular pathways are potent probes to decipher intricate physiological functions of biomolecules. The analysis of Arabidopsis thaliana sterol profiles upon treatment with a series of azasterols reveals a specific in vivo inhibition of SMT2, a plant sterol-C-methyltransferase acting as a branch point between the campesterol and sitosterol biosynthetic segments in the pathway. Side chain azasteroids that modify sitosterol homeostasis help to refine its particular function in plant development.

Biotic elicitors enhance diosgenin production in Helicteres isora L. suspension cultures via up-regulation of CAS and HMGR genes

In an attempt to find an alternative and potent source of diosgenin, a steroidal saponin in great demand for its pharmaceutical importance, Helicteres isora suspension cultures were explored for diosgenin extraction. The effect of biotic elicitors on the biosynthesis of diosgenin, in suspension cultures of H. isora was studied. Bacterial as well as fungal elicitors such as Escherichia coli, Bacillus subtilis, Saccharomyces cerevisiae and Aspergillus niger were applied at varying concentrations to investigate their effects on diosgenin content. The HPLC based quantification of the treated samples proved that amongst the biotic elicitors, E. coli (1.5%) proved best with a 9.1-fold increase in diosgenin content over respective control cultures. Further, the scaling-up of the suspension culture to shake-flask and ultimately to bioreactor level were carried out for production of diosgenin. During all the scaling-up stages, diosgenin yield obtained was in the range between 7.91 and 8.64 mg l^-1, where diosgenin content was increased with volume of the medium. The quantitative real-time PCR (qRT-PCR) analysis showed biotic elicitors induced the expression levels of regulatory genes in diosgenin biosynthetic pathway, the 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) and cycloartenol synthase (CAS), which can be positively correlated with elicited diosgenin contents in those cultures. The study holds significance as H. isora represents a cleaner and easy source of diosgenin where unlike other traditional sources, it is not admixed with other steroidal saponins, and the scaled-up levels of diosgenin achieved herein have the potential to be explored commercially.

Plant triterpenoids with bond-missing skeletons: biogenesis, distribution and bioactivity

Covering: up to December 2018 The polycyclic ABCD(E) framework of triterpenoids can miss a single endocyclic C-C bond as a result of a modification of the cyclization cascade that triggers their formation (interrupted- or diverted cascades), or can be the result of post-cyclization ring cleavage by late-stage oxidative modifications (seco-triterpenoids). Because of mechanistic and biogenetic differences, ring opening associated with loss of a skeletal fragment, as in nor-seco-triterpenoids (limonoids, quassinoids), will not be covered, nor will compounds where ring opening is part of a fragmentation cascade or of a multiple diversion from it. Even with these limitations, 342 bond-missing triterpenoids could be retrieved from the literature, with transversal distribution in the plant kingdom. Their structural diversity translates into a variety of biological targets, with dominance of potential applications in the realm of cancer, neuroprotection, and anti-infective therapy. In addition to the bioactivity and chemotaxonomic relevance of bond-missing triterpenoids, current knowledge on the genetic basis of interrupted- and diverted oxidosqualene cyclases will be summarized. This untapped source of enzymes could be useful to selectively modify triterpenoids by metabolic engineering, circumventing the bottlenecks of their isolation (poor yield or inadequate supply chain) to explore new areas of their chemical space.

Pale green mutant analyses reveal the importance of CsGLKs in chloroplast developmental regulation and their effects on flavonoid biosynthesis in tea plant

Tea cultivars with leaf color variation have attracted increasing attention in tea production and research due to their unusual appearances and appealing flavors. However, the molecular mechanism underlying this variation is little known due to the unavailability of genetic transformation and a highly complex genome. Here, a natural tea plant mutant producing pale green branches (pgb) was discovered and characterized. Ultrastructural and biochemical analyses showed that the leaves of the pgb mutant had defective chloroplast structure and significantly lower pigment content than the normal control. Comprehensive expression detection of chloroplast-development-related genes further indicated that a significant downregulation of CsGLKs in the pgb mutant likely caused the chloroplast defect. Transcriptome analyses and polyphenolic compound determination highlighted a tight correlation between photosynthesis and secondary metabolite biosynthesis in tea plant. These results provide useful information illuminating the mechanism of chloroplast development and leaf color variation in tea plant.

Regulation of Brassinosteroid Homeostasis in Higher Plants

Brassinosteroids (BRs) are known as one of the major classes of phytohormones essential for various processes during normal plant growth, development, and adaptations to biotic and abiotic stresses. Significant progress has been achieved on revealing mechanisms regulating BR biosynthesis, catabolism, and signaling in many crops and in model plant Arabidopsis. It is known that BRs control plant growth and development in a dosage-dependent manner. Maintenance of BR homeostasis is therefore critical for optimal functions of BRs. In this review, updated discoveries on mechanisms controlling BR homeostasis in higher plants in response to internal and external cues are discussed.

