The promoter of the strictosidine synthase gene from periwinkle confers elicitor-inducible expression in transgenic tobacco and binds nuclear factors GT-1 and GBF

Strictosidine synthase, an indispensable enzyme involved in the biosynthesis of terpenoid indole and β-carboline alkaloids

Terpenoid indole (TIAs) and β-carboline alkaloids (BCAs), such as suppressant reserpine, vasodilatory yohimbine, and antimalarial quinine, are natural compounds derived from strictosidine. These compounds can exert powerful pharmacological effects but be obtained from limited source in nature. the whole biosynthetic pathway of TIAs and BCAs, The Pictet-Spengler reaction catalyzed by strictosidine synthase (STR; EC: 4.3.3.2) is the rate-limiting step. Therefore, it is necessary to investigate their biosynthesis pathways, especially the role of STR, and related findings will support the biosynthetic generation of natural and unnatural compounds. This review summarizes the latest studies concerning the function of STR in TIA and BCA biosynthesis, and illustrates the compounds derived from strictosidine. The substrate specificity of STR based on its structure is also summarized. Proteins that contain six-bladed four-stranded β-propeller folds in many organisms, other than plants, are listed. The presence of these folds may lead to similar functions among organisms. The expression of STR gene can greatly influence the production of many compounds. STR is mainly applied to product various valuable drugs in plant cell suspension culture and biosynthesis in other carriers.

Systematic analysis of the Capsicum ERF transcription factor family: identification of regulatory factors involved in the regulation of species-specific metabolites

BACKGROUND

ERF transcription factors (TFs) belong to the Apetala2/Ethylene responsive Factor (AP2/ERF) TF family and play a vital role in plant growth and development processes. Capsorubin and capsaicinoids have relatively high economic and nutritional value, and they are specifically found in Capsicum. However, there is little understanding of how ERFs participate in the regulatory networks of capsorubin and capsaicinoids biosynthesis.

RESULTS

In this study, a total of 142 ERFs were identified in the Capsicum annuum genome. Subsequent phylogenetic analysis allowed us to divide ERFs into DREB (dehydration responsive element binding proteins) and ERF subfamilies, and further classify them into 11 groups with several subgroups. Expression analysis of biosynthetic pathway genes and CaERFs facilitated the identification of candidate genes related to the regulation of capsorubin and capsaicinoids biosynthesis; the candidates were focused in cluster C9 and cluster C10, as well as cluster L3 and cluster L4, respectively. The expression patterns of CaERF82, CaERF97, CaERF66, CaERF107 and CaERF101, which were found in cluster C9 and cluster C10, were consistent with those of accumulating of carotenoids (β-carotene, zeaxanthin and capsorubin) in the pericarp. In cluster L3 and cluster L4, the expression patterns of CaERF102, CaERF53, CaERF111 and CaERF92 were similar to those of the accumulating capsaicinoids. Furthermore, CaERF92, CaERF102 and CaERF111 were found to be potentially involved in temperature-mediated capsaicinoids biosynthesis.

CONCLUSION

This study will provide an extremely useful foundation for the study of candidate ERFs in the regulation of carotenoids and capsaicinoids biosynthesis in peppers.

Revisiting the ORCA gene cluster that regulates terpenoid indole alkaloid biosynthesis in Catharanthus roseus

Transcription factor (TF) gene clusters in plants, such as tomato, potato, petunia, tobacco, and almond, have been characterized for their roles in the biosynthesis of diverse array of specialized metabolites. In Catharanthus roseus, three AP2/ERF TFs, ORCA3, ORCA4, and ORCA5, have been shown to be present on the same genomic scaffold, forming a cluster that regulates the biosynthesis of pharmaceutically important terpenoid indole alkaloids (TIAs). Our analysis of the recently updated C. roseus genome sequence revealed that the ORCA cluster comprises two additional AP2/ERFs, the previously characterized ORCA2 and a newly identified member designated as ORCA6. Transcriptomic analysis revealed that the ORCAs are highly expressed in stems, followed by leaves, roots and flowers. Expression of ORCAs was differentially induced in response to methyl-jasmonate and ethylene treatment. In addition, ORCA6 activated the strictosidine synthase (STR) promoter in tobacco cells. Activation of the STR promoter was significantly higher when ORCA2 or ORCA6 was coexpressed with the mitogen-activated protein kinase kinase, CrMPKK1. Furthermore, transient overexpression of ORCA6 in C. roseus flower petals activated TIA pathway gene expression and TIA accumulation. The results described here advance our understanding of regulation of TIA pathway by the ORCA gene cluster and the evolution for plant ERF gene clusters.

