Inactivation of pea genes by RNAi supports the involvement of two similar O-methyltransferases in the biosynthesis of (+)-pisatin and of chiral intermediates with a configuration opposite that found in (+)-pisatin

Domestication has altered gene expression and secondary metabolites in pea seed coat

The mature seed in legumes consists of an embryo and seed coat. In contrast to knowledge about the embryo, we know relatively little about the seed coat. We analyzed the gene expression during seed development using a panel of cultivated and wild pea genotypes. Gene co-expression analysis identified gene modules related to seed development, dormancy, and domestication. Oxidoreductase genes were found to be important components of developmental and domestication processes. Proteomic and metabolomic analysis revealed that domestication favored proteins involved in photosynthesis and protein metabolism at the expense of seed defense. Seed coats of wild peas were rich in cell wall-bound metabolites and the protective compounds predominated in their seed coats. Altogether, we have shown that domestication altered pea seed development and modified (mostly reduced) the transcripts along with the protein and metabolite composition of the seed coat, especially the content of the compounds involved in defense. We investigated dynamic profiles of selected identified phenolic and flavonoid metabolites across seed development. These compounds usually deteriorated the palatability and processing of the seeds. Our findings further provide resources to study secondary metabolism and strategies for improving the quality of legume seeds which comprise an important part of the human protein diet.

Dirigent isoflavene-forming PsPTS2: 3D structure, stereochemical, and kinetic characterization comparison with pterocarpan-forming PsPTS1 homolog in pea

Pea phytoalexins (-)-maackiain and (+)-pisatin have opposite C6a/C11a configurations, but biosynthetically how this occurs is unknown. Pea dirigent-protein (DP) PsPTS2 generates 7,2'-dihydroxy-4',5'-methylenedioxyisoflav-3-ene (DMDIF), and stereoselectivity toward four possible 7,2'-dihydroxy-4',5'-methylenedioxyisoflavan-4-ol (DMDI) stereoisomers was investigated. Stereoisomer configurations were determined using NMR spectroscopy, electronic circular dichroism, and molecular orbital analyses. PsPTS2 efficiently converted cis-(3R,4R)-DMDI into DMDIF 20-fold faster than the trans-(3R,4S)-isomer. The 4R-configured substrate's near β-axial OH orientation significantly enhanced its leaving group abilities in generating A-ring mono-quinone methide (QM), whereas 4S-isomer's α-equatorial-OH was a poorer leaving group. Docking simulations indicated that the 4R-configured β-axial OH was closest to Asp51, whereas 4S-isomer's α-equatorial OH was further away. Neither cis-(3S,4S)- nor trans-(3S,4R)-DMDIs were substrates, even with the former having C3/C4 stereochemistry as in (+)-pisatin. PsPTS2 used cis-(3R,4R)-7,2'-dihydroxy-4'-methoxyisoflavan-4-ol [cis-(3R,4R)-DMI] and C3/C4 stereoisomers to give 2',7-dihydroxy-4'-methoxyisoflav-3-ene (DMIF). DP homologs may exist in licorice (Glycyrrhiza pallidiflora) and tree legume Bolusanthus speciosus, as DMIF occurs in both species. PsPTS1 utilized cis-(3R,4R)-DMDI to give (-)-maackiain 2200-fold more efficiently than with cis-(3R,4R)-DMI to give (-)-medicarpin. PsPTS1 also slowly converted trans-(3S,4R)-DMDI into (+)-maackiain, reflecting the better 4R configured OH leaving group. PsPTS2 and PsPTS1 provisionally provide the means to enable differing C6a and C11a configurations in (+)-pisatin and (-)-maackiain, via identical DP-engendered mono-QM bound intermediate generation, which PsPTS2 either re-aromatizes to give DMDIF or PsPTS1 intramolecularly cyclizes to afford (-)-maackiain. Substrate docking simulations using PsPTS2 and PsPTS1 indicate cis-(3R,4R)-DMDI binds in the anti-configuration in PsPTS2 to afford DMDIF, and the syn-configuration in PsPTS1 to give maackiain.

