Isoprenyl diphosphate synthases of terpenoid biosynthesis in rose-scented geranium (Pelargonium graveolens)

The essential oil of Pelargonium graveolens (rose-scented geranium), an important aromatic plant, comprising mainly mono- and sesqui-terpenes, has applications in food and cosmetic industries. This study reports the characterization of isoprenyl disphosphate synthases (IDSs) involved in P. graveolens terpene biosynthesis. The six identified PgIDSs belonged to different classes of IDSs, comprising homomeric geranyl diphosphate synthases (GPPSs; PgGPPS1 and PgGPPS2), the large subunit of heteromeric GPPS or geranylgeranyl diphosphate synthases (GGPPSs; PgGGPPS), the small subunit of heteromeric GPPS (PgGPPS.SSUI and PgGPPS.SSUII), and farnesyl diphosphate synthases (FPPS; PgFPPS).All IDSs exhibited maximal expression in glandular trichomes (GTs), the site of aroma formation, and their expression except PgGPPS.SSUII was induced upon treatment with MeJA. Functional characterization of recombinant proteins revealed that PgGPPS1, PgGGPPS and PgFPPS were active enzymes producing GPP, GGPP/GPP, and FPP respectively, whereas both PgGPPS.SSUs and PgGPPS2 were inactive. Co-expression of PgGGPPS (that exhibited bifunctional G(G)PPS activity) with PgGPPS.SSUs in bacterial expression system showed lack of interaction between the two proteins, however, PgGGPPS interacted with a phylogenetically distant Antirrhinum majus GPPS.SSU. Further, transient expression of AmGPPS.SSU in P. graveolens leaf led to a significant increase in monoterpene levels. These findings provide insight into the types of IDSs and their role in providing precursors for different terpenoid components of P. graveolens essential oil.

ROP GTPases with a geranylgeranylation motif modulate alkaloid biosynthesis in Catharanthus roseus

Rho of Plant (ROP) GTPases function as molecular switches that control signaling processes essential for growth, development, and defense. However, their role in specialized metabolism is poorly understood. Previously, we demonstrated that inhibition of protein geranylgeranyl transferase (PGGT-I) negatively impacts the biosynthesis of monoterpenoid indole alkaloids (MIA) in Madagascar periwinkle (Catharanthus roseus), indicating the involvement of prenylated proteins in signaling. Here, we show through biochemical, molecular, and in planta approaches that specific geranylgeranylated ROPs modulate C. roseus MIA biosynthesis. Among the six C. roseus ROP GTPases (CrROPs), only CrROP3 and CrROP5, having a C-terminal CSIL motif, were specifically prenylated by PGGT-I. Additionally, their transcripts showed higher expression in most parts than other CrROPs. Protein-protein interaction studies revealed that CrROP3 and CrROP5, but not ΔCrROP3, ΔCrROP5, and CrROP2 lacking the CSIL motif, interacted with CrPGGT-I. Further, CrROP3 and CrROP5 exhibited nuclear localization, whereas CrROP2 was localized to the plasma membrane. In planta functional studies revealed that silencing of CrROP3 and CrROP5 negatively affected MIA biosynthesis, while their overexpression upregulated MIA formation. In contrast, silencing and overexpression of CrROP2 had no effect on MIA biosynthesis. Moreover, overexpression of ΔCrROP3 and ΔCrROP5 mutants devoid of sequence coding for the CSIL motif failed to enhance MIA biosynthesis. These results implicate that CrROP3 and CrROP5 have a positive regulatory role on MIA biosynthesis and thus shed light on how geranylgeranylated ROP GTPases mediate the modulation of specialized metabolism in C. roseus.

Analysis of root volatiles and functional characterization of a root-specific germacrene A synthase in Artemisia pallens

MAIN CONCLUSION

The study demonstrated that Artemisia pallens roots can be a source of terpene-rich essential oil and root-specific ApTPS1 forms germacrene A contributing to major root volatiles. Davana (Artemisia pallens Bess) is a valuable aromatic herb within the Asteraceae family, highly prized for its essential oil (EO) produced in the aerial parts. However, the root volatile composition, and the genes responsible for root volatiles have remained unexplored until now. Here, we show that A. pallens roots possess distinct oil bodies and yields ~ 0.05% of EO, which is primarily composed of sesquiterpenes β-elemene, neryl isovalerate, β-selinene, and α-selinene, and trace amounts of monoterpenes β-myrcene, D-limonene. This shows that, besides aerial parts, roots of davana can also be a source of unique EO. Moreover, we functionally characterized a terpene synthase (ApTPS1) that exhibited high in silico expression in the root transcriptome. The recombinant ApTPS1 showed the formation of β-elemene and germacrene A with E,E-farnesyl diphosphate (FPP) as a substrate. Detailed analysis of assay products revealed that β-elemene was the thermal rearrangement product of germacrene A. The functional expression of ApTPS1 in Saccharomyces cerevisiae confirmed the in vivo germacrene A synthase activity of ApTPS1. At the transcript level, ApTPS1 displayed predominant expression in roots, with significantly lower level of expression in other tissues. This expression pattern of ApTPS1 positively correlated with the tissue-specific accumulation level of germacrene A. Overall, these findings provide fundamental insights into the EO profile of davana roots, and the contribution of ApTPS1 in the formation of a major root volatile.

Temperature-induced lipocalin-mediated membrane integrity: Possible implications for vindoline accumulation in Catharanthus roseus leaves

Plant lipocalins perform diverse functions. Recently, allene oxide cyclase, a lipocalin family member, has been shown to co-express with vindoline pathway genes in Catharanthus roseus under various biotic/abiotic stresses. This brought focus to another family member, a temperature-induced lipocalin (CrTIL), which was selected for full-length cloning, tissue-specific expression profiling, in silico characterization, and upstream genomic region analysis for cis-regulatory elements. Stress-mediated variations in CrTIL expression were reflected as disturbances in cell membrane integrity, assayed through measurement of electrolyte leakage and lipid peroxidation product, MDA, which implicated the role of CrTIL in maintaining cell membrane integrity. For ascertaining the function of CrTIL in maintaining membrane stability and elucidating the relationship between CrTIL expression and vindoline content, if any, a direct approach was adopted, whereby CrTIL was transiently silenced and overexpressed in C. roseus. CrTIL silencing and overexpression confirmed its role in the maintenance of membrane integrity and indicated an inverse relationship of its expression with vindoline content. GFP fusion-based subcellular localization indicated membrane localization of CrTIL, which was in agreement with its role in maintaining membrane integrity. Altogether, the role of CrTIL in maintaining membrane structure has possible implications for the intracellular sequestration, storage, and viability of vindoline.