EASI Transformation: An Efficient Transient Expression Method for Analyzing Gene Function in Catharanthus roseus Seedlings

The Catharanthus roseus plant is the exclusive source of the valuable anticancer terpenoid indole alkaloids, vinblastine (VB) and vincristine (VC). The recent availability of transcriptome and genome resources for C. roseus necessitates a fast and reliable method for studying gene function. In this study, we developed an Agrobacterium-mediated transient expression method to enable the functional study of genes rapidly in planta, conserving the compartmentalization observed in the VB and VC pathway. We focused on (1) improving the transformation method (syringe versus vacuum agroinfiltration) and cultivation conditions (seedling age, Agrobacterium density, and time point of maximum transgene expression), (2) improving transformation efficiency through the constitutive expression of the virulence genes and suppressing RNA silencing mechanisms, and (3) improving the vector design by incorporating introns, quantitative and qualitative reporter genes (luciferase and GUS genes), and accounting for transformation heterogeneity across the tissue using an internal control. Of all the parameters tested, vacuum infiltration of young seedlings (10-day-old, harvested 3 days post-infection) resulted in the strongest increase in transgene expression, at 18 - 57 fold higher than either vacuum or syringe infiltration of other seedling ages. Endowing the A. tumefaciens strain with the mutated VirGN54D or silencing suppressors within the same plasmid as the reporter gene further increased expression by 2 - 10 fold. For accurate measurement of promoter transactivation or activity, we included an internal control to normalize the differences in plant mass and transformation efficiency. Including the normalization gene (Renilla luciferase) on the same plasmid as the reporter gene (firefly luciferase) consistently yielded a high signal and a high correlation between RLUC and FLUC. As proof of principle, we applied this approach to investigate the regulation of the CroSTR1 promoter with the well-known activator ORCA3 and repressor ZCT1. Our method demonstrated the quantitative assessment of both the activation and repression of promoter activity in C. roseus. Our efficient Agrobacterium-mediated seedling infiltration (EASI) protocol allows highly efficient, reproducible, and homogenous transformation of C. roseus cotyledons and provides a timely tool for the community to rapidly assess the function of genes in planta, particularly for investigating how transcription factors regulate terpenoid indole alkaloid biosynthesis.

Cross-family transcription factor interaction between MYC2 and GBFs modulates terpenoid indole alkaloid biosynthesis

Biosynthesis of medicinally valuable terpenoid indole alkaloids (TIAs) in Catharanthus roseus is regulated by transcriptional activators such as the basic helix-loop-helix factor CrMYC2. However, the transactivation effects are often buffered by repressors, such as the bZIP factors CrGBF1 and CrGBF2, possibly to fine-tune the accumulation of cytotoxic TIAs. Questions remain as to whether and how these factors interact to modulate TIA production. We demonstrated that overexpression of CrMYC2 induces CrGBF expression and results in reduced alkaloid accumulation in C. roseus hairy roots. We found that CrGBF1 and CrGBF2 form homo- and heterodimers to repress the transcriptional activities of key TIA pathway gene promoters. We showed that CrGBFs dimerize with CrMYC2, and CrGBF1 binds to the same cis-elements (T/G-box) as CrMYC2 in the target gene promoters. Our findings suggest that CrGBFs antagonize CrMYC2 transactivation possibly by competitive binding to the T/G-box in the target promoters and/or protein-protein interaction that forms a non-DNA binding complex that prevents CrMYC2 from binding to its target promoters. Homo- and heterodimer formation allows fine-tuning of the amplitude of TIA gene expression. Our findings reveal a previously undescribed regulatory mechanism that governs the TIA pathway genes to balance metabolic flux for TIA production in C. roseus.