A simple method for creating transgenic pea hairy roots using a Japanese pea cultivar and a Japanese Rhizobium radiobacter strain

Pea (Pisum sativum) is an agriculturally important leguminous crop cultivated worldwide. It is also the plant from which phytoalexin was isolated for the first time. Several studies have investigated gene functions using pea hairy root culture systems. However, the procedures for producing hairy roots are relatively complicated and only a few pea cultivars and Rhizobium strains have been used. In this study, we established a simple method for generating transgenic hairy roots using a pea cultivar and a Rhizobium strain available in Japan. The transformation efficiency for the transgenic hairy roots (approximately 14%) was calculated on the basis of GFP fluorescence because the binary vector used in this study carried a GFP cassette as a marker. Furthermore, we confirmed that the production of the phytoalexin (+)-pisatin was induced by a copper dichloride treatment, indicating that this system can be used to characterize the biosynthesis of (+)-pisatin, which is a compound with a unique pterocarpan structure. Interestingly, some of the hairy roots turned into crown galls during the culture period. In summary, our simple method enables the production of transgenic pea hairy roots using biological materials accessible in Japan. The generated hairy roots can be used to elucidate the molecular mechanisms underlying (+)-pisatin biosynthesis as well as hairy root/crown gall formation.

Recent Advances in Heterologous Synthesis Paving Way for Future Green-Modular Bioindustries: A Review With Special Reference to Isoflavonoids

Isoflavonoids are well-known plant secondary metabolites that have gained importance in recent time due to their multiple nutraceutical and pharmaceutical applications. In plants, isoflavonoids play a role in plant defense and can confer the host plant a competitive advantage to survive and flourish under environmental challenges. In animals, isoflavonoids have been found to interact with multiple signaling pathways and have demonstrated estrogenic, antioxidant and anti-oncologic activities in vivo. The activity of isoflavonoids in the estrogen pathways is such that the class has also been collectively called phytoestrogens. Over 2,400 isoflavonoids, predominantly from legumes, have been identified so far. The biosynthetic pathways of several key isoflavonoids have been established, and the genes and regulatory components involved in the biosynthesis have been characterized. The biosynthesis and accumulation of isoflavonoids in plants are regulated by multiple complex environmental and genetic factors and interactions. Due to this complexity of secondary metabolism regulation, the export and engineering of isoflavonoid biosynthetic pathways into non-endogenous plants are difficult, and instead, the microorganisms Saccharomyces cerevisiae and Escherichia coli have been adapted and engineered for heterologous isoflavonoid synthesis. However, the current ex-planta production approaches have been limited due to slow enzyme kinetics and traditionally laborious genetic engineering methods and require further optimization and development to address the required titers, reaction rates and yield for commercial application. With recent progress in metabolic engineering and the availability of advanced synthetic biology tools, it is envisaged that highly efficient heterologous hosts will soon be engineered to fulfill the growing market demand.

Molecular cloning and biochemical characterization of isoflav-3-ene synthase, a key enzyme of the biosyntheses of (+)-pisatin and coumestrol

Most leguminous plants produce (-)-type enantiomers of pterocarpans as the phytoalexin, but pea (Pisum sativum L.) produces the opposite stereoisomer of pterocarpan, (+)-pisatin. Biosynthesis of (-)-pterocarpan skeleton is completely characterized at the molecular level, and pterocarpan synthase (PTS), a dirigent (DIR) domain-containing protein, participates in the last dehydration reaction. Similarly, isoflav-3-ene, a precursor of (+)-pisatin, is likely to be biosynthesized by the DIR-mediated dehydration reaction; however the biosynthesis is still unknown. In the present study, we screened PTS homologs based on RNA-sequence data from (+)-pisatin-producing pea seedlings and demonstrated that one of the candidates encodes isoflav-3-ene synthase (I3S). Real-time PCR analysis revealed that transcripts of I3S, in addition to other genes involved in the (+)-pisatin pathway, transiently accumulated in pea upon elicitation prior to the maximum accumulation of (+)-pisatin. I3S orthologs were also found in soybean and Lotus japonicus that are not known to accumulate (+)-pterocarpan, and the catalytic function of gene products was verified to be I3S by the in vitro enzyme assay. Incubation of the crude extract of elicited soybean cells with isoflav-3-ene yielded coumestrol, suggesting that isoflav-3-ene is a precursor of coumestrol biosynthesis in soybean.