Identification of a second 16-hydroxytabersonine-O-methyltransferase suggests an evolutionary relationship between alkaloid and flavonoid metabolisms in Catharanthus roseus

The medicinal plant Catharanthus roseus biosynthesizes many important drugs for human health, including the anticancer monoterpene indole alkaloids (MIAs) vinblastine and vincristine. Over the past decades, the continuous increase in pharmaceutical demand has prompted several research groups to characterize MIA biosynthetic pathways for considering future metabolic engineering processes of supply. In line with previous work suggesting that diversification can potentially occur at various steps along the vindoline branch, we were here interested in investigating the involvement of distinct isoforms of tabersonine-16-O-methyltransferase (16OMT) which plays a pivotal role in the MIA biosynthetic pathway. By combining homology searches based on the previously characterized 16OMT1, phylogenetic analyses, functional assays in yeast, and biochemical and in planta characterizations, we identified a second isoform of 16OMT, referred to as 16OMT2. 16OMT2 appears to be a multifunctional enzyme working on both MIA and flavonoid substrates, suggesting that a constrained evolution of the enzyme for accommodating the MIA substrate has probably occurred to favor the apparition of 16OMT2 from an ancestral specific flavonoid-O-methyltransferase. Since 16OMT1 and 16OMT2 displays a high sequence identity and similar kinetic parameters for 16-hydroxytabersonine, we postulate that 16OMT1 may result from a later 16OMT2 gene duplication accompanied by a continuous neofunctionalization leading to an almost complete loss of flavonoid O-methyltransferase activity. Overall, these results participate in increasing our knowledge on the evolutionary processes that have likely led to enzyme co-optation for MIA synthesis.

Betulinic acid mitigates oxidative stress-mediated apoptosis and enhances longevity in the yeast Saccharomyces cerevisiae model

Betulinic acid (BA), a pentacyclic triterpenoid found in certain plant species, has been reported to have several health benefits including antioxidant and anti-apoptotic properties. However, the mechanism by which BA confers these properties is currently unknown. Saccharomyces cerevisiae, a budding yeast with a short life cycle and conserved cellular mechanism with high homology to humans, was used as a model for determining the role of BA in aging and programmed cell death (PCD). Treatment with hydrogen peroxide (H₂O₂) exhibited significantly increased (30-35%) survivability of antioxidant (sod1Δ, sod2Δ, cta1Δ, ctt1Δ, and tsa1Δ) and anti-apoptotic (pep4Δ and fis1Δ) mutant strains when cells were pretreated with BA (30 µM) as demonstrated in spot and CFU (Colony forming units) assays. Measurement of intracellular oxidation level using the ROS-specific dye H₂DCF-DA showed that all tested BA-pretreated mutants exhibited decreased ROS than the control when exposed to H₂O₂. Similarly, when mutant strains were pretreated with BA and then exposed to H₂O₂, there was reduced lipid peroxidation as revealed by the reduced malondialdehyde content. Furthermore, BA-pretreated mutant cells showed significantly lower apoptotic activity by decreasing DNA/nuclear fragmentation and chromatin condensation under H₂O₂-induced stress as determined by DAPI and acridine orange/ethidium bromide staining. In addition, BA treatment also extended the life span of antioxidant and anti-apoptotic mutants by ∼10-25% by scavenging ROS and preventing apoptotic cell death. Our overall results suggest that BA extends the chronological life span of mutant strains lacking antioxidant and anti-apoptotic genes by lowering the impact of oxidative stress, ROS levels, and apoptotic activity. These properties of BA could be further explored for its use as a valuable nutraceutical.

An inducible potato (E,E)-farnesol synthase confers tolerance against bacterial pathogens in potato and tobacco

Terpene synthases (TPSs) have diverse biological functions in plants. Though the roles of TPSs in herbivore defense are well established in many plant species, their role in bacterial defense has been scarce and is emerging. Through functional genomics, here we report the in planta role of potato (Solanum tuberosum) terpene synthase (StTPS18) in bacterial defense. Expression of StTPS18 was highest in leaves and was induced in response to Pseudomonas syringae and methyl jasmonate treatments. The recombinant StTPS18 exhibited bona fide (E,E)-farnesol synthase activity forming a sesquiterpenoid, (E,E)-farnesol as the sole product, utilising (E,E)-farnesyl diphosphate (FPP). Subcellular localization of GFP fusion protein revealed that StTPS18 is localized to the cytosol. Silencing and overexpression of StTPS18 in potato resulted in reduced and enhanced tolerance, respectively, to bacterial pathogens P. syringae and Ralstonia solanacearum. Bacterial growth assay using medium containing (E,E)-farnesol significantly inhibited P. syringae growth. Moreover, StTPS18 overexpressing transgenic potato and Nicotiana tabacum leaves, and (E,E)-farnesol and P. syringae infiltrated potato leaves exhibited elevated expression of sterol pathway and members of pathogenesis-related genes with enhanced phytosterol accumulation. Interestingly, enhanced phytosterols in 13 C3 -(E,E)-farnesol infiltrated potato leaves were devoid of any noticeable 13 C labeling, indicating no direct utilization of (E,E)-farnesol in phytosterols formation. Furthermore, leaves of StTPS18 overexpressing transgenic lines had no detectable (E,E)-farnesol similar to the control plant, and emitted lower levels of sesquiterpenes than the control. These findings point towards an indirect involvement of StTPS18 and its product (E,E)-farnesol in bacterial defense through upregulation of phytosterol biosynthesis and defense genes.

Virus-Induced Gene Silencing for Functional Genomics of Specialized Metabolism in Medicinal Plants

Virus-induced gene silencing (VIGS) is a functional genomics tool to transiently downregulate the expression of target gene(s) by exploiting the plant's innate defense mechanism against invading RNA viruses. VIGS is a rapid and efficient approach to analyze the gene function, particularly, in the plants that are not amenable to stable genetic transformation. This strategy has been successfully used to decipher the function of several genes and transcription factors involved in the biosynthesis of specialized metabolites and regulation of specialized metabolism, respectively, in different medicinal and aromatic plants. Here, we describe a detailed Tobacco rattle virus (TRV)-mediated VIGS protocol for silencing of the gene encoding Phytoene desaturase (PDS) in important medicinal plants Catharanthus roseus, Calotropis gigantean, Rauwolfia serpentina, and Ocimum basilicum. Our methods allow the study of gene function within 3-4 weeks after agro-inoculation, and can be an easy and efficient approach for future studies on understanding of the biosynthesis of specialized metabolites in these important medicinal plants.

Agrobacterium-Mediated in Planta Transformation in Periwinkle

Madagascar periwinkle (Catharanthus roseus, family Apocynaceae) is a reservoir of more than 130 monoterpene indole alkaloids (MIAs) including the famous anti-neoplastic dimeric MIAs vinblastine and vincristine, and anti-hypertensive monomeric MIAs ajmalicine and serpentine. Understanding the biosynthetic steps and regulatory factors leading to the formation of MIAs is crucial for rational engineering to achieve targeted enhancement of different MIAs. Due to its highly recalcitrant nature, C. roseus is considered genetically non-tractable for transformation at the whole-plant level. Though few reports have demonstrated tissue culture-mediated regeneration and transformation of C. roseus at whole-plant level recently, the efficiency and reproducibility of these protocols have been a major challenge. To overcome this, we have developed a tissue-culture-independent Agrobacterium-mediated in planta transformation method in C. roseus. Using this method, we were able to efficiently generate stable transgenic plants without relying on the cumbersome methods of tissue-culture regeneration and transformation. Moreover, the transformed plants obtained through this in planta method exhibited stability in subsequent generations. Our method is useful not only for the elucidation of biosynthetic and regulatory steps involved in MIA formation through transgenic plant approach but also for metabolic engineering at the whole-plant level in C. roseus.

