Molecular characterization and expression of alfalfa (Medicago sativa L.) flavanone-3-hydroxylase and dihydroflavonol-4-reductase encoding genes

Transcriptional regulation mechanism of flavonoids biosynthesis gene during fruit development in astragalus membranaceus

Astragalus membranaceus, as an important medicinal plant, are an excellent source of flavonoids. Flavonoid compounds in A. membranaceus have been widely used in medicine and supplement, but known of the molecular mechanism of flavonoid biosynthesis is still very few. Here, we analyzed the association between flavonoid content and gene expression pattern during six different fruit developmental stages. Sixteen gene expression trends were significantly identified, involving 8,218 genes. The gene expression trend in profile 0 was positively correlated with flavonoid content, while the gene expression trend in profile 79 was negatively correlated with flavonoid content at six developmental stages. The expression level of genes involved in the general phenylpropane pathway was higher than that of genes involved in the flavonoid biosynthesis pathway. A total of 37 genes involved in flavonoid synthesis were identified in A. membranaceus. The expression pattern of flavonoid-related genes was highly correlated with flavonoid content. Our study deepened the understanding of the flavonoid synthesis mechanism and provided useful resources for future studies on the high flavonoid molecular breeding of A. membranaceus.

Multiomics Analyses of Two Sorghum Cultivars Reveal the Molecular Mechanism of Salt Tolerance

Sorghum [Sorghum bicolor (L.) Moench] is one of the most important cereal crops and contains many health-promoting substances. Sorghum has high tolerance to abiotic stress and contains a variety of flavonoids compounds. Flavonoids are produced by the phenylpropanoid pathway and performed a wide range of functions in plants resistance to biotic and abiotic stress. A multiomics analysis of two sorghum cultivars (HN and GZ) under different salt treatments time (0, 24, 48, and 72) was performed. A total of 45 genes, 58 secondary metabolites, and 246 proteins were recognized with significant differential abundances in different comparison models. The common differentially expressed genes (DEGs) were allocated to the "flavonoid biosynthesis" and "phenylpropanoid biosynthesis" pathways. The most enriched pathways of the common differentially accumulating metabolites (DAMs) were "flavonoid biosynthesis," followed by "phenylpropanoid biosynthesis" and "arginine and proline metabolism." The common differentially expressed proteins (DEPs) were mainly distributed in "phenylpropanoid biosynthesis," "biosynthesis of cofactors," and "RNA transport." Furthermore, considerable differences were observed in the accumulation of low molecular weight nonenzymatic antioxidants and the activity of antioxidant enzymes. Collectively, the results of our study support the idea that flavonoid biological pathways may play an important physiological role in the ability of sorghum to withstand salt stress.

Biosynthetic Pathway of Proanthocyanidins in Major Cash Crops

Proanthocyanidins (PAs) are a group of oligomers or polymers composed of monomeric flavanols. They offer many benefits for human fitness, such as antioxidant, anticancer, and anti-inflammatory activities. To date, three types of PA have been observed in nature: procyanidins, propelargonidins, and prodelphinidins. These are synthesized as some of the end-products of the flavonoid pathway by different consecutive enzymatic activities, from the same precursor—naringenin. Although the general biosynthetic pathways of PAs have been reported in a few model plant species, little is known about the species-specific pathways in major crops containing different types of PA. In the present study, we identified the species-specific pathways in 10 major crops, based on the presence/absence of flavanol-based intermediates in the metabolic pathway, and found 202 orthologous genes in the reference genomic database of each species, which may encode for key enzymes involved in the biosynthetic pathways of PAs. Parallel enzymatic reactions in the pathway are responsible for the ratio between PAs and anthocyanins, as well as among the three types of PAs. Our study suggests a promising strategy for molecular breeding, to regulate the content of PAs and anthocyanins and improve the nutritional quality of food sources globally.