NaCl stress induces CsSAMs gene expression in Cucumis sativus by mediating the binding of CsGT-3b to the GT-1 element within the CsSAMs promoter

MAIN CONCLUSION

The CsSAMs promoter is a salt-stress-inducible promoter containing three GT-1 elements that are sufficient for the salt-stress response. The transcription factor CsGT-3b was found to bind to the GT-1 element. The S-adenosyl-L-methionine synthase (SAMs) gene is among the functional genes induced during environmental stress. However, little is known about the regulatory mechanism and upstream regulators of this salt-inducible gene in cucumber plants. Thus, it is necessary to understand the characteristics of the SAMs gene by analyzing its promoter and transcription factors. In this study, we isolated and functionally analyzed a 1743-bp flanking fragment of the CsSAMs gene from Cucumis sativus. To examine promoter activity, the full-length promoter, as well as different promoter fragments, were fused to the β-glucuronidase (GUS) reporter gene and introduced into the tobacco genome. The full-length promoter displayed maximal promoter activity, whereas the P4 promoter, containing 321 bp of upstream sequence, showed no basal promoter activity. In addition, the CsSAMs promoter exhibited stress-inducible regulation rather than tissue-specific activity in transgenic tobacco. Histochemical analysis revealed strong GUS staining in leaves, stems, and roots, especially in the veins of leaves, the vascular bundle of stems, and root tip zones following NaCl stress. A transient expression assay confirmed that the 242-bp region (-1743 to -1500) was sufficient for the NaCl-stress response. Yeast one-hybrid assays further revealed interaction between the NaCl-response protein CsGT-3b and the GT-1 (GAAAAA) element within the 242-bp region. Taken together, we revealed the presence of four salt-stress-responsive elements (GT-1 cis-elements) in the CsSAMs promoter and identified a transcription factor, CsGT-3b, that specifically binds to this sequence. These results might help us better understand the intricate regulatory network of the cucumber SAMs gene.

Clade IVa Basic Helix-Loop-Helix Transcription Factors Form Part of a Conserved Jasmonate Signaling Circuit for the Regulation of Bioactive Plant Terpenoid Biosynthesis

Plants produce many bioactive, specialized metabolites to defend themselves when facing various stress situations. Their biosynthesis is directed by a tightly controlled regulatory circuit that is elicited by phytohormones such as jasmonate (JA). The basic helix-loop-helix (bHLH) transcription factors (TFs) bHLH iridoid synthesis 1 (BIS1) and Triterpene Saponin Activating Regulator (TSAR) 1 and 2, from Catharanthus roseus and Medicago truncatula, respectively, all belong to clade IVa of the bHLH protein family and activate distinct terpenoid pathways, thereby mediating monoterpenoid indole alkaloid (MIA) and triterpene saponin (TS) accumulation, respectively, in these two species. In this study, we report that promoters of the genes encoding the enzymes involved in the specific terpenoid pathway of one of these species can be transactivated by the orthologous bHLH factor from the other species through recognition of the same cis-regulatory elements. Accordingly, ectopic expression of CrBIS1 in M. truncatula hairy roots up-regulated the expression of all genes required for soyasaponin production, resulting in strongly increased levels of soyasaponins in the transformed roots. Likewise, transient expression of MtTSAR1 and MtTSAR2 in C. roseus petals led to up-regulation of the genes involved in the iridoid branch of the MIA pathway. Together, our data illustrate the functional similarity of these JA-inducible TFs and indicate that recruitment of defined cis-regulatory elements constitutes an important aspect of the evolution of conserved regulatory modules for the activation of species-specific terpenoid biosynthesis pathways by common signals such as the JA phytohormones.

Fruit specific variability in capsaicinoid accumulation and transcription of structural and regulatory genes in Capsicum fruit

Accumulation of capsaicinoids in the placental tissue of ripening chile (Capsicum spp.) fruit follows the coordinated expression of multiple biosynthetic enzymes producing the substrates for capsaicin synthase. Transcription factors are likely agents to regulate expression of these biosynthetic genes. Placental RNAs from habanero fruit (Capsicum chinense) were screened for expression of candidate transcription factors; with two candidate genes identified, both in the ERF family of transcription factors. Characterization of these transcription factors, Erf and Jerf, in nine chile cultivars with distinct capsaicinoid contents demonstrated a correlation of expression with pungency. Amino acid variants were observed in both ERF and JERF from different chile cultivars; none of these changes involved the DNA binding domains. Little to no transcription of Erf was detected in non-pungent Capsium annuum or C. chinense mutants. This correlation was characterized at an individual fruit level in a set of jalapeño (C. annuum) lines again with distinct and variable capsaicinoid contents. Both Erf and Jerf are expressed early in fruit development, 16-20 days post-anthesis, at times prior to the accumulation of capsaicinoids in the placental tissues. These data support the hypothesis that these two members of the complex ERF family participate in regulation of the pungency phenotype in chile.

