The 'ins' and 'outs' of flavonoid transport

Using genome-wide associations to identify metabolic pathways involved in maize aflatoxin accumulation resistance

BACKGROUND

Aflatoxin is a potent carcinogen that can contaminate grain infected with the fungus Aspergillus flavus. However, resistance to aflatoxin accumulation in maize is a complex trait with low heritability. Here, two complementary analyses were performed to better understand the mechanisms involved. The first coupled results of a genome-wide association study (GWAS) that accounted for linkage disequilibrium among single nucleotide polymorphisms (SNPs) with gene-set enrichment for a pathway-based approach. The rationale was that the cumulative effects of genes in a pathway would give insight into genetic differences that distinguish resistant from susceptible lines of maize. The second involved finding non-pathway genes close to the most significant SNP-trait associations with the greatest effect on reducing aflatoxin in multiple environments. Unlike conventional GWAS, the latter analysis emphasized multiple aspects of SNP-trait associations rather than just significance and was performed because of the high genotype x environment variability exhibited by this trait.

RESULTS

The most significant metabolic pathway identified was jasmonic acid (JA) biosynthesis. Specifically, there was at least one allelic variant for each step in the JA biosynthesis pathway that conferred an incremental decrease to the level of aflatoxin observed among the inbred lines in the GWAS panel. Several non-pathway genes were also consistently associated with lowered aflatoxin levels. Those with predicted functions related to defense were: leucine-rich repeat protein kinase, expansin B3, reversion-to-ethylene sensitivity1, adaptor protein complex2, and a multidrug and toxic compound extrusion protein.

CONCLUSIONS

Our genetic analysis provided strong evidence for several genes that were associated with aflatoxin resistance. Inbred lines that exhibited lower levels of aflatoxin accumulation tended to share similar haplotypes for genes specifically in the pathway of JA biosynthesis, along with several non-pathway genes with putative defense-related functions. Knowledge gained from these two complementary analyses has improved our understanding of population differences in aflatoxin resistance.

De novo sequencing and analysis of the cranberry fruit transcriptome to identify putative genes involved in flavonoid biosynthesis, transport and regulation

BACKGROUND

Cranberries (Vaccinium macrocarpon Ait.), renowned for their excellent health benefits, are an important berry crop. Here, we performed transcriptome sequencing of one cranberry cultivar, from fruits at two different developmental stages, on the Illumina HiSeq 2000 platform. Our main goals were to identify putative genes for major metabolic pathways of bioactive compounds and compare the expression patterns between white fruit (W) and red fruit (R) in cranberry.

RESULTS

In this study, two cDNA libraries of W and R were constructed. Approximately 119 million raw sequencing reads were generated and assembled de novo, yielding 57,331 high quality unigenes with an average length of 739 bp. Using BLASTx, 38,460 unigenes were identified as putative homologs of annotated sequences in public protein databases, including NCBI NR, NT, Swiss-Prot, KEGG, COG and GO. Of these, 21,898 unigenes mapped to 128 KEGG pathways, with the metabolic pathways, secondary metabolites, glycerophospholipid metabolism, ether lipid metabolism, starch and sucrose metabolism, purine metabolism, and pyrimidine metabolism being well represented. Among them, many candidate genes were involved in flavonoid biosynthesis, transport and regulation. Furthermore, digital gene expression (DEG) analysis identified 3,257 unigenes that were differentially expressed between the two fruit developmental stages. In addition, 14,473 simple sequence repeats (SSRs) were detected.

CONCLUSIONS

Our results present comprehensive gene expression information about the cranberry fruit transcriptome that could facilitate our understanding of the molecular mechanisms of fruit development in cranberries. Although it will be necessary to validate the functions carried out by these genes, these results could be used to improve the quality of breeding programs for the cranberry and related species.

Rethinking how volatiles are released from plant cells

For plant volatile organic compounds (VOCs) to be emitted, they must cross membrane(s), the aqueous cell wall, and sometimes the cuticle, before moving into the gas phase. It is presumed that VOC movement through each barrier occurs via passive diffusion. However, VOCs, which are primarily nonpolar compounds, will preferentially partition into membranes, making diffusion into aqueous compartments slow. Using Fick's first law, we calculated that to achieve observed VOC emission rates by diffusion alone would necessitate toxic VOC levels in membranes. Here, we propose that biological mechanisms, such as those involved in trafficking other hydrophobic compounds, must contribute to VOC emission. Such parallel biological pathways would lower barrier resistances and, thus, steady-state emission rates could be maintained with significantly reduced intramembrane VOC concentrations.

Flavonoid transport mechanisms: how to go, and with whom

Subcellular flavonoid transport and its underlying regulatory mechanisms are still poorly understood, but are fascinating research frontiers in plant science. Recent studies support and further extend previous hypotheses indicating that vacuolar sequestration of flavonoids involves vesicle trafficking, membrane transporters, and glutathione S-transferase (GST). However, the question remains to be addressed of how three distinct but nonexclusive mechanisms are functionally integrated into diverse but redundant transport routes for vacuolar sequestration or extracellular secretion of flavonoids. In this review, I highlight recent progress in understanding flavonoid-transporting vesicle behavior and properties, GST and membrane transporter functions and mechanisms, and flavonoid transport substrate specificity and preference.

Natural variation in flavonol and anthocyanin metabolism during cold acclimation in Arabidopsis thaliana accessions

In plants from temperate climates such as Arabidopsis thaliana, low, non-freezing temperatures lead to increased freezing tolerance in a process termed cold acclimation. During cold acclimation, massive changes in gene expression and in the content of primary metabolites and lipids have been observed. Here, we have analysed the influence of cold acclimation on flavonol and anthocyanin content and on the expression of genes related to flavonoid metabolism in 54 Arabidopsis accessions covering a wide range of freezing tolerance. Most flavonols and anthocyanins accumulated upon cold exposure, but the extent of accumulation varied strongly among the accessions. This was also true for most of the investigated transcripts. Correlation analyses revealed a high degree of coordination among metabolites and among transcripts, but only little correlation between metabolites and transcripts, indicating an important role of post-transcriptional regulation in flavonoid metabolism. Similarly, levels of many flavonoid biosynthesis genes were correlated with freezing tolerance after cold acclimation, but only the pool sizes of a few flavonols and anthocyanins. Collectively, our data provide evidence for an important role of flavonoid metabolism in Arabidopsis freezing tolerance and point to the importance of post-transcriptional mechanisms in the regulation of flavonoid metabolism in response to cold.

Regulation of flavonol content and composition in (Syrah×Pinot Noir) mature grapes: integration of transcriptional profiling and metabolic quantitative trait locus analyses

Flavonols are a ubiquitous class of flavonoids that accumulate preferentially in flowers and mature berries. Besides their photo-protective function, they play a fundamental role during winemaking, stabilizing the colour by co-pigmentation with anthocyanins and contributing to organoleptic characteristics. Although the general flavonol pathway has been genetically and biochemically elucidated, the genetic control of flavonol content and composition at harvest is still not clear. To this purpose, the grapes of 170 segregating F1 individuals from a 'Syrah'×'Pinot Noir' population were evaluated at the mature stage for the content of six flavonol aglycons in four seasons. Metabolic data in combination with genetic data enabled the identification of 16 mQTLs (metabolic quantitative trait loci). For the first time, major genetic control by the linkage group 2 (LG 2)/MYBA region on flavonol variation, in particular of tri-hydroxylated flavonols, is demonstrated. Moreover, seven regions specifically associated with the fine control of flavonol biosynthesis are identified. Gene expression profiling of two groups of individuals significantly divergent for their skin flavonol content identified a large set of differentially modulated transcripts. Among these, the transcripts coding for MYB and bZIP transcription factors, methyltranferases, and glucosyltranferases specific for flavonols, proteins, and factors belonging to the UV-B signalling pathway and co-localizing with the QTL regions are proposed as candidate genes for the fine regulation of flavonol content and composition in mature grapes.