Genetic Analysis of Walnut (Juglans regia L.) Pellicle Pigment Variation Through a Novel, High-Throughput Phenotyping Platform

Walnut pellicle color is a key quality attribute that drives consumer preference and walnut sales. For the first time a high-throughput, computer vision-based phenotyping platform using a custom algorithm to quantitatively score each walnut pellicle in L* a* b* color space was deployed at large-scale. This was compared to traditional qualitative scoring by eye and was used to dissect the genetics of pellicle pigmentation. Progeny from both a bi-parental population of 168 trees (‘Chandler’ × ‘Idaho’) and a genome-wide association (GWAS) with 528 trees of the UC Davis Walnut Improvement Program were analyzed. Color phenotypes were found to have overlapping regions in the ‘Chandler’ genetic map on Chr01 suggesting complex genetic control. In the GWAS population, multiple, small effect QTL across Chr01, Chr07, Chr08, Chr09, Chr10, Chr12 and Chr13 were discovered. Marker trait associations were co-localized with QTL mapping on Chr01, Chr10, Chr14, and Chr16. Putative candidate genes controlling walnut pellicle pigmentation were postulated.

Evolutionary and functional analyses of the 2-oxoglutarate-dependent dioxygenase genes involved in the flavonoid biosynthesis pathway in tobacco

MAIN CONCLUSION

This study illustrates the differences in the gene structure of 2-oxoglutarate-dependent oxygenase involved in flavonoid biosynthesis (2ODD-IFB), and their potential roles in regulating tobacco flavonoid biosynthesis and plant growth. Flavonol synthase (FLS), anthocyanidin synthase (ANS), and flavanone 3β-hydroxylase belong to the 2-oxoglutarate-dependent (2ODD) oxygenase family, and each performs crucial functions in the biosynthesis of flavonoids. We identified two NtFLS genes, two NtANS genes, and four NtF3H genes from Nicotiana tabacum genome, as well as their homologous genes in the N. sylvestris and N. tomentosiformis genomes. Our phylogenetic analysis indicated that these three types of genes split from each other before the divergence of gymnosperms and angiosperms. FLS evolved faster in the eudicot plants, whereas ANS evolved faster in the monocot plants. Gene structure analysis revealed two fragment insertions occurred at different times in the intron one position of tobacco FLS genes. Homologous protein modeling revealed distinct structures in the N terminus of the tobacco 2ODD oxygenases. We found that the expression patterns of genes encoding tobacco 2ODD oxygenases in flavonoids biosynthesis (2ODD-IFB) did not determine the accumulation patterns of flavonoids among various tobacco tissues, but strongly affected the concentration of flavonoids in the tissues, where they were biosynthesized. More carbon resource flowed to the flavonol biosynthesis when NtANS gene was silenced, otherwise more anthocyanidin accumulated when NtFLS gene was repressed. This study illustrates the 2ODD-IFB gene structure evolution, differences among their protein structures, and provides a foundation for regulating plant development and altering flavonoid content and/or composition through the manipulation of plant 2ODD-IFB genes.

High-density SNP linkage map construction and QTL mapping for flavonoid-related traits in a tea plant (Camellia sinensis) using 2b-RAD sequencing

BACKGROUND

Flavonoids are important components that confer upon tea plants a unique flavour and health functions. However, the traditional breeding method for selecting a cultivar with a high or unique flavonoid content is time consuming and labour intensive. High-density genetic map construction associated with quantitative trait locus (QTL) mapping provides an effective way to facilitate trait improvement in plant breeding. In this study, an F1 population (LJ43×BHZ) was genotyped using 2b-restriction site-associated DNA (2b-RAD) sequencing to obtain massive single nucleotide polymorphism (SNP) markers to construct a high-density genetic map for a tea plant. Furthermore, QTLs related to flavonoids were identified using our new genetic map.

RESULTS

A total of 13,446 polymorphic SNP markers were developed using 2b-RAD sequencing, and 4,463 of these markers were available for constructing the genetic linkage map. A 1,678.52-cM high-density map at an average interval of 0.40 cM with 4,217 markers, including 427 frameset simple sequence repeats (SSRs) and 3,800 novel SNPs, mapped into 15 linkage groups was successfully constructed. After QTL analysis, a total of 27 QTLs related to flavonoids or caffeine content (CAF) were mapped to 8 different linkage groups, LG01, LG03, LG06, LG08, LG10, LG11, LG12, and LG13, with an LOD from 3.14 to 39.54, constituting 7.5% to 42.8% of the phenotypic variation.

CONCLUSIONS

To our knowledge, the highest density genetic map ever reported was constructed since the largest mapping population of tea plants was adopted in present study. Moreover, novel QTLs related to flavonoids and CAF were identified based on the new high-density genetic map. In addition, two markers were located in candidate genes that may be involved in flavonoid metabolism. The present study provides valuable information for gene discovery, marker-assisted selection breeding and map-based cloning for functional genes that are related to flavonoid content in tea plants.