Alternative splicing creates a pseudo-strictosidine β-d-glucosidase modulating alkaloid synthesis in Catharanthus roseus

Deglycosylation is a key step in the activation of specialized metabolites involved in plant defense mechanisms. This reaction is notably catalyzed by β-glucosidases of the glycosyl hydrolase 1 (GH1) family such as strictosidine β-d-glucosidase (SGD) from Catharanthus roseus. SGD catalyzes the deglycosylation of strictosidine, forming a highly reactive aglycone involved in the synthesis of cytotoxic monoterpene indole alkaloids (MIAs) and in the crosslinking of aggressor proteins. By exploring C. roseus transcriptomic resources, we identified an alternative splicing event of the SGD gene leading to the formation of a shorter isoform of this enzyme (shSGD) that lacks the last 71-residues and whose transcript ratio with SGD ranges from 1.7% up to 42.8%, depending on organs and conditions. Whereas it completely lacks β-glucosidase activity, shSGD interacts with SGD and causes the disruption of SGD multimers. Such disorganization drastically inhibits SGD activity and impacts downstream MIA synthesis. In addition, shSGD disrupts the metabolic channeling of downstream biosynthetic steps by hampering the recruitment of tetrahydroalstonine synthase in cell nuclei. shSGD thus corresponds to a pseudo-enzyme acting as a regulator of MIA biosynthesis. These data shed light on a peculiar control mechanism of β-glucosidase multimerization, an organization common to many defensive GH1 members.

Non-radioactive Assay to Determine Product Profile of Short-chain Isoprenyl Diphosphate Synthases

Isoprenoids represent the largest class of metabolites with amazing diversities in structure and function. They are involved in protecting plants against pathogens or herbivores or involved in attracting pollinators. Isoprenoids are derived from geranyl diphosphate (GPP; C₁₀), farnesyl diphosphate (FPP; C₁₅), geranylgeranyl diphosphate (GGPP; C₂₀), and geranylfarnesyl diphosphate (GFPP; C₂₅) that are in turn formed by sequential condensations of isopentenyl diphosphate (IPP; C₅) with an allylic acceptor such as dimethylallyl diphosphate (DMAPP; C₅), GPP, FPP, or GGPP in a reaction catalyzed by isoprenyl diphosphate synthases (IDSs). IDS enzyme assay for determination of prenyl diphosphate products is generally performed using radiolabelled substrates, and the products formed are identified by employing expensive instruments such as phosphor imager, radio-GC, or radio-HPLC. Though a non-radioactive assay for measuring IDS activity in crude plant extract has been reported, it requires a complex methodology utilizing chromatography coupled with tandem mass spectrometry (LC/MS-MS). Here, we describe a non-radioactive and simple inexpensive assay for determining the IDS assay products using non-radiolabeled IPP and its co-allylic substrates DMAPP, GPP, and FPP. The detection of prenyl diphosphate products generated in the assay was highly efficient and spots corresponding to prenyl alcohols were visible at >40 µM concentrations of IPP and DMAPP/GPP/FPP substrates. The protocol described here is sensitive, reliable, and technically simple, which could be used for functional characterization of IDS candidates.

Functional characterization of a defense-responsive bulnesol/elemol synthase from potato

Terpene synthases (TPSs) produce a variety of terpenoids that play numerous functional roles in primary and secondary metabolism, as well as in ecological interactions. Here, we report the functional characterization of an inducible potato TPS gene encoding bulnesol/elemol synthase (StBUS/ELS). The expression of StBUS/ELS in potato leaves was significantly induced in response to both bacterial (Pseudomonas syringae) and fungal (Alternaria solani) infection as well as methyl jasmonate treatment, indicating its role in defense. The leaves exhibited the highest StBUS/ELS expression followed by the stem with least and similar expression in tuber, sprout and root. Recombinant StBUS/ELS catalyzed the formation of different sesquiterpenes by utilizing farnesyl diphosphate as substrate, and the monoterpene geraniol from geranyl diphosphate. Among the sesquiterpenes formed by StBUS/ELS, elemol was the predominant product followed by α-bulnesene, bulnesol and β-elemene. Further gas chromatography-mass spectrometry (GC-MS) analysis of StBUS/ELS assay products at different injection temperatures revealed elemol and bulnesol as the major products at 275 and 200/150°C, respectively, without much change in the levels of minor products. This indicated thermal rearrangement of bulnesol into elemol at higher temperatures. Transient overexpression of StBUS/ELS in potato leaves conferred tolerance against the growth of bacteria P. syringae and Ralstonia solanacearum, and the fungus A. solani. Further, expression analysis of pathogenesis-related (PR) genes in StBUS/ELS overexpressing leaves showed no significant change in comparison to control, indicating a direct involvement of StBUS/ELS enzymatic products against pathogens. Overall, our study suggested that StBUS/ELS is a pathogen-inducible TPS encoding bulnesol/elemol synthase and could provide a direct role in defense against biotic stress in potato.

A plastid-localized bona fide geranylgeranyl diphosphate synthase plays a necessary role in monoterpene indole alkaloid biosynthesis in Catharanthus roseus

In plants, geranylgeranyl diphosphate (GGPP, C₂₀ ) synthesized by GGPP synthase (GGPPS) serves as precursor for vital metabolic branches including specialized metabolites. Here, we report the characterization of a GGPPS (CrGGPPS2) from the Madagascar periwinkle (Catharanthus roseus) and demonstrate its role in monoterpene (C₁₀ )-indole alkaloids (MIA) biosynthesis. The expression of CrGGPPS2 was not induced in response to methyl jasmonate (MeJA), and was similar to the gene encoding type-I protein geranylgeranyltransferase_β subunit (CrPGGT-I_β), which modulates MIA formation in C. roseus cell cultures. Recombinant CrGGPPS2 exhibited a bona fide GGPPS activity by catalyzing the formation of GGPP as the sole product. Co-localization of fluorescent protein fusions clearly showed CrGGPPS2 was targeted to plastids. Downregulation of CrGGPPS2 by virus-induced gene silencing (VIGS) significantly decreased the expression of transcription factors and pathway genes related to MIA biosynthesis, resulting in reduced MIA. Chemical complementation of CrGGPPS2-vigs leaves with geranylgeraniol (GGol, alcoholic form of GGPP) restored the negative effects of CrGGPPS2 silencing on MIA biosynthesis. In contrast to VIGS, transient and stable overexpression of CrGGPPS2 enhanced the MIA biosynthesis. Interestingly, VIGS and transgenic-overexpression of CrGGPPS2 had no effect on the main GGPP-derived metabolites, cholorophylls and carotenoids in C. roseus leaves. Moreover, silencing of CrPGGT-I_β, similar to CrGGPPS2-vigs, negatively affected the genes related to MIA biosynthesis resulting in reduced MIA. Overall, this study demonstrated that plastidial CrGGPPS2 plays an indirect but necessary role in MIA biosynthesis. We propose that CrGGPPS2 might be involved in providing GGPP for modifying proteins of the signaling pathway involved in MIA biosynthesis.