New candidate genes for the fine regulation of the colour of grapes

In the last decade, great progress has been made in clarifying the main determinants of anthocyanin accumulation in grape berry skin. However, the molecular details of the fine variation among cultivars, which ultimately contributes to wine typicity, are still not completely understood. To shed light on this issue, the grapes of 170 F1 progeny from the cross 'Syrah'×'Pinot Noir' were characterized at the mature stage for the content of 15 anthocyanins during four growing seasons. This huge data set was used in combination with a dense genetic map to detect genomic regions controlling the anthocyanin pathway both at key enzymatic points and at particular branches. Genes putatively involved in fine tuning the global regulation of anthocyanin biosynthesis were identified by exploring the gene predictions in the QTL (quantitative trait locus) confidence intervals and their expression profile during berry development in offspring with contrasting anthocyanin accumulation. New information on some aspects which had scarcely been investigated so far, such as anthocyanin transport into the vacuole, or completely neglected, such as acylation, is provided. These genes represent a valuable resource in grapevine molecular-based breeding programmes to improve both fruit and wine quality and to tailor wine sensory properties according to consumer demand.

TRANSPARENT TESTA 13 is a tonoplast P3A -ATPase required for vacuolar deposition of proanthocyanidins in Arabidopsis thaliana seeds

Intracellular pH homeostasis is essential for all living cells. In plants, pH is usually maintained by three structurally distinct and differentially localized types of proton pump: P-type H(+) -ATPases in the plasma membrane, and multimeric vacuolar-type H(+) -ATPases (V-ATPases) and vacuolar H(+) -pyrophosphatases (H(+) -PPases) in endomembranes. Here, we show that reduced accumulation of proanthocyanidins (PAs) and hence the diminished brown seed coloration found in the Arabidopsis thaliana mutant transparent testa 13 (tt13) is caused by disruption of the gene encoding the P3A -ATPase AHA10. Identification of the gene encoded by the tt13 locus completes the molecular characterization of the classical set of transparent testa mutants. Cells of the tt13 seed coat endothelium do not contain PA-filled central vacuoles as observed in the wild-type. tt13 phenocopies tt12, a mutant that is defective in vacuolar import of the PA precursor epicatechin. Our data show that vacuolar loading with PA precursors depends on TT13. Consistent with the tt13 phenotype, but in contrast to other isoforms of P-type H(+) -ATPases, TT13 localizes to the tonoplast. PA accumulation in tt13 is partially restored by expression of the tonoplast localized H(+) -PPase VHP1. Our findings indicate that the P3A -ATPase TT13 functions as a proton pump in the tonoplast of seed coat endothelium cells, and generates the driving force for TT12-mediated transport of PA precursors to the vacuole.

Metabolomic profiling in tomato reveals diel compositional changes in fruit affected by source-sink relationships

A detailed study of the diurnal compositional changes was performed in tomato (Solanum lycopersicum cv. Moneymaker) leaves and fruits. Plants were cultivated in a commercial greenhouse under two growth conditions: control and shaded. Expanding fruits and the closest mature leaves were harvested during two different day/night cycles (cloudy or sunny day). High-throughput robotized biochemical phenotyping of major compounds, as well as proton nuclear magnetic resonance and mass spectrometry metabolomic profiling, were used to measure the contents of about 70 metabolites in the leaves and 60 metabolites in the fruits, in parallel with ecophysiological measurements. Metabolite data were processed using multivariate, univariate, or clustering analyses and correlation networks. The shaded carbon-limited plants adjusted their leaf area, decreased their sink carbon demand and showed subtle compositional modifications. For source leaves, several metabolites varied along a diel cycle, including those directly linked to photosynthesis and photorespiration. These metabolites peaked at midday in both conditions and diel cycles as expected. However, transitory carbon storage was limited in tomato leaves. In fruits, fewer metabolites showed diel fluctuations, which were also of lower amplitude. Several organic acids were among the fluctuating metabolites. Diel patterns observed in leaves and especially in fruits differed between the cloudy and sunny days, and between the two conditions. Relationships between compositional changes in leaves and fruits are in agreement with the fact that several metabolic processes of the fruit appeared linked to its momentary supply of sucrose.

The State of the Art in Biosynthesis of Anthocyanins and Its Regulation in Pigmented Sweet Oranges [(Citrus sinensis) L. Osbeck]

Anthocyanins are water-soluble pigments belonging to the flavonoid compound family involved in nature in several aspects of plant development and defense. By bestowing much of the color and flavor on fruits and vegetables, they are components of the human diet and, thanks to their radical-scavenging properties, are not considered exclusively as food products but also as therapeutic agents. Several cultivars of red (or blood) oranges [Citrus sinensis (L.) Osbeck], such as Tarocco, Moro, and Sanguinello, are characterized by the presence of anthocyanins in both the rind and fruit juice vesicles. The amount and composition of anthocyanins in the pigmented orange cultivar vary greatly depending on variety, maturity, region of cultivation, and many other environmental conditions. Most of the blood orange varieties require a wide day-night thermal range to maximize color formation. Therefore, the production of red oranges characterized by high anthocyanin levels is limited to a few regions and in particular to the Sicilian area around Mount Etna in Italy, where the characteristic climate conditions yield fruits of unique color intensity and quality. In this review, both the basic information and the most recent advances in red orange anthocyanins are reported, with intense attention given to their biosynthesis and regulation.

Transcriptional control of flavonoid biosynthesis by MYB-bHLH-WDR complexes

Flavonoids are widely known for the colors they confer to plant tissues, their contribution to plant fitness and health benefits, and impact on food quality. As convenient biological markers, flavonoids have been instrumental in major genetic and epigenetic discoveries. We review recent advances in the characterization of the underlying regulatory mechanisms of flavonoid biosynthesis, with a special focus on the MBW (MYB-bHLH-WDR) protein complexes. These proteins are well conserved in higher plants. They participate in different types of controls ranging from fine-tuned transcriptional regulation by environmental factors to the initiation of the flavonoid biosynthesis pathway by positive regulatory feedback. The MBW protein complexes provide interesting models for investigating developmentally or environmentally controlled transcriptional regulatory networks.

SNARE-RNAi results in higher terpene emission from ectopically expressed caryophyllene synthase in Nicotiana benthamiana

Plants produce numerous terpenes and much effort has been dedicated to the identification and characterization of the terpene biosynthetic genes. However, little is known about how terpenes are transported within the cell and from the cell into the apoplast. To investigate a putative role of vesicle fusion in this process, we used Agrobacterium tumefaciens-mediated transient coexpression in Nicotiana benthamiana of an MtVAMP721e-RNAi construct (Vi) with either a caryophyllene synthase or a linalool synthase, respectively. Headspace analysis of the leaves showed that caryophyllene or linalool emission increased about five-fold when N. benthamiana VAMP72 function was blocked. RNA sequencing and protein ubiquitination analysis of the agroinfiltrated N. benthamiana leaf extracts suggested that increased terpene emissions may be attributed to proteasome malfunction based on three observations: leaves with TPS+Vi showed (1) a higher level of a DsRed marker protein, (2) a higher level of ubiquitinated proteins, and (3) coordinated induced expression of multiple proteasome genes, presumably caused by the lack of proteasome-mediated feedback regulation. However, caryophyllene or linalool did not inhibit proteasome-related protease activity in the in vitro assays. While the results are not conclusive for a role of vesicle fusion in terpene transport, they do show a strong interaction between inhibition of vesicle fusion and ectopic expression of certain terpenes. The results have potential applications in metabolic engineering.

