The Arabidopsis MATE transporter TT12 acts as a vacuolar flavonoid/H+ -antiporter active in proanthocyanidin-accumulating cells of the seed coat

Abscisic acid stimulated ripening and gene expression in berry skins of the Cabernet Sauvignon grape

We investigated the effect of exogenous abscisic acid (ABA) application on the transcriptome as well as the phenolic profiles in the skins of Vitis vinifera cv. Cabernet Sauvignon grape berries grown on the vine and cultured in vitro. ABA application rapidly induced the accumulation of anthocyanin and flavonol. Correlatively, the structural genes in the phenylpropanoid and flavonoid pathways, their transcriptional regulators, as well as genes considered to be involved in the acylation and transport of anthocyanin into the vacuole, were upregulated by ABA treatment. The Genechip analysis showed that the ABA treatment significantly up- or downregulated a total of 345 and 1,482 transcripts in the skins of berries grown on the vine and cultured in vitro, respectively. Exogenous ABA modulated the transcripts associated with osmotic responses, stress responses, cell wall modification, auxin and ethylene metabolism and responses, in addition to the induction of anthocyanin biosynthetic genes, and reduced those associated with photosynthesis; approximately half of these transcripts were identical to the previously reported ripening-specific genes.

Pleiotropic changes in Arabidopsis f5h and sct mutants revealed by large-scale gene expression and metabolite analysis

Hydrocinnamic acid esters, lignin, flavonoids, glucosinolates, and salicylic acid protect plants against UV exposure, oxidative stress, diseases, and herbivores. Through the phenylpropanoid pathway, certain Brassicaceae family members, including Arabidopsis thaliana and Brassica napus, accumulate large amounts of the anti-nutritive sinapoylcholine (sinapine) in the seed. We successfully down-regulated activities of key enzymes in the pathway including F5H and SCT and achieved reduction of sinapine and lignin in B. napus seeds. Despite this success, it was unclear how multiple agronomic traits were affected in the transgenic plants. Here, we report altered large-scale gene expression of new alleles of f5h and sct mutants of A. thaliana and resultant accumulation of sinapoylglucose, disinapoylglucose, quercetin-3-O-rhamnoside, salicylic acid glucoside, and total indolyl glucosinolates in the two mutants. Expression of several flowering genes was altered in these mutants when grown under drought and NaCl treatments. Furthermore, both mutants were more susceptible to fungal infection than the wild type. Microarray experiments identified distinctive spatial and temporal expression patterns of gene clusters involved in silique/seed developmental processes and metabolite biosynthesis in these mutants. Taken together, these findings suggest that both f5h and sct mutants exhibit major differences in accumulation of diverse metabolites in the seed and profound changes in global large-scale gene expression, resulting in differential pleiotropic responses to environmental cues. Electronic supplementary material The online version of this article (doi:10.1007/s00425-009-1007-2) contains supplementary material, which is available to authorized users.

A survival strategy: the coevolution of the camptothecin biosynthetic pathway and self-resistance mechanism

A diverse array of secondary metabolites in plants represents the process of coevolution between the plants and their natural enemies including herbivores and pathogens. For defense, plants produce many toxic compounds that harm other organisms. However, if the target of these compounds is a fundamental biological process then the producing plant may also be harmed. In such cases self-resistance strategies must coevolve with the biosynthetic pathway of toxic metabolites. In this review, we discuss the recent elucidation of the self-resistance mechanism of camptothecin (CPT)-producing plants. In this case the target protein of CPT, topoisomerase (Top) 1, has been mutated in order to overcome the toxicity of the compound. Similar mechanisms might also be used by other plants producing different toxic compounds which target fundamental metabolism.

Molecular identification of 1-Cys peroxiredoxin and anthocyanidin/flavonol 3-O-galactosyltransferase from proanthocyanidin-rich young fruits of persimmon (Diospyros kaki Thunb.)

