Enzymatic Synthesis of Bioactive O-Glucuronides Using Plant Glucuronosyltransferases.

Many O-glucuronides exhibiting various pharmacological activities have been found in nature and in drug metabolism. The glucuronidation of bioactive natural products or drugs to generate glucuronides with better activity and druggability is important in drug discovery and research. In this study, by using two uridine diphosphate (UDP)-dependent glucuronosyltransferases (GATs, UGT88D4 and UGT88D7) from plants, we developed two glucuronidation approaches, pure enzyme catalysis in vitro and recombinant whole-cell catalysis in vivo, to efficiently synthesize bioactive O-glucuronides by the glucuronidation of natural products. In total, 14 O-glucuronides with different structures, including flavonoids, anthraquinones, coumarins, and lignans, were obtained, 7 of which were new compounds. Furthermore, one of the biosynthesized O-glucuronides, kaempferol-7- O-ß-d-glucuronide (3a), potently inhibited protein tyrosine phosphatase (PTP) 1B with an IC50 value of 8.02 × 10-6 M. Some of the biosynthesized O-glucuronides also exhibited significant antioxidant activities.

Physical interactions among flavonoid enzymes in snapdragon and torenia reveal the diversity in the flavonoid metabolon organization of different plant species.

Flavonoid metabolons (weakly-bound multi-enzyme complexes of flavonoid enzymes) are believed to occur in diverse plant species. However, how flavonoid enzymes are organized to form a metabolon is unknown for most plant species. We analyzed the physical interaction partnerships of the flavonoid enzymes from two lamiales plants (snapdragon and torenia) that produce flavones and anthocyanins. In snapdragon, protein-protein interaction assays using yeast and plant systems revealed the following binary interactions: flavone synthase II (FNSII)/chalcone synthase (CHS); FNSII/chalcone isomerase (CHI); FNSII/dihydroflavonol 4-reductase (DFR); CHS/CHI; CHI/DFR; and flavonoid 3'-hydroxylase/CHI. These results along with the subcellular localizations and membrane associations of snapdragon flavonoid enzymes suggested that FNSII serves as a component of the flavonoid metabolon tethered to the endoplasmic reticulum (ER). The observed interaction partnerships and temporal gene expression patterns of flavonoid enzymes in red snapdragon petal cells suggested the flower stage-dependent formation of the flavonoid metabolon, which accounted for the sequential flavone and anthocyanin accumulation patterns therein. We also identified interactions between FNSII and other flavonoid enzymes in torenia, in which the co-suppression of FNSII expression was previously reported to diminish petal anthocyanin contents. The observed physical interactions among flavonoid enzymes of these plant species provided further evidence supporting the long-suspected organization of flavonoid metabolons as enzyme complexes tethered to the ER via cytochrome P450, and illustrated how flavonoid metabolons mediate flower coloration. Moreover, the observed interaction partnerships were distinct from those previously identified in other plant species (Arabidopsis thaliana and soybean), suggesting that the organization of flavonoid metabolons may differ among plant species.

Inverted stereocontrol of iridoid synthase in snapdragon.

The natural product class of iridoids, found in various species of flowering plants, harbors astonishing chemical complexity. The discovery of iridoid biosynthetic genes in the medicinal plant Catharanthus roseus has provided insight into the biosynthetic origins of this class of natural product. However, not all iridoids share the exact five- to six-bicyclic ring scaffold of the Catharanthus iridoids. For instance, iridoids in the ornamental flower snapdragon (Antirrhinum majus, Plantaginaceae family) are derived from the C7 epimer of this scaffold. Here we have cloned and characterized the iridoid synthase enzyme from A. majus (AmISY), the enzyme that is responsible for converting 8-oxogeranial into the bicyclic iridoid scaffold in a two-step reduction-cyclization sequence. Chiral analysis of the reaction products reveals that AmISY reduces C7 to generate the opposite stereoconfiguration in comparison with the Catharanthus homologue CrISY. The catalytic activity of AmISY thus explains the biosynthesis of 7-epi-iridoids in Antirrhinum and related genera. However, although the stereoselectivity of the reduction step catalyzed by AmISY is clear, in both AmISY and CrISY, the cyclization step produces a diastereomeric mixture. Although the reduction of 8-oxogeranial is clearly enzymatically catalyzed, the cyclization step appears to be subject to less stringent enzyme control.

