Stagonospora avenae secretes multiple enzymes that hydrolyze oat leaf saponins

The Loss-of-Function Mutation aldA67 Leads to Enhanced α-L-Rhamnosidase Production by Aspergillus nidulans

In Aspergillus nidulans L-rhamnose is catabolised to pyruvate and L-lactaldehyde, and the latter ultimately to L-lactate, via the non-phosphorylated pathway (LRA) encoded by the genes lraA-D, and aldA that encodes a broad substrate range aldehyde dehydrogenase (ALDH) that also functions in ethanol utilisation. LRA pathway expression requires both the pathway-specific transcriptional activator RhaR (rhaR is expressed constitutively) and the presence of L-rhamnose. The deletion of lraA severely impairs growth when L-rhamnose is the sole source of carbon and in addition it abolishes the induction of genes that respond to L-rhamnose/RhaR, indicating that an intermediate of the LRA pathway is the physiological inducer likely required to activate RhaR. The loss-of-function mutation aldA67 also has a severe negative impact on growth on L-rhamnose but, in contrast to the deletion of lraA, the expression levels of L-rhamnose/RhaR-responsive genes under inducing conditions are substantially up-regulated and the production of α-L-rhamnosidase activity is greatly increased compared to the aldA⁺ control. These findings are consistent with accumulation of the physiological inducer as a consequence of the loss of ALDH activity. Our observations suggest that aldA loss-of-function mutants could be biotechnologically relevant candidates for the over-production of α-L-rhamnosidase activity or the expression of heterologous genes driven by RhaR-responsive promoters.

Metabolomic Characterisation of Discriminatory Metabolites Involved in Halo Blight Disease in Oat Cultivars Caused by Pseudomonas syringae pv. coronafaciens

The metabolome is the underlying biochemical layer of the phenotype and offers a functional readout of the cellular mechanisms involved in a biological system. Since metabolites are considered end-products of regulatory processes at a cellular level, their levels are considered the definitive response of the biological system to genetic or environmental variations. The metabolome thus serves as a metabolic fingerprint of the biochemical events that occur in a biological system under specific conditions. In this study, an untargeted metabolomics approach was applied to elucidate biochemical processes implicated in oat plant responses to Pseudomonas syringae pv. coronafaciens (Ps-c) infection, and to identify signatory markers related to defence responses and disease resistance against halo blight. Metabolic changes in two oat cultivars (“Dunnart” and “SWK001”) responding to Ps-c, were examined at the three-leaf growth stage and metabolome changes monitored over a four-day post-inoculation period. Hydromethanolic extracts were analysed using an ultra-high-performance liquid chromatography (UHPLC) system coupled to a high-definition mass spectrometer (MS) analytical platform. The acquired multi-dimensional data were processed using multivariate statistical analysis and chemometric modelling. The validated chemometric models indicated time- and cultivar-related metabolic changes, defining the host response to the bacterial inoculation. Further multivariate analyses of the data were performed to profile differential signatory markers, putatively associated with the type of launched defence response. These included amino acids, phenolics, phenolic amides, fatty acids, flavonoids, alkaloids, terpenoids, lipids, saponins and plant hormones. Based on the results, metabolic alterations involved in oat defence responses to Ps-c were elucidated and key signatory metabolic markers defining the defence metabolome were identified. The study thus contributes toward a more holistic understanding of the oat metabolism under biotic stress.

Analysis of saponins detoxification genes in Ilyonectria mors-panacis G3B inducing root rot of Panax notoginseng by RNA-Seq

Saponins are kinds of antifungal compounds produced by Panax notoginseng to resist invasion by pathogens. Ilyonectria mors-panacis G3B was the dominant pathogen inducing root rot of P. notoginseng, and the abilities to detoxify saponins were the key to infect P. notoginseng successfully. To research the molecular mechanisms of detoxifying saponins in I. mors-panacis G3B, we used high-throughput RNA-Seq to identify 557 and 1519 differential expression genes (DEGs) in I. mors-panacis G3B with saponins treatments for 4H (Hours) and 12H (Hours) compared with no saponins treatments, respectively. Among these DEGs, we found 93 genes which were simultaneously highly expressed in I. mors-panacis G3B with saponins treatments for 4H and 12H, they mainly belong to genes encoding transporters, glycoside hydrolases, oxidation-reduction enzymes, transcription factors and so on. In addition, there were 21 putative PHI (Pathogen-Host Interaction) genes out of those 93 up-regulated genes. In this report, we analyzed virulence-associated genes in I. mors-panacis G3B which may be related to detoxifying saponins to infect P. notoginseng successfully. They provided an excellent starting point for in-depth study on pathogenicity of I. mors-panacis G3B and developed appropriate root rot disease management strategies in the future.