Comparative analysis of anthocyanin biosynthesis during fruit development in two Lycium species

Dietary consumption of functional foods enriched in anthocyanins benefit for human health by protection against far-ranging human diseases. Delphinidin-derived anthocyanins (valuable as blue pigments and antioxidants) are accumulated specifically in the fruits of Lycium ruthenicum but not in the fruits of Lycium barbarum, suggesting the differences of anthocyanin biosynthesis between the two species. In this study, anthocyanin profiling confirmed that anthocyanins were increasingly accumulated during fruit ripening in L. ruthenicum, and sharply increased at full expanded mature fruit, while no anthocyanin were detected at any stage of L. barbarum fruit development. Several genes involved in anthocyanin biosynthesis were characterized in L. ruthenicum and L. barbarum fruits. Expression profiling of these genes during fruit development showed a significant positive correlation between transcript abundance and anthocyanin accumulation in L. ruthenicum fruit. Meanwhile, transcripts in L. barbarum fruit were either undetectable or were downregulated during fruit ripening, before increasing slightly in the final stages of maturation. In addition, the ratio of LrF3'5H/LrF3'H transcription showed a gradual increase before 6 days after breaker (DAB) and a sharp enhancement at 10 DAB. Our results suggest that the expression patterns of both regulatory and structural genes and the transcriptional ratio of branch-node structural genes F3'5'H/F3'H may determine the phenotypic difference in anthocyanin biosynthesis between L. ruthenicum and L. barbarum fruits.

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.

Crystal structure of grape dihydroflavonol 4-reductase, a key enzyme in flavonoid biosynthesis

The nicotinamide adenine dinucleotide phosphate (NADPH)-dependent enzyme dihydroflavonol 4-reductase (DFR) catalyzes a late step in the biosynthesis of anthocyanins and condensed tannins, two flavonoid classes of importance to plant survival and human nutrition. This enzyme has been widely investigated in many plant species, but little is known about its structural and biochemical properties. To provide a basis for detailed structure-function studies, the crystal structure of Vitis vinifera DFR, heterologously expressed in Escherichia coli, has been determined at 1.8 A resolution. The 3D structure of the ternary complex obtained with the oxidized form of nicotinamide adenine dinucleotide phosphate and dihydroquercetin, one of the DFR substrates, presents common features with the short-chain dehydrogenase/reductase family, i.e., an N-terminal domain adopting a Rossmann fold and a variable C-terminal domain, which participates in substrate binding. The structure confirms the importance of the 131-156 region, which lines the substrate binding site and enlightens the role of a specific residue at position 133 (Asn or Asp), assumed to control substrate recognition. The activity of the wild-type enzyme and its variant N133D has been quantified in vitro, using dihydroquercetin or dihydrokaempferol. Our results demonstrate that position 133 cannot be solely responsible for the recognition of the B-ring hydroxylation pattern of dihydroflavonols.

Cloning and characterization of a flavanone 3-hydroxylase gene from Ginkgo biloba

Flavanone 3-hydroxylase (F3H) activity is necessary for the biosynthesis of flavonoids, the main ingredients of Gingko biloba extract. The full-length cDNA and genomic DNA sequences of F3H gene were isolated from G. biloba for the first time. The full-length cDNA of G. biloba F3H gene (designated as GbF3H) contained a 1071 bp open reading frame (ORF) encoding a 357-amino-acid protein with a calculated molecular weight of about 40 kDa and isoelectric point (pI) of 5.57. The genomic DNA analysis showed that GbF3H gene had three exons and two introns. The deduced GbF3H protein showed high identities to other plant F3Hs. The conserved amino acids ligating ferrous iron and residues participating in 2-oxoglutarate binding (R-X-S) were found in GbF3H at the similar positions like other F3Hs. Three-dimensional structure modeling showed that GbF3H had a jerry roll in the enzyme core consisted of beta-sheet, a typical structure shared by all 2-oxoglutarate-dependent dioxygenases including F3Hs. Phylogenetic tree analysis revealed that GbF3H shared the same ancestor in evolution with other F3Hs and had a further relationship with other angiosperms species. Southern blot analysis indicated that GbF3H belonged to a multi-gene family. Transcription analysis revealed that GbF3H expressed in stem and leaf with the highest transcription level in leaf. The isolation and characterization of GbF3H gene will be helpful to further study the role of GbF3H gene in the biosynthesis of flavonoids in G. biloba.