Electrophoretic mobility shift assay for the analysis of interactions of jasmonic acid-responsive transcription factors with DNA

The electrophoretic mobility shift assay based on nondenaturing polyacrylamide gel electrophoresis is a simple, rapid, and sensitive method for the study of the interaction of transcription factors with DNA in vitro. It relies on a change in the electrophoretic mobility of a DNA fragment when bound to an interacting protein. The assay can be used to test DNA binding of either purified or recombinant proteins or uncharacterized binding activities present in crude protein extracts from plant cells or nuclei. It allows the determination of the abundance, affinity, association rate constants, dissociation rate constants, and binding specificity of DNA-binding proteins.

Transcription factors in alkaloid biosynthesis

Higher plants produce a large variety of low-molecular weight secondary compounds. Among them, nitrogen-containing alkaloids are the most biologically active and are often used pharmaceutically. Whereas alkaloid chemistry has been intensively investigated, alkaloid biosynthesis, including the relevant biosynthetic enzymes, genes and their regulation, and especially transcription factors, is largely unknown, as only a limited number of plant species produce certain types of alkaloids and they are difficult to study. Recently, however, several groups have succeeded in isolating the transcription factors that are involved in the biosynthesis of several types of alkaloids, including bHLH, ERF, and WRKY. Most of them show Jasmonate (JA) responsiveness, which suggests that the JA signaling cascade plays an important role in alkaloid biosynthesis. Here, we summarize the types and functions of transcription factors that have been isolated in alkaloid biosynthesis, and characterize their similarities and differences compared to those in other secondary metabolite pathways, such as phenylpropanoid and terpenoid biosyntheses. The evolution of this biosynthetic pathway and regulatory network, as well as the application of these transcription factors to metabolic engineering, is discussed.

The gene encoding Arabidopsis acyl-CoA-binding protein 3 is pathogen inducible and subject to circadian regulation

In Arabidopsis thaliana, acyl-CoA-binding protein 3 ( ACBP3), one of six ACBPs, is unique in terms of the C-terminal location of its acyl-CoA-binding domain. It promotes autophagy-mediated leaf senescence and confers resistance to Pseudomonas syringae pv. tomato DC3000. To understand the regulation of ACBP3, a 1.7 kb 5'-flanking region of ACBP3 and its deletion derivatives were characterized using β-glucuronidase (GUS) fusions. A 374 bp minimal fragment (-151/+223) could drive GUS expression while a 1698 bp fragment (-1475/+223) conferred maximal activity. Further, histochemical analysis on transgenic Arabidopsis harbouring the largest (1698 bp) ACBP3pro::GUS fusion displayed ubiquitous expression in floral organs and vegetative tissues (vascular bundles of leaves and stems), consistent with previous results showing that extracellularly localized ACBP3 functions in plant defence. A 160 bp region (-434/-274) induced expression in extended darkness and caused down-regulation in extended light. Electrophoretic mobility shift assay (EMSA) and DNase I footprinting assay showed that the DNA-binding with one finger box (Dof-box, -341/-338) interacted specifically with leaf nuclear proteins from dark-treated Arabidopsis, while GT-1 (-406/-401) binds both dark- and light-treated Arabidopsis, suggesting that Dof and GT-1 motifs are required to mediate circadian regulation of ACBP3. Moreover, GUS staining and fluorometric measurements revealed that a 109 bp region (-543/-434) was responsive to phytohormones and pathogens. An S-box of AT-rich sequence (-516/-512) was identified to bind nuclear proteins from pathogen-infected Arabidopsis leaves, providing the basis for pathogen-inducible regulation of ACBP3 expression. Thus, three cis-responsive elements (Dof, GT-1, and the S-box) in the 5'-flanking region of ACBP3 are proven functional in the regulation of ACBP3.