Metabolomics for Biomarker Discovery: Key Signatory Metabolic Profiles for the Identification and Discrimination of Oat Cultivars

The first step in crop introduction-or breeding programmes-requires cultivar identification and characterisation. Rapid identification methods would therefore greatly improve registration, breeding, seed, trade and inspection processes. Metabolomics has proven to be indispensable in interrogating cellular biochemistry and phenotyping. Furthermore, metabolic fingerprints are chemical maps that can provide detailed insights into the molecular composition of a biological system under consideration. Here, metabolomics was applied to unravel differential metabolic profiles of various oat (Avena sativa) cultivars (Magnifico, Dunnart, Pallinup, Overberg and SWK001) and to identify signatory biomarkers for cultivar identification. The respective cultivars were grown under controlled conditions up to the 3-week maturity stage, and leaves and roots were harvested for each cultivar. Metabolites were extracted using 80% methanol, and extracts were analysed on an ultra-high performance liquid chromatography (UHPLC) system coupled to a quadrupole time-of-flight (qTOF) high-definition mass spectrometer analytical platform. The generated data were processed and analysed using multivariate statistical methods. Principal component analysis (PCA) models were computed for both leaf and root data, with PCA score plots indicating cultivar-related clustering of the samples and pointing to underlying differential metabolic profiles of these cultivars. Further multivariate analyses were performed to profile differential signatory markers, which included carboxylic acids, amino acids, fatty acids, phenolic compounds (hydroxycinnamic and hydroxybenzoic acids, and associated derivatives) and flavonoids, among the respective cultivars. Based on the key signatory metabolic markers, the cultivars were successfully distinguished from one another in profiles derived from both leaves and roots. The study demonstrates that metabolomics can be used as a rapid phenotyping tool for cultivar differentiation.

Disarming the Host: Detoxification of Plant Defense Compounds During Fungal Necrotrophy

While fungal biotrophs are dependent on successfully suppressing/subverting host defenses during their interaction with live cells, necrotrophs, due to their lifestyle are often confronted with a suite of toxic metabolites. These include an assortment of plant defense compounds (PDCs) which can demonstrate broad antifungal activity. These PDCs can be either constitutively present in plant tissue or induced in response to infection, but are nevertheless an important obstacle which needs to be overcome for successful pathogenesis. Fungal necrotrophs have developed a number of strategies to achieve this goal, from the direct detoxification of these compounds through enzymatic catalysis and modification, to the active transport of various PDCs to achieve toxin sequestration and efflux. Studies have shown across multiple pathogens that the efficient detoxification of host PDCs is both critical for successful infection and often a determinant factor in pathogen host range. Here, we provide a broad and comparative overview of the various mechanisms for PDC detoxification which have been identified in both fungal necrotrophs and fungal pathogens which depend on detoxification during a necrotrophic phase of infection. Furthermore, the effect that these mechanisms have on fungal host range, metabolism, and disease control will be discussed.

Whole grain oats, more than just a fiber: Role of unique phytochemicals

Oats are a good source of soluble dietary fiber, especially β-glucan, which has outstanding functional and nutritional properties. β-Glucan is considered to be the major active component of oats because of its cholesterol-lowering and antidiabetic effects. However, the nutritional benefits of oats appear to go beyond fiber to bioactive phytochemicals with strong antioxidant and anti-inflammatory effects. In this review, we summarize current knowledge on the chemistry, stability, bioavailability, and health effects of two unique phytochemicals in oats, avenanthramides, and avenacosides A and B. We conclude that studies on the beneficial effects of avenanthramides and avenacosides A and B are still in their infancy, and additional health benefits of these unique oat components may yet be identified.