The Effects of Domestication on Secondary Metabolite Composition in Legumes

Legumes are rich in secondary metabolites, such as polyphenols, alkaloids, and saponins, which are important defense compounds to protect the plant against herbivores and pathogens, and act as signaling molecules between the plant and its biotic environment. Legume-sourced secondary metabolites are well known for their potential benefits to human health as pharmaceuticals and nutraceuticals. During domestication, the color, smell, and taste of crop plants have been the focus of artificial selection by breeders. Since these agronomic traits are regulated by secondary metabolites, the basis behind the genomic evolution was the selection of the secondary metabolite composition. In this review, we will discuss the classification, occurrence, and health benefits of secondary metabolites in legumes. The differences in their profiles between wild legumes and their cultivated counterparts will be investigated to trace the possible effects of domestication on secondary metabolite compositions, and the advantages and drawbacks of such modifications. The changes in secondary metabolite contents will also be discussed at the genetic level to examine the genes responsible for determining the secondary metabolite composition that might have been lost due to domestication. Understanding these genes would enable breeding programs and metabolic engineering to produce legume varieties with favorable secondary metabolite profiles for facilitating adaptations to a changing climate, promoting beneficial interactions with biotic factors, and enhancing health-beneficial secondary metabolite contents for human consumption.

Pterocarpan synthase (PTS) structures suggest a common quinone methide-stabilizing function in dirigent proteins and proteins with dirigent-like domains

The biochemical activities of dirigent proteins (DPs) give rise to distinct complex classes of plant phenolics. DPs apparently began to emerge during the aquatic-to-land transition, with phylogenetic analyses revealing the presence of numerous DP subfamilies in the plant kingdom. The vast majority (>95%) of DPs in these large multigene families still await discovery of their biochemical functions. Here, we elucidated the 3D structures of two pterocarpan-forming proteins with dirigent-like domains. Both proteins stereospecifically convert distinct diastereomeric chiral isoflavonoid precursors to the chiral pterocarpans, (-)- and (+)-medicarpin, respectively. Their 3D structures enabled comparisons with stereoselective lignan- and aromatic terpenoid-forming DP orthologs. Each protein provides entry into diverse plant natural products classes, and our experiments suggest a common biochemical mechanism in binding and stabilizing distinct plant phenol-derived mono- and bis-quinone methide intermediates during different C-C and C-O bond-forming processes. These observations provide key insights into both their appearance and functional diversification of DPs during land plant evolution/adaptation. The proposed biochemical mechanisms based on our findings provide important clues to how additional physiological roles for DPs and proteins harboring dirigent-like domains can now be rationally and systematically identified.

Flavonoids as key bioactive components of Oxytropis falcata bunge, a traditional anti-inflammatory and analgesic Tibetan medicine

The traditional Tibetan medicine Oxytropis falcata Bunge, in the Leguminosae family, is widely used in the west area owing to its significant anti-inflammatory and analgesic activities. O. falcata is rich in flavonoids, which are the main secondary metabolites and key bioactive components of this plant. Up to now, 91 flavonoids have been isolated from O. falcata, including isoflavone, flavone, flavonone, flavonol, homoisoflavonoid, chalcone, dihydrochalcone, chalcone dimers, and pterocarpans. The flavonoids in O. falcata have good anti-inflammatory and analgesic activities, which are comparable to those of a positive drug control (indomethacin). Furthermore, these flavonoids exhibit antibacterial, antioxidant, antitumour, anti-cardiovascular disease, and haemostatic activities. However, to date, O. falcata has not been reviewed comprehensively. Herein, the main secondary metabolites, biosynthetic pathways, and bioactivities of O. falcata are discussed.