Advances in biosynthesis, regulation, and metabolic engineering of plant specialized terpenoids

Plant specialized terpenoids are natural products that have no obvious role in growth and development, but play many important functional roles to improve the plant's overall fitness. Besides, plant specialized terpenoids have immense value to humans due to their applications in fragrance, flavor, cosmetic, and biofuel industries. Understanding the fundamental aspects involved in the biosynthesis and regulation of these high-value molecules in plants not only paves the path to enhance plant traits, but also facilitates homologous or heterologous engineering for overproduction of target molecules of importance. Recent developments in functional genomics and high-throughput analytical techniques have led to unraveling of several novel aspects involved in the biosynthesis and regulation of plant specialized terpenoids. The knowledge thus derived has been successfully utilized to produce target specialized terpenoids of plant origin in homologous or heterologous host systems by metabolic engineering and synthetic biology approaches. Here, we provide an overview and highlights on advances related to the biosynthetic steps, regulation, and metabolic engineering of plant specialized terpenoids.

Virus-Induced Gene Silencing for Functional Genomics in Withania somnifera, an Important Indian Medicinal Plant

Virus-induced gene silencing (VIGS) has emerged as a fast and efficient reverse and forward genetics tool to study gene function in model plants as well as in agriculturally important plants. In addition, VIGS approach has been successfully used to provide insights into the role of several genes and regulators involved in plant secondary metabolism. Ashwagandha (Withania somnifera) is an important Indian medicinal plant that accumulates pharmacologically important triterpenoid steroidal lactones, which are collectively termed as withanolides. W. somnifera being a highly recalcitrant plant for genetic transformation, Tobacco rattle virus (TRV)-mediated VIGS was established by our group to facilitate understanding of withanolides' pathway. Here, we describe a detailed procedure to carry out VIGS for gene function studies in W. somnifera.

Characterization of a class III peroxidase from Artemisia annua: relevance to artemisinin metabolism and beyond

A class III peroxidase from Artemisia annua has been shown to indicate the possibility of cellular localization-based role diversity, which may have implications in artemisinin catabolism as well as lignification. Artemisia annua derives its importance from the antimalarial artemisinin. The -O-O- linkage in artemisinin makes peroxidases relevant to its metabolism. Earlier, we identified three peroxidase-coding genes from A. annua, whereby Aa547 showed higher expression in the low-artemisinin plant stage whereas Aa528 and Aa540 showed higher expression in the artemisinin-rich plant stage. Here we carried out tertiary structure homology modelling of the peroxidases for docking studies. Maximum binding affinity for artemisinin was shown by Aa547. Further, Aa547 showed greater binding affinity for post-artemisinin metabolite, deoxyartemisinin, as compared to pre-artemisinin metabolites (dihydroartemisinic hydroperoxide, artemisinic acid, dihydroartemisinic acid). It also showed significant binding affinity for the monolignol, coniferyl alcohol. Moreover, Aa547 expression was related inversely to artemisinin content and directly to total lignin content as indicated by its transient silencing and overexpression in A. annua. Artemisinin reduction assay also indicated inverse relationship between Aa547 expression and artemisinin content. Subcellular localization using GFP fusion suggested that Aa547 is peroxisomal. Nevertheless, dual localization (intracellular/extracellular) of Aa547 could not be ruled out due to its effect on both, artemisinin and lignin. Taken together, this indicates possibility of localization-based role diversity for Aa547, which may have implications in artemisinin catabolism as well as lignification in A. annua.

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.

Terpene Moiety Enhancement by Overexpression of Geranyl(geranyl) Diphosphate Synthase and Geraniol Synthase Elevates Monomeric and Dimeric Monoterpene Indole Alkaloids in Transgenic Catharanthus roseus

Catharanthus roseus is the sole source of two of the most important anticancer monoterpene indole alkaloids (MIAs), vinblastine and vincristine and their precursors, vindoline and catharanthine. The MIAs are produced from the condensation of precursors derived from indole and terpene secoiridoid pathways. It has been previously reported that the terpene moiety limits MIA biosynthesis in C. roseus. Here, to overcome this limitation and enhance MIAs levels in C. roseus, bifunctional geranyl(geranyl) diphosphate synthase [G(G)PPS] and geraniol synthase (GES) that provide precursors for early steps of terpene moiety (secologanin) formation, were overexpressed transiently by agroinfiltration and stably by Agrobacterium-mediated transformation. Both transient and stable overexpression of G(G)PPS and co-expression of G(G)PPS+GES significantly enhanced the accumulation of secologanin, which in turn elevated the levels of monomeric MIAs. In addition, transgenic C. roseus plants exhibited increased levels of root alkaloid ajmalicine. The dimeric alkaloid vinblastine was enhanced only in G(G)PPS but not in G(G)PPS+GES transgenic lines that correlated with transcript levels of peroxidase-1 (PRX1) involved in coupling of vindoline and catharanthine into 3',4'-anhydrovinblastine, the immediate precursor of vinblastine. Moreover, first generation (T₁) lines exhibited comparable transcript and metabolite levels to that of T₀ lines. In addition, transgenic lines displayed normal growth similar to wild-type plants indicating that the bifunctional G(G)PPS enhanced flux toward both primary and secondary metabolism. These results revealed that improved availability of early precursors for terpene moiety biosynthesis enhanced production of MIAs in C. roseus at the whole plant level. This is the first report demonstrating enhanced accumulation of monomeric and dimeric MIAs including root MIA ajmalicine in C. roseus through transgenic approaches.

A WRKY transcription factor from Withania somnifera regulates triterpenoid withanolide accumulation and biotic stress tolerance through modulation of phytosterol and defense pathways

Withania somnifera produces pharmacologically important triterpenoid withanolides that are derived via phytosterol pathway; however, their biosynthesis and regulation remain to be elucidated. A jasmonate- and salicin-inducible WRKY transcription factor from W. somnifera (WsWRKY1) exhibiting correlation with withaferin A accumulation was functionally characterized employing virus-induced gene silencing and overexpression studies combined with transcript and metabolite analyses, and chromatin immunoprecipitation assay. WsWRKY1 silencing resulted in stunted plant growth, reduced transcripts of phytosterol pathway genes with corresponding reduction in phytosterols and withanolides in W. somnifera. Its overexpression elevated the biosynthesis of triterpenoids in W. somnifera (phytosterols and withanolides), as well as tobacco and tomato (phytosterols). Moreover, WsWRKY1 binds to W-box sequences in promoters of W. somnifera genes encoding squalene synthase and squalene epoxidase, indicating its direct regulation of triterpenoid pathway. Furthermore, while WsWRKY1 silencing in W. somnifera compromised the tolerance to bacterial growth, fungal infection, and insect feeding, its overexpression in tobacco led to improved biotic stress tolerance. Together these findings demonstrate that WsWRKY1 has a positive regulatory role on phytosterol and withanolides biosynthesis, and defense against biotic stress, highlighting its importance as a metabolic engineering tool for simultaneous improvement of triterpenoid biosynthesis and plant defense.