Metabolic engineering of Escherichia coli for the biosynthesis of flavonoid-O-glucuronides and flavonoid-O-galactoside.

Most flavonoids are glycosylated and the nature of the attached sugar can strongly affect their physiological properties. Although many flavonoid glycosides have been synthesized in Escherichia coli, most of them are glucosylated. In order to synthesize flavonoids attached to alternate sugars such as glucuronic acid and galactoside, E. coli was genetically modified to express a uridine diphosphate (UDP)-dependent glycosyltransferase (UGT) specific for UDP-glucuronic acid (AmUGT10 from Antirrhinum majus or VvUGT from Vitis vinifera) and UDP-galactoside (PhUGT from Petunia hybrid) along with the appropriate nucleotide biosynthetic genes to enable simultaneous production of their substrates, UDP-glucuronic acid and UDP-galactose. To engineer UDP-glucuronic acid biosynthesis, the araA gene encoding UDP-4-deoxy-4-formamido-L-arabinose formyltransferase/UDP-glucuronic acid C-4″ decarboxylase, which also used UDP-glucuronic acid as a substrate, was deleted in E. coli, and UDP-glucose dehydrogenase (ugd) gene was overexpressed to increase biosynthesis of UDP-glucuronic acid. Using these strategies, luteolin-7-O-glucuronide and quercetin-3-O-glucuronide were biosynthesized to levels of 300 and 687 mg/L, respectively. For the synthesis of quercetin 3-O-galactoside, UGE (encoding UDP-glucose epimerase from Oryza sativa) was overexpressed along with a glycosyltransferase specific for quercetin and UDP-galactose. Using this approach, quercetin 3-O-galactoside was successfully synthesized to a level of 280 mg/L.

Biolistics-based gene silencing in plants using a modified particle inflow gun.

RNA interference (RNAi) is one of the most commonly used techniques for examining the function of genes of interest. In this chapter we present two examples of RNAi that use the particle inflow gun for delivery of the DNA constructs. In one example transient RNAi is used to show the function of an anthocyanin regulatory gene in flower petals. In the second example stably transformed cell cultures are produced with an RNAi construct that results in a change in the anthocyanin hydroxylation pattern.

Betalain production is possible in anthocyanin-producing plant species given the presence of DOPA-dioxygenase and L-DOPA.

BACKGROUND: Carotenoids and anthocyanins are the predominant non-chlorophyll pigments in plants. However, certain families within the order Caryophyllales produce another class of pigments, the betalains, instead of anthocyanins. The occurrence of betalains and anthocyanins is mutually exclusive. Betalains are divided into two classes, the betaxanthins and betacyanins, which produce yellow to orange or violet colours, respectively. In this article we show betalain production in species that normally produce anthocyanins, through a combination of genetic modification and substrate feeding. RESULTS: The biolistic introduction of DNA constructs for transient overexpression of two different dihydroxyphenylalanine (DOPA) dioxygenases (DODs), and feeding of DOD substrate (L-DOPA), was sufficient to induce betalain production in cell cultures of Solanum tuberosum (potato) and petals of Antirrhinum majus. HPLC analysis showed both betaxanthins and betacyanins were produced. Multi-cell foci with yellow, orange and/or red colours occurred, with either a fungal DOD (from Amanita muscaria) or a plant DOD (from Portulaca grandiflora), and the yellow/orange foci showed green autofluorescence characteristic of betaxanthins. Stably transformed Arabidopsis thaliana (arabidopsis) lines containing 35S: AmDOD produced yellow colouration in flowers and orange-red colouration in seedlings when fed L-DOPA. These tissues also showed green autofluorescence. HPLC analysis of the transgenic seedlings fed L-DOPA confirmed betaxanthin production. CONCLUSIONS: The fact that the introduction of DOD along with a supply of its substrate (L-DOPA) was sufficient to induce betacyanin production reveals the presence of a background enzyme, possibly a tyrosinase, that can convert L-DOPA to cyclo-DOPA (or dopaxanthin to betacyanin) in at least some anthocyanin-producing plants. The plants also demonstrate that betalains can accumulate in anthocyanin-producing species. Thus, introduction of a DOD and an enzyme capable of converting tyrosine to L-DOPA should be sufficient to confer both betaxanthin and betacyanin production to anthocyanin-producing species. The requirement for few novel biosynthetic steps may have assisted in the evolution of the betalain biosynthetic pathway in the Caryophyllales, and facilitated multiple origins of the pathway in this order and in fungi. The stably transformed 35S: AmDOD arabidopsis plants provide material to study, for the first time, the physiological effects of having both betalains and anthocyanins in the same plant tissues.