Fruits of persimmon (Diospyros kaki Thunb.) accumulate large amounts of proanthocyanidins (PAs) in the early stages of development. Astringent (A)-type fruits remain rich in soluble PAs even after they reach full-mature stage, whereas non-astringent (NA)-type fruits lose these compounds before full maturation. As a first step to elucidate the mechanism of PA accumulation in this non-model species, we used suppression subtractive hybridization to identify transcripts accumulating differently in young fruits of A- and NA-type. Interestingly, only a few clones involved in PA biosynthesis were identified in A-NA libraries. Represented by multiple clones were those encoding a novel 1-Cys peroxiredoxin and a new member of family 1 glycosyltransferases. Quantitative RT-PCR analyses confirmed correlation of the amount of PAs and accumulation of transcripts encoding these proteins in young persimmon fruits. Furthermore, the new family 1 glycosyltransferase was produced in Escherichia coli and shown to efficiently catalyze galactosylation at 3-hydroxyl groups of several anthocyanidins and flavonols. These findings suggest a complex mechanism of PA accumulation in persimmon fruits.

Full-size ABC transporters from the ABCG subfamily in medicago truncatula

Full-size ATP-binding cassette (ABC) transporters belonging to the ABCG subfamily are unique for plants and fungi. There is growing evidence that certain of these proteins play a role in plant defense or signaling systems. As yet, a complete set of full-size ABCG protein genes has been inventoried and classified in only two plants: Arabidopsis thaliana and Oryza sativa. Recently, a domain-based clustering analysis has predicted the presence of at least 12 genes encoding such proteins in the Lotus japonicus genome. Here, we identify and classify 19 genes coding full-size ABCG proteins in Medicago truncatula, a model legume plant. We have found that the majority of these genes are expressed in roots and flowers whereas only a few are expressed in leaves. Expression of several has been induced upon pathogenic infection in both roots and leaves. ABCG messenger RNAs have been detected in root nodules forming during symbiosis of legume plants and nitrogen-fixing bacteria. The data presented provide a scaffold for further studies of the physiological function of Medicago ABCG transporters and their possible role in modulating plant-microorganism interactions.

MATE transporters facilitate vacuolar uptake of epicatechin 3'-O-glucoside for proanthocyanidin biosynthesis in Medicago truncatula and Arabidopsis

Expression of the Arabidopsis thaliana MYB transcription factor TRANSPARENT TESTA 2 (TT2) in Medicago trunculata hairy roots induces both proanthocyanidin accumulation and the ATP-dependent vacuolar/vesicular uptake of epicatechin 3'-O-glucoside; neither process is active in control roots that do, however, possess anthocyanidin 3-O-glucoside vacuolar uptake activity. A vacuolar membrane-localized multidrug and toxic compound extrusion (MATE) transporter, Medicago MATE1, was identified at the molecular level and shown to preferentially transport epicatechin 3'-O-glucoside. Genetic evidence has implicated TT12, a tonoplastic MATE transporter from Arabidopsis, in the transport of precursors for proanthocyanidin biosynthesis in the seed coat. However, although Arabidopsis TT12 facilitates the transport of cyanidin 3-O-glucoside into membrane vesicles when expressed in yeast, there is no evidence that cyanidin 3-O-glucoside is converted to proanthocyanidins after transport into the vacuole. Here, we show that Arabidopsis TT12, like Medicago MATE1, functions to transport epicatechin 3'-O-glucoside as a precursor for proanthocyanidin biosynthesis, and Medicago MATE1 complements the seed proanthocyanidin phenotype of the Arabidopsis tt12 mutant both quantitatively and qualitatively. On the basis of biochemical properties, tissue-specific expression pattern, and genetic loss-of-function analysis, we conclude that MATE1 is an essential membrane transporter for proanthocyanidin biosynthesis in the Medicago seed coat. Implications of these findings for the assembly of oligomeric proanthocyanidins are discussed.