The wp mutation of Glycine max carries a gene-fragment-rich transposon of the CACTA superfamily

We used soybean (Glycine max) cDNA microarrays to identify candidate genes for a stable mutation at the Wp locus in soybean, which changed a purple-flowered phenotype to pink, and found that flavanone 3-hydroxylase cDNAs were overexpressed in purple flower buds relative to the pink. Restriction fragment length polymorphism analysis and RNA gel blots of purple and pink flower isolines, as well as the presence of a 5.7-kb transposon insertion in the wp mutant allele, have unequivocally shown that flavanone 3-hydroxylase gene 1 is the Wp locus. Moreover, the 5.7-kb insertion in wp represents a novel transposable element (termed Tgm-Express1) with inverted repeats closely related to those of other Tgms (transposable-like elements, G. max) but distinct in several characteristics, including the lack of subterminal inverted repeats. More significantly, Tgm-Express1 contains four truncated cellular genes from the soybean genome, resembling the Pack-MULEs (Mutator-like transposable elements) found in maize (Zea mays), rice (Oryza sativa), and Arabidopsis thaliana and the Helitrons of maize. The presence of the Tgm-Express1 element causing the wp mutation, as well as a second Tgm-Express2 element elsewhere in the soybean genome, extends the ability to acquire and transport host DNA segments to the CACTA family of elements, which includes both Tgm and the prototypical maize Spm/En.

A comprehensive analysis of six dihydroflavonol 4-reductases encoded by a gene cluster of the Lotus japonicus genome

Dihydroflavonol 4-reductase (DFR) is the first committed enzyme of the anthocyanin and condensed tannin pathways. Several DFR cDNAs have been cloned, and different specificities of DFR isozymes in the substrate hydroxylation patterns have been reported, but only fragmentary knowledge of DFR gene organization is available. Reported here is a comprehensive analysis of DFRs of a model legume, Lotus japonicus. A total of five DFR genes were found to form a cluster within a 38 kb region in the L. japonicus genome, whereas six cDNAs, including two splicing variants resulting from a transversion at a splicing acceptor site, were cloned. All the genes were expressed, with different organ specificities, in the mature plant. Three of the DFR proteins heterologously expressed in Escherichia coli showed catalytic activity, and their substrate preferences agreed with the variation of a specific active site residue (Asp or Asn) reported to control the specificity. The hydroxylation patterns of anthocyanidins and condensed tannin units in the stems did not reflect the substrate specificity of the expressed isozymes, implying complex regulation mechanisms in the biosynthesis. The two splicing variants and one DFR with Ser at the specificity-controlling position failed to show the activity, but a revertant protein replacing the unusual splicing restored the activity. The phylogenetic tree, constructed with known DFR sequences, showed evolutionary divergence of some of the DFR genes prior to the plant speciation. This work affords the basis for genetic and biochemical studies on the diversity of DFR and the flavonoid products.

Proanthocyanidin biosynthesis--still more questions than answers?

Proanthocyanidins, also known as condensed tannins, are oligomers or polymers of flavan-3-ol units. In spite of important breakthroughs in our understanding of the biosynthesis of the major building blocks of proanthocyanidins, (+)-catechin and (-)-epicatechin, important questions still remain to be answered as to the exact nature of the molecular species that undergo polymerization, and the mechanisms of assembly. We review the structures of proanthocyanidins reported over the past 12 years in the context of biosynthesis, and summarize the outstanding questions concerning synthesis of proanthocyanidins from the chemical, biochemical and molecular genetic perspectives.

Metabolic engineering of isoflavonoid biosynthesis in alfalfa

The potential health benefits of dietary isoflavones have generated considerable interest in engineering the synthesis of these phytoestrogens into plants. Genistein glucoside production (up to 50 nmol g(-1) fresh weight) was engineered in alfalfa (Medicago sativa) leaves by constitutive expression of isoflavone synthase from Medicago truncatula (MtIFS1). Glucosides of biochanin A (4'-O-methylgenistein) and pratensein (3'-hydroxybiochanin A) also accumulated. Although MtIFS1 was highly expressed in all organs examined, genistein accumulation was limited to leaves. MtIFS1-expressing lines accumulated several additional isoflavones, including formononetin and daidzein, in response to UV-B or Phoma medicaginis, whereas the chalcone and flavanone precursors of these compounds accumulated in control lines. Enhanced accumulation of the phytoalexin medicarpin was observed in P. medicaginis-infected leaves of MtIFS1-expressing plants. Microarray profiling indicated that MtIFS1 expression does not significantly alter global gene expression in the leaves. Our results highlight some of the challenges associated with metabolic engineering of plant natural products, including tissue-specific accumulation, potential for further modification by endogenous enzyme activities (hydroxylation, methylation, and glycosylation), and the differential response of engineered plants to environmental factors.

