Isolation of the cDNAs encoding (+)6a-hydroxymaackiain 3-O-methyltransferase, the terminal step for the synthesis of the phytoalexin pisatin in Pisum sativum

Bioprospecting microbes and enzymes for the production of pterocarpans and coumestans

The isoflavonoid derivatives, pterocarpans and coumestans, are explored for multiple clinical applications as osteo-regenerative, neuroprotective and anti-cancer agents. The use of plant-based systems to produce isoflavonoid derivatives is limited due to cost, scalability, and sustainability constraints. Microbial cell factories overcome these limitations in which model organisms such as Saccharomyces cerevisiae offer an efficient platform to produce isoflavonoids. Bioprospecting microbes and enzymes can provide an array of tools to enhance the production of these molecules. Other microbes that naturally produce isoflavonoids present a novel alternative as production chassis and as a source of novel enzymes. Enzyme bioprospecting allows the complete identification of the pterocarpans and coumestans biosynthetic pathway, and the selection of the best enzymes based on activity and docking parameters. These enzymes consolidate an improved biosynthetic pathway for microbial-based production systems. In this review, we report the state-of-the-art for the production of key pterocarpans and coumestans, describing the enzymes already identified and the current gaps. We report available databases and tools for microbial bioprospecting to select the best production chassis. We propose the use of a holistic and multidisciplinary bioprospecting approach as the first step to identify the biosynthetic gaps, select the best microbial chassis, and increase productivity. We propose the use of microalgal species as microbial cell factories to produce pterocarpans and coumestans. The application of bioprospecting tools provides an exciting field to produce plant compounds such as isoflavonoid derivatives, efficiently and sustainably.

Characterization of field pea (Pisum sativum) resistance against Peyronellaea pinodes and Didymella pinodella that cause ascochyta blight

Ascochyta blight is one of the most destructive diseases in field pea and is caused by either individual or combined infections by the necrotrophic pathogens Peyronellaea pinodes, Didymella pinodella, Ascochyta pisi and Ascochyta koolunga. Knowledge of disease epidemiology will help in understanding the resistance mechanisms, which, in turn, is beneficial in breeding for disease resistance. A pool of breeding lines and cultivars were inoculated with P. pinodes and D. pinodella to study the resistance responses and to characterize the underlying resistance reactions. In general, phenotypic analysis of controlled environment disease assays showed clear differential responses among genotypes against the two pathogens. The released variety PBA Wharton and the breeding line 11HP302-12HO-1 showed high levels of resistance against both pathogens whereas PBA Twilight and 10HP249-11HO-7 showed differential responses between the two pathogens, showing higher resistance against D. pinodella as compared to P. pinodes. OZP1604 had high infection levels against both pathogens. Histochemical analysis of leaves using diamino benzidine (DAB) showed the more resistant genotypes had lower accumulation of hydrogen peroxide compared to susceptible genotypes. The digital images of DAB staining were analyzed using ImageJ, an image analysis software. The image analysis results showed that quantification of leaf disease infection through image analysis is a useful tool in estimating the level of cell death in biotic stress studies. The qRT-PCR analysis of defense related genes showed that partially resistant genotypes had significantly higher expression of PsOXII and Pshmm6 in the P. pinodes treated plants, whereas expression of PsOXII, PsAPX1, PsCHS3 and PsOPR1 increased in partially resistant plants inoculated with D. pinodella. The differential timing and intensity of expression of a range of genes between resistant lines challenged with the same pathogen, or challenged with different pathogens, suggests that there are multiple pathways that restrict infection in this complex pathogen-host interaction. The combination of phenotypic, histochemical and molecular approaches provide a comprehensive picture of the infection process and resistance mechanism of pea plants against these pathogens.

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.

Identification of a Unique Type of Isoflavone O-Methyltransferase, GmIOMT1, Based on Multi-Omics Analysis of Soybean under Biotic Stress