An archived activation tagged population of Arabidopsis thaliana to facilitate forward genetics approaches

BACKGROUND

Functional genomics tools provide researchers with the ability to apply high-throughput techniques to determine the function and interaction of a diverse range of genes. Mutagenized plant populations are one such resource that facilitate gene characterisation. They allow complex physiological responses to be correlated with the expression of single genes in planta, through either reverse genetics where target genes are mutagenized to assay the affect, or through forward genetics where populations of mutant lines are screened to identify those whose phenotype diverges from wild type for a particular trait. One limitation of these types of populations is the prevalence of gene redundancy within plant genomes, which can mask the affect of individual genes. Activation or enhancer populations, which not only provide knock-out but also dominant activation mutations, can facilitate the study of such genes.

RESULTS

We have developed a population of almost 50,000 activation tagged A. thaliana lines that have been archived as individual lines to the T3 generation. The population is an excellent tool for both reverse and forward genetic screens and has been used successfully to identify a number of novel mutants. Insertion site sequences have been generated and mapped for 15,507 lines to enable further application of the population, while providing a clear distribution of T-DNA insertions across the genome. The population is being screened for a number of biochemical and developmental phenotypes, provisional data identifying novel alleles and genes controlling steps in proanthocyanidin biosynthesis and trichome development is presented.

CONCLUSION

This publicly available population provides an additional tool for plant researcher's to assist with determining gene function for the many as yet uncharacterised genes annotated within the Arabidopsis genome sequence http://aafc-aac.usask.ca/FST. The presence of enhancer elements on the inserted T-DNA molecule allows both knock-out and dominant activation phenotypes to be identified for traits of interest.

Identification and localization of the bilitranslocase homologue in white grape berries (Vitis vinifera L.) during ripening

A homologue of the mammalian bilirubin transporter bilitranslocase (BTL) (TCDB 2.A.65.1.1), able to perform an apparent secondary active transport of flavonoids, has previously been found in carnation petals and red grape berries. In the present work, a BTL homologue was also shown in white berries from Vitis vinifera L. cv. Tocai/Friulano, using anti-sequence antibodies specific for rat liver BTL. This transporter, similarly to what found in red grape, was localized in the first layers of the epidermal tissue and in the vascular bundle cells of the mesocarp. In addition, a strong immunochemical reaction was detected in the placental tissue and particularly in peripheral integuments of the seed. The protein was expressed during the last maturation stages in both skin and pulp tissues and exhibited an apparent molecular mass of c. 31 kDa. Furthermore, the transport activity of such a carrier, measured as bromosulphophthalein (BSP) uptake, was detected in berry pulp microsomes, where it was inhibited by specific anti-BTL antibodies. The BTL homologue activity exhibited higher values, for both K(m) and V(max), than those found in the red cultivar. Moreover, two non-pigmented flavonoids, such as quercetin (a flavonol) and eriodictyol (a flavanone), inhibited the uptake of BSP in an uncompetitive manner. Such results strengthen the hypothesis that this BTL homologue acts as a carrier involved also in the membrane transport of colourless flavonoids and demonstrate the presence of such a carrier in different organs and tissues.

Grapevine MATE-type proteins act as vacuolar H+-dependent acylated anthocyanin transporters

In grapevine (Vitis vinifera), anthocyanins are responsible for most of the red, blue, and purple pigmentation found in the skin of berries. In cells, anthocyanins are synthesized in the cytoplasm and accumulated into the vacuole. However, little is known about the transport of these compounds through the tonoplast. Recently, the sequencing of the grapevine genome allowed us to identify genes encoding proteins with high sequence similarity to the Multidrug And Toxic Extrusion (MATE) family. Among them, we selected two genes as anthocyanin transporter candidates and named them anthoMATE1 (AM1) and AM3. The expression of both genes was mainly fruit specific and concomitant with the accumulation of anthocyanin pigment. Subcellular localization assays in grapevine hairy roots stably transformed with AM1 or AM3green fluorescent protein fusion protein revealed that AM1 and AM3 are primarily localized to the tonoplast. Yeast vesicles expressing anthoMATEs transported acylated anthocyanins in the presence of MgATP. Inhibitor studies demonstrated that AM1 and AM3 proteins act in vitro as vacuolar H(+)-dependent acylated anthocyanin transporters. By contrast, under our experimental conditions, anthoMATEs could not transport malvidin 3-O-glucoside or cyanidin 3-O-glucoside, suggesting that the acyl conjugation was essential for the uptake. Taken together, these results provide evidence that in vitro the two grapevine AM1 and AM3 proteins mediate specifically acylated anthocyanin transport.