Molecular evolution of flavonoid dioxygenases in the family Apiaceae

Plant species of the family Apiaceae are known to accumulate flavonoids mainly in the form of flavones and flavonols. Three 2-oxoglutarate-dependent dioxygenases, flavone synthase or flavanone 3 beta-hydroxylase and flavonol synthase are involved in the biosynthesis of these secondary metabolites. The corresponding genes were cloned recently from parsley (Petroselinum crispum) leaves. Flavone synthase I appears to be confined to the Apiaceae, and the unique occurrence as well as its high sequence similarity to flavanone 3beta-hydroxylase laid the basis for evolutionary studies. In order to examine the relationship of these two enzymes throughout the Apiaceae, RT-PCR based cloning and functional identification of flavone synthases I or flavanone 3beta-hydroxylases were accomplished from Ammi majus, Anethum graveolens, Apium graveolens, Pimpinella anisum, Conium maculatum and Daucus carota, yielding three additional synthase and three additional hydroxylase cDNAs. Molecular and phylogenetic analyses of these sequences were compatible with the phylogeny based on morphological characteristics and suggested that flavone synthase I most likely resulted from gene duplication of flavanone 3beta-hydroxylase, and functional diversification at some point during the development of the apiaceae subfamilies. Furthermore, the genomic sequences from Petroselinum crispum and Daucus carota revealed two introns in each of the synthases and a lack of introns in the hydroxylases. These results might be explained by intron losses from the hydroxylases occurring at a later stage of evolution.

Partial reconstruction of flavonoid and isoflavonoid biosynthesis in yeast using soybean type I and type II chalcone isomerases

Flavonoids and isoflavonoids are major plant secondary metabolites that mediate diverse biological functions and exert significant ecological impacts. These compounds play important roles in many essential physiological processes. In addition, flavonoids and isoflavonoids have direct but complex effects on human health, ranging from reducing cholesterol levels and preventing certain cancers to improving women's health. In this study, we cloned and functionally characterized five soybean (Glycine max) chalcone isomerases (CHIs), key enzymes in the phenylpropanoid pathway that produces flavonoids and isoflavonoids. Gene expression and kinetics analysis suggest that the soybean type I CHI, which uses naringenin chalcone as substrate, is coordinately regulated with other flavonoid-specific genes, while the type II CHIs, which use a variety of chalcone substrates, are coordinately regulated with an isoflavonoid-specific gene and specifically activated by nodulation signals. Furthermore, we found that some of the newly identified soybean CHIs do not require the 4'-hydroxy moiety on the substrate for high enzyme activity. We then engineered yeast (Saccharomyces cerevisiae) to produce flavonoid and isoflavonoid compounds. When one of the type II CHIs was coexpressed with an isoflavone synthase, the enzyme catalyzing the first committed step of isoflavonoid biosynthesis, various chalcone substrates added to the culture media were converted to an assortment of isoflavanones and isoflavones. We also reconstructed the flavonoid pathway by coexpressing CHI with either flavanone 3beta-hydroxylase or flavone synthase II. The in vivo reconstruction of the flavonoid and isoflavonoid pathways in yeast provides a unique platform to study enzyme interactions and metabolic flux.

Proanthocyanidins--a final frontier in flavonoid research?

Proanthocyanidins are oligomeric and polymeric end products of the flavonoid biosynthetic pathway. They are present in the fruits, bark, leaves and seeds of many plants, where they provide protection against predation. At the same time they give flavor and astringency to beverages such as wine, fruit juices and teas, and are increasingly recognized as having beneficial effects on human health. The presence of proanthocyanidins is also a major quality factor for forage crops. The past 2 years have seen important breakthroughs in our understanding of the biosynthesis of the building blocks of proanthocyanidins, the flavan-3-ols (+)-catechin and (-)-epicatechin. However, virtually nothing is known about the ways in which these units are assembled into the corresponding oligomers in vivo. Molecular genetic approaches are leading to an understanding of the regulatory genes that control proanthocyanidin biosynthesis, and this information, together with increased knowledge of the enzymes specific for the pathway, will facilitate the genetic engineering of plants for introduction of value-added nutraceutical and forage quality traits.