Arabidopsis thaliana β-glucosidase BGLU15 attacks flavonol 3-O-β-glucoside-7-O-α-rhamnosides

Kaempferol and quercetin 3-O-β-glucoside-7-O-α-rhamnoside (K3G7R and Q3G7R, respectively) are major flavonol bisglycosides accumulating in Arabidopsis thaliana with synergistic abiotic stresses (i.e., nitrogen deficiency and low temperature, NDLT). However, these molecules disappear rapidly during recovery from NDLT. Typically, catabolism of related chemicals relies on β-glucosidase (BGLU) action. Evidence for flavonol 3-O-β-glucoside-7-O-α-rhamnoside BGLU activity is provided here. Major losses of Q3G7R and K3G7R coincided with an approximate 250% induction in flavonol 3-O-β-glucoside-7-O-α-rhamnoside BGLU activity within 2days of NDLT recovery relative to plants cultured under nitrogen sufficiency and high temperature (NSHT, control). QTOF-MS/MS established the product of Q3G7R hydrolysis in the presence of Arabidopsis cell free extracts was quercetin 7-O-α-rhamnoside. A phylogenetic analysis of the Arabidopsis glycoside hydrolase family 1 identified BGLU15 (At2g44450) and five other members that cluster with Fabaceae hydrolases known to attack isoflavones and isoflavonoids, which are structurally somewhat related to flavonol 3-O-β-glucoside-7-O-α-rhamnosides. Real time quantitative PCR analysis established a 300% higher expression of BGLU15 within 1day of the recovery from NDLT relative to control plants; lower or negligible changes in expression were evident for the remaining BGLUs. Recombinant thioredoxin-His6-tagged mature BGLU15 protein was expressed in Escherichia coli and purified to homogeneity. A comparison of a wide spectrum of β-glucosides showed that recombinant BGLU15 preferentially hydrolyses the 3-O-β-glucosides of flavonols, but does not attack quercetin 3-O-α-rhamnoside, quercetin 3-O-β-galactoside and rutin. BGLU15 displayed the highest catalytic efficiency for Q3G7R and K3G7R yielding their respective 7-O-rhamnosides as products; flavonol 3-O-glucosides were also attacked, albeit with lower efficiency. Together, it appears the loss of flavonol 3-O-β-glucoside-7-O-α-rhamnosides in Arabidopsis is dependent upon the enzyme-mediated cleavage of the 3-O-β linked glucose moiety.

Integration of a Decrescent Transcriptome and Metabolomics Dataset of Peucedanum praeruptorum to Investigate the CYP450 and MDR Genes Involved in Coumarins Biosynthesis and Transport

Peucedanum praeruptorum Dunn is well-known traditional Chinese medicine. However, little is known in the biosynthesis and the transport mechanisms of its coumarin compounds at the molecular level. Although transcriptomic sequence is playing an increasingly significant role in gene discovery, it is not sufficient in predicting the specific function of target gene. Furthermore, there is also a huge database to be analyzed. In this study, RNA sequencing assisted transcriptome dataset and high-performance liquid chromatography (HPLC) coupled with electrospray-ionization quadrupole time-of-flight mass spectrometry (Q-TOF MS)-based metabolomics dataset of P. praeruptorum were firstly constructed for gene discovery and compound identification. Subsequently, methyl jasmonate (MeJA)-induced gene expression analysis and metabolomics analysis were conducted to narrow-down the dataset for selecting the candidate genes and the potential marker metabolites. Finally, the genes involved in coumarins biosynthesis and transport were predicted with parallel analysis of transcript and metabolic profiles. As a result, a total of 40,952 unigenes and 19 coumarin compounds were obtained. Based on the results of gene expression and metabolomics analysis, 7 cytochrome-P450 and 8 multidrug resistance transporter unigenes were selected as candidate genes and 8 marker compounds were selected as biomarkers, respectively. The parallel analysis of gene expression and metabolites accumulation indicated that the gene labeled as 23,746, 228, and 30,922 were related to the formation of the coumarin core compounds whereas 36,276 and 9533 participated in the prenylation, hydroxylation, cyclization or structural modification. Similarly, 1462, 20,815, and 15,318 participated in the transport of coumarin core compounds while 124,029 and 324,293 participated in the transport of the modified compounds. This finding suggested that integration of a decrescent transcriptome and metabolomics dataset could largely narrow down the number of gene to be investigated and significantly improve the efficiency of functional gene predication. In addition, the large amount of transcriptomic data produced from P. praeruptorum and the genes discovered in this study would provide useful information in investigating the biosynthesis and transport mechanism of coumarins.

Metabolic engineering to enhance the value of plants as green factories

The promise of plants to serve as the green factories of the future is ever increasing. Plants have been used traditionally for construction, energy, food and feed. Bioactive compounds primarily derived from specialized plant metabolism continue to serve as important scaffold molecules for pharmaceutical drug production. Yet, the past few years have witnessed a growing interest on plants as the ultimate harvesters of carbon and energy from the sun, providing carbohydrate and lipid biofuels that would contribute to balancing atmospheric carbon. How can the metabolic output from plants be increased even further, and what are the bottlenecks? Here, we present what we perceive to be the main opportunities and challenges associated with increasing the efficiency of plants as chemical factories. We offer some perspectives on when it makes sense to use plants as production systems because the amount of biomass needed makes any other system unfeasible. However, there are other instances in which plants serve as great sources of biological catalysts, yet are not necessarily the best-suited systems for production. We also present emerging opportunities for manipulating plant genomes to make plant synthetic biology a reality.

Transient silencing of CHALCONE SYNTHASE during fruit ripening modifies tomato epidermal cells and cuticle properties

Tomato (Solanum lycopersicum) fruit ripening is accompanied by an increase in CHALCONE SYNTHASE (CHS) activity and flavonoid biosynthesis. Flavonoids accumulate in the cuticle, giving its characteristic orange color that contributes to the eventual red color of the ripe fruit. Using virus-induced gene silencing in fruits, we have down-regulated the expression of SlCHS during ripening and compared the cuticles derived from silenced and nonsilenced regions. Silenced regions showed a pink color due to the lack of flavonoids incorporated to the cuticle. This change in color was accompanied by several other changes in the cuticle and epidermis. The epidermal cells displayed a decreased tangential cell width; a decrease in the amount of cuticle and its main components, cutin and polysaccharides, was also observed. Flavonoids dramatically altered the cuticle biomechanical properties by stiffening the elastic and viscoelastic phase and by reducing the ability of the cuticle to deform. There seemed to be a negative relation between SlCHS expression and wax accumulation during ripening that could be related to the decreased cuticle permeability to water observed in the regions silencing SlCHS. A reduction in the overall number of ester linkages present in the cutin matrix was also dependent on the presence of flavonoids.