Functional Gene Network of Prenyltransferases in Arabidopsis thaliana

Prenyltransferases (PTs) are enzymes that catalyze prenyl chain elongation. Some are highly similar to each other at the amino acid level. Therefore, it is difficult to assign their function based solely on their sequence homology to functional orthologs. Other experiments, such as in vitro enzymatic assay, mutant analysis, and mutant complementation are necessary to assign their precise function. Moreover, subcellular localization can also influence the functionality of the enzymes within the pathway network, because different isoprenoid end products are synthesized in the cytosol, mitochondria, or plastids from prenyl diphosphate (prenyl-PP) substrates. In addition to in vivo functional experiments, in silico approaches, such as co-expression analysis, can provide information about the topology of PTs within the isoprenoid pathway network. There has been huge progress in the last few years in the characterization of individual Arabidopsis PTs, resulting in better understanding of their function and their topology within the isoprenoid pathway. Here, we summarize these findings and present the updated topological model of PTs in the Arabidopsis thaliana isoprenoid pathway.

Plastoglobular protein 18 is involved in chloroplast function and thylakoid formation

Plastoglobules are lipoprotein particles that are found in different types of plastids. They contain a very specific and specialized set of lipids and proteins. Plastoglobules are highly dynamic in size and shape, and are therefore thought to participate in adaptation processes during either abiotic or biotic stresses or transitions between developmental stages. They are suggested to function in thylakoid biogenesis, isoprenoid metabolism, and chlorophyll degradation. While several plastoglobular proteins contain identifiable domains, others provide no structural clues to their function. In this study, we investigate the role of plastoglobular protein 18 (PG18), which is conserved from cyanobacteria to higher plants. Analysis of a PG18 loss-of-function mutant in Arabidopsis thaliana demonstrated that PG18 plays an important role in thylakoid formation; the loss of PG18 results in impaired accumulation, assembly, and function of thylakoid membrane complexes. Interestingly, the mutant accumulated less chlorophyll and carotenoids, whereas xanthophyll cycle pigments were increased. Accumulation of photosynthetic complexes is similarly affected in both a Synechocystis and an Arabidopsis PG18 mutant. However, the ultrastructure of cyanobacterial thylakoids is not compromised by the lack of PG18, probably due to its less complex architecture.

Distinct triterpene synthases in the laticifers of Euphorbia lathyris

Euphorbia lathyris was proposed about fifty years ago as a potential agroenergetic crop. The tremendous amounts of triterpenes present in its latex has driven investigations for transforming this particular biological fluid into an industrial hydrocarbon source. The huge accumulation of terpenes in the latex of many plant species represent a challenging question regarding cellular homeostasis. In fact, the enzymes, the mechanisms and the controllers that tune the amount of products accumulated in specialized compartments (to fulfill ecological roles) or deposited at important sites (as essential factors) are not known. Here, we have isolated oxidosqualene cyclases highly expressed in the latex of Euphorbia lathyris. This triterpene biosynthetic machinery is made of distinct paralogous enzymes responsible for the massive accumulation of steroidal and non-steroidal tetracyclic triterpenes. More than eighty years after the isolation of butyrospermol from shea butter (Heilbronn IM, Moffet GL, and Spring FS J. Chem. Soc. 1934, 1583), a butyrospermol synthase is characterized in this work using yeast and in folia heterologous expression assays.

Analysis of the mechanisms regulating the expression of isoprenoid biosynthesis genes in hydroponically-grown Nicotiana benthamiana plants using virus-induced gene silencing

Secondary metabolites in plants play important roles in defence against biotic and abiotic stresses. Although the biosynthesis pathways of secondary metabolites have been extensively studied, the regulatory mechanism of gene expression involved in these pathways remains poorly understood. In this study, we develop a virus-induced gene silencing (VIGS) system that enables a rapid analysis of the regulatory mechanism of genes involved in the biosynthesis of isoprenoids, one of the largest groups in secondary metabolites, using hydroponically-grown Nicotiana benthamiana. Using VIGS, we successfully reduced the transcript levels of 3-hydroxy-3-methylglutaryl-CoA reductase 1 (HMGR1), cycloartenol synthase 1 (CAS1), sterol side chain reductase 2 (SSR2) and S-adenosyl-L-Met-dependent C-24 sterol methyltransferase 1 (SMT1) in leaf, stem and root tissues in approximately 2 weeks. We identified novel feedback and feed-forward regulation of isoprenoid biosynthesis genes when CAS1, which encodes a key enzyme involved in the biosynthesis of sterols and steroidal glycoalkaloids, was down-regulated. Furthermore, the regulation of these genes differed among different tissues. These results demonstrate that our system can rapidly analyse the regulatory mechanisms involved in the biosynthesis of secondary metabolites.

Improved fruit α-tocopherol, carotenoid, squalene and phytosterol contents through manipulation of Brassica juncea 3-HYDROXY-3-METHYLGLUTARYL-COA SYNTHASE1 in transgenic tomato