Isoflavonoids are commonly found in leguminous plants. Glycitein is one of the isoflavones produced by soybean. The genes encoding the enzymes in the isoflavone biosynthetic pathway have mostly been identified and characterized. However, the gene(s) for isoflavone O-methyltransferase (IOMT), which catalyzes the last step of glycitein biosynthesis, has not yet been identified. In this study, we conducted multi-omics analyses of fungal-inoculated soybean and indicated that glycitein biosynthesis was induced in response to biotic stress. Moreover, we identified a unique type of IOMT, which participates in glycitein biosynthesis. Soybean seedlings were inoculated with Aspergillus oryzae or Rhizopus oligosporus and sampled daily for 8 d. Multi-omics analyses were conducted using liquid chromatography-tandem mass spectrometry and RNA sequencing. Metabolome analysis revealed that glycitein derivatives increased following fungal inoculation. Transcriptome co-expression analysis identified two candidate IOMTs that were co-expressed with the gene encoding flavonoid 6-hydroxylase (F6H), the key enzyme in glycitein biosynthesis. The enzymatic assay of the two IOMTs using respective recombinant proteins showed that one IOMT, named as GmIOMT1, produced glycitein. Unlike other IOMTs, GmIOMT1 belongs to the cation-dependent OMT family and exhibited the highest activity with Zn2+ among cations tested. Moreover, we demonstrated that GmIOMT1 overexpression increased the levels of glycitein derivatives in soybean hairy roots when F6H was co-expressed. These results strongly suggest that GmIOMT1 participates in inducing glycitein biosynthesis in response to biotic stress.

An Alternative Pathway for Formononetin Biosynthesis in Pueraria lobata

The O-methylation is an important tailing process in Pueraria lobata isoflavone metabolism, but the molecular mechanism governing it remains not elucidated. This manuscript describes the mining of key O-methyltransferases (OMTs) involved in the process. Using our previously constructed P. lobata transcriptome, the OMT candidates were searched, extensively analyzed, and their functions were investigated by expression in yeast, Escherichia coli, or Glycine max hairy roots. Here, we report the identification of the key OMT gene responsible for formononetin production in P. lobata (designated as PlOMT9). PlOMT9 primarily functions as an isoflavone-specific 4'-O-methyltransferase, although it shows high sequence identities with isoflavone 7-O-methyltransferases. Moreover, unlike the previously reported OMTs that catalyze the 4'-O-methylation for formononetin biosynthesis at the isoflavanone stage, PlOMT9 performs this modifying step at the isoflavone level, using daidzein rather than 2,7,4'-trihydroxy-isoflavanone as the substrate. Gene expression analyses and metabolite profiling supported its proposed roles in P. lobata. Using the system of transgenic G. max hairy roots, the role of PlOMT9 in the biosynthesis of formononetin was further demonstrated in vivo.

The role of strigolactones and ethylene in disease caused by Pythium irregulare

Plant hormones play key roles in defence against pathogen attack. Recent work has begun to extend this role to encompass not just the traditional disease/stress hormones, such as ethylene, but also growth-promoting hormones. Strigolactones (SLs) are the most recently defined group of plant hormones with important roles in plant-microbe interactions, as well as aspects of plant growth and development, although the knowledge of their role in plant-pathogen interactions is extremely limited. The oomycete Pythium irregulare is a poorly controlled pathogen of many crops. Previous work has indicated an important role for ethylene in defence against this oomycete. We examined the role of ethylene and SLs in response to this pathogen in pea (Pisum sativum L.) at the molecular and whole-plant levels using a set of well-characterized hormone mutants, including an ethylene-insensitive ein2 mutant and SL-deficient and insensitive mutants. We identified a key role for ethylene signalling in specific cell types that reduces pathogen invasion, extending the work carried out in other species. However, we found no evidence that SL biosynthesis or response influences the interaction of pea with P. irregulare or that synthetic SL influences the growth or hyphal branching of the oomycete in vitro. Future work should seek to extend our understanding of the role of SLs in other plant interactions, including with other fungal, bacterial and viral pathogens, nematodes and insect pests.

Molecular Cloning and Functional Characterization of a Novel Isoflavone 3'-O-methyltransferase from Pueraria lobata

Pueraria lobata roots accumulate 3'-, 4'- and 7-O-methylated isoflavones and many of these methylated compounds exhibit various pharmacological activities. Either the 4'- or 7-O-methylation activity has been investigated at molecular levels in several legume species. However, the gene encoding the isoflavone 3'-O-methyltransferase (OMT) has not yet been isolated from any plant species. In this study, we reported the first cDNA encoding the isoflavone 3'-OMT from P. lobata (designated PlOMT4). Heterologous expressions in yeast and Escherichia coli cells showed that the gene product exhibits an enzyme activity to methylate the 3'-hydroxy group of the isoflavone substrate. The transcript abundance of PlOMT4 matches well with its enzymatic product in different organs of P. lobata and in the plant roots in response to methyl jasmonate elicitation. Integration of the biochemical with metabolic and transcript data supported the proposed function of PlOMT4. The identification of PlOMT4 would not only help to understand the isoflavonoid metabolism in P. lobata but also potentially provide an enzyme catalyst for methylating existing drug candidates to improve their hydrophobicity.