CYP82E4-mediated nicotine to nornicotine conversion in tobacco is regulated by a senescence-specific signaling pathway

Nicotine to nornicotine conversion in tobacco (Nicotiana tabacum L.) is regulated by an unstable converter locus which in its activated state gives rise to a high nornicotine, low nicotine phenotype in the senescing leaves. In plants that carry the high nornicotine trait, nicotine conversion is primarily catalyzed by a cytochrome P450 protein, designated CYP82E4 whose transcription is strongly upregulated during leaf senescence. To further investigate the regulation of CYP82E4 expression, we examined the spatiotemporal distribution and the stress- and signaling molecule-elicited expression patterns of CYP82E4 using alkaloid analysis and a fusion construct between the 2.2 kb upstream regulatory region of CYP82E4 and the beta-glucurodinase (GUS) gene. Histochemical and fluorometric analyses of GUS expression revealed that the CYP82E4 promoter confers high levels of expression in the senescing leaves and flowers, and in the green stems of young and mature plants, but only very low activity was detected in the roots. In the leaves, GUS activity was strongly correlated with the progression of senescence. Treatments of leaf tissue with various signaling molecules including abscisic acid, ethylene, jasmonic acid, salicylic acid and yeast extract; and stresses, such as drought, wounding and tobacco mosaic virus infection did not enhance nicotine conversion or GUS activity in the green leaves, but an increase in CYP82E4 expression was observed in response to ethylene- or tobacco mosaic virus-induced senescence. These results suggest that the expression of CYP82E4 is senescence-specific in the leaves and the use of the CYP82E4 promoter could provide a valuable tool for regulating gene expression in the senescing leaves.

Enhancement of alkaloid production in opium and California poppy by transactivation using heterologous regulatory factors

Genes encoding regulatory factors isolated from Arabidopsis, soybean and corn have been screened to identify those that modulate the expression of genes encoding for enzymes involved in the biosynthesis of morphinan alkaloids in opium poppy (Papaver somniferum) and benzophenanthridine alkaloids in California poppy (Eschscholzia californica). In opium poppy, the over-expression of selected regulatory factors increased the levels of PsCOR (codeinone reductase), Ps4'OMT (S-adenosyl-l-methionine:3'-hydroxy-N-methylcoclaurine 4'-O-methyltransferase) and Ps6OMT [(R,S)-norcoclaurine 6-O-methyltransferase] transcripts by 10- to more than 100-fold. These transcriptional activations translated into an enhancement of alkaloid production in opium poppy of up to at least 10-fold. In California poppy, the transactivation effect of regulatory factor WRKY1 resulted in an increase of up to 60-fold in the level of EcCYP80B1 [(S)-N-methylcoclaurine 3'-hydroxylase] and EcBBE (berberine bridge enzyme) transcripts. As a result, the accumulations of selected alkaloid intermediates were enhanced up to 30-fold. The transactivation effects of other regulatory factors led to the accumulation of the same intermediates. These regulatory factors also led to the production of new alkaloids in California poppy callus culture.

Structural biology in plant natural product biosynthesis--architecture of enzymes from monoterpenoid indole and tropane alkaloid biosynthesis

Several cDNAs of enzymes catalyzing biosynthetic pathways of plant-derived alkaloids have recently been heterologously expressed, and the production of appropriate enzymes from ajmaline and tropane alkaloid biosynthesis in bacteria allows their crystallization. This review describes the architecture of these enzymes with and without their ligands.