Nitrogen treatment enhances sterols and withaferin A through transcriptional activation of jasmonate pathway, WRKY transcription factors, and biosynthesis genes in Withania somnifera (L.) Dunal

The medicinal plant Withania somnifera is researched extensively to increase the quantity of withanolides and specifically withaferin A, which finds implications in many pharmacological activities. Due to insufficient knowledge on biosynthesis and unacceptability of transgenic approach, it is preferred to follow alternative physiological methods to increase the yield of withanolides. Prior use of elicitors like salicylic acid, methyl jasmonate, fungal extracts, and even mechanical wounding have shown to increase the withanolide biosynthesis with limited success; however, the commercial viability and logistics of application are debatable. In this investigation, we tested the simple nitrogeneous fertilizers pertaining to the enhancement of withaferin A biosynthesis. Application of ammonium sulfate improved the sterol contents required for the withanolide biosynthesis and correlated to higher expression of pathway genes like FPPS, SMT1, SMT2, SMO1, SMO2, and ODM. Increased expression of a gene homologous to allene oxide cyclase, crucial in jasmonic acid biosynthetic pathway, suggested the involvement of jasmonate signaling. High levels of WRKY gene transcripts indicated transcriptional regulation of the pathway genes. Increase in transcript level could be correlated with a corresponding increase in the protein levels for WsSMT1 and WsWRKY1. The withaferin A increase was also demonstrated in the potted plants growing in the glasshouse and in the open field. These results implicated simple physiological management of nitrogen fertilizer signal to improve the yield of secondary metabolite through probable involvement of jasmonate signal and WRKY transcription factor for the first time, in W. somnifera besides improving the foliage.

De Novo Sequencing and Analysis of Lemongrass Transcriptome Provide First Insights into the Essential Oil Biosynthesis of Aromatic Grasses

Aromatic grasses of the genus Cymbopogon (Poaceae family) represent unique group of plants that produce diverse composition of monoterpene rich essential oils, which have great value in flavor, fragrance, cosmetic, and aromatherapy industries. Despite the commercial importance of these natural aromatic oils, their biosynthesis at the molecular level remains unexplored. As the first step toward understanding the essential oil biosynthesis, we performed de novo transcriptome assembly and analysis of C. flexuosus (lemongrass) by employing Illumina sequencing. Mining of transcriptome data and subsequent phylogenetic analysis led to identification of terpene synthases, pyrophosphatases, alcohol dehydrogenases, aldo-keto reductases, carotenoid cleavage dioxygenases, alcohol acetyltransferases, and aldehyde dehydrogenases, which are potentially involved in essential oil biosynthesis. Comparative essential oil profiling and mRNA expression analysis in three Cymbopogon species (C. flexuosus, aldehyde type; C. martinii, alcohol type; and C. winterianus, intermediate type) with varying essential oil composition indicated the involvement of identified candidate genes in the formation of alcohols, aldehydes, and acetates. Molecular modeling and docking further supported the role of identified protein sequences in aroma formation in Cymbopogon. Also, simple sequence repeats were found in the transcriptome with many linked to terpene pathway genes including the genes potentially involved in aroma biosynthesis. This work provides the first insights into the essential oil biosynthesis of aromatic grasses, and the identified candidate genes and markers can be a great resource for biotechnological and molecular breeding approaches to modulate the essential oil composition.

Virus-induced gene silencing of Withania somnifera squalene synthase negatively regulates sterol and defence-related genes resulting in reduced withanolides and biotic stress tolerance

Withania somnifera (L.) Dunal is an important Indian medicinal plant that produces withanolides, which are triterpenoid steroidal lactones having diverse biological activities. To enable fast and efficient functional characterization of genes in this slow-growing and difficult-to-transform plant, a virus-induced gene silencing (VIGS) was established by silencing phytoene desaturase (PDS) and squalene synthase (SQS). VIGS of the gene encoding SQS, which provides precursors for triterpenoids, resulted in significant reduction of squalene and withanolides, demonstrating its application in studying withanolides biosynthesis in W. somnifera leaves. A comprehensive analysis of gene expression and sterol pathway intermediates in WsSQS-vigs plants revealed transcriptional modulation with positive feedback regulation of mevalonate pathway genes, and negative feed-forward regulation of downstream sterol pathway genes including DWF1 (delta-24-sterol reductase) and CYP710A1 (C-22-sterol desaturase), resulting in significant reduction of sitosterol, campesterol and stigmasterol. However, there was little effect of SQS silencing on cholesterol, indicating the contribution of sitosterol, campesterol and stigmasterol, but not of cholesterol, towards withanolides formation. Branch-point oxidosqualene synthases in WsSQS-vigs plants exhibited differential regulation with reduced CAS (cycloartenol synthase) and cycloartenol, and induced BAS (β-amyrin synthase) and β-amyrin. Moreover, SQS silencing also led to the down-regulation of brassinosteroid-6-oxidase-2 (BR6OX2), pathogenesis-related (PR) and nonexpressor of PR (NPR) genes, resulting in reduced tolerance to bacterial and fungal infection as well as to insect feeding. Taken together, SQS silencing negatively regulated sterol and defence-related genes leading to reduced phytosterols, withanolides and biotic stress tolerance, thus implicating the application of VIGS for functional analysis of genes related to withanolides formation in W. somnifera leaves.

Precursor feeding studies and molecular characterization of geraniol synthase establish the limiting role of geraniol in monoterpene indole alkaloid biosynthesis in Catharanthus roseus leaves

The monoterpene indole alkaloids (MIAs) are generally derived from strictosidine, which is formed by condensation of the terpene moiety secologanin and the indole moiety tryptamine. There are conflicting reports on the limitation of either terpene or indole moiety in the production of MIAs in Catharanthus roseus cell cultures. Formation of geraniol by geraniol synthase (GES) is the first step in secologanin biosynthesis. In this study, feeding of C. roseus leaves with geraniol, but not tryptophan (precursor for tryptamine), increased the accumulation of the MIAs catharanthine and vindoline, indicating the limitation of geraniol in MIA biosynthesis. This was further validated by molecular and in planta characterization of C. roseus GES (CrGES). CrGES transcripts exhibited leaf and shoot specific expression and were induced by methyl jasmonate. Virus-induced gene silencing (VIGS) of CrGES significantly reduced the MIA content, which was restored to near-WT levels upon geraniol feeding. Moreover, over-expression of CrGES in C. roseus leaves increased MIA content. Further, CrGES exhibited correlation with MIA levels in leaves of different C. roseus cultivars and has significantly lower expression relative to other pathway genes. These results demonstrated that the transcriptional regulation of CrGES and thus, the in planta geraniol availability plays crucial role in MIA biosynthesis.