GFS9/TT9 contributes to intracellular membrane trafficking and flavonoid accumulation in Arabidopsis thaliana

Flavonoids are the most important pigments for the coloration of flowers and seeds. In plant cells, flavonoids are synthesized by a multi-enzyme complex located on the cytosolic surface of the endoplasmic reticulum, and they accumulate in vacuoles. Two non-exclusive pathways have been proposed to mediate flavonoid transport to vacuoles: the membrane transporter-mediated pathway and the vesicle trafficking-mediated pathway. No molecules involved in the vesicle trafficking-mediated pathway have been identified, however. Here, we show that a membrane trafficking factor, GFS9, has a role in flavonoid accumulation in the vacuole. We screened a library of Arabidopsis thaliana mutants with defects in vesicle trafficking, and isolated the gfs9 mutant with abnormal pale tan-colored seeds caused by low flavonoid accumulation levels. gfs9 is allelic to the unidentified transparent testa mutant tt9. The responsible gene for these phenotypes encodes a previously uncharacterized protein containing a region that is conserved among eukaryotes. GFS9 is a peripheral membrane protein localized at the Golgi apparatus. GFS9 deficiency causes several membrane trafficking defects, including the mis-sorting of vacuolar proteins, vacuole fragmentation, the aggregation of enlarged vesicles, and the proliferation of autophagosome-like structures. These results suggest that GFS9 is required for vacuolar development through membrane fusion at vacuoles. Our findings introduce a concept that plants use GFS9-mediated membrane trafficking machinery for delivery of not only proteins but also phytochemicals, such as flavonoids, to vacuoles.

Unexpected roles for ancient proteins: flavone 8-hydroxylase in sweet basil trichomes is a Rieske-type, PAO-family oxygenase

Most elucidated hydroxylations in plant secondary metabolism are catalyzed by oxoglutarate- or cytochrome P450-dependent oxygenases. Numerous hydroxylations still evade clarification, suggesting that they might be performed by alternative enzyme types. Here, we report the identification of the flavone 8-hydroxylase (F8H) in sweet basil (Ocimum basilicum L.) trichomes as a Rieske-type oxygenase. Several features of the F8H activity in trichome protein extracts helped to differentiate it from a cytochrome P450-catalyzed reaction and identify candidate genes in the basil trichome EST database. The encoded ObF8H proteins share approximately 50% identity with Rieske-type protochlorophyllide a oxygenases (PTC52) from higher plants. Homology cloning and DNA blotting revealed the presence of several PTC52-like genes in the basil genome. The transcripts of the candidate gene designated ObF8H-1 are strongly enriched in trichomes compared to whole young leaves, indicating trichome-specific expression. The full-length ObF8H-1 protein possesses a predicted N-terminal transit peptide, which directs green fluorescent protein at least in part to chloroplasts. The F8H activity in crude trichome protein extracts correlates well with the abundance of ObF8H peptides. The purified recombinant ObF8H-1 displays high affinity for salvigenin and is inactive with other tested flavones except cirsimaritin, which is 8-hydroxylated with less than 0.2% relative activity. The efficiency of in vivo 8-hydroxylation by engineered yeast was improved by manipulation of protein subcellular targeting. blast searches showed that occurrence of several PTC52-like genes is rather common in sequenced plant genomes. The discovery of ObF8H suggests that Rieske-type oxygenases may represent overlooked candidate catalysts for oxygenations in specialized plant metabolism.

Multidrug and toxic compound extrusion protein-1 (MATE1/SLC47A1) is a novel flavonoid transporter

Dietary flavonoids have various biological functions. However, their cellular transport mechanisms are largely unknown. We have determined that the multidrug and toxic compound extrusion transporter-1 (MATE1) is a membrane transporter for flavonoids and has a high affinity for quercetin. HEK293T cells overexpressing MATE1 exhibited increased intracellular quercetin accumulation. This effect disappeared in the presence of a MATE1 inhibitor and after MATE1 gene knockdown. HepG2 cells expressed MATE1 significantly, with the uptake quercetin of which was dramatically reduced with MATE1 inhibition. On the basis of immunofluorescence analysis, MATE1 was highly expressed in peroxisomes and the endoplasmic reticulum (ER) as well as in plasma membranes in the liver and intestine, which suggests potential accumulation of quercetin in peroxisomes and the ER in these tissues. Fluorescent microscopic analysis confirmed selective accumulation of qurcetin in peroxisome. The effects of quercetin on cellular lipid reduction and glucose uptake were exaggerated with MATE1 overexpression. In conclusion, MATE1 is a membrane transporter for quercetin; its overexpression enhances the hypolipidemic activity of quercetin and cellular glucose transport. Considering the low bioavailability of quercetin, appropriate regulation of MATE1 expression may optimize cellular quercetin concentrations and promote health benefits.

Involvement of the leaf-specific multidrug and toxic compound extrusion (MATE) transporter Nt-JAT2 in vacuolar sequestration of nicotine in Nicotiana tabacum

Alkaloids play a key role in higher plant defense against pathogens and herbivores. Following its biosynthesis in root tissues, nicotine, the major alkaloid of Nicotiana species, is translocated via xylem transport toward the accumulation sites, leaf vacuoles. Our transcriptome analysis of methyl jasmonate-treated tobacco BY-2 cells identified several multidrug and toxic compound extrusion (MATE) transporter genes. In this study, we characterized a MATE gene, Nicotiana tabacum jasmonate-inducible alkaloid transporter 2 (Nt-JAT2), which encodes a protein that has 32% amino acid identity with Nt-JAT1. Nt-JAT2 mRNA is expressed at a very low steady state level in whole plants, but is rapidly upregulated by methyl jasmonate treatment in a leaf-specific manner. To characterize the function of Nt-JAT2, yeast cells were used as the host organism in a cellular transport assay. Nt-JAT2 was localized at the plasma membrane in yeast cells. When incubated in nicotine-containing medium, the nicotine content in Nt-JAT2-expressing cells was significantly lower than in control yeast. Nt-JAT2-expressing cells also showed lower content of other alkaloids like anabasine and anatabine, but not of flavonoids, suggesting that Nt-JAT2 transports various alkaloids including nicotine. Fluorescence assays in BY-2 cells showed that Nt-JAT2-GFP was localized to the tonoplast. These findings indicate that Nt-JAT2 is involved in nicotine sequestration in leaf vacuoles following the translocation of nicotine from root tissues.

Comprehensive profiling and natural variation of flavonoids in rice

Flavonoids constitute a major group of plant phenolic compounds. While extensively studied in Arabidopsis, profiling and naturally occurring variation of these compounds in rice (Oryza sativa), the monocot model plant, are less reported. Using a collection of rice germplasm, comprehensive profiling and natural variation of flavonoids were presented in this report. Application of a widely targeted metabolomics method facilitated the simultaneous identification and quantification of more than 90 flavonoids using liquid chromatography tandem mass spectrometry (LC-MS/MS). Comparing flavonoid contents in various tissues during different developmental stages revealed tissue-specific accumulation of most flavonoids. Further investigation indicated that flavone mono-C-glycosides, malonylated flavonoid O-hexosides, and some flavonoid O-glycosides accumulated at significantly higher levels in indica than in japonica, while the opposite was observed for aromatic acylated flavone C-hexosyl-O-hexosides. In contrast to the highly differential accumulation between the two subspecies, relatively small variations within subspecies were detected for most flavonoids. Besides, an association analysis between flavonoid accumulation and its biosynthetic gene sequence polymorphisms disclosed that natural variation of flavonoids was probably caused by sequence polymorphisms in the coding region of flavonoid biosynthetic genes. Our work paves the way for future dissection of biosynthesis and regulation of flavonoid pathway in rice.