The promoter of the TLC1.1 retrotransposon from Solanum chilense is activated by multiple stress-related signaling molecules

The LTR retrotransposons are the most abundant mobile elements in the plant genome and seem to play an important role in genome reorganization induced by environmental challenges. Their success in this function depends on the ability of their promoters to respond to different signaling pathways that regulate plant adaptation to biotic and abiotic stresses. The promoter of the TLC1.1 retrotransposon from Solanum chilense contains two primary ethylene-responsive elements (PERE boxes) that are essential for its response to ethylene and for the stress-induced expression. Here, we describe that a 270 bp fragment (P270), derivative of this retroelement promoter, is also able to activate the transcription of the GUS reporter gene in transgenic plants in response to salicylic acid (SA), abscisic acid (ABA), methyl jasmonate (MeJA), hydrogen peroxide (H2O2) and the synthetic auxin 2,4-D. PERE box-dependent and independent routes are involved in the response of P270 to these signal molecules. MeJA, H2O2 and 2,4-D activate this promoter through cis-acting elements other than PERE boxes, whereas ABA and SA act via a PERE box-independent pathway but require this element for maximal activation. Three putative cis-acting elements MRE, GCN4 and GT1/TCA identified in the P270 promoter may be involved in the PERE box-independent activation pathway. These results suggest that the promoter of TLC1.1 may act as an integrator of different signal transduction pathways, allowing this member of the TLC1 retrotransposon family to be activated in response to multiples challenges.

Transcript profiling of terpenoid indole alkaloid pathway genes and regulators reveals strong expression of repressors in Catharanthus roseus cell cultures

The understanding of the complexities and molecular events regulating genes and the activators involved in terpenoid indole alkaloid (TIA) metabolism is known to a certain extent in cell cultures of an important TIA yielding plant, Catharanthus roseus, though it is not yet complete. Recently, the repressors of early TIA pathway genes have also been identified. However, their roles in the regulation of TIA pathway in C. roseus cell cultures remains yet unknown. We have made a comparative profiling of genes catalyzing the important steps of 2-C methyl-D-erythritol-4-phosphate (MEP), shikimate and TIA biosynthetic pathways, their activator and repressors using macroarray, semiquantitative RT-PCR and northern analyses in a rotation culture system of C. roseus comprising differentiated and proliferated cells. Our results demonstrate that TIA biosynthetic pathway genes and their activators show variable expression pattern, which was correlated with the changes in the cellular conditions in these systems. Under similar conditions, TIA pathway repressors show strong and consistent expression. The role of repressors in the complex regulation of the TIA pathway in C. roseus cell cultures is discussed. The results were supported by HPLC data, which demonstrated that the molecular program of cellular differentiation is intimately linked with TIA pathway gene expression and TIA production in C. roseus cell cultures.

Promoter analysis of the Catharanthus roseus geraniol 10-hydroxylase gene involved in terpenoid indole alkaloid biosynthesis

Geraniol 10-hydroxylase (G10H) is an important enzyme in the biosynthetic pathway of monoterpenoid alkaloids found in diverse plant species. The Catharanthus roseus G10H controls the first committed step in biosynthesis of terpenoid indole alkaloids (TIA). The C. roseus G10H promoter sequence was isolated by a PCR-based genome walking method. Sequence analysis revealed that the G10H promoter contains several potential eukaryotic regulatory elements involved in regulation of gene expression. The major transcription start site of the promoter was mapped to an adenine 31 bp downstream of the TATA-box. For functional characterization, transcriptional fusions between the G10H promoter fragments with 5' or 3' deletions and the GUS reporter gene were generated and their expressions were analyzed in a tobacco protoplast transient expression assay. Deletion of the promoter down to -318 bp had little effect on GUS activity. However, further deletion of the promoter to position -103 resulted in approximately 5-fold reduction of GUS activity. Gain-of-function experiments revealed the presence of three potential transcriptional enhancers located in regions between -191 and -147, -266 and -188, and -318 and -266, respectively. The G10H promoter was capable of conferring stable GUS expression in transgenic tobacco plants and C. roseus hairy roots. In transgenic tobacco seedlings GUS expression was tissue-specific, restricted to leaf and actively growing cells around the root tip, and not detected in the hypocotyls, root cap and older developing areas of the root. The GUS expression in both transgenic C. roseus hairy roots and tobacco seedlings were responsive to fungal elicitor and methyljasmonate. Compared to other known promoters of TIA pathway genes, the G10H promoter contains unique binding sites for several transcription factors, suggesting that the G10H promoter may be regulated by a different transcriptional cascade.