The Missing Link in Leguminous Pterocarpan Biosynthesis is a Dirigent Domain-Containing Protein with Isoflavanol Dehydratase Activity

Pterocarpan forms the basic structure of leguminous phytoalexins, and most of the isoflavonoid pathway genes encoding the enzymes responsible for its biosynthesis have been identified. However, the last step of pterocarpan biosynthesis is a ring closure reaction, and the enzyme that catalyzes this step, 2'-hydroxyisoflavanol 4,2'-dehydratase or pterocarpan synthase (PTS), remains as an unidentified 'missing link'. This last ring formation is assumed to be the key step in determining the stereochemistry of pterocarpans, which plays a role in their antimicrobial activity. In this study, a cDNA clone encoding PTS from Glycyrrhiza echinata (GePTS1) was identified through functional expression fractionation screening of a cDNA library, which requires no sequence information, and orthologs from soybean (GmPTS1) and Lotus japonicus (LjPTS1) were also identified. These proteins were heterologously expressed in Escherichia coli and biochemically characterized. Surprisingly, the proteins were found to include amino acid motifs characteristic of dirigent proteins, some of which control stereospecific phenoxy radical coupling in lignan biosynthesis. The stereospecificity of substrates and products was examined using four substrate stereoisomers with hydroxy and methoxy derivatives at C-4'. The results showed that the 4R configuration was essential for the PTS reaction, and (-)- and (+)-pterocarpans were produced depending on the stereochemistry at C-3. In suspension-cultured soybean cells, levels of the GmPTS1 transcript increased temporarily prior to the peak in phytoalexin accumulation, strongly supporting the possible involvement of PTS in pterocarpan biosynthesis.

Deciphering common and specific transcriptional immune responses in pea towards the oomycete pathogens Aphanomyces euteiches and Phytophthora pisi

BACKGROUND

Root rot caused by Aphanomyces euteiches is one of the most destructive pea diseases while a distantly related species P. pisi has been recently described as the agent of pea and faba bean root rot. These two oomycete pathogens with different pathogenicity factor repertories have both evolved specific mechanisms to infect pea. However, little is known about the genes and mechanisms of defence against these pathogens in pea. In the present study, the transcriptomic response of pea to these two pathogens was investigated at two time points during early phase of infection using a Medicago truncatula microarray.

RESULTS

Of the 37,976 genes analysed, 574 and 817 were differentially expressed in response to A. euteiches at 6 hpi and 20 hpi, respectively, while 544 and 611 genes were differentially regulated against P. pisi at 6 hpi and 20 hpi, respectively. Differentially expressed genes associated with plant immunity responses were involved in cell wall reinforcement, hormonal signalling and phenylpropanoid metabolism. Activation of cell wall modification, regulation of jasmonic acid biosynthesis and induction of ethylene signalling pathway were among the common transcriptional responses to both of these oomycetes. However, induction of chalcone synthesis and the auxin pathway were specific transcriptional changes against A. euteiches.

CONCLUSIONS

Our results demonstrate a global view of differentially expressed pea genes during compatible interactions with P. pisi and A. euteiches at an early phase of infection. The results suggest that distinct signalling pathways are triggered in pea by these two pathogens that lead to common and specific immune mechanisms in response to these two oomycetes. The generated knowledge may eventually be used in breeding pea varieties with resistance against root rot disease.

Deciphering the role of phytoalexins in plant-microorganism interactions and human health

Phytoalexins are low molecular weight antimicrobial compounds that are produced by plants as a response to biotic and abiotic stresses. As such they take part in an intricate defense system which enables plants to control invading microorganisms. In this review we present the key features of this diverse group of molecules, namely their chemical structures, biosynthesis, regulatory mechanisms, biological activities, metabolism and molecular engineering.

(+)-Pisatin biosynthesis: from (-) enantiomeric intermediates via an achiral 7,2'-dihydroxy-4',5'-methylenedioxyisoflav-3-ene