Functional analysis of Antirrhinum kelloggii flavonoid 3'-hydroxylase and flavonoid 3',5'-hydroxylase genes; critical role in flower color and evolution in the genus Antirrhinum.

The enzymes flavonoid 3'-hydroxylase (F3'H) and flavonoid 3',5'-hydroxylase (F3'5'H) play an important role in flower color by determining the B-ring hydroxylation pattern of anthocyanins, the major floral pigments. F3'5'H is necessary for biosynthesis of the delphinidin-based anthocyanins that confer a violet or blue color to most plants. Antirrhinum majus does not produce delphinidin and lacks violet flower colour while A. kelloggii produces violet flowers containing delphinidin. To understand the cause of this inter-specific difference in the Antirrhinum genus, we isolated one F3'H and two F3'5'H homologues from the A. kelloggii petal cDNA library. Their amino acid sequences showed high identities to F3'Hs and F3'5'Hs of closely related species. Transgenic petunia expressing these genes had elevated amounts of cyanidin and delphinidin respectively, and flower color changes in the transgenics reflected the type of accumulated anthocyanidins. The results indicate that the homologs encode F3'H and F3'5'H, respectively, and that the ancestor of A. majus lost F3'5'H activity after its speciation from the ancestor of A. kelloggii.

Homology modeling and dynamics study of aureusidin synthase--an important enzyme in aurone biosynthesis of snapdragon flower.

Aurones, a class of plant flavonoids, provide bright yellow color on some important ornamental flowers, such as cosmos, coreopsis, and snapdragon (Antirrhinum majus). Recently, it has been elucidated that aureusidin synthase (AUS), a homolog of plant polyphenol oxidase (PPO), plays a key role in the yellow coloration of snapdragon flowers. In addition, it has been shown that AUS is a chalcone-specific PPO specialized for aurone biosynthesis. AUS gene has been successfully demonstrated as an attractive tool to engineer yellow flowers in blue flowers. Despite these biological studies, the structural basis for the specificity of substrate interactions of AUS remains elusive. In this study, we performed homology modeling of AUS using Grenache PPO and Sweet potato catechol oxidase (CO). An AUS-inhibitor was then developed from the initial homology model based on the CO and subsequently validated. We performed a thorough study between AUS and PTU inhibitor by means of interaction energy, which indicated the most important residues in the active site that are highly conserved. Analysis of the molecular dynamics simulations of the apo enzyme and ligand-bound complex showed that complex is relatively stable than apo and the active sites of both systems are flexible. The results from this study provide very helpful information to understand the structure-function relationships of AUS.

Temperature controls nuclear import of Tam3 transposase in Antirrhinum.

It has been proposed that environmental stimuli can activate transposable elements (TEs), whereas few substantial mechanisms have been shown so far. The class-II element Tam3 from Antirrhinum majus exhibits a unique property of low-temperature-dependent transposition (LTDT). LTDT has proved invaluable in developing the gene isolation technologies that have underpinned much of modern plant developmental biology. Here, we reveal that LTDT involves differential subcellular localization of the Tam3 transposase (TPase) in cells grown at low (15°C) and high (25°C) temperatures. The mechanism is associated with the nuclear import of Tam3 TPase in Antirrhinum cells. At high temperature, the nuclear import of Tam3 TPase is severely restricted in Antirrhinum cells, whereas at low temperature, the nuclear localization of Tam3 TPase is observed in about 20% of the cells. However, in tobacco BY-2 and Allium cepa (onion) cells, Tam3 TPase is transported into most nuclei. In addition to three nuclear localization signals (NLSs), the Tam3 TPase is equipped with a nuclear localization inhibitory domain (NLID), which functions to abolish nuclear import of the TPase at high temperature in Antirrhinum. NLID in Tam3 TPase is considered to interact with Antirrhinum-specific factor(s). The host-specific regulation of the nuclear localization of transposase represents a new repertoire controlling class-II TEs.