Identification of a WRKY protein as a transcriptional regulator of benzylisoquinoline alkaloid biosynthesis in Coptis japonica

Selected cultured Coptis japonica cells produce a large amount of the benzylisoquinoline alkaloid berberine. Previous studies have suggested that berberine productivity is controlled at the transcript level of biosynthetic genes. We have identified a regulator of transcription in berberine biosynthesis using functional genomics with a transient RNA interference (RNAi) and overexpression of the candidate gene. The 24 primary candidate clones were selected from 1,014 expressed sequence tags (ESTs) that were obtained from a C. japonica cell line producing high levels of berberine. Further characterization of the expression profiles of these ESTs suggested that five ESTs would be good candidates as regulators of berberine production. A newly developed transient RNAi system with C. japonica protoplasts indicated that double-stranded RNA of an EST clone significantly reduced the level of transcripts of 3'-hydroxy N-methylcoclaurine 4'-O-methyltransferase. Sequence analysis showed that this EST encoded a group-II WRKY, and we named it CjWRKY1. When the effects of double-stranded RNA of the CjWRKY1 gene were examined in detail, a marked reduction in the transcripts of all genes involved in berberine biosynthesis was detected, whereas little effect was found in the transcript levels of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and chorismate mutase (CM) that are associated with primary metabolism. Ectopic expression of CjWRKY1 cDNA in C. japonica protoplasts clearly increased the level of transcripts of all berberine biosynthetic genes examined compared with control treatment, whereas the levels of GAPDH and CM were not affected. The functional role of CjWRKY1 as a specific and comprehensive regulator of berberine biosynthesis is discussed.

Pathogen- and NaCl-induced expression of the SCaM-4 promoter is mediated in part by a GT-1 box that interacts with a GT-1-like transcription factor

The Ca(2+)-binding protein calmodulin mediates cellular Ca(2+) signals in response to a wide array of stimuli in higher eukaryotes. Plants express numerous CaM isoforms. Transcription of one soybean (Glycine max) CaM isoform, SCaM-4, is dramatically induced within 30 min of pathogen or NaCl stresses. To characterize the cis-acting element(s) of this gene, we isolated an approximately 2-kb promoter sequence of the gene. Deletion analysis of the promoter revealed that a 130-bp region located between nucleotide positions -858 and -728 is required for the stressors to induce expression of SCaM-4. A hexameric DNA sequence within this region, GAAAAA (GT-1 cis-element), was identified as a core cis-acting element for the induction of the SCaM-4 gene. The GT-1 cis-element interacts with an Arabidopsis GT-1-like transcription factor, AtGT-3b, in vitro and in a yeast selection system. Transcription of AtGT-3b is also rapidly induced within 30 min after pathogen and NaCl treatment. These results suggest that an interaction between a GT-1 cis-element and a GT-1-like transcription factor plays a role in pathogen- and salt-induced SCaM-4 gene expression in both soybean and Arabidopsis.

Methyl jasmonate induced expression of the tobacco putrescine N -methyltransferase genes requires both G-box and GCC-motif elements

Putrescine N-methyltransferase (PMT) catalyzes the first committed step of nicotine biosynthesis, converting putrescine into N-methylputrescine. A variety of chemical, environmental, and developmental cues have been implicated in its regulation. Here we have examined the differential expression of beta-glucuronidase (GUS) transgenes under the control of the transcriptional regulatory sequences of four distinct members of the NtPMT gene family from tobacco (Nicotiana tabacum L.). BY-2 cell cultures expressing various NtPMT promoter-GUS constructs were examined for their response to treatment with various combinations of methyl jasmonate (MeJA), auxin (AUX), and ethylene (ETH). All four NtPMT gene promoters examined were inducible by MeJA, although the extent of the induction varied dramatically, with the NtPMT1a promoter being the most responsive. High AUX levels in the cell growth media repressed NtPMT::GUS transgene expression and inhibited their MeJA-induced transcription. Treatment of BY-2 cells with ETH alone did not result in a significant alteration in NtPMT::GUS expression. However, similar to AUX, ETH treatment led to the suppression of MeJA-induced transcription. Detailed deletion analysis of the NtPMT1a gene promoter showed that as little as 111 bp upstream of the transcriptional start site were sufficient to confer MeJA-responsiveness. Deletion of a conserved G-box element (GCACGTTG) at -103 to -96 bp completely abolished MeJA-responsiveness. Further mutagenesis studies revealed that in addition to a functional G-box, MeJA-responsiveness of the NtPMT1a promoter also required a TA-rich region and a GCC-motif (TGCGCCC) located at -80 to -69 bp and -62 to -56 bp relative to the start site, respectively. A synthetic G-box tetramer (4 X syn G-box) fused to a -83 bp fragment from the NtPMT1a promoter (containing the TA-rich region, GCC-box, and TATA-box) displayed a 30-fold induction by MeJA treatment, whereas when the 4 X syn G-box was fused to a minimal (-46 bp) promoter fragment derived from the CaMV 35S gene, no induction by MeJA treatment was detected. Our results indicate that multiple intersecting signal transduction pathways and different transcriptional regulatory factors are involved in mediating JA-responsiveness of NtPMT expression in tobacco.