Ectopic expression of VlmybA1 in grapevine activates a narrow set of genes involved in anthocyanin synthesis and transport

The colour of the red wine is essentially due to the release of anthocyanins from the red skin of grape berries during the process of wine making. Anthocyanins are synthesized during ripening of the berries under the control of VvMYBA1 transcription factor that controls the expression of UFGT. In order to identify the whole set of downstream regulated genes, we targeted constitutive ectopic expression of VlmybA1-2 into grapevine hairy roots and plants. The ectopic expression of VlmybA1-2 triggered de novo production and storage of anthocyanins in all transgenic vegetative organs, leading to a very intense red coloration, and did not interfere with proanthocyanidin (PA) biosynthesis. The ectopic red pigmentation was due to the accumulation of anthocyanins in vacuoles and anthocyanin vacuolar inclusion (AVIs) in all organs but only in specific tissues. A transcriptomic analysis using a 14 K oligoarray revealed that the ectopic expression of VlmybA1-2 activated only few genes, most of which are involved in both PA and anthocyanin biosynthesis, while the expression of BAN and LAR (two specific genes of the PA biosynthesis pathway) was unaffected. Among these, 4 genes emerged given the amplitude of their up-regulation, quantitatively similar to VlmybA1-2 itself. In addition to the previously described UFGT, this set comprised an isogen of GST, an O-methyltransferase, both of which are supposed to play a role in the anthocyanin biosynthesis pathway, as well as a candidate gene putatively involved in the vacuolar anthocyanin transport in grapevine (anthoMATE). Together, these results suggest that MybA1 activates the last steps of anthocyanin synthesis and transport through the regulation of a narrow, specific spectrum of genes regulated as a cluster.

Vacuolar transport of nicotine is mediated by a multidrug and toxic compound extrusion (MATE) transporter in Nicotiana tabacum

Alkaloids play a key role in plant defense mechanisms against pathogens and herbivores, but the plants themselves need to cope with their toxicity as well. The major alkaloid of the Nicotiana species, nicotine, is translocated via xylem transport from the root tissues where it is biosynthesized to the accumulation sites, the vacuoles of leaves. To unravel the molecular mechanisms behind this membrane transport, we characterized one transporter, the tobacco (Nicotiana tabacum) jasmonate-inducible alkaloid transporter 1 (Nt-JAT1), whose expression was coregulated with that of nicotine biosynthetic genes in methyl jasmonate-treated tobacco cells. Nt-JAT1, belonging to the family of multidrug and toxic compound extrusion transporters, was expressed in roots, stems, and leaves, and localized in the tonoplast of leaf cells. When produced in yeast cells, Nt-JAT1 occurred mainly in the plasma membrane and showed nicotine efflux activity. Biochemical analysis with proteoliposomes reconstituted with purified Nt-JAT1 and bacterial F(0)F(1)-ATPase revealed that Nt-JAT1 functioned as a proton antiporter and recognized endogenous tobacco alkaloids, such as nicotine and anabasine, and other alkaloids, such as hyoscyamine and berberine, but not flavonoids. These findings strongly suggest that Nt-JAT1 plays an important role in the nicotine translocation by acting as a secondary transporter responsible for unloading of alkaloids in the aerial parts and deposition in the vacuoles.