(+)-Pisatin, produced by peas (Pisum sativum L.), is an isoflavonoid derivative belonging to the pterocarpan family. It was the first chemically identified phytoalexin, and subsequent research has demonstrated that most legumes produce pterocarpans with the opposite stereochemistry. Studies on the biosynthesis of (+)-pisatin have shown that (-) enantiomeric compounds are intermediates in (+)-pisatin synthesis. However, the steps from the (-)-7,2'-dihydroxy-4',5'-methylenedioxyisoflavanone [(-)-sophorol] intermediate to (+)-6a-hydroxymaackiain intermediate are undetermined. Chemical reduction of (-)-sophorol using sodium borohydride (NaBH4) produced two isomers of (-)-7,2'-dihydroxy-4',5'-methylenedioxyisoflavanol [(-)-DMDI] with optimal UV absorbance at 299.3 and 300.5 nm, respectively. In contrast, enzymatic reduction of (-)-sophorol by the pea enzyme sophorol reductase (SOR) produced only the 299.3 nm (-)-DMDI isomer. Proton nuclear magnetic resonance ((1)H NMR) analysis of the 299.3 nm (-)-DMDI isomer demonstrated that this isomer had the same NMR spectrum as previously reported for cis-isoflavanol isomers, indicating that cis-(-)-DMDI is an intermediate in (+)-pisatin biosynthesis. Enzyme assays using protein extracts from pea tissue treated with CuCl2 as an elicitor converted the cis-(-)-DMDI isomer into an achiral isoflavene, 7,2'-dihydroxy-4',5'-methylenedioxyisoflav-3-ene (DMDIF), and the trans-(-)-DMDI isomer was not metabolized by the same protein preparation. A comparison of the enzyme activities on cis-(-)-DMDI with protein preparations from elicited tissue versus non-elicited tissue showed a threefold increase in the amount of activity in the proteins from the elicited tissue. Proteins from the elicited tissues of alfalfa, bean, and chickpea converted cis-(-)-DMDI into either (-)-maackiain and/or (-)-sophorol, while proteins from the elicited tissues of broccoli and pepper produced no detectable product. These results are consistent with the involvement of cis-(-)-DMDI and the achiral DMDIF as intermediates in (+)-pisatin biosynthesis.

Modulation of phytoalexin biosynthesis in engineered plants for disease resistance

Phytoalexins are antimicrobial substances of low molecular weight produced by plants in response to infection or stress, which form part of their active defense mechanisms. Starting in the 1950's, research on phytoalexins has begun with biochemistry and bio-organic chemistry, resulting in the determination of their structure, their biological activity as well as mechanisms of their synthesis and their catabolism by microorganisms. Elucidation of the biosynthesis of numerous phytoalexins has permitted the use of molecular biology tools for the exploration of the genes encoding enzymes of their synthesis pathways and their regulators. Genetic manipulation of phytoalexins has been investigated to increase the disease resistance of plants. The first example of a disease resistance resulting from foreign phytoalexin expression in a novel plant has concerned a phytoalexin from grapevine which was transferred to tobacco. Transformations were then operated to investigate the potential of other phytoalexin biosynthetic genes to confer resistance to pathogens. Unexpectedly, engineering phytoalexins for disease resistance in plants seem to have been limited to exploiting only a few phytoalexin biosynthetic genes, especially those encoding stilbenes and some isoflavonoids. Research has rather focused on indirect approaches which allow modulation of the accumulation of phytoalexin employing transcriptional regulators or components of upstream regulatory pathways. Genetic approaches using gain- or less-of functions in phytoalexin engineering together with modulation of phytoalexin accumulation through molecular engineering of plant hormones and defense-related marker and elicitor genes have been reviewed.

Phytoalexin transgenics in crop protection--fairy tale with a happy end?

Phytoalexins are pathogen induced low molecular weight compounds with antimicrobial activities derived from secondary metabolism. Following their identification, phytoalexins were directly incorporated into the network of plant defense responses. Due to their heterogeneity, the metabolic pathways involved in phytoalexin formation and in particular the regulatory mechanisms remained elusive. Consequently, research focus shifted to the characterization of other components of plant immunity such as defense signaling and resistance mechanisms, including components of systemic acquired and induced systemic resistance, effector and pathogen-associated molecular pattern triggered immunity as well as R-gene resistance. Despite the obtained knowledge on these immunity mechanisms, genetic engineering employing these mechanisms and classical breeding reached too low improvements in crop protection, probably because classical breeding focused on yield performance and taste, rather than pathogen resistance. The increasing demand for disease resistant crop species and the aim to reduce pesticide application therefore requires alternative approaches. Recent advances in the understanding of phytoalexin function, biosynthesis and regulation, in combination with novel methods of molecular engineering and advances in instrumental analysis, returned attention to phytoalexins as a potent target for improving crop protection. Based on this, the advantages as well as potential bottlenecks for molecular approaches of modulating inducible phytoalexins to improve crop protection are discussed.