Steroidal Saponins in Oat Bran

Saponins are one type of widespread defense compound in the plant kingdom and have been exploited for the production of lead compounds with diverse pharmacological properties in drug discovery. Oats contain two unique steroidal saponins, avenacoside A, 1, and avenacoside B, 2. However, the chemical composition, the levels of these saponins in commercial oat products, and their health effects are still largely unknown. In this study, we directly purified 5 steroidal saponins (1-5) from a methanol extract of oat bran, characterized their structures by analyzing their MS and NMR spectra, and also tentatively identified 11 steroidal saponins (6-16) on the basis of their tandem mass spectra (MS(n), n = 2-3). Among the five purified saponins, 5 is a new compound and 4 is purified from oats for the first time. Using HPLC-MS techniques, a complete profile of oat steroidal saponins was determined, and the contents of the two primary steroidal saponins, 1 and 2, were quantitated in 15 different commercial oat products. The total levels of these two saponins vary from 49.6 to 443.0 mg/kg, and oat bran or oatmeal has higher levels of these two saponins than cold oat cereal. Furthermore, our results on the inhibitory effects of 1 and 2 against the growth of human colon cancer cells HCT-116 and HT-29 showed that both had weak activity, with 2 being more active than 1.

The Aspergillus nidulans Zn(II)2Cys6 transcription factor AN5673/RhaR mediates L-rhamnose utilization and the production of α-L-rhamnosidases

BACKGROUND

Various plant-derived substrates contain L-rhamnose that can be assimilated by many fungi and its liberation is catalyzed by α-L-rhamnosidases. Initial data obtained in our laboratory focussing on two Aspergillus nidulans α-L-rhamnosidase genes (rhaA and rhaE) showed α-L-rhamnosidase production to be tightly controlled at the level of transcription by the carbon source available. Whilst induction is effected by L-rhamnose, unlike many other glycosyl hydrolase genes repression by glucose and other carbon sources occurs in a manner independent of CreA. To date regulatory genes affecting L-rhamnose utilization and the production of enzymes that yield L-rhamnose as a product have not been identified in A. nidulans. The purpose of the present study is to characterize the corresponding α-L-rhamnosidase transactivator.

RESULTS

In this study we have identified the rhaR gene in A. nidulans and Neurospora crassa (AN5673, NCU9033) encoding a putative Zn(II)2Cys6 DNA-binding protein. Genetic evidence indicates that its product acts in a positive manner to induce transcription of the A. nidulans L-rhamnose regulon. rhaR-deleted mutants showed reduced ability to induce expression of the α-L-rhamnosidase genes rhaA and rhaE and concomitant reduction in α-L-rhamnosidase production. The rhaR deletion phenotype also results in a significant reduction in growth on L-rhamnose that correlates with reduced expression of the L-rhamnonate dehydratase catabolic gene lraC (AN5672). Gel mobility shift assays revealed RhaR to be a DNA binding protein recognizing a partially symmetrical CGG-X11-CCG sequence within the rhaA promoter. Expression of rhaR alone is insufficient for induction since its mRNA accumulates even in the absence of L-rhamnose, therefore the presence of both functional RhaR and L-rhamnose are absolutely required. In N. crassa, deletion of rhaR also impairs growth on L-rhamnose.

CONCLUSIONS

To define key elements of the L-rhamnose regulatory circuit, we characterized a DNA-binding Zn(II)2Cys6 transcription factor (RhaR) that regulates L-rhamnose induction of α-L-rhamnosidases and the pathway for its catabolism in A. nidulans, thus extending the list of fungal regulators of genes encoding plant cell wall polysaccharide degrading enzymes. These findings can be expected to provide valuable information for modulating α-L-rhamnosidase production and L-rhamnose utilization in fungi and could eventually have implications in fungal pathogenesis and pectin biotechnology.