Natural product biosynthesis in Medicago species

The genus Medicago, a member of the legume (Fabaceae) family, comprises 87 species of flowering plants, including the forage crop M. sativa (alfalfa) and the model legume M. truncatula (barrel medic). Medicago species synthesize a variety of bioactive natural products that are used to engage into symbiotic interactions but also serve to deter pathogens and herbivores. For humans, these bioactive natural products often possess promising pharmaceutical properties. In this review, we focus on the two most interesting and well characterized secondary metabolite classes found in Medicago species, the triterpene saponins and the flavonoids, with a detailed overview of their biosynthesis, regulation, and profiling methods. Furthermore, their biological role within the plant as well as their potential utility for human health or other applications is discussed. Finally, we give an overview of the advances made in metabolic engineering in Medicago species and how the development of novel molecular and omics toolkits can influence a better understanding of this genus in terms of specialized metabolism and chemistry. Throughout, we critically analyze the current bottlenecks and speculate on future directions and opportunities for research and exploitation of Medicago metabolism.

A bulk segregant gene expression analysis of a peach population reveals components of the underlying mechanism of the fruit cold response

Peach fruits subjected for long periods of cold storage are primed to develop chilling injury once fruits are shelf ripened at room temperature. Very little is known about the molecular changes occurring in fruits during cold exposure. To get some insight into this process a transcript profiling analyses was performed on fruits from a PopDG population segregating for chilling injury CI responses. A bulked segregant gene expression analysis based on groups of fruits showing extreme CI responses indicated that the transcriptome of peach fruits was modified already during cold storage consistently with eventual CI development. Most peach cold-responsive genes have orthologs in Arabidopsis that participate in cold acclimation and other stresses responses, while some of them showed expression patterns that differs in fruits according to their susceptibility to develop mealiness. Members of ICE1, CBF1/3 and HOS9 regulons seem to have a prominent role in differential cold responses between low and high sensitive fruits. In high sensitive fruits, an alternative cold response program is detected. This program is probably associated with dehydration/osmotic stress and regulated by ABA, auxins and ethylene. In addition, the observation that tolerant siblings showed a series of genes encoding for stress protective activities with higher expression both at harvest and during cold treatment, suggests that preprogrammed mechanisms could shape fruit ability to tolerate postharvest cold-induced stress. A number of genes differentially expressed were validated and extended to individual genotypes by medium-throughput RT-qPCR. Analyses presented here provide a global view of the responses of peach fruits to cold storage and highlights new peach genes that probably play important roles in the tolerance/sensitivity to cold storage. Our results provide a roadmap for further experiments and would help to develop new postharvest protocols and gene directed breeding strategies to better cope with chilling injury.

A putative lambda class glutathione S-transferase enhances plant survival under salinity stress

In a survey of candidate genes located in the salinity tolerance locus of soybean, we identified a putative glutathione S-transferase (GST) gene (GmGSTL1) which was up-regulated in response to salt treatment. Phylogenetic analyses revealed that this putative GST belongs to the Lambda class, a plant-specific group with unknown functions. We expressed GmGSTL1 in heterologous systems, including tobacco BY-2 cells and Arabidopsis thaliana, to test its ability to protect the cell/plant against salinity stress. Compared with the control, we observed a marked reduction of reactive oxygen species (ROS) accumulation in transgenic cells under salt treatment, and their survival rate was also improved. Similarly, expression of GmGST1 in transgenic A. thaliana also alleviated stress symptoms under salt treatment. To address further the possible protective mechanisms of GmGSTL1, we identified two candidate flavonoid interactants (quercetin and kaemferol) of the GmGSTL1 protein from soybean leaf extract. Exogenous application of quercetin could reduce salinity-induced ROS accumulation in BY-2 cells and leaf chlorosis in A. thaliana.

Transport of flavonolignans to the culture medium of elicited cell suspensions of Silybum marianum

Cell suspension cultures of Silybum marianum are able to excrete silymarin compounds into the medium upon elicitation with methyl jasmonate or cyclodextrins. Knowledge of transport mechanism is important to understand Sm metabolism and to develop strategies aimed at increasing production by means of cell cultures. For these reasons, a pharmacological approach was undertaken in this work in order to elucidate the possible mechanism involved in the release of this class of secondary metabolites into the extracellular medium of suspensions. Treatment with an ionophore or NH4Cl displayed little effect in elicited cultures, thus indicating that secondary transport, which uses electrochemical gradients, is not involved in the release. Several inhibitors of ABC transporters showed differential effects. Sodium ortho-vanadate, a typical suppressor of ATPase activity, was highly toxic to cultures even at very low concentrations. The common Ca-channel blocker verapamil did not influence extracellular secondary metabolite accumulation. Glybenclamide and probenecid, both effective inhibitors of ABCC-type ABC transporters, strongly reduced silymarin secretion. A partial cDNA, SmABC1, which showed similarity to ABCC-type ABC transporters, was isolated by RT-PCR from silymarin-producing cultures. SmABC1 expression was enhanced by methyljasmonate and cyclodextrins. Brefeldin A, a fungal metabolite which affects vesicular trafficking by preventing GTP/GDP exchange, inhibited release in a dose dependent manner. These results suggest that excretion of silymarin and their precursors is a transporter-dependent active transport and that yet another mechanism involving a vesicle trafficking system seems to participate in driving this class of secondary metabolites to the extracellular compartment.

Transporters in plant sulfur metabolism

Sulfur is an essential nutrient, necessary for synthesis of many metabolites. The uptake of sulfate, primary and secondary assimilation, the biosynthesis, storage, and final utilization of sulfur (S) containing compounds requires a lot of movement between organs, cells, and organelles. Efficient transport systems of S-containing compounds across the internal barriers or the plasma membrane and organellar membranes are therefore required. Here, we review a current state of knowledge of the transport of a range of S-containing metabolites within and between the cells as well as of their long distance transport. An improved understanding of mechanisms and regulation of transport will facilitate successful engineering of the respective pathways, to improve the plant yield, biotic interaction and nutritional properties of crops.

Biosynthesis and metabolic engineering of anthocyanins in Arabidopsis thaliana

Arabidopsis thaliana is the first model plant, the genome of which has been sequenced. In general, intensive studies on this model plant over the past nearly 30 years have led to many new revolutionary understandings in every single aspect of plant biology. Here, we review the current understanding of anthocyanin biosynthesis in this model plant. Although the investigation of anthocyanin structures in this model plant was not performed until 2002, numerous studies over the past three decades have been conducted to understand the biosynthesis of anthocyanins. To date, it appears that all pathway genes of anthocyanins have been molecularly, genetically and biochemically characterized in this plant. These fundamental accomplishments have made Arabidopsis an ideal model to understand the regulatory mechanisms of anthocyanin pathway. Several studies have revealed that the biosynthesis of anthocyanins is controlled by WD40-bHLH-MYB (WBM) transcription factor complexes under lighting conditions. However, how different regulatory complexes coordinately and specifically regulate the pathway genes of anthocyanins remains unclear. In this review, we discuss current progresses and findings including structural diversity, regulatory properties and metabolic engineering of anthocyanins in Arabidopsis thaliana.