3-Hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS) in the mevalonate (MVA) pathway generates isoprenoids including phytosterols. Dietary phytosterols are important because they can lower blood cholesterol levels. Previously, the overexpression of Brassica juncea wild-type (wt) and mutant (S359A) BjHMGS1 in Arabidopsis up-regulated several genes in sterol biosynthesis and increased sterol content. Recombinant S359A had earlier displayed a 10-fold higher in vitro enzyme activity. Furthermore, tobacco HMGS overexpressors (OEs) exhibited improved sterol content, plant growth and seed yield. Increased growth and seed yield in tobacco OE-S359A over OE-wtBjHMGS1 coincided with elevations in NtSQS expression and sterol content. Herein, the overexpression of wt and mutant (S359A) BjHMGS1 in a crop plant, tomato (Solanum lycopersicum), caused an accumulation of MVA-derived squalene and phytosterols, as well as methylerythritol phosphate (MEP)-derived α-tocopherol (vitamin E) and carotenoids, which are important to human health as antioxidants. In tomato HMGS-OE seedlings, genes associated with the biosyntheses of C10, C15 and C20 universal precursors of isoprenoids, phytosterols, brassinosteroids, dolichols, methylerythritol phosphate, carotenoid and vitamin E were up-regulated. In OE-S359A tomato fruits, increased squalene and phytosterol contents over OE-wtBjHMGS1 were attributed to heightened SlHMGR2, SlFPS1, SlSQS and SlCYP710A11 expression. In both tomato OE-wtBjHMGS1 and OE-S359A fruits, the up-regulation of SlGPS and SlGGPPS1 in the MEP pathway that led to α-tocopherol and carotenoid accumulation indicated cross-talk between the MVA and MEP pathways. Taken together, the manipulation of BjHMGS1 represents a promising strategy to simultaneously elevate health-promoting squalene, phytosterols, α-tocopherol and carotenoids in tomato, an edible fruit.

The photosynthetic bacteria Rhodobacter capsulatus and Synechocystis sp. PCC 6803 as new hosts for cyclic plant triterpene biosynthesis

Cyclic triterpenes constitute one of the most diverse groups of plant natural products. Besides the intriguing biochemistry of their biosynthetic pathways, plant triterpenes exhibit versatile bioactivities, including antimicrobial effects against plant and human pathogens. While prokaryotes have been extensively used for the heterologous production of other classes of terpenes, the synthesis of cyclic triterpenes, which inherently includes the two-step catalytic formation of the universal linear precursor 2,3-oxidosqualene, is still a major challenge. We thus explored the suitability of the metabolically versatile photosynthetic α-proteobacterium Rhodobacter capsulatus SB1003 and cyanobacterium Synechocystis sp. PCC 6803 as alternative hosts for biosynthesis of cyclic plant triterpenes. Therefore, 2,3-oxidosqualene production was implemented and subsequently combined with different cyclization reactions catalyzed by the representative oxidosqualene cyclases CAS1 (cycloartenol synthase), LUP1 (lupeol synthase), THAS1 (thalianol synthase) and MRN1 (marneral synthase) derived from model plant Arabidopsis thaliana. While successful accumulation of 2,3-oxidosqualene could be detected by LC-MS analysis in both hosts, cyclase expression resulted in differential production profiles. CAS1 catalyzed conversion to only cycloartenol, but expression of LUP1 yielded lupeol and a triterpenoid matching an oxidation product of lupeol, in both hosts. In contrast, THAS1 expression did not lead to cyclic product formation in either host, whereas MRN1-dependent production of marnerol and hydroxymarnerol was observed in Synechocystis but not in R. capsulatus. Our findings thus indicate that 2,3-oxidosqualene cyclization in heterologous phototrophic bacteria is basically feasible but efficient conversion depends on both the respective cyclase enzyme and individual host properties. Therefore, photosynthetic α-proteo- and cyanobacteria are promising alternative candidates for providing new bacterial access to the broad class of triterpenes for biotechnological applications.

Correlated production and consumption of chloromethane in the Arabidopsis thaliana phyllosphere

Chloromethane (CH₃Cl) is a toxic gas mainly produced naturally, in particular by plants, and its emissions contribute to ozone destruction in the stratosphere. Conversely, CH₃Cl can be degraded and used as the sole carbon and energy source by specialised methylotrophic bacteria, isolated from a variety of environments including the phyllosphere, i.e. the aerial parts of vegetation. The potential role of phyllospheric CH₃Cl-degrading bacteria as a filter for plant emissions of CH₃Cl was investigated using variants of Arabidopsis thaliana with low, wild-type and high expression of HOL1 methyltransferase previously shown to be responsible for most of CH₃Cl emissions by A. thaliana. Presence and expression of the bacterial chloromethane dehalogenase cmuA gene in the A. thaliana phyllosphere correlated with HOL1 genotype, as shown by qPCR and RT-qPCR. Production of CH₃Cl by A. thaliana paralleled HOL1 expression, as assessed by a fluorescence-based bioreporter. The relation between plant production of CH₃Cl and relative abundance of CH₃Cl-degrading bacteria in the phyllosphere suggests that CH₃Cl-degrading bacteria co-determine the extent of plant emissions of CH₃Cl to the atmosphere.

Two Cycloartenol Synthases for Phytosterol Biosynthesis in Polygala tenuifolia Willd

Oxidosqualene cyclases (OSCs) are enzymes that play a key role in control of the biosynthesis of phytosterols and triterpene saponins. In order to uncover OSC genes from Polygala tenuifolia seedlings induced by methyl jasmonate (MeJA), RNA-sequencing analysis was performed using the Illumina sequencing platform. A total of 148,488,632 high-quality reads from two samples (control and the MeJA treated) were generated. We screened genes related to phytosterol and triterpene saponin biosynthesis and analyzed the transcriptional changes of differentially expressed unigene (DEUG) values calculated by fragments per kilobase million (FPKM). In our datasets, two full-length cDNAs of putative OSC genes, PtCAS1, and PtCAS2, were found, in addition to the PtBS (β-amyrin synthase) gene reported in our previous studies and the two cycloartenol synthase genes of P. tenuifolia. All genes were isolated and characterized in yeast cells. The functional expression of the two PtCAS genes in yeast cells showed that the genes all produce a cycloartenol as the sole product. When qRT-PCR analysis from different tissues was performed, the expressions of PtCAS1 and PtCAS2 were highest in flowers and roots, respectively. After MeJA treatment, the transcripts of PtCAS1 and PtCAS2 genes increased by 1.5- and 2-fold, respectively. Given these results, we discuss the potential roles of the two PtCAS genes in relation to triterpenoid biosynthesis.