De novo sequencing and analysis of Lophophora williamsii transcriptome, and searching for putative genes involved in mescaline biosynthesis

Background

Lophophora williamsii (commonly named peyote) is a small, spineless cactus with psychoactive alkaloids, particularly mescaline. Peyote utilizes crassulacean acid metabolism (CAM), an alternative form of photosynthesis that exists in succulents such as cacti and other desert plants. Therefore, its transcriptome can be considered an important resource for future research focused on understanding how these plants make more efficient use of water in marginal environments and also for research focused on better understanding of the overall mechanisms leading to production of plant natural products and secondary metabolites.

Results

In this study, two cDNA libraries were generated from L. williamsii. These libraries, representing buttons (tops of stems) and roots were sequenced using different sequencing platforms (GS-FLX, GS-Junior and PGM, respectively). A total of 5,541,550 raw reads were generated, which were assembled into 63,704 unigenes with an average length of 564.04 bp. A total of 25,149 unigenes (62.19 %) was annotated using public databases. 681 unigenes were found to be differentially expressed when comparing the two libraries, where 400 were preferentially expressed in buttons and 281 in roots. Some of the major alkaloids, including mescaline, were identified by GC-MS and relevant metabolic pathways were reconstructed using the Kyoto encyclopedia of genes and genomes database (KEGG). Subsequently, the expression patterns of preferentially expressed genes putatively involved in mescaline production were examined and validated by qRT-PCR.

Conclusions

High throughput transcriptome sequencing (RNA-seq) analysis allowed us to efficiently identify candidate genes involved in mescaline biosynthetic pathway in L. williamsii; these included tyrosine/DOPA decarboxylase, hydroxylases, and O-methyltransferases. This study sets the theoretical foundation for bioassay design directed at confirming the participation of these genes in mescaline production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1821-9) contains supplementary material, which is available to authorized users.

(+)-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.

Structure, function, and evolution of plant O-methyltransferases

Plant O-methyltransferases (OMTs) constitute a large family of enzymes that methylate the oxygen atom of a variety of secondary metabolites including phenylpropanoids, flavonoids, and alkaloids. O-Methylation plays a key role in lignin biosynthesis, stress tolerance, and disease resistance in plants. To gain insights into the evolution of the extraordinary diversity of plant O-methyltransferases, and to develop a framework phylogenetic tree for improved prediction of the putative function of newly identified OMT-like gene sequences, we performed a comparative and phylogenetic analysis of 61 biochemically characterized plant OMT protein sequences. The resulting phylogenetic tree revealed two major groups. One of the groups included two sister clades, one comprising the caffeoyl CoA OMTs (CCoA OMTs) that methylate phenolic hydroxyl groups of hydroxycinnamoyl CoA esters, and the other containing the carboxylic acid OMTs that methylate aliphatic carboxyl groups. The other group comprised the remaining OMTs, which act on a diverse group of metabolites including hydroxycinnamic acids, flavonoids, and alkaloids. The results suggest that some OMTs may have undergone convergent evolution, while others show divergent evolution. The high number of unique conserved regions within the CCoA OMTs and carboxylic acid OMTs provide an opportunity to design oligonucleotide primers to selectively amplify and characterize similar OMT genes from many plant species.