Keel petal incision: a simple and efficient method for genetic crossing in Medicago truncatula

BACKGROUND

Genetic crossing is an essential tool in both forward and reverse genetic approaches to understand the biological functions of genes. For Medicago truncatula (barrel medic) various crossing techniques have been used which differ in the methods used to dissect the female parent's unopened flower bud to remove immature anthers for prevention of self-pollination. Previously described methods including front, side or back incision methods may damage the flower bud, impeding successful fertilization and/or seed development because they may allow pollen to dislodge and floral organs to desiccate after crossing, all of which diminish the success rates of crossing.

RESULTS

We report the keel petal incision method for genetic crossing in M. truncatula ecotype R108 and demonstrate successful crosses with two other M. truncatula ecotypes, A17 and A20. In the method presented here, an incision is made along the central line of the keel petal from the bottom 1/3rd of the female parent's flower bud to its distal end. This allows easy removal of anthers from the flower bud and access for cross-pollination. After pollination, the stigma and the deposited pollen from the male donor are covered by the keel petal, wing petals and standard petal, forming a natural pouch. The pouch prevents dislodging of deposited pollen from the stigma and protects the internal floral organs from drying out, without using cling-film or water-containing chambers to maintain a humid environment. The keel petal incision method showed an approximate 80% success rate in the M. truncatula R108 ecotype and also in other ecotypes including Jemalong A17 and A20.

CONCLUSIONS

Our keel petal incision protocol shows marked improvement over existing methods with respect to the ease of crossing and the percentage of successful crosses. Developed for the M. truncatula R108 ecotype, the protocol has been demonstrated with A17 and A20 ecotypes and is expected to work with other ecotypes. Investigators of varying experience have achieved genetic crosses in M. truncatula using this method.

Membrane transporters and drought resistance - a complex issue

Land plants have evolved complex adaptation strategies to survive changes in water status in the environment. Understanding the molecular nature of such adaptive changes allows the development of rapid innovations to improve crop performance. Plant membrane transport systems play a significant role when adjusting to water scarcity. Here we put proteins participating in transmembrane allocations of various molecules in the context of stomatal, cuticular, and root responses, representing a part of the drought resistance strategy. Their role in the transport of signaling molecules, ions or osmolytes is summarized and the challenge of the forthcoming research, resulting from the recent discoveries, is highlighted.

Chemistry and biological activities of flavonoids: an overview

There has been increasing interest in the research on flavonoids from plant sources because of their versatile health benefits reported in various epidemiological studies. Since flavonoids are directly associated with human dietary ingredients and health, there is need to evaluate structure and function relationship. The bioavailability, metabolism, and biological activity of flavonoids depend upon the configuration, total number of hydroxyl groups, and substitution of functional groups about their nuclear structure. Fruits and vegetables are the main dietary sources of flavonoids for humans, along with tea and wine. Most recent researches have focused on the health aspects of flavonoids for humans. Many flavonoids are shown to have antioxidative activity, free radical scavenging capacity, coronary heart disease prevention, hepatoprotective, anti-inflammatory, and anticancer activities, while some flavonoids exhibit potential antiviral activities. In plant systems, flavonoids help in combating oxidative stress and act as growth regulators. For pharmaceutical purposes cost-effective bulk production of different types of flavonoids has been made possible with the help of microbial biotechnology. This review highlights the structural features of flavonoids, their beneficial roles in human health, and significance in plants as well as their microbial production.

The strawberry pathogenesis-related 10 (PR-10) Fra a proteins control flavonoid biosynthesis by binding to metabolic intermediates

Pathogenesis-related 10 (PR-10) proteins are involved in many aspects of plant biology but their molecular function is still unclear. They are related by sequence and structural homology to mammalian lipid transport and plant abscisic acid receptor proteins and are predicted to have cavities for ligand binding. Recently, three new members of the PR-10 family, the Fra a proteins, have been identified in strawberry, where they are required for the activity of the flavonoid biosynthesis pathway, which is essential for the development of color and flavor in fruits. Here, we show that Fra a proteins bind natural flavonoids with different selectivity and affinities in the low μm range. The structural analysis of Fra a 1 E and a Fra a 3-catechin complex indicates that loops L3, L5, and L7 surrounding the ligand-binding cavity show significant flexibility in the apo forms but close over the ligand in the Fra a 3-catechin complex. Our findings provide mechanistic insight on the function of Fra a proteins and suggest that PR-10 proteins, which are widespread in plants, may play a role in the control of secondary metabolic pathways by binding to metabolic intermediates.

Reprogramming cells to study vacuolar development

During vegetative and embryonic developmental transitions, plant cells are massively reorganized to support the activities that will take place during the subsequent developmental phase. Studying cellular and subcellular changes that occur during these short transitional periods can sometimes present challenges, especially when dealing with Arabidopsis thaliana embryo and seed tissues. As a complementary approach, cellular reprogramming can be used as a tool to study these cellular changes in another, more easily accessible, tissue type. To reprogram cells, genetic manipulation of particular regulatory factors that play critical roles in establishing or repressing the seed developmental program can be used to bring about a change of cell fate. During different developmental phases, vacuoles assume different functions and morphologies to respond to the changing needs of the cell. Lytic vacuoles (LVs) and protein storage vacuoles (PSVs) are the two main vacuole types found in flowering plants such as Arabidopsis. Although both are morphologically distinct and carry out unique functions, they also share some similar activities. As the co-existence of the two vacuole types is short-lived in plant cells, how they replace each other has been a long-standing curiosity. To study the LV to PSV transition, LEAFY COTYLEDON2, a key transcriptional regulator of seed development, was overexpressed in vegetative cells to activate the seed developmental program. At the cellular level, Arabidopsis leaf LVs were observed to convert to PSV-like organelles. This presents the opportunity for further research to elucidate the mechanism of LV to PSV transitions. Overall, this example demonstrates the potential usefulness of cellular reprogramming as a method to study cellular processes that occur during developmental transitions.

Reverted glutathione S-transferase-like genes that influence flower color intensity of carnation (Dianthus caryophyllus L.) originated from excision of a transposable element

A glutathione S-transferase-like gene, DcGSTF2, is responsible for carnation (Dianthus caryophyllus L.) flower color intensity. Two defective genes, DcGSTF2mu with a nonsense mutation and DcGSTF2-dTac1 containing a transposable element dTac1, have been characterized in detail in this report. dTac1 is an active element that produces reverted functional genes by excision of the element. A pale-pink cultivar 'Daisy' carries both defective genes, whereas a spontaneous deep-colored mutant 'Daisy-VPR' lost the element from DcGSTF2-dTac1. This finding confirmed that dTac1 is active and that the resulting reverted gene, DcGSTF2rev1, missing the element is responsible for this color change. Crosses between the pale-colored cultivar '06-LA' and a deep-colored cultivar 'Spectrum' produced segregating progeny. Only the deep-colored progeny had DcGSTF2rev2 derived from the 'Spectrum' parent, whereas progeny with pale-colored flowers had defective forms from both parents, DcGSTF2mu and DcGSTF2-dTac1. Thus, DcGSTF2rev2 had functional activity and likely originated from excision of dTac1 since there was a footprint sequence at the vacated site of the dTac1 insertion. Characterizing the DcGSTF2 genes in several cultivars revealed that the two functional genes, DcGSTF2rev1 and DcGSTF2rev2, have been used for some time in carnation breeding with the latter in use for more than half a century.