Phytoalexins in defense against pathogens

Plants use an intricate defense system against pests and pathogens, including the production of low molecular mass secondary metabolites with antimicrobial activity, which are synthesized de novo after stress and are collectively known as phytoalexins. In this review, we focus on the biosynthesis and regulation of camalexin, and its role in plant defense. In addition, we detail some of the phytoalexins produced by a range of crop plants from Brassicaceae, Fabaceae, Solanaceae, Vitaceae and Poaceae. This includes the very recently identified kauralexins and zealexins produced by maize, and the biosynthesis and regulation of phytoalexins produced by rice. Molecular approaches are helping to unravel some of the mechanisms and reveal the complexity of these bioactive compounds, including phytoalexin action and metabolism.

Association between border cell responses and localized root infection by pathogenic Aphanomyces euteiches

BACKGROUND AND AIMS

The oomycete Aphanomyces euteiches causes up to 80 % crop loss in pea (Pisum sativum). Aphanomyces euteiches invades the root system leading to a complete arrest of root growth and ultimately to plant death. To date, disease control measures are limited to crop rotation and no resistant pea lines are available. The present study aims to get a deeper understanding of the early oomycete-plant interaction at the tissue and cellular levels.

METHODS

Here, the process of root infection by A. euteiches on pea is investigated using flow cytometry and microscopic techniques. Dynamic changes in secondary metabolism are analysed with high-performance liquid chromatography with diode-array detection.

KEY RESULTS

Root infection is initiated in the elongation zone but not in the root cap and border cells. Border-cell production is significantly enhanced in response to root inoculation with changes in their size and morphology. The stimulatory effect of A. euteiches on border-cell production is dependent on the number of oospores inoculated. Interestingly, border cells respond to pathogen challenge by increasing the synthesis of the phytoalexin pisatin.

CONCLUSIONS

Distinctive responses to A. euteiches inoculation occur at the root tissue level. The findings suggest that root border cells in pea are involved in local defence of the root tip against A. euteiches. Root border cells constitute a convenient quantitative model to measure the molecular and cellular basis of plant-microbe interactions.

Sub-lethal levels of electric current elicit the biosynthesis of plant secondary metabolites

Many secondary metabolites that are normally undetectable or in low amounts in healthy plant tissue are synthesized in high amounts in response to microbial infection. Various abiotic and biotic agents have been shown to mimic microorganisms and act as elicitors of the synthesis of these plant compounds. In the present study, sub-lethal levels of electric current are shown to elicit the biosynthesis of secondary metabolites in transgenic and non-transgenic plant tissue. The production of the phytoalexin (+)-pisatin by pea was used as the main model system. Non-transgenic pea hairy roots treated with 30-100 mA of electric current produced 13 times higher amounts of (+)-pisatin than did the non-elicited controls. Electrically elicited transgenic pea hairy root cultures blocked at various enzymatic steps in the (+)-pisatin biosynthetic pathway also accumulated intermediates preceding the blocked enzymatic step. Secondary metabolites not usually produced by pea accumulated in some of the transgenic root cultures after electric elicitation due to the diversion of the intermediates into new pathways. The amount of pisatin in the medium bathing the roots of electro-elicited roots of hydroponically cultivated pea plants was 10 times higher 24 h after elicitation than in the medium surrounding the roots of non-elicited control plants, showing not only that the electric current elicited (+)-pisatin biosynthesis but also that the (+)-pisatin was released from the roots. Seedlings, intact roots or cell suspension cultures of fenugreek (Trigonella foenum-graecum), barrel medic, (Medicago truncatula), Arabidopsis thaliana, red clover (Trifolium pratense) and chickpea (Cicer arietinum) also produced increased levels of secondary metabolites in response to electro-elicitation. On the basis of our results, electric current would appear to be a general elicitor of plant secondary metabolites and to have potential for application in both basic and commercial research.