Cloning and Characterization of Saponin Hydrolases fromAspergillus oryzaeandEupenicillium brefeldianum

We purified saponin hydrolases from Aspergillus oryzae PF1224 and Eupenicillium brefeldianum PF1226. It was confirmed that the enzymes from A. oryzae PF1224 (Sda1) and E. brefeldianum PF1226 (Sde1) are glycoproteins with molecular masses of 82 and 90 kDa respectively. The deduced amino acid sequences of each enzyme from the cloned genes (sda1 or sde1) showed approximately 50% homology with that of the saponin hydrolase Sdn1 from Neocosmospora vasinfecta var. vasinfecta PF1225 (DDBJ accession no. AB110615). When sda1 and sde1 were expressed in the host Trichoderma viride under the control of the cellobiohydrolase I gene promoter, recombinant proteins were secreted with molecular masses of 77 and 67 kDa respectively. These recombinant enzymes hydrolyzed soyasaponin I to soyasapogenol B and triose, and its substrate specificities for glycosides were similar to that of Sdn1, but the specific activities of these enzymes were lower than that of Sdn1.

A novel β-glucosidase from Aspergillus fumigates releases diosgenin from spirostanosides of Dioscorea zingiberensis C. H. Wright (DZW)

A β-glucosidase effectively releasing diosgenin from spirostanosides of Dioscorea zingiberensis C. H. Wright (DZW), named AfG, was purified from a strain of Aspergillus fumigates. The molecular weight of AfG was 113 kDa. Analysis of protein fragments by ESI-Q-TOF indicated that AfG was a β-glucosidase. The circular dichroism spectrum suggested that the main secondary structure of AfG in Milli-Q water was α-helixes. Atomic force microscopy revealed that it was a globular protein. AfG maintained high activity from pH 3.6 to 5.0 and from 50 to 90°C. With the strong heat stability, AfG retained 55% of its original activity at 65°C for 120 h. AfG utilized muti-3-O-glycosides of various steroidal saponins from DZW as substrate, such as trillin, diosgenin diglucoside, dioscin, deltonin and gracillin, to yield diosgenin, suggesting the possibility of producing diosgenin from total saponins of DZW using a single enzyme.

Antifungal activity and fungal metabolism of steroidal glycosides of Easter lily (Lilium longiflorum Thunb.) by the plant pathogenic fungus, Botrytis cinerea

Botrytis cinerea Pers. Fr. is a plant pathogenic fungus and the causal organism of blossom blight of Easter lily (Lilium longiflorum Thunb.). Easter lily is a rich source of steroidal glycosides, compounds which may play a role in the plant-pathogen interaction of Easter lily. Five steroidal glycosides, including two steroidal glycoalkaloids and three furostanol saponins, were isolated from L. longiflorum and evaluated for fungal growth inhibition activity against B. cinerea, using an in vitro plate assay. All of the compounds showed fungal growth inhibition activity; however, the natural acetylation of C-6''' of the terminal glucose in the steroidal glycoalkaloid, (22R,25R)-spirosol-5-en-3β-yl O-α-L-rhamnopyranosyl-(1→2)-[6-O-acetyl-β-D-glucopyranosyl-(1→4)]-β-D-glucopyranoside (2), increased antifungal activity by inhibiting the rate of metabolism of the compound by B. cinerea. Acetylation of the glycoalkaloid may be a plant defense response to the evolution of detoxifying mechanisms by the pathogen. The biotransformation of the steroidal glycoalkaloids by B. cinerea led to the isolation and characterization of several fungal metabolites. The fungal metabolites that were generated in the model system were also identified in Easter lily tissues infected with the fungus by LC-MS. In addition, a steroidal glycoalkaloid, (22R,25R)-spirosol-5-en-3β-yl O-α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside (6), was identified as both a fungal metabolite of the steroidal glycoalkaloids and as a natural product in L. longiflorum for the first time.