Complex interplays between phytosterols and plastid development

Isoprenoids comprise the largest class of natural compounds and are found in all kinds of organisms. In plants, they participate in both primary and specialized metabolism, playing essential roles in nearly all aspects of growth and development. The enormous diversity of this family of compounds is extensively exploited for biotechnological and biomedical applications as biomaterials, biofuels or drugs. Despite their variety of structures, all isoprenoids derive from the common C₅ precursor isopentenyl diphosphate (IPP). Plants synthesize IPP through two different metabolic pathways, the mevalonic acid (MVA) and the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathways that operate in the cytosol-RE and plastids, respectively. MEP-derived isoprenoids include important compounds for chloroplast function and as such, knock-out mutant plants affected in different steps of this pathway display important alterations in plastid structure. These alterations often lead to albino phenotypes and lethality at seedling stage. MVA knock-out mutant plants show, on the contrary, lethal phenotypes already exhibited at the gametophyte or embryo developmental stage. However, the recent characterization of conditional knock-down mutant plants of farnesyl diphosphate synthase (FPS), a central enzyme in cytosolic and mitochondrial isoprenoid biosynthesis, revealed an unexpected role of this pathway in chloroplast development and plastidial isoprenoid metabolism in post-embryonic stages. Upon FPS silencing, chloroplast structure is severely altered, together with a strong reduction in the levels of MEP pathway-derived major end products. This phenotype is associated to misregulation of genes involved in stress responses predominantly belonging to JA and Fe homeostasis pathways. Transcriptomic experiments and analysis of recent literature indicate that sterols are the cause of the observed alterations through an as yet undiscovered mechanism.

Plant Glandular Trichomes: Natural Cell Factories of High Biotechnological Interest

Multicellular glandular trichomes are epidermal outgrowths characterized by the presence of a head made of cells that have the ability to secrete or store large quantities of specialized metabolites. Our understanding of the transcriptional control of glandular trichome initiation and development is still in its infancy. This review points to some central questions that need to be addressed to better understand how such specialized cell structures arise from the plant protodermis. A key and unique feature of glandular trichomes is their ability to synthesize and secrete large amounts, relative to their size, of a limited number of metabolites. As such, they qualify as true cell factories, making them interesting targets for metabolic engineering. In this review, recent advances regarding terpene metabolic engineering are highlighted, with a special focus on tobacco (Nicotiana tabacum). In particular, the choice of transcriptional promoters to drive transgene expression and the best ways to sink existing pools of terpene precursors are discussed. The bioavailability of existing pools of natural precursor molecules is a key parameter and is controlled by so-called cross talk between different biosynthetic pathways. As highlighted in this review, the exact nature and extent of such cross talk are only partially understood at present. In the future, awareness of, and detailed knowledge on, the biology of plant glandular trichome development and metabolism will generate new leads to tap the largely unexploited potential of glandular trichomes in plant resistance to pests and lead to the improved production of specialized metabolites with high industrial or pharmacological value.

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.

Virus-induced gene silencing of the two squalene synthase isoforms of apple tree (Malus × domestica L.) negatively impacts phytosterol biosynthesis, plastid pigmentation and leaf growth

The use of a VIGS approach to silence the newly characterized apple tree SQS isoforms points out the biological function of phytosterols in plastid pigmentation and leaf development. Triterpenoids are beneficial health compounds highly accumulated in apple; however, their metabolic regulation is poorly understood. Squalene synthase (SQS) is a key branch point enzyme involved in both phytosterol and triterpene biosynthesis. In this study, two SQS isoforms were identified in apple tree genome. Both isoforms are located at the endoplasmic reticulum surface and were demonstrated to be functional SQS enzymes using an in vitro activity assay. MdSQS1 and MdSQS2 display specificities in their expression profiles with respect to plant organs and environmental constraints. This indicates a possible preferential involvement of each isoform in phytosterol and/or triterpene metabolic pathways as further argued using RNAseq meta-transcriptomic analyses. Finally, a virus-induced gene silencing (VIGS) approach was used to silence MdSQS1 and MdSQS2. The concomitant down-regulation of both MdSQS isoforms strongly affected phytosterol synthesis without alteration in triterpene accumulation, since triterpene-specific oxidosqualene synthases were found to be up-regulated to compensate metabolic flux reduction. Phytosterol deficiencies in silenced plants clearly disturbed chloroplast pigmentation and led to abnormal development impacting leaf division rather than elongation or differentiation. In conclusion, beyond the characterization of two SQS isoforms in apple tree, this work brings clues for a specific involvement of each isoform in phytosterol and triterpene pathways and emphasizes the biological function of phytosterols in development and chloroplast integrity. Our report also opens the door to metabolism studies in Malus domestica using the apple latent spherical virus-based VIGS method.