De novo sequencing and comparative analysis of holy and sweet basil transcriptomes

Background

Ocimum L. of family Lamiaceae is a well known genus for its ethnobotanical, medicinal and aromatic properties, which are attributed to innumerable phenylpropanoid and terpenoid compounds produced by the plant. To enrich genomic resources for understanding various pathways, de novo transcriptome sequencing of two important species, O. sanctum and O. basilicum, was carried out by Illumina paired-end sequencing.

Results

The sequence assembly resulted in 69117 and 130043 transcripts with an average length of 1646 ± 1210.1 bp and 1363 ± 1139.3 bp for O. sanctum and O. basilicum, respectively. Out of the total transcripts, 59648 (86.30%) and 105470 (81.10%) from O. sanctum and O. basilicum, and respectively were annotated by uniprot blastx against Arabidopsis, rice and lamiaceae. KEGG analysis identified 501 and 952 transcripts from O. sanctum and O. basilicum, respectively, related to secondary metabolism with higher percentage of transcripts for biosynthesis of terpenoids in O. sanctum and phenylpropanoids in O. basilicum. Higher digital gene expression in O. basilicum was validated through qPCR and correlated to higher essential oil content and chromosome number (O. sanctum, 2n = 16; and O. basilicum, 2n = 48). Several CYP450 (26) and TF (40) families were identified having probable roles in primary and secondary metabolism. Also SSR and SNP markers were identified in the transcriptomes of both species with many SSRs linked to phenylpropanoid and terpenoid pathway genes.

Conclusion

This is the first report of a comparative transcriptome analysis of Ocimum species and can be utilized to characterize genes related to secondary metabolism, their regulation, and breeding special chemotypes with unique essential oil composition in Ocimum.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-588) contains supplementary material, which is available to authorized users.

Past achievements, current status and future perspectives of studies on 3-hydroxy-3-methylglutaryl-CoA synthase (HMGS) in the mevalonate (MVA) pathway

HMGS functions in phytosterol biosynthesis, development and stress responses. F-244 could specifically-inhibit HMGS in tobacco BY-2 cells and Brassica seedlings. An update on HMGS from higher plants is presented. 3-Hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS) is the second enzyme in the mevalonate pathway of isoprenoid biosynthesis and catalyzes the condensation of acetoacetyl-CoA and acetyl-CoA to produce S-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). Besides HMG-CoA reductase (HMGR), HMGS is another key enzyme in the regulation of cholesterol and ketone bodies in mammals. In plants, it plays an important role in phytosterol biosynthesis. Here, we summarize the past investigations on eukaryotic HMGS with particular focus on plant HMGS, its enzymatic properties, gene expression, protein structure, and its current status of research in China. An update of the findings on HMGS from animals (human, rat, avian) to plants (Brassica juncea, Hevea brasiliensis, Arabidopsis thaliana) will be discussed. Current studies on HMGS have been vastly promoted by developments in biochemistry and molecular biology. Nonetheless, several limitations have been encountered, thus some novel advances in HMGS-related research that have recently emerged will be touched on.

4-coumarate: CoA ligase partitions metabolites for eugenol biosynthesis

Biosynthesis of eugenol shares its initial steps with that of lignin, involving conversion of hydroxycinnamic acids to their corresponding coenzyme A (CoA) esters by 4-coumarate:CoA ligases (4CLs). In this investigation, a 4CL (OS4CL) was identified from glandular trichome-rich tissue of Ocimum sanctum with high sequence similarity to an isoform (OB4CL_ctg4) from Ocimum basilicum. The levels of OS4CL and OB4CL_ctg4-like transcripts were highest in O. sanctum trichome, followed by leaf, stem and root. The eugenol content in leaf essential oil was positively correlated with the expression of OS4CL in the leaf at different developmental stages. Recombinant OS4CL showed the highest activity with p-coumaric acid, followed by ferulic, caffeic and trans-cinnamic acids. Transient RNA interference (RNAi) suppression of OS4CL in O. sanctum leaves caused a reduction in leaf eugenol content and trichome transcript level, with a considerable increase in endogenous p-coumaric, ferulic, trans-cinnamic and caffeic acids. A significant reduction in the expression levels was observed for OB4CL_ctg4-related transcripts in suppressed trichome compared with transcripts similar to the other four isoforms (OB4CL_ctg1, 2, 3 and 5). Sinapic acid and lignin content were also unaffected in RNAi suppressed leaf samples. Transient expression of OS4CL-green fluorescent protein fusion protein in Arabidopsis protoplasts was associated with the cytosol. These results indicate metabolite channeling of intermediates towards eugenol by a specific 4CL and is the first report demonstrating the involvement of 4CL in creation of virtual compartments through substrate utilization and committing metabolites for eugenol biosynthesis at an early stage of the pathway.

Contribution of CoA ligases to benzenoid biosynthesis in petunia flowers

Biosynthesis of benzoic acid from Phe requires shortening of the side chain by two carbons, which can occur via the β-oxidative or nonoxidative pathways. The first step in the β-oxidative pathway is cinnamoyl-CoA formation, likely catalyzed by a member of the 4-coumarate:CoA ligase (4CL) family that converts a range of trans-cinnamic acid derivatives into the corresponding CoA thioesters. Using a functional genomics approach, we identified two potential CoA-ligases from petunia (Petunia hybrida) petal-specific cDNA libraries. The cognate proteins share only 25% amino acid identity and are highly expressed in petunia corollas. Biochemical characterization of the recombinant proteins revealed that one of these proteins (Ph-4CL1) has broad substrate specificity and represents a bona fide 4CL, whereas the other is a cinnamate:CoA ligase (Ph-CNL). RNA interference suppression of Ph-4CL1 did not affect the petunia benzenoid scent profile, whereas downregulation of Ph-CNL resulted in a decrease in emission of benzylbenzoate, phenylethylbenzoate, and methylbenzoate. Green fluorescent protein localization studies revealed that the Ph-4CL1 protein is localized in the cytosol, whereas Ph-CNL is in peroxisomes. Our results indicate that subcellular compartmentalization of enzymes affects their involvement in the benzenoid network and provide evidence that cinnamoyl-CoA formation by Ph-CNL in the peroxisomes is the committed step in the β-oxidative pathway.