Molecular and biochemical analysis of two cDNA clones encoding dihydroflavonol-4-reductase from Medicago truncatula

Dihydroflavonol-4-reductase (DFR; EC1.1.1.219) catalyzes a key step late in the biosynthesis of anthocyanins, condensed tannins (proanthocyanidins), and other flavonoids important to plant survival and human nutrition. Two DFR cDNA clones (MtDFR1 and MtDFR2) were isolated from the model legume Medicago truncatula cv Jemalong. Both clones were functionally expressed in Escherichia coli, confirming that both encode active DFR proteins that readily reduce taxifolin (dihydroquercetin) to leucocyanidin. M. truncatula leaf anthocyanins were shown to be cyanidin-glucoside derivatives, and the seed coat proanthocyanidins are known catechin and epicatechin derivatives, all biosynthesized from leucocyanidin. Despite high amino acid similarity (79% identical), the recombinant DFR proteins exhibited differing pH and temperature profiles and differing relative substrate preferences. Although no pelargonidin derivatives were identified in M. truncatula, MtDFR1 readily reduced dihydrokaempferol, consistent with the presence of an asparagine residue at a location known to determine substrate specificity in other DFRs, whereas MtDFR2 contained an aspartate residue at the same site and was only marginally active on dihydrokaempferol. Both recombinant DFR proteins very efficiently reduced 5-deoxydihydroflavonol substrates fustin and dihydrorobinetin, substances not previously reported as constituents of M. truncatula. Transcript accumulation for both genes was highest in young seeds and flowers, consistent with accumulation of condensed tannins and leucoanthocyanidins in these tissues. MtDFR1 transcript levels in developing leaves closely paralleled leaf anthocyanin accumulation. Overexpression of MtDFR1 in transgenic tobacco (Nicotiana tabacum) resulted in visible increases in anthocyanin accumulation in flowers, whereas MtDFR2 did not. The data reveal unexpected properties and differences in two DFR proteins from a single species.

Proteomic analysis on symbiotic differentiation of mitochondria in soybean nodules

Symbiotic interactions between legume plants and rhizobia induce specific metabolisms and intracellular organelles in nodules. For surveying symbiotic differentiation of a key organelle, mitochondria, protein constituents of soybean nodule and root mitochondria were compared after two-dimensional (2-D) electrophoresis, and the proteins were characterized in combination with matrix-assisted desorption/ionization time-of-flight mass spectrometry, electrospray ionization mass spectrometry and N-terminal amino acid sequencing. Of the proteins that were detected only in nodule mitochondria, phosphoserine aminotransferase, flavanone 3-hydroxylase, coproporphyrinogen III oxidase, one ribonucleoprotein and three unknown proteins were identified. Seven up-regulated, eight down-regulated and two strongly suppressed protein spots in nodule mitochondria were also assigned protein identities. The physiological roles of these differential expressions were discussed in relation to nodule-specific metabolisms in soybean nodules.

Key role of bacterial NH(4)(+) metabolism in Rhizobium-plant symbiosis

Symbiotic nitrogen fixation is carried out in specialized organs, the nodules, whose formation is induced on leguminous host plants by bacteria belonging to the family Rhizobiaceae: Nodule development is a complex multistep process, which requires continued interaction between the two partners and thus the exchange of different signals and metabolites. NH(4)(+) is not only the primary product but also the main regulator of the symbiosis: either as ammonium and after conversion into organic compounds, it regulates most stages of the interaction, from the production of nodule inducers to the growth, function, and maintenance of nodules. This review examines the adaptation of bacterial NH(4)(+) metabolism to the variable environment generated by the plant, which actively controls and restricts bacterial growth by affecting oxygen and nutrient availability, thereby allowing a proficient interaction and at the same time preventing parasitic invasion. We describe the regulatory circuitry responsible for the downregulation of bacterial genes involved in NH(4)(+) assimilation occurring early during nodule invasion. This is a key and necessary step for the differentiation of N(2)-fixing bacteroids (the endocellular symbiotic form of rhizobia) and for the development of efficient nodules.