Analysis of CFB, a cytokinin-responsive gene of Arabidopsis thaliana encoding a novel F-box protein regulating sterol biosynthesis

Protein degradation by the ubiquitin-26S proteasome pathway is important for the regulation of cellular processes, but the function of most F-box proteins relevant to substrate recognition is unknown. We describe the analysis of the gene Cytokinin-induced F-box encoding (CFB, AT3G44326), identified in a meta-analysis of cytokinin-related transcriptome studies as one of the most robust cytokinin response genes. F-box domain-dependent interaction with the E3 ubiquitin ligase complex component ASK1 classifies CFB as a functional F-box protein. Apart from F-box and transmembrane domains, CFB contains no known functional domains. CFB is expressed in all plant tissues, predominantly in root tissue. A ProCFB:GFP-GUS fusion gene showed strongest expression in the lateral root cap and during lateral root formation. CFB-GFP fusion proteins were mainly localized in the nucleus and the cytosol but also at the plasma membrane. cfb mutants had no discernible phenotype, but CFB overexpressing plants showed several defects, such as a white upper inflorescence stem, similar to the hypomorphic cycloartenol synthase mutant cas1-1. Both CFB overexpressing plants and cas1-1 mutants accumulated the CAS1 substrate 2,3-oxidosqualene in the white stem tissue, the latter even more after cytokinin treatment, indicating impairment of CAS1 function. This suggests that CFB may link cytokinin and the sterol biosynthesis pathway.

Plastoglobuli: Plastid Microcompartments with Integrated Functions in Metabolism, Plastid Developmental Transitions, and Environmental Adaptation

Plastoglobuli (PGs) are plastid lipoprotein particles surrounded by a membrane lipid monolayer. PGs contain small specialized proteomes and metabolomes. They are present in different plastid types (e.g., chloroplasts, chromoplasts, and elaioplasts) and are dynamic in size and shape in response to abiotic stress or developmental transitions. PGs in chromoplasts are highly enriched in carotenoid esters and enzymes involved in carotenoid metabolism. PGs in chloroplasts are associated with thylakoids and contain ∼30 core proteins (including six ABC1 kinases) as well as additional proteins recruited under specific conditions. Systems analysis has suggested that chloroplast PGs function in metabolism of prenyl lipids (e.g., tocopherols, plastoquinone, and phylloquinone); redox and photosynthetic regulation; plastid biogenesis; and senescence, including recycling of phytol, remobilization of thylakoid lipids, and metabolism of jasmonate. These functionalities contribute to chloroplast PGs' role in responses to stresses such as high light and nitrogen starvation. PGs are thus lipid microcompartments with multiple functions integrated into plastid metabolism, developmental transitions, and environmental adaptation. This review provides an in-depth overview of PG experimental observations, summarizes the present understanding of PG features and functions, and provides a conceptual framework for PG research and the realization of opportunities for crop improvement.

Tomato UDP-Glucose Sterol Glycosyltransferases: A Family of Developmental and Stress Regulated Genes that Encode Cytosolic and Membrane-Associated Forms of the Enzyme

Sterol glycosyltransferases (SGTs) catalyze the glycosylation of the free hydroxyl group at C-3 position of sterols to produce sterol glycosides. Glycosylated sterols and free sterols are primarily located in cell membranes where in combination with other membrane-bound lipids play a key role in modulating their properties and functioning. In contrast to most plant species, those of the genus Solanum contain very high levels of glycosylated sterols, which in the case of tomato may account for more than 85% of the total sterol content. In this study, we report the identification and functional characterization of the four members of the tomato (Solanum lycopersicum cv. Micro-Tom) SGT gene family. Expression of recombinant SlSGT proteins in E. coli cells and N. benthamiana leaves demonstrated the ability of the four enzymes to glycosylate different sterol species including cholesterol, brassicasterol, campesterol, stigmasterol, and β-sitosterol, which is consistent with the occurrence in their primary structure of the putative steroid-binding domain found in steroid UDP-glucuronosyltransferases and the UDP-sugar binding domain characteristic for a superfamily of nucleoside diphosphosugar glycosyltransferases. Subcellular localization studies based on fluorescence recovery after photobleaching and cell fractionation analyses revealed that the four tomato SGTs, like the Arabidopsis SGTs UGT80A2 and UGT80B1, localize into the cytosol and the PM, although there are clear differences in their relative distribution between these two cell fractions. The SlSGT genes have specialized but still largely overlapping expression patterns in different organs of tomato plants and throughout the different stages of fruit development and ripening. Moreover, they are differentially regulated in response to biotic and abiotic stress conditions. SlSGT4 expression increases markedly in response to osmotic, salt, and cold stress, as well as upon treatment with abscisic acid and methyl jasmonate. Stress-induced SlSGT2 expression largely parallels that of SlSGT4. On the contrary, SlSGT1 and SlSGT3 expression remains almost unaltered under the tested stress conditions. Overall, this study contributes to broaden the current knowledge on plant SGTs and provides support to the view that tomato SGTs play overlapping but not completely redundant biological functions involved in mediating developmental and stress responses.