Production of tetraketide lactones by mutated Antirrhinum majus chalcone synthases (AmCHS1).

Chalcone synthase (CHS) is a key enzyme of flavonoid biosynthesis in higher plants, catalyzing the stepwise decarboxylative condensation of three acetate units from malonyl-CoA with p-coumaroyl-CoA to yield 2',4,4',6'-tetrahydroxychalcone (THC). Reaction (at pH 7.5) of a mutant (V196M/T197A) of Antirrhinum majus CHS (AmCHS1) with p-coumaroyl-CoA and malonyl-CoA yielded a significant amount of a non-chalcone product, along with a small amount of THC. The non-chalcone product was identified as p-coumaroyltriacetic acid lactone (CTAL), a tetraketide lactone produced due to derailment from the canonical THC-producing reaction pathway. In vitro, the wild-type AmCHS1 showed low CTAL-producing activity at pH 7.5, but an appreciable level at pH 10. Each of the amino acid substitutions, V196M, T197A and V196M/T197A, caused a shift toward neutrality of the optimum pH for CTAL-producing activity. The V196M substitution resulted in a loss of THC-producing activity, as well as a 12.6-fold enhancement of CTAL-producing activity (at pH 7.5); hence, AmCHS1 was converted to a p-coumaroyltriacetic acid synthase by this single amino acid substitution. The THC-producing activity of the V196M mutant appeared to be restored by additional T197A substitution, although a single T197A substitution caused no substantial enhancement of the CTAL-producing activity of the wild-type enzyme. The enhancement of the tetraketide producing activity upon V196M and V196M/T197A substitutions was most markedly observed when p-coumaroyl-CoA was used as the starter substrate, and only slightly with benzoyl-, caffeoyl- and hexanoyl-CoAs. These results show the importance of the two contiguous amino acids at positions 196 and 197 for product specificity of an AmCHS1-catalyzed reaction.

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.

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.

Genetic diversity and the reproductive system in related species of antirrhinum.

BACKGROUND AND AIMS: Seven related species of Antirrhinum (A. siculum, A. majus, A. latifolium, A. linkianum, A. litigiosum, A. cirrhigherum and A. tortuosum) were studied in order to compare levels of genetic variation and its partitioning in them, and to check relationships between genetic patterns and the reproductive system. METHODS: Eight hundred and fifty-one plants were screened for variability at 13 allozyme loci by means of horizontal starch gel electrophoresis. Parameters of genetic diversity and its partitioning, the inbreeding coefficient as well as an indirect estimate of gene flow based on the equation: Nm = (1 - G(ST))/4G(ST), were calculated. KEY RESULTS: Genetic variability in A. siculum was found to be the lowest known in the genus. Mean values of F(IT) and F(IS) were mostly positive and not significantly different from zero. Population differentiation (F(ST)) ranged between 6.1 in A. tortuosum and 17.6 in A. linkianum. The inbreeding coefficient within populations ranged between F(IS) = -0.5 in A. tortuosum and F(IS) = 1 in A. siculum. Estimates of gene flow ranged between Nm = 15 in A. majus (considered as very high) to Nm = 0.42 in A. siculum (considered as low). CONCLUSIONS: Correlation was found between levels of diversity and differentiation on one hand, and the reproductive system of the studied taxa on the other. Striking differences among species in the inbreeding coefficient (F(IS)) show different reproductive systems, which mostly support previous reports. Strategies for the conservation of A. siculum are recommended, such as preservation of natural populations as well as ex situ preservation of seeds from different populations.

Regulation of terpenoid and benzenoid production in flowers.

The production and emission of fragrant molecules by flowers are strictly regulated during the floral lifespan and often peak when pollinators are active. The best-studied classes of floral volatiles are benzenoids and terpenoids. The production of these molecules appears to be primarily regulated at the level of precursor biosynthesis. The genes from the petunia floral shikimate pathway, which provides the precursors for the formation of benzenoids, have recently been shown to be regulated by a MYB transcription factor. The floral terpenoids of snapdragon appear to be derived exclusively from the methyl-erythritol-phosphate pathway in plastids. This pathway controls precursor levels for geranyl diphosphate synthase, which in turn is transcriptionally regulated.