Ectopic expression of VvMybPA2 promotes proanthocyanidin biosynthesis in grapevine and suggests additional targets in the pathway

Grapevine (Vitis vinifera) proanthocyanidins contribute to plant defense mechanisms against biotic stress and also play a critical role in organoleptic properties of wine. In grapevine berry, these compounds are mainly accumulated in exocarps and seeds in the very early stages of development. A previous study has already identified VvMybPA1 as the first transcription factor involved in the regulation of the proanthocyanidin pathway during seed development in grapevine. A novel Myb factor, VvMybPA2, which is described in this study, is in contrast mainly expressed in the exocarp of young berries and in the leaves. This transcription factor shows very high protein sequence homology with other plant Myb factors, which regulate flavonoid biosynthesis. Ectopic expression of either VvMybPA1 or VvMybPA2 in grapevine hairy roots induced qualitative and quantitative changes of the proanthocyanidin profiles. High-throughput transcriptomic analyses of transformed grapevine organs identified a large set of putative targets of the VvMybPA1 and VvMybPA2 transcription factors. Both genes significantly activated enzymes of the flavonoid pathway, including anthocyanidin reductase and leucoanthocyanidin reductase 1, the specific terminal steps in the biosynthesis of epicatechin and catechin, respectively, but not leucoanthocyanidin reductase 2. The functional annotation of the genes whose expression was modified revealed putative new actors of the proanthocyanidin pathway, such as glucosyltransferases and transporters.

Aluminum-activated citrate and malate transporters from the MATE and ALMT families function independently to confer Arabidopsis aluminum tolerance

Aluminum-activated root malate and citrate exudation play an important role in plant Al tolerance. This paper characterizes AtMATE, a homolog of the recently discovered sorghum and barley Al-tolerance genes, shown here to encode an Al-activated citrate transporter in Arabidopsis. Together with the previously characterized Al-activated malate transporter, AtALMT1, this discovery allowed us to examine the relationship in the same species between members of the two gene families for which Al-tolerance genes have been identified. AtMATE is expressed primarily in roots and is induced by Al. An AtMATE T-DNA knockdown line exhibited very low AtMATE expression and Al-activated root citrate exudation was abolished. The AtALMT1 AtMATE double mutant lacked both Al-activated root malate and citrate exudation and showed greater Al sensitivity than the AtALMT1 mutant. Therefore, although AtALMT1 is a major contributor to Arabidopsis Al tolerance, AtMATE also makes a significant but smaller contribution. The expression patterns of AtALMT1 and AtMATE and the profiles of Al-activated root citrate and malate exudation are not affected by the presence or absence of the other gene. These results suggest that AtALMT1-mediated malate exudation and AtMATE-mediated citrate exudation evolved independently to confer Al tolerance in Arabidopsis. However, a link between regulation of expression of the two transporters in response to Al was identified through work on STOP1, a transcription factor that was previously shown to be necessary for AtALMT1 expression. Here we show that STOP1 is also required for AtMATE expression and Al-activated citrate exudation.

Multidrug and toxic compound extrusion-type transporters implicated in vacuolar sequestration of nicotine in tobacco roots

Nicotine is a major alkaloid accumulating in the vacuole of tobacco (Nicotiana tabacum), but the transporters involved in the vacuolar sequestration are not known. We here report that tobacco genes (NtMATE1 and NtMATE2) encoding transporters of the multidrug and toxic compound extrusion (MATE) family are coordinately regulated with structural genes for nicotine biosynthesis in the root, with respect to spatial expression patterns, regulation by NIC regulatory loci, and induction by methyl jasmonate. Subcellular fractionation, immunogold electron microscopy, and expression of a green fluorescent protein fusion protein all suggested that these transporters are localized to the vacuolar membrane. Reduced expression of the transporters rendered tobacco plants more sensitive to the application of nicotine. In contrast, overexpression of NtMATE1 in cultured tobacco cells induced strong acidification of the cytoplasm after jasmonate elicitation or after the addition of nicotine under nonelicited conditions. Expression of NtMATE1 in yeast (Saccharomyces cerevisiae) cells compromised the accumulation of exogenously supplied nicotine into the yeast cells. The results imply that these MATE-type proteins transport tobacco alkaloids from the cytosol into the vacuole in exchange for protons in alkaloid-synthesizing root cells.