Sequence Polymorphisms and Structural Variations among Four Grapevine (Vitis vinifera L.) Cultivars Representing Sardinian Agriculture

The genetic diversity among grapevine (Vitis vinifera L.) cultivars that underlies differences in agronomic performance and wine quality reflects the accumulation of single nucleotide polymorphisms (SNPs) and small indels as well as larger genomic variations. A combination of high throughput sequencing and mapping against the grapevine reference genome allows the creation of comprehensive sequence variation maps. We used next generation sequencing and bioinformatics to generate an inventory of SNPs and small indels in four widely cultivated Sardinian grape cultivars (Bovale sardo, Cannonau, Carignano and Vermentino). More than 3,200,000 SNPs were identified with high statistical confidence. Some of the SNPs caused the appearance of premature stop codons and thus identified putative pseudogenes. The analysis of SNP distribution along chromosomes led to the identification of large genomic regions with uninterrupted series of homozygous SNPs. We used a digital comparative genomic hybridization approach to identify 6526 genomic regions with significant differences in copy number among the four cultivars compared to the reference sequence, including 81 regions shared between all four cultivars and 4953 specific to single cultivars (representing 1.2 and 75.9% of total copy number variation, respectively). Reads mapping at a distance that was not compatible with the insert size were used to identify a dataset of putative large deletions with cultivar Cannonau revealing the highest number. The analysis of genes mapping to these regions provided a list of candidates that may explain some of the phenotypic differences among the Bovale sardo, Cannonau, Carignano and Vermentino cultivars.

Cloning and Functional Characterization of Cycloartenol Synthase from the Red Seaweed Laurencia dendroidea

The red seaweed Laurencia dendroidea belongs to the Rhodophyta, a phylum of eukaryotic algae that is widely distributed across the oceans and that constitute an important source of bioactive specialized metabolites. Laurencia species have been studied since 1950 and were found to contain a plethora of specialized metabolites, mainly halogenated sesquiterpenes, diterpenes and triterpenes that possess a broad spectrum of pharmacological and ecological activities. The first committed step in the biosynthesis of triterpenes is the cyclization of 2,3-oxidosqualene, an enzymatic reaction carried out by oxidosqualene cyclases (OSCs), giving rise to a broad range of different compounds, such as the sterol precursors cycloartenol and lanosterol, or triterpene precursors such as cucurbitadienol and β-amyrin. Here, we cloned and characterized the first OSC from a red seaweed. The OSC gene was identified through mining of a L. dendroidea transcriptome dataset and subsequently cloned and heterologously expressed in yeast for functional characterization, which indicated that the corresponding enzyme cyclizes 2,3-oxidosqualene to the sterol precursor cycloartenol. Accordingly, the gene was named L. dendroidea cycloartenol synthase (LdCAS). A phylogenetic analysis using OSCs genes from plants, fungi and algae revealed that LdCAS grouped together with OSCs from other red algae, suggesting that cycloartenol could be the common product of the OSC in red seaweeds. Furthermore, profiling of L. dendroidea revealed cholesterol as the major sterol accumulating in this species, implicating red seaweeds contain a ‘hybrid’ sterol synthesis pathway in which the phytosterol precursor cycloartenol is converted into the major animal sterol cholesterol.

Suppressing Farnesyl Diphosphate Synthase Alters Chloroplast Development and Triggers Sterol-Dependent Induction of Jasmonate- and Fe-Related Responses

Farnesyl diphosphate synthase (FPS) catalyzes the synthesis of farnesyl diphosphate from isopentenyl diphosphate and dimethylallyl diphosphate. Arabidopsis (Arabidopsis thaliana) contains two genes (FPS1 and FPS2) encoding FPS. Single fps1 and fps2 knockout mutants are phenotypically indistinguishable from wild-type plants, while fps1/fps2 double mutants are embryo lethal. To assess the effect of FPS down-regulation at postembryonic developmental stages, we generated Arabidopsis conditional knockdown mutants expressing artificial microRNAs devised to simultaneously silence both FPS genes. Induction of silencing from germination rapidly caused chlorosis and a strong developmental phenotype that led to seedling lethality. However, silencing of FPS after seed germination resulted in a slight developmental delay only, although leaves and cotyledons continued to show chlorosis and altered chloroplasts. Metabolomic analyses also revealed drastic changes in the profile of sterols, ubiquinones, and plastidial isoprenoids. RNA sequencing and reverse transcription-quantitative polymerase chain reaction transcriptomic analysis showed that a reduction in FPS activity levels triggers the misregulation of genes involved in biotic and abiotic stress responses, the most prominent one being the rapid induction of a set of genes related to the jasmonic acid pathway. Down-regulation of FPS also triggered an iron-deficiency transcriptional response that is consistent with the iron-deficient phenotype observed in FPS-silenced plants. The specific inhibition of the sterol biosynthesis pathway by chemical and genetic blockage mimicked these transcriptional responses, indicating that sterol depletion is the primary cause of the observed alterations. Our results highlight the importance of sterol homeostasis for normal chloroplast development and function and reveal important clues about how isoprenoid and sterol metabolism is integrated within plant physiology and development.