Synthesis of the food flavoring methyl benzoate by genetically engineered Saccharomyces cerevisiae.

Current means of production for plant-derived aroma compounds include chemical synthesis and extraction from plant material. Both methods are environmentally detrimental and relatively expensive: plant material is only seasonally available and only a small subset of the plant biomass produces the desired aroma compounds, while organic synthesis inevitably involves waste byproducts with a negative ecological impact. Benzenoids are a class of plant metabolites that includes a number of aroma compounds. This paper explores, for the first time, the feasibility of producing benzenoids in yeast. We present a method for the production of the phenylpropanoid methyl benzoate in Saccharomyces cerevisiae using benzoic acid as a substrate, by heterologous expression of Antirrhinum majus benzoic acid methyl transferase. Production was pH dependent with a maximal yield of approximately 50 microg of methyl benzoate per liter of culture per hour, and with linear kinetics over at least 24 h. In addition, we have analyzed two alternative expression vectors for the production of benzoic acid methyl transferase in S. cerevisiae: a constitutive triosephosphate isomerase promoter-based system was compared with a copper-inducible CUP1 promoter system. We find major differences in the amounts of methylbenzoate produced by these respective systems. Potential applications are discussed.

An auxin-responsive 1-aminocyclopropane-1-carboxylate synthase is responsible for differential ethylene production in gravistimulated Antirrhinum majus L. flower stems.

The regulation of gravistimulation-induced ethylene production and its role in gravitropic bending was studied in Antirrhinum majus L. cut flower stems. Gravistimulation increased ethylene production in both lower and upper halves of the stems with much higher levels observed in the lower half. Expression patterns of three different 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) genes, an ACC oxidase (ACO) and an ethylene receptor (ETR/ERS homolog) gene were studied in the bending zone of gravistimulated stems and in excised stem sections following treatment with different chemicals. One of the ACS genes (Am-ACS3) was abundantly expressed in the bending zone cortex at the lower side of the stems within 2 h of gravistimulation. Am-ACS3 was not expressed in vertical stems or in other parts of (gravistimulated) stems, leaves or flowers. Am-ACS3 was strongly induced by indole-3-acetic acid (IAA) but not responsive to ethylene. The Am-ACS3 expression pattern strongly suggests that Am-ACS3 is responsible for the observed differential ethylene production in gravistimulated stems; its responsiveness to IAA suggests that Am-ACS3 expression reflects changes in auxin signalling. Am-ACS1 also showed increased expression in gravistimulated and IAA-treated stems although to a much lesser extent than Am-ACS3. In contrast to Am-ACS3, Am-ACS1 was also expressed in non-bending regions of vertical and gravistimulated stems and in leaves, and Am-ACS1 expression was not confined to the lower side cortex but evenly distributed over the diameter of the stem. Am-ACO and Am-ETR/ERS expression was increased in both the lower and upper halves of gravistimulated stems. Expression of both Am-ACO and Am-ETR/ERS was responsive to ethylene, suggesting regulation by IAA-dependent differential ethylene production. Am-ACO expression and in vivo ACO activity, in addition, were induced by IAA, independent of the IAA-induced ethylene. IAA-induced growth of vertical stem sections and bending of gravistimulated flowering stems were little affected by ethylene or 1-methylcyclopropene treatments, indicating that the differential ethylene production plays no pivotal role in the kinetics of gravitropic bending.

Allozymic differentiation of the Antirrhinum majus and A. siculum species groups.

BACKGROUND AND AIMS: Fifty-two populations were sampled in order to establish the taxonomic delimitation and relationships of eight taxa belonging to the A. majus L. and A. siculum Miller groups. METHODS: Data on 13 allozyme loci were recorded after extraction of fresh leaves and electrophoresis on horizontal 10% starch gels. KEY RESULTS: Genetic distances between conspecific populations are lower than for other species of the genus. CONCLUSIONS: These results support the recognition of A. majus, A. tortuosum, A. linkianum, A. cirrigherum, A. litigiosum and A. barrelieri at specific rank. The genetic distances, together with the lack of morphological differences and the sympatric distribution ranges, support the inclusion of A. australe into A. tortuosum, A. dielsianum into A. siculum, and A. latifolium subsp. intermedium as a synonym of A. latifolium. The results support separation of the taxa studied into two groups, coinciding with series Sicula Rothm. and Majora, but disagreeing with the arrangement of species into them. According to our results, Sicula consist of A. siculum and Majora consists of A. latifolium, A. majus, A. tortuosum, A. linkianum, A. cirrigherum, A. litigiosum and A. barrelieri.