OsFRDL1 is a citrate transporter required for efficient translocation of iron in rice

Multidrug and toxic compound extrusion (MATE) transporters represent a large family in plants, but their functions are poorly understood. Here, we report the function of a rice (Oryza sativa) MATE gene (Os03g0216700, OsFRDL1), the closest homolog of barley (Hordeum vulgare) HvAACT1 (aluminum [Al]-activated citrate transporter 1), in terms of metal stress (iron [Fe] deficiency and Al toxicity). This gene was mainly expressed in the roots and the expression level was not affected by either Fe deficiency or Al toxicity. Knockout of this gene resulted in leaf chlorosis, lower leaf Fe concentration, higher accumulation of zinc and manganese concentration in the leaves, and precipitation of Fe in the root's stele. The concentration of citrate and ferric iron in the xylem sap was lower in the knockout line compared to the wild-type rice. Heterologous expression of OsFRDL1 in Xenopus oocytes showed transport activity for citrate. Immunostaining showed that OsFRDL1 was localized at the pericycle cells of the roots. On the other hand, there was no difference in the Al-induced secretion of citrate from the roots between the knockout line and the wild-type rice. Taken together, our results indicate that OsFRDL1 is a citrate transporter localized at the pericycle cells, which is necessary for efficient translocation of Fe to the shoot as a Fe-citrate complex.

TRANSPARENT TESTA 12 genes from Brassica napus and parental species: cloning, evolution, and differential involvement in yellow seed trait

Molecular dissection of the Brassica yellow seed trait has been the subject of intense investigation. Arabidopsis thaliana TRANSPARENT TESTA 12 (AtTT12) encodes a multidrug and toxic compound extrusion (MATE) transporter involved in seed coat pigmentation. Two, one, and one full-length TT12 genes were isolated from B. napus, B. oleracea, and B. rapa, respectively, and Southern hybridization confirmed these gene numbers, implying loss of some of the triplicated TT12 genes in Brassica. BnTT12-1, BnTT12-2, BoTT12, and BrTT12 are 2,714, 3,062, 4,760, and 2,716 bp, with the longest mRNAs of 1,749, 1,711, 1,739, and 1,752 bp, respectively. All genes contained alternative transcriptional start and polyadenylation sites. BrTT12 and BoTT12 are the progenitors of BnTT12-1 and BnTT12-2, respectively, validating B. napus as an amphidiploid. All Brassica TT12 proteins displayed high levels of identity (>99%) to each other and to AtTT12 (>92%). Brassica TT12 genes resembled AtTT12 in such basic features as MatE/NorM CDs, subcellular localization, transmembrane helices, and phosphorylation sites. Plant TT12 orthologs differ from other MATE proteins by two specific motifs. Like AtTT12, all Brassica TT12 genes are most highly expressed in developing seeds. However, a range of organ specificity was observed with BnTT12 genes being less organ-specific. TT12 expression is absent in B. rapa yellow-seeded line 06K124, but not downregulated in B. oleracea yellow-seeded line 06K165. In B. napus yellow-seeded line L2, BnTT12-2 expression is absent, whereas BnTT12-1 is expressed normally. Among Brassica species, TT12 genes are differentially related to the yellow seed trait. The molecular basis for the yellow seed trait, in Brassica, and the theoretical and practical implications of the highly variable intron 1 of these TT12 genes are discussed.

Biosynthesis and genetic regulation of proanthocyanidins in plants

Proanthocyanidins (PAs), also known as condensed tannins, are a group of polyphenolic secondary metabolites synthesized in plants as oligomers or polymers of flavan-3-ol units via the flavonoid pathway. Due to their structural complexity and varied composition, only in the recent years has the study on the biosynthesis and regulation of PAs in plants taken off, although some details of the synthetic mechanism remain unclear. This paper aims to summarize the status of research on the structures of PAs in plants, the genes encoding key enzymes of biosynthetic pathway, the transport factors, the transcriptional regulation of PA biosynthesis and the genetic manipulation of PAs. The problems of this field were also discussed, including the nature of the final "enzyme" which catalyzes the polymerization reaction of PAs and the possible mechanism of how the elementary units of flavanols are assembled in vivo.