Gene coexpression network analysis of oil biosynthesis in an interspecific backcross of oil palm

Global demand for vegetable oils is increasing at a dramatic rate, while our understanding of the regulation of oil biosynthesis in plants remains limited. To gain insights into the mechanisms that govern oil synthesis and fatty acid (FA) composition in the oil palm fruit, we used a multilevel approach combining gene coexpression analysis, quantification of allele-specific expression and joint multivariate analysis of transcriptomic and lipid data, in an interspecific backcross population between the African oil palm, Elaeis guineensis, and the American oil palm, Elaeis oleifera, which display contrasting oil contents and FA compositions. The gene coexpression network produced revealed tight transcriptional coordination of fatty acid synthesis (FAS) in the plastid with sugar sensing, plastidial glycolysis, transient starch storage and carbon recapture pathways. It also revealed a concerted regulation, along with FAS, of both the transfer of nascent FA to the endoplasmic reticulum, where triacylglycerol assembly occurs, and of the production of glycerol-3-phosphate, which provides the backbone of triacylglycerols. Plastid biogenesis and auxin transport were the two other biological processes most tightly connected to FAS in the network. In addition to WRINKLED1, a transcription factor (TF) known to activate FAS genes, two novel TFs, termed NF-YB-1 and ZFP-1, were found at the core of the FAS module. The saturated FA content of palm oil appeared to vary above all in relation to the level of transcripts of the gene coding for β-ketoacyl-acyl carrier protein synthase II. Our findings should facilitate the development of breeding and engineering strategies in this and other oil crops.

Lipids in pollen - They are different

During evolution, the male gametophyte of Angiosperms has been severely reduced to the pollen grain, consisting of a vegetative cell containing two sperm cells. This vegetative cell has to deliver the sperm cells from the stigma through the style to the ovule. It does so by producing a pollen tube and elongating it to many centimeters in length in some species, requiring vast amounts of fatty acid and membrane lipid synthesis. In order to optimize this polar tip growth, a unique lipid composition in the pollen has evolved. Pollen tubes produce extraplastidial galactolipids and store triacylglycerols in lipid droplets, probably needed as precursors of glycerolipids or for acyl editing. They also possess special sterol and sphingolipid moieties that might together form microdomains in the membranes. The individual lipid classes, the proteins involved in their synthesis as well as the corresponding Arabidopsis knockout mutant phenotypes are discussed in this review. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Inhibition of Cycloartenol Synthase (CAS) Function in Tobacco BY-2 Cells

Tobacco BY-2 cell suspensions are our preferred model for studying isoprenoid biosynthesis pathways, due to their easy genetic transformation and the efficient absorption of metabolic precursors, intermediates, and/or inhibitors. Using this model system, we have analyzed the effects of chemical and genetic blockage of cycloartenol synthase (CAS, EC 5.4.99.8), an oxidosqualene cyclase that catalyzes the first committed step in the sterol pathway of plants. BY-2 cells were treated with RO 48-8071, a potent inhibitor of oxidosqualene cyclization. Short-term treatments (24 h) resulted in accumulation of oxidosqualene with no changes in the final sterol products. Interestingly, long-term treatments (6 days) induced down-regulation in gene expression not only of CAS but also of the SMT2 gene coding sterol methyltransferase 2 (EC 2.1.1.41). This explains some of the increase in 24-methyl sterols at the expense of the 24-ethyl sterols stigmasterol and sitosterol. In our alternative strategy, CAS gene expression was partially blocked by using an inducible artificial microRNA. The limited effectiveness of this approach might be explained by some dependence of the machinery for RNAi formation on an operating MVA/sterol pathway. For comparison we checked the effect of RO 48-8071 on a green cell suspension of Arabidopsis and on seedlings, containing a small spectrum of triterpenes besides phytosterols. Triterpenes remained essentially unaffected, but phytosterol accumulation was clearly diminished.

Transcriptome Profiling of Rust Resistance in Switchgrass Using RNA-Seq Analysis

Switchgrass rust caused by Puccinia emaculata is a major limiting factor for switchgrass (Panicum virgatum L.) production, especially in monoculture. Natural populations of switchgrass displayed diverse reactions to P. emaculata when evaluated in an Ardmore, OK, field. To identify the differentially expressed genes during the rust infection process and the mechanisms of switchgrass rust resistance, transcriptome analysis using RNA-Seq was conducted in two pseudo-F₁ parents ('PV281' and 'NFGA472'), and three moderately resistant and three susceptible progenies selected from a three-generation, four-founder switchgrass population (K5 × A4) × (AP13 × VS16). On average, 23.5 million reads per sample (leaf tissue was collected at 0, 24, and 60 h post-inoculation (hpi)) were obtained from paired-end (2 × 100 bp) sequencing on the Illumina HiSeq2000 platform. Mapping of the RNA-Seq reads to the switchgrass reference genome (AP13 ver. 1.1 assembly) constructed a total of 84,209 transcripts from 98,007 gene loci among all of the samples. Further analysis revealed that host defense-related genes, including the nucleotide binding site-leucine-rich repeat domain containing disease resistance gene analogs, play an important role in resistance to rust infection. Rust-induced gene (RIG) transcripts inherited across generations were identified. The rust-resistant gene transcripts can be a valuable resource for developing molecular markers for rust resistance. Furthermore, the rust-resistant genotypes and gene transcripts identified in this study can expedite rust-resistant cultivar development in switchgrass.

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.