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

(+)-Pisatin, the major phytoalexin of pea (Pisum sativum L.), is believed to be synthesized via two chiral intermediates, (-)-7,2'-dihydroxy-4',5'-methylenedioxyisoflavanone [(-)-sophorol] and (-)-7,2'-dihydroxy-4',5'-methylenedioxyisoflavanol [(-)-DMDI]; both have an opposite C-3 absolute configuration to that found at C-6a in (+)-pisatin. The expression of isoflavone reductase (IFR), which converts 7,2'-dihydroxy-4',5'-methylenedioxyisoflavone (DMD) to (-)-sophorol, sophorol reductase (SOR), which converts (-)-sophorol to (-)-DMDI, and hydroxymaackiain-3-O-methyltransferase (HMM), believed to be the last step of (+)-pisatin biosynthesis, were inactivated by RNA-mediated genetic interference (RNAi) in pea hairy roots. Some hairy root lines containing RNAi constructs of IFR and SOR accumulated DMD or (-)-sophorol, respectively, and were deficient in (+)-pisatin biosynthesis supporting the involvement of chiral intermediates with a configuration opposite to that found in (+)-pisatin in the biosynthesis of (+)-pisatin. Pea proteins also converted (-)-DMDI to an achiral isoflavene suggesting that an isoflavene might be the intermediate through which the configuration is changed to that found in (+)-pisatin. Hairy roots containing RNAi constructs of HMM also were deficient in (+)-pisatin biosynthesis, but did not accumulate (+)-6a-hydroxymaackiain, the proposed precursor to (+)-pisatin. Instead, 2,7,4'-trihydroxyisoflavanone (TIF), daidzein, isoformononetin, and liquiritigenin accumulated. HMM has a high amino acid similarity to hydroxyisoflavanone-4'-O-methyltransferase (HI4'OMT), an enzyme that methylates TIF, an early intermediate in the isoflavonoid pathway. The accumulation of these four compounds is consistent with the blockage of the synthesis of (+)-pisatin at the HI4'OMT catalyzed step resulting in the accumulation of liquiritigenin and TIF and the diversion of the pathway to produce daidzein and isoformononetin, compounds not normally made by pea. Previous results have identified two highly similar "HMMs" in pea. The current results suggest that both of these O-methyltransferases are involved in (+)-pisatin biosynthesis and that one functions early in the pathway as HI4'OMT and the second acts at the terminal step of the pathway.

Candidate genes for quantitative resistance to Mycosphaerella pinodes in pea (Pisum sativum L.)

Partial resistance to Mycosphaerella pinodes in pea is quantitatively inherited. Genomic regions involved in resistance (QTLs) have been previously identified in the pea genome, but the molecular basis of the resistance is still unknown. The objective of this study was to map resistance gene analogs (RGA) and defense-related (DR) genes in the JI296 x DP RIL population that has been used for mapping QTLs for resistance to M. pinodes, and identify co-localizations between candidate genes and QTLs. Using degenerate oligonucleotide primers designed on the conserved motifs P-loop and GLPL of cloned resistance genes, we isolated and cloned 16 NBS-LRR sequences, corresponding to five distinct classes of RGAs. Specific second-generation primers were designed for each class. RGAs from two classes were located on the linkage group (LG) VII. Another set of PCR-based markers was designed for four RGA sequences previously isolated in pea and 12 previously cloned DR gene sequences available in databases. Out of the 16 sequences studied, the two RGAs RGA-G3A and RGA2.97 were located on LG VII, PsPRP4A was located on LG II, Peachi21, PsMnSOD, DRR230-b and PsDof1 were mapped on LG III and peabetaglu and DRR49a were located on LG VI. Two co-localizations between candidate genes and QTLs for resistance to M. pinodes were observed on LG III, between the putative transcription factor PsDof1 and the QTL mpIII-1 and between the pea defensin DRR230-b gene and the QTL mpIII-4. Another co-localization was observed on LG VII between a cluster of RGAs and the QTL mpVII-1. The three co-localizations appear to be located in chromosomal regions containing other disease resistance or DR genes, suggesting an important role of these genomic regions in defense responses against pathogens in pea.

Isoflavonoids of the leguminosae

Isoflavonoids are found predominantly in subfamily Papilionoideae of the Leguminosae. This review describes more than 420 new examples of Leguminosae isoflavonoids, giving details of their source, identification, biological activity, synthesis, and ecological or chemosystematic significance. Other topics addressed include the application of hyphenated analytical techniques to the characterisation of legume-derived isoflavonoids, and advances made in biosynthetic studies. A checklist of new compounds by species is given, and 404 references are cited.