Involvement of anthocyanins in the resistance to chilling-induced oxidative stress in Saccharum officinarum L. leaves

Whether anthocyanins elevate resistance to chilling-induced oxidative stress in Saccharum officinarum L. cv Badila seedlings is investigated. Plants with four fully expanded leaves were exposed to chilling stress (8 °C/4 °C, 11 h photoperiod) for 3 days and then transferred to rewarming condition (25 °C/20 °C, 11 h photoperiod) for another 2 days. At the end of the chilling period, H2O2 and superoxide radical (O2-) levels increased sharply and were near the same in the central (CL) and the final fully expanded leaves (FL). Moreover, the degree of chilling injury indicated by malonaldehyde concentration and percent of ion leakage also was near the same. Most of the tested parameters returned near to the control level after 2 days of rewarming. With further analyzing, we found that superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), glutathione reductase (EC 1.6.4.2) activities increased much higher and catalase (EC 1.11.1.6) activity and ascorbate/dehydroascorbate ratio decreased much more in FL than CL in response to chilling. However, anthocyanins concentration coupling with glutathione/oxidized glutathione increased much higher in CL than FL under chilling stress. These finds suggest that anthocyanins at least partially compensate the relative deficiency of antioxidants in CL compared with FL. α,α-Diphenyl-β-picrylhydrazyl assays further confirmed this idea. The relationships between anthocyanins and antioxidants were analyzed and the possible mechanisms of the affection of anthocyanins on antioxidant metabolism were discussed.

Functional roles of flavonoids in photoprotection: new evidence, lessons from the past

We discuss on the relative significance of different functional roles potentially served by flavonoids in photoprotection, with special emphasis to their ability to scavenge reactive oxygen species (ROS) and control the development of individual organs and whole plant. We propose a model in which chloroplast-located flavonoids scavenge H2O2 and singlet oxygen generated under excess light-stress, thus avoiding programmed cell death. We also draw a picture in which vacuolar flavonoids in conjunction with peroxidases and ascorbic acid constitute a secondary antioxidant system aimed at detoxifying H2O2, which may diffuse out of the chloroplast at considerable rates and enter the vacuole following excess light stress-induced depletion of ascorbate peroxidase. We hypothesize for flavonols key roles as developmental regulators in early and current-day land-plants, based on their ability to modulate auxin movement and auxin catabolism. We show that antioxidant flavonoids display the greatest capacity to regulate key steps of cell growth and differentiation in eukaryotes. These regulatory functions of flavonoids, which are shared by plants and animals, are fully accomplished in the nM concentration range, as likely occurred in early land plants. We therefore conclude that functions of flavonoids as antioxidants and/or developmental regulators flavonoids are of great value in photoprotection. We also suggest that UV-B screening was just one of the multiple functions served by flavonoids when early land-plants faced an abrupt increase in sunlight irradiance.

Insight into the role of anthocyanin biosynthesis-related genes in Medicago truncatula mutants impaired in pigmentation in leaves

Flavonoids are the most common antioxidant compounds produced in plants. In this study, two wild types and two independent mutants of Medicago truncatula with altered anthocyanin content in leaves were characterized at the phenotype, metabolite profile, gene structure and transcript levels. Flavonoid profiles showed conserved levels of dihydroflavonols, leucoanthocyanidins and flavonols, while anthocyanidin, anthocyanin and isoflavone levels were lower in the mutants (up to 90% less) compared with the wild types. Genes encoding key enzymes of the anthocyanin pathway and transcriptional factors were analyzed by RT-PCR. Genes involved in the later steps of the anthocyanin pathway (dihydrokaempferol reductase 2, UDP-glucose:anthocyanin 3-O-glucosyltransferase and glutathione S-transferase) were found under-expressed in both mutants. Dihydrokaempferol reductase 1 was downregulated two-fold in the anthocyanin-less mutant while the UDP-glucose:anthocyanin 5-O-glucosyltransferase was strongly repressed only in the mutant with low pigmentation, suggesting a different regulation in the two genotypes. The common feature was that the first enzymes of the flavonoid biosynthesis pathway were not altered in rate of expression. A very high reduction in transcript accumulation was also found for two homologous R2R3 MYB genes, namely MtMYBA and AN2, suggesting that these genes have a role in anthocyanin accumulation in leaves. More evidence was found on analyzing their nucleotide sequence: several SNPs, insertions and deletions in the coding and non-coding regions of both MYB genes were found between mutants and wild types that could influence anthocyanin biosynthesis. Moreover, a subfamily of eight MYB genes with a high homology to MtMYBA was discovered in tandem on chromosome 5 of M. truncatula.

Plant flavonoids--biosynthesis, transport and involvement in stress responses

This paper aims at analysing the synthesis of flavonoids, their import and export in plant cell compartments, as well as their involvement in the response to stress, with particular reference to grapevine (Vitis vinifera L.). A multidrug and toxic compound extrusion (MATE) as well as ABC transporters have been demonstrated in the tonoplast of grape berry, where they perform a flavonoid transport. The involvement of a glutathione S-transferase (GST) gene has also been inferred. Recently, a putative flavonoid carrier, similar to mammalian bilitranslocase (BTL), has been identified in both grape berry skin and pulp. In skin the pattern of BTL expression increases from véraison to harvest, while in the pulp its expression reaches the maximum at the early ripening stage. Moreover, the presence of BTL in vascular bundles suggests its participation in long distance transport of flavonoids. In addition, the presence of a vesicular trafficking in plants responsible for flavonoid transport is discussed. Finally, the involvement of flavonoids in the response to stress is described.

Alteration of flavonoid accumulation patterns in transparent testa mutants disturbs auxin transport, gravity responses, and imparts long-term effects on root and shoot architecture

Flavonoids have broad cross-kingdom biological activity. In Arabidopsis, flavonoid accumulation in specific tissues, notably the root elongation zone and root/shoot junction modulate auxin transport, affect root gravitropism, and influence overall plant architecture. The relative contribution made by aglycones and their glycosides remains undetermined, and the longer-term phenotypic effects of altered flavonoid accumulation are not fully assessed. We tested Arabidopsis thaliana mutants that accumulate different flavonoids to determine which flavonoids were causing these affects. Tandem mass spectrometry and in situ fluorescence localisation were used to determine the in vivo levels of aglycones in specific tissues of 11 transparent testa mutants. We measured rootward and shootward auxin transport, gravitropic responses, and identified the long-term changes to root and shoot architecture. Unexpected aglycone species accumulated in vivo in several flavonoid-pathway mutants, and lower aglycone levels occurred in transcription factor mutants. Mutants accumulating more quercetin and quercetin-glycosides changed the greatest in auxin transport, gravitropism, and aerial tissue growth. Early flavonoid-pathway mutants showed aberrant lateral root initiation patterns including clustered lateral root initiations at a single site. Transcription factor mutants had multiple phenotypes including shallow root systems. These results confirm that aglycones are present at very low levels, show that lateral root initiation is perturbed in early flavonoid-pathway mutants, and indicate that altered flavonoid accumulation affects multiple aspects of plant architecture.