Functional analysis of LjSUT4, a vacuolar sucrose transporter from Lotus japonicus

Sucrose transporters in the SUT family are important for phloem loading and sucrose uptake into sink tissues. The recent localization of type III SUTs AtSUT4 and HvSUT2 to the vacuole membrane suggests that SUTs also function in vacuolar sucrose transport. The transport mechanism of type III SUTs has not been analyzed in detail. LjSUT4, a type III sucrose transporter homolog from Lotus japonicus, is expressed in nodules and its transport activity has not been previously investigated. In this report, LjSUT4 was expressed in Xenopus oocytes and its transport activity assayed by two-electrode voltage clamping. LjSUT4 transported a range of glucosides including sucrose, salicin, helicin, maltose, sucralose and both alpha- and beta-linked synthetic phenyl glucosides. In contrast to other sucrose transporters, LjSUT4 did not transport the plant glucosides arbutin, fraxin and esculin. LjSUT4 showed a low affinity for sucrose (K(0.5)=16 mM at pH 5.3). In addition to inward currents induced by sucrose, other evidence also indicated that LjSUT4 is a proton-coupled symporter: (14)C-sucrose uptake into LjSUT4-expressing oocytes was inhibited by CCCP and sucrose induced membrane depolarization in LjSUT4-expressing oocytes. A GFP-fusion of LjSUT4 localized to the vacuole membrane in Arabidopsis thaliana and in the roots and nodules of Medicago truncatula. Based on these results we propose that LjSUT4 functions in the proton-coupled uptake of sucrose and possibly other glucosides into the cytoplasm from the vacuole.

A transcript profiling approach reveals an epicatechin-specific glucosyltransferase expressed in the seed coat of Medicago truncatula

Expression of the Arabidopsis TRANSPARENT TESTA 2 (TT2) MYB family transcription factor leads to massive accumulation of proanthocyanidins (PAs) in hairy roots of Medicago truncatula. Microarray analysis showed that TT2 induces genes for flavonoid/PA biosynthesis, transcription factors, and a large number of genes of unknown function. A second microarray dataset identified genes that were preferentially expressed in the M. truncatula seed coat. Comparison of the two datasets defines target genes for steps that are yet unidentified in PA biosynthesis and accumulation. Of these genes, a glycosyltransferase, UGT72L1, was active specifically toward the PA precursor (-)-epicatechin, and its expression pattern in developing seeds correlated with the presence of epicatechin glucoside and accumulation of PAs. UGT72L1 may be involved in the production of epicatechin 3'-O-glucoside in the seed coat as a key step in PA biosynthesis or its regulation.

Trafficking and Sequestration of Anthocyanins

Flavonoids are synthesized on the cytoplasmic surface of the endoplasmic reticulum (ER). As is the case for several other phytochemicals, anthocyanins and other products of the pathway often accumulate in the large central vacuole. This review summarizes recent findings on the possible mechanisms by which flavonoids traffic between the ER and the vacuole, and discusses the frequent localization of anthocyanins in sub-vacuolar structures with variable characteristics.

Evidence for a putative flavonoid translocator similar to mammalian bilitranslocase in grape berries (Vitis vinifera L.) during ripening

During maturation, Vitis vinifera berries accumulate a large amount of several anthocyanins in the epidermal tissue, whereas their precursors and intermediates are ubiquitously synthesized within the fruit. Up to date, several mechanisms of flavonoid transport at subcellular level have been hypothesized, but it is not possible to identify a general model applicable in every plant tissue and organ. Recently, a putative anthocyanin carrier, homologue to mammalian bilitranslocase (BTL) (TC 2.A.65.1.1), was found in Dianthus caryophyllus petal microsomes. In the present paper, an immunohistochemical and immunochemical analysis, using an antibody raised against a BTL epitope, evidences the expression and function of such a transporter in V. vinifera berries (cv. Merlot). Specific localisations of the putative carrier within berry tissues together with expression changes during different developmental stages are shown. Water stress induces an increase in protein expression in both skin and pulp samples. A bromosulfalein (BSP) uptake activity, inhibitable by the BTL antibody, is detected in berry mesocarp microsomes, with K (m) = 2.39 microM BSP and V (max) = 0.29 micromol BSP min(-1) mg(-1) protein. This BSP uptake is also competitively inhibited by quercetin (K (i) = 4 microM). A putative role for this carrier is discussed in relation to the membrane transport of secondary metabolites.