Flavan-containing cells delimit Frankia-infected compartments in Casuarina glauca nodules

We investigated the involvement of polyphenols in the Casuarina glauca-Frankia symbiosis. Histological analysis revealed a cell-specific accumulation of phenolics in C. glauca nodule lobes, creating a compartmentation in the cortex. Histochemical and biochemical analyses indicated that these phenolic compounds belong to the flavan class of flavonoids. We show that the same compounds were synthesized in nodules and uninfected roots. However, the amount of each flavan was dramatically increased in nodules compared with uninfected roots. The use of in situ hybridization established that chalcone synthase transcripts accumulate in flavan-containing cells at the apex of the nodule lobe. Our findings are discussed in view of the possible role of flavans in plant-microbe interactions.

Bigfoot. a new family of MITE elements characterized from the Medicago genus

We have characterized from the legume plant Medicago a new family of miniature inverted-repeat transposable elements (MITE), called the Bigfoot transposable elements. Two of these insertion elements are present only in a single allele of two different M. sativa genes. Using a PCR strategy we have isolated 19 other Bigfoot elements from the M. sativa and M. truncatula genomes. They differ from the previously characterized MITEs by their sequence, a target site of 9 bp and a partially clustered genomic distribution. In addition, we show that they exhibit a significantly stable secondary structure. These elements may represent up to 0.1% of the genome of the outcrossing Medicago sativa but are present at a reduced copy number in the genome of the autogamous M. truncatula plant, revealing major differences in the genome organization of these two plants.

Genetic manipulation of condensed tannins in higher plants. Ii. Analysis Of birdsfoot trefoil plants harboring antisense dihydroflavonol reductase constructs

We have produced and analyzed transgenic birdsfoot trefoil (Lotus corniculatus L.) plants harboring antisense dihydroflavonol reductase (AS-DFR) sequences. In initial experiments the effect of introducing three different antisense Antirrhinum majus L. DFR constructs into a single recipient genotype (S50) was assessed. There were no obvious effects on plant biomass, but levels of condensed tannins showed a statistical reduction in leaf, stem, and root tissues of some of the antisense lines. Transformation events were also found, which resulted in increased levels of condensed tannins. In subsequent experiments a detailed study of AS-DFR phenotypes was carried out in genotype S33 using pMAJ2 (an antisense construct comprising the 5' half of the A. majus cDNA). In this case, reduced tannin levels were found in leaf and stem tissues and in juvenile shoot tissues. Analysis of soluble flavonoids and isoflavonoids in tannin down-regulated shoot tissues indicated few obvious default products. When two S33 AS-DFR lines were outcrossed, there was an underrepresentation of transgene sequences in progeny plants and no examples of inheritance of an antisense phenotype were observed. To our knowledge, this is the first report of the genetic manipulation of condensed tannin biosynthesis in higher plants.

The expression pattern of alfalfa flavanone 3-hydroxylase promoter-gus fusion in Nicotiana benthamiana correlates with the presence of flavonoids detected in situ

Flavanone 3-hydroxylase is an enzyme acting in the central part of the flavonoid biosynthesis pathway. It is generally encoded by a single gene and seems to have a key position for the regulation in this pathway. These two features make a single f3h promoter-gus fusion a suitable tool to study both the f3h expression and the regulation of this pathway. We present here the spatial and temporal analysis of the expression of an alfalfa flavanone 3-hydroxylase (f3h) promoter-gus fusion introduced into Nicotiana benthamiana. The Medicago sativa (alfalfa) f3h promoter directed gus expression in flowers, stems, leaves and roots. In flowers, GUS activity was observed in pollen grains, in ovules, in ovary placenta and in the epidermis, medullary parenchyma, trichomes and second cortical cellular layer surrounding the vascular bundles of the peduncle. In stems, GUS activity was detected at the same places as in the peduncle except for the medullary parenchyma. In roots, we found GUS staining in root hairs, epidermis and in the vascular bundles of the elongated zone. Finally, in leaves, the f3h promoter expressed essentially in the stalk cells of the multicellular trichomes. The expression pattern of the f3h-gus fusion was correlated to the presence of flavonoids in situ. These data indicate that this construct can be very useful to study factors controlling the production of flavonoids.