Low levels of allozyme variability in the threatened species Antirrhinum subbaeticum and A. pertegasii (Scrophulariaceae): implications for conservation of the species.

AIMS: This study was designed to compare levels of genetic variation and its partitioning in three related species of Antirrhinum, A. subbaeticum, A. pertegasii and A. pulverulentum, and to check the hypothesis that species with small total population size have lower levels of genetic variability than those with bigger ones. This information should contribute to the development of conservation strategies of rare endemic species of Antirrhinum. METHODS: One hundred and seventy-seven plants were screened for variability at 14 allozyme loci by means of horizontal starch gel. Parameters of genetic diversity, and its partitioning, were calculated. An indirect estimate of gene flow was based on the equation: Nm = (1 - GST)/4GST. KEY RESULTS: Genetic variabilities in A. subbaeticum and A. pertegasii were found to be the lowest known for the genus, the within-population genetic diversity being correlated with population size in both species. The distribution of genetic diversity is strikingly different among species, with 85 % of the total variation distributed among populations in A. subbaeticum, 6 % in A. pertegasii and 23 % in A. pulverulentum. Estimated levels of gene flow were negligible for A. subbaeticum (0.04), high for A. pertegasii (3.92), and substantial for A. pulverulentum (0.83). Genetic and geographic distances were negatively correlated in A. pertegasii, whereas no significant correlation was found in the other two species. CONCLUSIONS: Levels of total genetic diversity agree with the hypothesis that species with small total population size have lower levels of genetic variability than those with bigger ones. Strategies for the conservation of the species are recommended, such as preservation of natural populations and avoidance of possible causes of threat, as well as ex situ preservation of seeds, reinforcement of small populations of A. subbaeticum with plants or seeds from the same population, and avoidance of translocations among populations.

Formation of monoterpenes in Antirrhinum majus and Clarkia breweri flowers involves heterodimeric geranyl diphosphate synthases.

The precursor of all monoterpenes is the C10 acyclic intermediate geranyl diphosphate (GPP), which is formed from the C5 compounds isopentenyl diphosphate and dimethylallyl diphosphate by GPP synthase (GPPS). We have discovered that Antirrhinum majus (snapdragon) and Clarkia breweri, two species whose floral scent is rich in monoterpenes, both possess a heterodimeric GPPS like that previously reported from Mentha piperita (peppermint). The A. majus and C. breweri cDNAs encode proteins with 53% and 45% amino acid sequence identity, respectively, to the M. piperita GPPS small subunit (GPPS.SSU). Expression of these cDNAs in Escherichia coli yielded no detectable prenyltransferase activity. However, when each of these cDNAs was coexpressed with the M. piperita GPPS large subunit (GPPS.LSU), which shares functional motifs and a high level of amino acid sequence identity with geranylgeranyl diphosphate synthases (GGPPS), active GPPS was obtained. Using a homology-based cloning strategy, a GPPS.LSU cDNA also was isolated from A. majus. Its coexpression in E. coli with A. majus GPPS.SSU yielded a functional heterodimer that catalyzed the synthesis of GPP as a main product. The expression in E. coli of A. majus GPPS.LSU by itself yielded active GGPPS, indicating that in contrast with M. piperita GPPS.LSU, A. majus GPPS.LSU is a functional GGPPS on its own. Analyses of tissue-specific, developmental, and rhythmic changes in the mRNA and protein levels of GPPS.SSU in A. majus flowers revealed that these levels correlate closely with monoterpene emission, whereas GPPS.LSU mRNA levels did not, indicating that the levels of GPPS.SSU, but not GPPS.LSU, might play a key role in regulating the formation of GPPS and, thus, monoterpene biosynthesis.