Plant oxidosqualene metabolism: cycloartenol synthase-dependent sterol biosynthesis in Nicotiana benthamiana

The plant sterol pathway exhibits a major biosynthetic difference as compared with that of metazoans. The committed sterol precursor is the pentacyclic cycloartenol (9β,19-cyclolanost-24-en-3β-ol) and not lanosterol (lanosta-8,24-dien-3β-ol), as it was shown in the late sixties. However, plant genome mining over the last years revealed the general presence of lanosterol synthases encoding sequences (LAS1) in the oxidosqualene cyclase repertoire, in addition to cycloartenol synthases (CAS1) and to non-steroidal triterpene synthases that contribute to the metabolic diversity of C30H50O compounds on earth. Furthermore, plant LAS1 proteins have been unambiguously identified by peptidic signatures and by their capacity to complement the yeast lanosterol synthase deficiency. A dual pathway for the synthesis of sterols through lanosterol and cycloartenol was reported in the model Arabidopsis thaliana, though the contribution of a lanosterol pathway to the production of 24-alkyl-Δ(5)-sterols was quite marginal (Ohyama et al. (2009) PNAS 106, 725). To investigate further the physiological relevance of CAS1 and LAS1 genes in plants, we have silenced their expression in Nicotiana benthamiana. We used virus induced gene silencing (VIGS) based on gene specific sequences from a Nicotiana tabacum CAS1 or derived from the solgenomics initiative (http://solgenomics.net/) to challenge the respective roles of CAS1 and LAS1. In this report, we show a CAS1-specific functional sterol pathway in engineered yeast, and a strict dependence on CAS1 of tobacco sterol biosynthesis.

ATP citrate lyase activity is post-translationally regulated by sink strength and impacts the wax, cutin and rubber biosynthetic pathways

Cytosolic acetyl-CoA is involved in the synthesis of a variety of compounds, including waxes, sterols and rubber, and is generated by the ATP citrate lyase (ACL). Plants over-expressing ACL were generated in an effort to understand the contribution of ACL activity to the carbon flux of acetyl-CoA to metabolic pathways occurring in the cytosol. Transgenic Arabidopsis plants synthesizing the polyester polyhydroxybutyrate (PHB) from cytosolic acetyl-CoA have reduced growth and wax content, consistent with a reduction in the availability of cytosolic acetyl-CoA to endogenous pathways. Increasing the ACL activity via the over-expression of the ACLA and ACLB subunits reversed the phenotypes associated with PHB synthesis while maintaining polymer synthesis. PHB production by itself was associated with an increase in ACL activity that occurred in the absence of changes in steady-state mRNA or protein level, indicating a post-translational regulation of ACL activity in response to sink strength. Over-expression of ACL in Arabidopsis was associated with a 30% increase in wax on stems, while over-expression of a chimeric homomeric ACL in the laticifer of roots of dandelion led to a four- and two-fold increase in rubber and triterpene content, respectively. Synthesis of PHB and over-expression of ACL also changed the amount of the cutin monomer octadecadien-1,18-dioic acid, revealing an unsuspected link between cytosolic acetyl-CoA and cutin biosynthesis. Together, these results reveal the complexity of ACL regulation and its central role in influencing the carbon flux to metabolic pathways using cytosolic acetyl-CoA, including wax and polyisoprenoids.

Transgenic tobacco overexpressing Brassica juncea HMG-CoA synthase 1 shows increased plant growth, pod size and seed yield

Seeds are very important not only in the life cycle of the plant but they represent food sources for man and animals. We report herein a mutant of 3-hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS), the second enzyme in the mevalonate (MVA) pathway that can improve seed yield when overexpressed in a phylogenetically distant species. In Brassica juncea, the characterisation of four isogenes encoding HMGS has been previously reported. Enzyme kinetics on recombinant wild-type (wt) and mutant BjHMGS1 had revealed that S359A displayed a 10-fold higher enzyme activity. The overexpression of wt and mutant (S359A) BjHMGS1 in Arabidopsis had up-regulated several genes in sterol biosynthesis, increasing sterol content. To quickly assess the effects of BjHMGS1 overexpression in a phylogenetically more distant species beyond the Brassicaceae, wt and mutant (S359A) BjHMGS1 were expressed in tobacco (Nicotiana tabacum L. cv. Xanthi) of the family Solanaceae. New observations on tobacco OEs not previously reported for Arabidopsis OEs included: (i) phenotypic changes in enhanced plant growth, pod size and seed yield (more significant in OE-S359A than OE-wtBjHMGS1) in comparison to vector-transformed tobacco, (ii) higher NtSQS expression and sterol content in OE-S359A than OE-wtBjHMGS1 corresponding to greater increase in growth and seed yield, and (iii) induction of NtIPPI2 and NtGGPPS2 and downregulation of NtIPPI1, NtGGPPS1, NtGGPPS3 and NtGGPPS4. Resembling Arabidopsis HMGS-OEs, tobacco HMGS-OEs displayed an enhanced expression of NtHMGR1, NtSMT1-2, NtSMT2-1, NtSMT2-2 and NtCYP85A1. Overall, increased growth, pod size and seed yield in tobacco HMGS-OEs were attributed to the up-regulation of native NtHMGR1, NtIPPI2, NtSQS, NtSMT1-2, NtSMT2-1, NtSMT2-2 and NtCYP85A1. Hence, S359A has potential in agriculture not only in improving phytosterol content but also seed yield, which may be desirable in food crops. This work further demonstrates HMGS function in plant reproduction that is reminiscent to reduced fertility of hmgs RNAi lines in let-7 mutants of Caenorhabditis elegans.