Catalytic specificity of pea O-methyltransferases suggests gene duplication for (+)-pisatin biosynthesis

S-adenosyl-l-methionine: 2-hydroxyisoflavanone 4'-O-methyltransferase (HI4'OMT) methylates 2,7, 4'-trihydroxyisoflavanone to produce formononetin, an essential intermediate in the synthesis of isoflavonoids with methoxy or methylenedioxy groups at carbon 4' (isoflavone numbering). HI4'OMT is highly similar (83% amino acid identity) to (+)-6a-hydroxymaackiain 3-O-methyltransferase (HMM), which catalyzes the last step of (+)-pisatin biosynthesis in pea. Pea contains two linked copies of HMM with 96% amino acid identity. In this report, the catalytic activities of the licorice HI4'OMT protein and of extracts of Escherichia coli containing the pea HMM1 or HMM2 protein are compared on 2,7,4'-trihydroxyisoflavanone and enantiomers of 6a-hydroxymaackiain. All these enzymes produced radiolabelled 2,7-dihydroxy-4'-methoxyisoflavanone or (+)-pisatin from 2,7,4'-trihydroxyisoflavanone or (+)-6a-hydroxymaakiain when incubated with [methyl-(14)C]-S-adenosyl-l-methionine. No product was detected when (-)-6a-hydroxymaackiain was used as the substrate. HI4'OMT and HMM1 showed efficiencies (relative V(max)/K(m)) for the methylation of 2,7,4'-trihydroxyisoflavanone 20 and 4 times higher than for the methylation of (+)-6a-hydroxymaackiain, respectively. In contrast, HMM2 had a higher V(max) and lower K(m) on (+)-6a-hydroxymaackiain, and had a 67-fold higher efficiency for the methylation of (+)-6a-hydroxymaackiain than that for 2,7,4'-trihydroxyisoflavanone. Among the 15 sites at which HMM1 and HMM2 have different amino acid residues, 11 of the residues in HMM1 are the same as found in HI4'OMTs from three plant species. Modeling of the HMM proteins identified three or four putative active site residues responsible for their different substrate preferences. It is proposed that HMM1 is the pea HI4'OMT and that HMM2 evolved by the duplication of a gene encoding a general biosynthetic enzyme (HI4'OMT).

Introduction of plant and fungal genes into pea (Pisum sativum L.) hairy roots reduces their ability to produce pisatin and affects their response to a fungal pathogen

Pisatin is an isoflavonoid phytoalexin synthesized by pea (Pisum sativum L.). Previous studies have identified two enzymes apparently involved in the synthesis of this phytoalexin, isoflavone reductase (IFR), which catalyzes an intermediate step in pisatin biosynthesis, and (+)6a-hydroxymaackiain 3-O-methyltransferase (HMM), an enzyme catalyzing the terminal step. To further evaluate the involvement of these enzymes in pisatin biosynthesis, sense- and antisense-oriented cDNAs of Ifr and Hmm fused to the 35s CaMV promoter, and Agrobacterium rhizogenes, were used to produce transgenic pea hairy root cultures. PDA, a gene encoding pisatin demethylating activity (pda) in the pea-pathogenic fungus Nectria haematococca, also was used in an attempt to reduce pisatin levels. Although hairy root tissue with either sense or antisense Ifr cDNA produced less pisatin, the greatest reduction occurred with sense or antisense Hmm cDNA. The reduced pisatin production in these lines was associated with reduced amounts of Hmm transcripts, HMM protein, and HMM enzyme activity. Hairy roots containing the PDA gene also produced less pisatin. To evaluate the role of pisatin in disease resistance, the virulence of N. haematococca on the transgenic roots that produced the lowest levels of pisatin was tested. Hairy roots expressing antisense Hmm were more susceptible than the control hairy roots to isolates of N. haematococca that are either virulent or nonvirulent on wild-type pea plants. This appears to be the first case of producing transgenic plant tissue with a reduced ability to produce a phytoalexin and demonstrating that such tissue is less resistant to fungal infection: these results support the hypothesis that phytoalexin production is a disease resistance mechanism.

cDNA cloning and biochemical characterization of S-adenosyl-L-methionine: 2,7,4'-trihydroxyisoflavanone 4'-O-methyltransferase, a critical enzyme of the legume isoflavonoid phytoalexin pathway