Biosynthesis of plant pigments: anthocyanins, betalains and carotenoids

Plant compounds that are perceived by humans to have color are generally referred to as 'pigments'. Their varied structures and colors have long fascinated chemists and biologists, who have examined their chemical and physical properties, their mode of synthesis, and their physiological and ecological roles. Plant pigments also have a long history of use by humans. The major classes of plant pigments, with the exception of the chlorophylls, are reviewed here. Anthocyanins, a class of flavonoids derived ultimately from phenylalanine, are water-soluble, synthesized in the cytosol, and localized in vacuoles. They provide a wide range of colors ranging from orange/red to violet/blue. In addition to various modifications to their structures, their specific color also depends on co-pigments, metal ions and pH. They are widely distributed in the plant kingdom. The lipid-soluble, yellow-to-red carotenoids, a subclass of terpenoids, are also distributed ubiquitously in plants. They are synthesized in chloroplasts and are essential to the integrity of the photosynthetic apparatus. Betalains, also conferring yellow-to-red colors, are nitrogen-containing water-soluble compounds derived from tyrosine that are found only in a limited number of plant lineages. In contrast to anthocyanins and carotenoids, the biosynthetic pathway of betalains is only partially understood. All three classes of pigments act as visible signals to attract insects, birds and animals for pollination and seed dispersal. They also protect plants from damage caused by UV and visible light.

Functional and physiological characterization of Arabidopsis INOSITOL TRANSPORTER1, a novel tonoplast-localized transporter for myo-inositol

Arabidopsis thaliana INOSITOL TRANSPORTER1 (INT1) is a member of a small gene family with only three more genes (INT2 to INT4). INT2 and INT4 were shown to encode plasma membrane-localized transporters for different inositol epimers, and INT3 was characterized as a pseudogene. Here, we present the functional and physiological characterization of the INT1 protein, analyses of the tissue-specific expression of the INT1 gene, and analyses of phenotypic differences observed between wild-type plants and mutant lines carrying the int1.1 and int1.2 alleles. INT1 is a ubiquitously expressed gene, and Arabidopsis lines with T-DNA insertions in INT1 showed increased intracellular myo-inositol concentrations and reduced root growth. In Arabidopsis, tobacco (Nicotiana tabacum), and Saccharomyces cerevisiae, fusions of the green fluorescent protein to the C terminus of INT1 were targeted to the tonoplast membranes. Finally, patch-clamp analyses were performed on vacuoles from wild-type plants and from both int1 mutant lines to study the transport properties of INT1 at the tonoplast. In summary, the presented molecular, physiological, and functional studies demonstrate that INT1 is a tonoplast-localized H(+)/inositol symporter that mediates the efflux of inositol that is generated during the degradation of inositol-containing compounds in the vacuolar lumen.

Different mechanisms for phytoalexin induction by pathogen and wound signals in Medicago truncatula

Cell suspensions of the model legume Medicago truncatula accumulated the isoflavonoid phytoalexin medicarpin in response to yeast elicitor or methyl jasmonate (MJ), accompanied by decreased levels of isoflavone glycosides in MJ-treated cells. DNA microarray analysis revealed rapid, massive induction of early (iso)flavonoid pathway gene transcripts in response to yeast elicitor, but not MJ, and differential induction by the two elicitors of sets of genes encoding transcription factors, ABC transporters, and beta-glucosidases. In contrast, both elicitors induced genes encoding enzymes for conversion of the isoflavone formononetin to medicarpin. Four MJ-induced beta-glucosidases were expressed as recombinant enzymes in yeast, and three were active with isoflavone glucosides. The most highly induced beta-glucosidase was nuclear localized and preferred flavones to isoflavones. The results indicate that the genetic and biochemical mechanisms underlying accumulation of medicarpin differ depending on the nature of the stimulus and suggest a role for MJ as a signal for rapid hydrolysis of preformed, conjugated intermediates for antimicrobial biosynthesis during wound responses.