Medicago glucosyltransferase UGT72L1: potential roles in proanthocyanidin biosynthesis

In the first reaction specific for proanthocyanidin (PA) biosynthesis in Arabidopsis thaliana and Medicago truncatula, anthocyanidin reductase (ANR) converts cyanidin to (-)-epicatechin. The glucosyltransferase UGT72L1 catalyzes formation of epicatechin 3'-O-glucoside (E3'OG), the preferred substrate for MATE transporters implicated in PA biosynthesis in both species. The mechanism of PA polymerization is still unclear, but may involve the laccase-like polyphenol oxidase TRANSPARENT TESTA 10 (TT10). We have employed a combination of cell biological, biochemical and genetic approaches to evaluate this PA pathway model. The promoter regions of UGT72L1 and MtANR share common cis-acting elements and direct overlapping, but partially distinct, expression patterns. UGT72L1 and MtANR are localized in the cytosol, whereas TT10 is localized to the vacuole. Over-expression of UGT72L1 in M. truncatula hairy roots results in increased accumulation of PA-like compounds, and loss of function of UGT72L1 partially reduces epicatechin, E3'OG and extractable PA levels in M. truncatula seeds. Expression of UGT72L1 in A. thaliana leads to a massive increase in E3'OG in immature seed, but reduced levels of extractable PAs. However, when UGT72L1 was expressed in the Arabidopsis tt10 mutant, extractable PA levels increased and seed coat browning was delayed. Our results suggest that glycosylation of epicatechin is important for both PA precursor transport and assembly, but that additional redundant pathways may exist.

Proteome alterations monitored by DIGE analysis in Silybum marianum cell cultures elicited with methyl jasmonate and methyl B cyclodextrin

Elicitation with methyl jasmonate (MeJA) or/and cyclodextrin (CD) strongly induced silymarin (Sm) accumulation in suspensions of Silybum marianum, with most of Sm isomers being detected in the culture medium. This induction provides a model platform to characterize the regulation of flavonolignan accumulation and release in response to elicitors and, with this aim, changes in the S. marianum cell proteome were investigated. The DIGE technique was used to detect statistically significant changes in the cell's proteome. A total number of 1269 unique spots were detected, 67 of which were de-regulated upon elicitation. Nineteen spots were identified by nLC-MS/MS database search analysis. Identified proteins belong to a few categories, including metabolism, stress and defense responses and transport processes. The most abundant group was represented by pathogenesis-related (PR) proteins and heat shock proteins. Two proteins related to transport process were identified and both were upregulated by elicitation. One was identified as Ras-related protein Rab11C of the Rab family of small ATPase superfamily. A second protein was identified as an ABC transporter. Some of the identified proteins are discussed with respect to their putative role in the extracellular flavonolignan accumulation in S. marianum cultures.

BIOLOGICAL SIGNIFICANCE

Most approaches to increase secondary metabolite yields using plant cell cultures have been focused on the optimization of its biosynthesis. The study of other post biosynthetic events, like chemical or enzymatic modifications, transport, storage/secretion and catabolism/degradation are also biotechnologically relevant. Secretion is of particular interest since if cell cultures are to be used routinely for the commercial production, they must release the targeted metabolites into the extracellular medium. Elicitor-induced silymarin accumulation and release in S. marianum cell cultures provide a responsive model system to profile both alterations in proteins related to monolignol/flavonoid biosynthesis and to identify potential systems involved in secretion of secondary metabolites. The proteomic approach undertaken in this work has permitted identify some of the events occurring in elicited S. marianum cell cultures. One attainment of this study is that a vesicular transport mechanism could be involved in the release of this class of secondary metabolites to the extracellular compartment. This finding forms a baseline for future research on a non-sequenced medicinal plant S. marianum at molecular level.

Glycyrrhiza uralensis transcriptome landscape and study of phytochemicals

Medicinal and industrial properties of phytochemicals (e.g. glycyrrhizin) from the root of Glycyrrhiza uralensis (licorice plant) made it an attractive, multimillion-dollar trade item. Bioengineering is one of the solutions to overcome such high market demand and to protect plants from extinction. Unfortunately, limited genomic information on medicinal plants restricts their research and thus biosynthetic mechanisms of many important phytochemicals are still poorly understood. In this work we utilized the de novo (no reference genome sequence available) assembly of Illumina RNA-Seq data to study the transcriptome of the licorice plant. Our analysis is based on sequencing results of libraries constructed from samples belonging to different tissues (root and leaf) and collected in different seasons and from two distinct strains (low and high glycyrrhizin producers). We provide functional annotations and the expression profile of 43,882 assembled unigenes, which are suitable for various further studies. Here, we searched for G. uralensis-specific enzymes involved in isoflavonoid biosynthesis as well as elucidated putative cytochrome P450 enzymes and putative vacuolar saponin transporters involved in glycyrrhizin production in the licorice root. To disseminate the data and the analysis results, we constructed a publicly available G. uralensis database. This work will contribute to a better understanding of the biosynthetic pathways of secondary metabolites in licorice plants, and possibly in other medicinal plants, and will provide an important resource to further advance transcriptomic studies in legumes.

Flavonoids as antioxidants and developmental regulators: relative significance in plants and humans

Phenylpropanoids, particularly flavonoids have been recently suggested as playing primary antioxidant functions in the responses of plants to a wide range of abiotic stresses. Furthermore, flavonoids are effective endogenous regulators of auxin movement, thus behaving as developmental regulators. Flavonoids are capable of controlling the development of individual organs and the whole-plant; and, hence, to contribute to stress-induced morphogenic responses of plants. The significance of flavonoids as scavengers of reactive oxygen species (ROS) in humans has been recently questioned, based on the observation that the flavonoid concentration in plasma and most tissues is too low to effectively reduce ROS. Instead, flavonoids may play key roles as signaling molecules in mammals, through their ability to interact with a wide range of protein kinases, including mitogen-activated protein kinases (MAPK), that supersede key steps of cell growth and differentiation. Here we discuss about the relative significance of flavonoids as reducing agents and signaling molecules in plants and humans. We show that structural features conferring ROS-scavenger ability to flavonoids are also required to effectively control developmental processes in eukaryotic cells.

A Medicago truncatula ABC transporter belonging to subfamily G modulates the level of isoflavonoids

Full-sized ATP-binding cassette (ABC) transporters of the G subfamily (ABCG) are considered to be essential components of the plant immune system. These proteins have been proposed to be implicated in the active transmembrane transport of various secondary metabolites. Despite the importance of ABCG-based transport for plant-microbe interactions, these proteins are still poorly recognized in legumes. The experiments described here demonstrated that the level of Medicago truncatula ABCG10 (MtABCG10) mRNA was elevated following application of fungal oligosaccharides to plant roots. Spatial expression pattern analysis with a reporter gene revealed that the MtABCG10 promoter was active in various organs, mostly within their vascular tissues. The corresponding protein was located in the plasma membrane. Silencing of MtABCG10 in hairy roots resulted in lower accumulation of the phenylpropanoid pathway-derived medicarpin and its precursors. PCR-based experiments indicated that infection with Fusarium oxysporum, a root-infecting pathogen, progressed faster in MtABCG10-silenced composite plants (consisting of wild-type shoots on transgenic roots) than in the corresponding controls. Based on the presented data, it is proposed that in Medicago, full-sized ABCG transporters might modulate isoflavonoid levels during the defence response associated with de novo synthesis of phytoalexins.

New insights into the transport mechanisms in plant vacuoles

The vacuole is the largest compartment in plant cells, often occupying more than 80% of the total cell volume. This organelle accumulates a large variety of endogenous ions, metabolites, and xenobiotics. The compartmentation of divergent substances is relevant for a wide range of biological processes, such as the regulation of stomata movement, defense mechanisms against herbivores, flower coloration, etc. Progress in molecular and cellular biology has revealed that a large number of transporters and channels exist at the tonoplast. In recent years, various biochemical and physiological functions of these proteins have been characterized in detail. Some are involved in maintaining the homeostasis of ions and metabolites, whereas others are related to defense mechanisms against biotic and abiotic stresses. In this review, we provide an updated inventory of vacuolar transport mechanisms and a comprehensive summary of their physiological functions.