Overexpression of Brassica juncea wild-type and mutant HMG-CoA synthase 1 in Arabidopsis up-regulates genes in sterol biosynthesis and enhances sterol production and stress tolerance

Brassica juncea 3-hydroxy-3-methylglutaryl-CoA synthase (HMGS) is encoded by four isogenes (BjHMGS1-BjHMGS4). In vitro enzyme assays had indicated that the recombinant BjHMGS1 H188N mutant lacked substrate inhibition by acetoacetyl-CoA (AcAc-CoA) and showed 8-fold decreased enzyme activity. The S359A mutant demonstrated 10-fold higher activity, while the H188N/S359A double mutant displayed a 10-fold increased enzyme activity and lacked inhibition by AcAc-CoA. Here, wild-type and mutant BjHMGS1 were overexpressed in Arabidopsis to examine their effects in planta. The expression of selected genes in isoprenoid biosynthesis, isoprenoid content, seed germination and stress tolerance was analysed in HMGS overexpressors (OEs). Those mRNAs encoding enzymes 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), sterol methyltransferase 2 (SMT2), delta-24 sterol reductase (DWF1), C-22 sterol desaturase (CYP710A1) and brassinosteroid-6-oxidase 2 (BR6OX2) were up-regulated in HMGS-OEs. The total sterol content in leaves and seedlings of OE-wtBjHMGS1, OE-S359A and OE-H188N/S359A was significantly higher than OE-H188N. HMGS-OE seeds germinated earlier than wild-type and vector-transformed controls. HMGS-OEs further displayed reduced hydrogen peroxide (H(2) O(2) )-induced cell death and constitutive expression of salicylic acid (SA)-dependent pathogenesis-related genes (PR1, PR2 and PR5), resulting in an increased resistance to Botrytis cinerea, with OE-S359A showing the highest and OE-H188N the lowest tolerance. These results suggest that overexpression of HMGS up-regulates HMGR, SMT2, DWF1, CYP710A1 and BR6OX2, leading to enhanced sterol content and stress tolerance in Arabidopsis.

Role of aromatic aldehyde synthase in wounding/herbivory response and flower scent production in different Arabidopsis ecotypes

Aromatic L-amino acid decarboxylases (AADCs) are key enzymes operating at the interface between primary and secondary metabolism. The Arabidopsis thaliana genome contains two genes, At2g20340 and At4g28680, encoding pyridoxal 5'-phosphate-dependent AADCs with high homology to the recently identified Petunia hybrida phenylacetaldehyde synthase involved in floral scent production. The At4g28680 gene product was recently biochemically characterized as an L-tyrosine decarboxylase (AtTYDC), whereas the function of the other gene product remains unknown. The biochemical and functional characterization of the At2g20340 gene product revealed that it is an aromatic aldehyde synthase (AtAAS), which catalyzes the conversion of phenylalanine and 3,4-dihydroxy-L-phenylalanine to phenylacetaldehyde and dopaldehyde, respectively. AtAAS knock-down and transgenic AtAAS RNA interference (RNAi) lines show significant reduction in phenylacetaldehyde levels and an increase in phenylalanine, indicating that AtAAS is responsible for phenylacetaldehyde formation in planta. In A. thaliana ecotype Columbia (Col-0), AtAAS expression was highest in leaves, and was induced by methyl jasmonate treatment and wounding. Pieris rapae larvae feeding on Col-0 leaves resulted in increased phenylacetaldehyde emission, suggesting that the emitted aldehyde has a defensive activity against attacking herbivores. In the ecotypes Sei-0 and Di-G, which emit phenylacetaldehyde as a predominant flower volatile, the highest expression of AtAAS was found in flowers and RNAi AtAAS silencing led to a reduction of phenylacetaldehyde formation in this organ. In contrast to ecotype Col-0, no phenylacetaldehyde accumulation was observed in Sei-0 upon wounding, suggesting that AtAAS and subsequently phenylacetaldehyde contribute to pollinator attraction in this ecotype.

The small subunit of snapdragon geranyl diphosphate synthase modifies the chain length specificity of tobacco geranylgeranyl diphosphate synthase in planta

Geranyl diphosphate (GPP), the precursor of many monoterpene end products, is synthesized in plastids by a condensation of dimethylallyl diphosphate and isopentenyl diphosphate (IPP) in a reaction catalyzed by homodimeric or heterodimeric GPP synthase (GPPS). In the heterodimeric enzymes, a noncatalytic small subunit (GPPS.SSU) determines the product specificity of the catalytic large subunit, which may be either an active geranylgeranyl diphosphate synthase (GGPPS) or an inactive GGPPS-like protein. Here, we show that expression of snapdragon (Antirrhinum majus) GPPS.SSU in tobacco (Nicotiana tabacum) plants increased the total GPPS activity and monoterpene emission from leaves and flowers, indicating that the introduced catalytically inactive GPPS.SSU found endogenous large subunit partner(s) and formed an active snapdragon/tobacco GPPS in planta. Bimolecular fluorescence complementation and in vitro enzyme analysis of individual and hybrid proteins revealed that two of four GGPPS-like candidates from tobacco EST databases encode bona fide GGPPS that can interact with snapdragon GPPS.SSU and form a functional GPPS enzyme in plastids. The formation of chimeric GPPS in transgenic plants also resulted in leaf chlorosis, increased light sensitivity, and dwarfism due to decreased levels of chlorophylls, carotenoids, and gibberellins. In addition, these transgenic plants had reduced levels of sesquiterpene emission, suggesting that the export of isoprenoid intermediates from the plastids into the cytosol was decreased. These results provide genetic evidence that GPPS.SSU modifies the chain length specificity of phylogenetically distant GGPPS and can modulate IPP flux distribution between GPP and GGPP synthesis in planta.

Involvement of snapdragon benzaldehyde dehydrogenase in benzoic acid biosynthesis

Benzoic acid (BA) is an important building block in a wide spectrum of compounds varying from primary metabolites to secondary products. Benzoic acid biosynthesis from L-phenylalanine requires shortening of the propyl side chain by two carbons, which can occur via a beta-oxidative pathway or a non-beta-oxidative pathway, with benzaldehyde as a key intermediate. The non-beta-oxidative route requires benzaldehyde dehydrogenase (BALDH) to convert benzaldehyde to BA. Using a functional genomic approach, we identified an Antirrhinum majus (snapdragon) BALDH, which exhibits 40% identity to bacterial BALDH. Transcript profiling, biochemical characterization of the purified recombinant protein, molecular homology modeling, in vivo stable isotope labeling, and transient expression in petunia flowers reveal that BALDH is capable of oxidizing benzaldehyde to BA in vivo. GFP localization and immunogold labeling studies show that this biochemical step occurs in the mitochondria, raising a question about the role of subcellular compartmentalization in BA biosynthesis.

Involvement of snapdragon benzaldehyde dehydrogenase in benzoic acid biosynthesis

Benzoic acid (BA) is an important building block in a wide spectrum of compounds varying from primary metabolites to secondary products. Benzoic acid biosynthesis from L-phenylalanine requires shortening of the propyl side chain by two carbons, which can occur via a beta-oxidative pathway or a non-beta-oxidative pathway, with benzaldehyde as a key intermediate. The non-beta-oxidative route requires benzaldehyde dehydrogenase (BALDH) to convert benzaldehyde to BA. Using a functional genomic approach, we identified an Antirrhinum majus (snapdragon) BALDH, which exhibits 40% identity to bacterial BALDH. Transcript profiling, biochemical characterization of the purified recombinant protein, molecular homology modeling, in vivo stable isotope labeling, and transient expression in petunia flowers reveal that BALDH is capable of oxidizing benzaldehyde to BA in vivo. GFP localization and immunogold labeling studies show that this biochemical step occurs in the mitochondria, raising a question about the role of subcellular compartmentalization in BA biosynthesis.