Formononetin (7-hydroxy-4'-methoxyisoflavone, also known as 4'-O-methyldaidzein) is an essential intermediate of ecophysiologically active leguminous isoflavonoids. The biosynthetic pathway to produce 4'-methoxyl of formononetin has been unknown because the methyl transfer from S-adenosyl-L-methionine (SAM) to 4'-hydroxyl of daidzein has never been detected in any plants. A hypothesis that SAM: daidzein 7-O-methyltransferase (D7OMT), an enzyme with a different regiospecificity, is involved in formononetin biosynthesis through its intracellular compartmentation with other enzymes recently prevails, but no direct evidence has been presented. We proposed a new scheme of formononetin biosynthesis involving 2,7,4'-trihydroxyisoflavanone as the methyl acceptor and subsequent dehydration. We now cloned a cDNA encoding SAM: 2,7,4'-trihydroxyisoflavanone 4'-O-methyltransferase (HI4'OMT) through the screening of functionally expressed Glycyrrhiza echinata (Fabaceae) cDNAs. The reaction product, 2,7-dihydroxy-4'-methoxyisoflavanone, was unambiguously identified. Recombinant G. echinata D7OMT did not show HI4'OMT activity, and G. echinata HI4'OMT protein free from D7OMT was partially purified. HI4'OMT is thus concluded to be distinct from D7OMT, and their distant phylogenetic relationship was further presented. HI4'OMT may be functionally identical to (+)-6a-hydroxymaackiain 3-OMT of pea. Homologous cDNAs were found in several legumes, and the catalytic function of the Lotus japonicus HI4'OMT was verified, indicating that HI4'OMT is the enzyme of formononetin biosynthesis in general legumes.

Tissue specific localization of root infection by fungal pathogens: role of root border cells

When roots of pea seedlings were inoculated uniformly with spores of Nectria haematocca or other pea pathogenic fungi, more than 90% developed lesions in the region of elongation within 3 days. More mature regions of most roots as well as the tip showed no visible signs of infection. Yet, microscopic observation revealed that 'mantles,' comprised of fungal hyphae intermeshed with populations of border cells, covered the tips of most roots. After physical detachment of the mantle, the underlying tip of most roots was found to be free of infection. Mantle-covered root tips did not respond to invasion of their border cells by activation of known defense genes unless there was invasion of the tip itself, as revealed by the presence of a lesion. Concomitant with the activation of defense genes was the induction of a cell-wall degrading enzyme whose expression is a marker for renewed production of border cells. Mantle formation did not occur in response to nonpathogens. The data are consistent with the hypothesis that border cells serve as a host-specific 'decoy' that protects root meristems by inhibiting fungal infection of the root tip.

Molecular cloning of columbamine O-methyltransferase from cultured Coptis japonica cells

To identify all of the O-methyltransferase genes involved in isoquinoline alkaloid biosynthesis in Coptis japonica cells, we sequenced 1014 cDNA clones isolated from high-alkaloid-producing cultured cells of C. japonica. Among them, we found all three reported O-methyltransferases and an O-methyltransferase-like cDNA clone (CJEST64). This cDNA was quite similar to S-adenosyl-l-methionine:coclaurine 6-O-methyltransferase and S-adenosyl-l-methionine:isoflavone 7-O-methyltransferase. As S-adenosyl-l-methionine:columbamine O-methyltransferase, which catalyzes the conversion of columbamine to palmatine, is one of the remaining unelucidated components in isoquinoline alkaloid biosynthesis in C. japonica, we heterologously expressed the protein in Escherichia coli and examined the activity of columbamine O-methyltransferase. The recombinant protein clearly showed O-methylation activity using columbamine, as well as (S)-tetrahydrocolumbamine, (S)-, (R,S)-scoulerine and (R,S)-2,3,9,10-tetrahydroxyprotoberberine as substrates. This result clearly indicated that EST analysis was useful for isolating the candidate gene in a relatively well-characterized biosynthetic pathway. The relationship between the structure and substrate recognition of the O-methyltransferases involved in isoquinoline alkaloid biosynthesis, and a reconsideration of the biosynthetic pathway to palmatine are discussed.

Molecular characterization of the S-adenosyl-L-methionine:3'-hydroxy-N-methylcoclaurine 4'-O-methyltransferase involved in isoquinoline alkaloid biosynthesis in Coptis japonica