Transcription factors: tools to engineer the production of pharmacologically active plant metabolites

Plants produce a variety of secondary metabolites, some of which are used as pharmaceuticals or are health promoting as food components. Recent genetic studies on the flavonoid biosynthetic pathway show that transcription factors are efficient new molecular tools for plant metabolic engineering to increase the production of valuable compounds. The use of specific transcription factors would avoid the time-consuming step of acquiring knowledge about all enzymatic steps of a poorly characterized biosynthetic pathway. Although genetic approaches are difficult for most plant species, promoter studies of single-pathway genes and T-DNA activation tagging are feasible alternative approaches for isolating transcription factors, as illustrated for terpenoid indole alkaloid biosynthesis in Catharanthus roseus.

Genetic modification of plant secondary metabolite pathways using transcriptional regulators

Plant secondary metabolism is the source of many natural products with diverse applications, including pharmaceuticals, food colors, dyes and fragrances. Functions in plants include attraction of pollinating insects and protection against pests and pathogens. An important regulatory step in secondary metabolism is transcription of the biosynthetic genes. The aim of this chapter is to discuss results and opportunities concerning modification of secondary metabolism using transcriptional regulators. The transcriptional regulation of two well-studied secondary pathways, the phenylpropanoid pathway and its flavonoid branch, and the terpenoid indole alkaloid biosynthetic pathway, are reviewed. Some examples of successful engineering of these pathways via transcriptional regulators are discussed.

The expanding universe of alkaloid biosynthesis

Characterization of many of the major gene families responsible for the generation of central intermediates and for their decoration, together with the development of large genomics and proteomics databases, has revolutionized our capability to identify exotic and interesting natural-product pathways. Over the next few years, these tools will facilitate dramatic advances in our knowledge of the biosynthesis of alkaloids, which will far surpass that which we have learned in the past 50 years. These tools will also be exploited for the rapid characterization of regulatory genes, which control the development of specialized cell factories for alkaloid biosynthesis.

ALKALOID BIOSYNTHESIS IN PLANTS: Biochemistry, Cell Biology, Molecular Regulation, and Metabolic Engineering Applications

Recent advances in the cell, developmental, and molecular biology of alkaloid biosynthesis have heightened our appreciation for the complexity and importance of plant secondary pathways. Several biosynthetic genes involved in the formation of tropane, benzylisoquinoline, and terpenoid indole alkaloids have now been isolated. The early events of signal perception, the pathways of signal transduction, and the function of gene promoters have been studied in relation to the regulation of alkaloid metabolism. Enzymes involved in alkaloid biosynthesis are associated with diverse subcellular compartments including the cytosol, vacuole, tonoplast membrane, endoplasmic reticulum, chloroplast stroma, thylakoid membranes, and perhaps unique "biosynthetic" or transport vesicles. Localization studies have shown that sequential alkaloid biosynthetic enzymes can also occur in distinct cell types, suggesting the intercellular transport of pathway intermediates. Isolated genes have also been used to genetically alter the accumulation of specific alkaloids and other plant secondary metabolites. Metabolic modifications include increased indole alkaloid levels, altered tropane alkaloid accumulation, elevated serotonin synthesis, reduced indole glucosinolate production, redirected shikimate metabolism, and increased cell wall-bound tyramine formation. This review discusses the biochemistry, cell biology, molecular regulation, and metabolic engineering of alkaloid biosynthesis in plants.