Two terpene synthases are responsible for the major sesquiterpenes emitted from the flowers of kiwifruit (Actinidia deliciosa)

Kiwifruit vines rely on bees for pollen transfer between spatially separated male and female individuals and require synchronized flowering to ensure pollination. Volatile terpene compounds, which are important cues for insect pollinator attraction, were studied by dynamic headspace sampling in the major green-fleshed kiwifruit (Actinidia deliciosa) cultivar 'Hayward' and its male pollinator 'Chieftain'. Terpene volatile levels showed a profile dominated by the sesquiterpenes alpha-farnesene and germacrene D. These two compounds were emitted by all floral tissues and could be observed throughout the day, with lower levels at night. The monoterpene (E)-beta-ocimene was also detected in flowers but was emitted predominantly during the day and only from petal tissue. Using a functional genomics approach, two terpene synthase (TPS) genes were isolated from a 'Hayward' petal EST library. Bacterial expression and transient in planta data combined with analysis by enantioselective gas chromatography revealed that one TPS produced primarily (E,E)-alpha-farnesene and small amounts of (E)-beta-ocimene, whereas the second TPS produced primarily (+)-germacrene D. Subcellular localization using GFP fusions showed that both enzymes were localized in the cytoplasm, the site for sesquiterpene production. Real-time PCR analysis revealed that both TPS genes were expressed in the same tissues and at the same times as the corresponding floral volatiles. The results indicate that two genes can account for the major floral sesquiterpene volatiles observed in both male and female A. deliciosa flowers.

Two nearly identical terpene synthases catalyze the formation of nerolidol and linalool in snapdragon flowers

Terpenoids emitted from snapdragon flowers include three monoterpenes derived from geranyl diphosphate (GPP), myrcene, (E)-beta-ocimene and linalool, and a sesquiterpene, nerolidol, derived from farnesyl diphosphate (FPP). Using a functional genomics approach, we have isolated and biochemically characterized two nearly identical nerolidol/linalool synthases, AmNES/LIS-1 and AmNES/LIS-2, two enzymes responsible for the terpenoid profile of snapdragon scent remaining to be characterized. The AmNES/LIS-2 protein has an additional 30 amino acids in the N-terminus, and shares 95% amino acid sequence identity with AmNES/LIS-1, with only 23 amino acid substitutions distributed across the homologous regions of the proteins. Although these two terpene synthases have very similar catalytic properties, and synthesize linalool and nerolidol as specific products from GPP and FPP, respectively, they are compartmentally segregated. GFP localization studies and analysis of enzyme activities in purified leucoplasts, together with our previous feeding experiments, revealed that AmNES/LIS-1 is localized in cytosol, and is responsible for nerolidol biosynthesis, whereas AmNES/LIS-2 is located in plastids, and accounts for linalool formation. Our results show that subcellular localization of bifunctional enzymes, in addition to the availability of substrate, controls the type of product formed. By directing nearly identical bifunctional enzymes to more than one cellular compartment, plants extend the range of available substrates for enzyme utilization, thus increasing the diversity of the metabolites produced.

Characterization of a petunia acetyltransferase involved in the biosynthesis of the floral volatile isoeugenol

Petunia flower petals emit large amounts of isoeugenol, which has been shown to be synthesized by isoeugenol synthase (PhIGS1) from an ester of coniferyl alcohol, hypothesized to be coniferyl acetate. This paper describes the identification and characterization of a novel petunia gene encoding an enzyme belonging to the BAHD acyltransferase family whose expression correlates with isoeugenol biosynthesis. RNAi suppression of this gene results in inhibition of isoeugenol biosynthesis. Biochemical characterization of the protein encoded by this gene showed that it has acetyltransferase activity and is most efficient with coniferyl alcohol among the alcohol substrates tested. Overall, these data support the conclusion that coniferyl acetate is the substrate of isoeugenol synthase.

Structural basis for the design of potent and species-specific inhibitors of 3-hydroxy-3-methylglutaryl CoA synthases

3-Hydroxy-3-methylglutaryl CoA synthase (HMGS) catalyzes the first committed step in the mevalonate metabolic pathway for isoprenoid biosynthesis and serves as an alternative target for cholesterol-lowering and antibiotic drugs. We have determined a previously undescribed crystal structure of a eukaryotic HMGS bound covalently to a potent and specific inhibitor F-244 [(E,E)-11-[3-(hydroxymethyl)-4-oxo-2-oxytanyl]-3,5,7-trimethyl-2,4-undecadienenoic acid]. Given the accessibility of synthetic analogs of the F-244 natural product, this inhibited eukaryotic HMGS structure serves as a necessary starting point for structure-based methods that may improve the potency and species-specific selectivity of the next generation of F-244 analogs designed to target particular eukaryotic and prokaryotic HMGS.

Brassica juncea HMG-CoA synthase: localization of mRNA and protein

3-Hydroxy-3-methylglutaryl-coenzyme-A (HMG-CoA) synthase (HMGS; EC 2.3.3.10) synthesizes HMG-CoA, a substrate for mevalonate biosynthesis in the isoprenoid pathway. It catalyzes the condensation of acetyl-CoA with acetoacetyl-CoA (AcAc-CoA) to yield S-HMG-CoA and HS-CoA. In Brassica juncea (Indian mustard), HMGS is encoded by four isogenes (BjHMGS1-BjHMGS4). We have already enzymatically characterized recombinant BjHMGS1 expressed in Escherichia coli, and have identified its residues that are significant in catalysis. To further study HMGS mRNA expression that is developmentally regulated in flowers and seedlings, we have examined its mRNA distribution by in situ hybridization and reverse transcriptase-polymerase chain reaction (RT-PCR). We observed predominant localization of HMGS mRNA in the stigmas and ovules of flower buds and in the piths of seedling hypocotyls. RT-PCR analysis revealed that BjHMGS1 and BjHMGS2 but not BjHMGS3 and BjHMGS4were expressed in floral buds. To investigate the subcellular localization of BjHMGS1, we fused BjHMGS1 translationally in-frame either to the N- or C-terminus of green fluorescent protein (GFP). BjHMGS1-GFP and GFP-BjHMGS1 fusions were used in particle gun bombardment of onion epidermal cells and tobacco BY-2 cells. The GFP-BjHMGS1 construct was also used in agroinfiltration of tobacco leaves. Both GFP-fusion proteins were observed transiently expressed in the cytosol on confocal microscopy of onion epidermal cells, tobacco BY-2 cells, and agroinfiltrated tobacco leaves. Further, subcellular fractionation of total proteins from transgenic plants expressing GFP-BjHMGS1 derived from Agrobacterium-mediated transformation confirmed that BjHMGS1 is a cytosolic enzyme. We suggest that the presence of BjHMGS isoforms is likely related to the specialization of each in different cellular and metabolic processes rather than to a different intracellular compartmentation of the enzyme.