S-adenosyl-L-methionine:3'-hydroxy-N-methylcoclaurine 4'-O-methyltransferase (4'-OMT) catalyzes the conversion of 3'-hydroxy-N-methylcoclaurine to reticuline, an important intermediate in synthesizing isoquinoline alkaloids. In an earlier step in the biosynthetic pathway to reticuline, another O-methyltransferase, S-adenosyl-L-methionine:norcoclaurine 6-O-methyltransferase (6-OMT), catalyzes methylation of the 6-hydroxyl group of norcoclaurine. We isolated two kinds of cDNA clones that correspond to the internal amino acid sequences of a 6-OMT/4'-OMT preparation from cultured Coptis japonica cells. Heterologously expressed proteins had 6-OMT or 4'-OMT activities, indicative that each cDNA encodes a different enzyme. 4'-OMT was purified using recombinant protein, and its enzymological properties were characterized. It had enzymological characteristics similar to those of 6-OMT; the active enzyme was the dimer of the subunit, no divalent cations were required for activity, and there was inhibition by Fe(2+), Cu(2+), Co(2+), Zn(2+), or Ni(2+), but none by the SH reagent. 4'-OMT clearly had different substrate specificity. It methylated (R,S)-6-O-methylnorlaudanosoline, as well as (R, S)-laudanosoline and (R,S)-norlaudanosoline. Laudanosoline, an N-methylated substrate, was a much better substrate for 4'-OMT than norlaudanosoline. 6-OMT methylated norlaudanosoline and laudanosoline equally. Further characterization of the substrate saturation and product inhibition kinetics indicated that 4'-OMT follows an ordered Bi Bi mechanism, whereas 6-OMT follows a Ping-Pong Bi Bi mechanism. The molecular evolution of these two related O-methyltransferases is discussed.

Functional expression of an Arabidopsis cDNA clone encoding a flavonol 3'-O-methyltransferase and characterization of the gene product

We report that the cDNA clone (Accession No. U70424), previously isolated from Arabidopsis thaliana as encoding a caffeic acid/5-hydroxyferulic acid O-methyltransferase (OMT) (1), has now been overexpressed in Escherichia coli BL21 and its recombinant protein identified as a novel flavonol 3'-OMT. It is, therefore, renamed AtOMT1. This cDNA clone has previously been identified on the basis of its 88% amino acid sequence similarity and 80% identity to the aspen bispecific lignin OMT (2), the type member of the group involved in lignin biosynthesis. Our data indicate that this novel OMT uses the flavonol quercetin as the preferred substrate, but neither of the hydroxycinnamic acids, caffeic or 5-hydroxyferulic, to any significant extent. This indicates that the high sequence similarity/identity of AtOMT1 to that of the aspen lignin OMT (2) is not sufficient to assign the function of this gene product.

Molecular markers for UV-B stress in plants: alteration of the expression of four classes of genes in Pisum sativum and the formation of high molecular mass RNA adducts

Sixteen ultraviolet-B radiation-regulated pea genes were identified. Functionally, the corresponding proteins were divided into four groups. (i) Chloroplast-localized proteins. Genes for these proteins were down-regulated, underlining the deleterious effects of UV-B on this organelle. A novel down-regulated photosystem I light-harvesting chlorophyll a/b-binding protein gene (PsLhcA4), was cloned and sequenced. (ii) Protein turnover enzymes. Levels of mature mRNAs for the PU1 and PsUBC4 genes, encoding proteins of the ubiquitin protein degradation pathway, were up- and down-regulated, respectively, implying alteration of plant cell protein content by changes in both gene expression and protein degradation. (iii) Proteins involved in intracellular signalling. Expression of genes for small GTPases, rab and rho homologues, were altered. (iv) Phenylpropanoid or flavonoid biosynthesis. Expression of three genes encoding enzymes in these pathways were up-regulated and one of them, the novel PsC450R1, was cloned and sequenced. Moreover, unexpected high molecular mass psbA RNA adducts were found to appear after UV-B exposure. In addition, a large increase in corresponding high molecular mass adducts were also found for PsLhcA4, and PsUBC4 mRNA and 23S rRNA. These RNA species do not contain protein and probably appear due to cross-linking of two or more RNA molecules, or are the result of UV-B-induced failure of transcription termination.

PHYTOALEXINS: What Have We Learned After 60 Years?

One of the best and longest-studied defense response of plants to infection is the induced accumulation of antimicrobial, low-molecular-weight secondary metabolites known as phytoalexins. Since the phytoalexin hypothesis was first proposed in 1940, a role for these compounds in defense has been revealed through several experimental approaches. Support has come, for example, through studies on the rate of phytoalexins in relation to cessation of pathogen development, quantification of phytoalexins at the infection site, and relationship of pathogen virulence to the phytoalexin tolerance. Evidence in support of phytoalexins in resistance as well some recent advances in phytoalexin biosynthesis are reviewed. Criteria for evaluating a role for phytoalexins in disease resistance are also discussed.