Quantitative analysis of steroidal glycosides in different organs of Easter lily (Lilium longiflorum Thunb.) by LC-MS/MS

The bulbs of the Easter lily ( Lilium longiflorum Thunb.) are regularly consumed in Asia as both food and medicine, and the beautiful white flowers are appreciated worldwide as an attractive ornamental. The Easter lily is a rich source of steroidal glycosides, a group of compounds that may be responsible for some of the traditional medicinal uses of lilies. Since the appearance of recent reports on the role steroidal glycosides in animal and human health, there is increasing interest in the concentration of these natural products in plant-derived foods. A LC-MS/MS method performed in multiple reaction monitoring (MRM) mode was used for the quantitative analysis of two steroidal glycoalkaloids and three furostanol saponins, in the different organs of L. longiflorum. The highest concentrations of the total five steroidal glycosides were 12.02 ± 0.36, 10.09 ± 0.23, and 9.36 ± 0.27 mg/g dry weight in flower buds, lower stems, and leaves, respectively. The highest concentrations of the two steroidal glycoalkaloids were 8.49 ± 0.3, 6.91 ± 0.22, and 5.83 ± 0.15 mg/g dry weight in flower buds, leaves, and bulbs, respectively. In contrast, the highest concentrations of the three furostanol saponins were 4.87 ± 0.13, 4.37 ± 0.07, and 3.53 ± 0.06 mg/g dry weight in lower stems, fleshy roots, and flower buds, respectively. The steroidal glycoalkaloids were detected in higher concentrations as compared to the furostanol saponins in all of the plant organs except the roots. The ratio of the steroidal glycoalkaloids to furostanol saponins was higher in the plant organs exposed to light and decreased in proportion from the aboveground organs to the underground organs. Additionally, histological staining of bulb scales revealed differential furostanol accumulation in the basal plate, bulb scale epidermal cells, and vascular bundles, with little or no staining in the mesophyll of the bulb scale. An understanding of the distribution of steroidal glycosides in the different organs of L. longiflorum is the first step in developing insight into the role these compounds play in plant biology and chemical ecology and aids in the development of extraction and purification methodologies for food, health, and industrial applications. In the present study, (22R,25R)-spirosol-5-en-3β-yl O-α-l-rhamnopyranosyl-(1→2)-β-d-glucopyranosyl-(1→4)-β-d-glucopyranoside, (22R,25R)-spirosol-5-en-3β-yl O-α-l-rhamnopyranosyl-(1→2)-[6-O-acetyl-β-d-glucopyranosyl-(1→4)]-β-d-glucopyranoside, (25R)-26-O-(β-d-glucopyranosyl)furost-5-ene-3β,22α,26-triol 3-O-α-l-rhamnopyranosyl-(1→2)-β-d-glucopyranosyl-(1→4)-β-d-glucopyranoside, (25R)-26-O-(β-d-glucopyranosyl)furost-5-ene-3β,22α,26-triol 3-O-α-l-rhamnopyranosyl-(1→2)-α-l-arabinopyranosyl-(1→3)-β-d-glucopyranoside, and (25R)-26-O-(β-d-glucopyranosyl)furost-5-ene-3β,22α,26-triol 3-O-α-l-rhamnopyranosyl-(1→2)-α-l-xylopyranosyl-(1→3)-β-d-glucopyranoside were quantified in the different organs of L. longiflorum for the first time.

The twin arginine protein transport pathway exports multiple virulence proteins in the plant pathogen Streptomyces scabies

Summary Streptomyces scabies is one of a group of organisms that causes the economically important disease potato scab. Analysis of the S. scabies genome sequence indicates that it is likely to secrete many proteins via the twin arginine protein transport (Tat) pathway, including several proteins whose coding sequences may have been acquired through horizontal gene transfer and share a common ancestor with proteins in other plant pathogens. Inactivation of the S. scabies Tat pathway resulted in pleiotropic phenotypes including slower growth rate and increased permeability of the cell envelope. Comparison of the extracellular proteome of the wild type and DeltatatC strains identified 73 predicted secretory proteins that were present in reduced amounts in the tatC mutant strain, and 47 Tat substrates were verified using a Tat reporter assay. The DeltatatC strain was almost completely avirulent on Arabidopsis seedlings and was delayed in attaching to the root tip relative to the wild-type strain. Genes encoding 14 candidate Tat substrates were individually inactivated, and seven of these mutants were reduced in virulence compared with the wild-type strain. We conclude that the Tat pathway secretes multiple proteins that are required for full virulence.

Resistance in the plant, Barbarea vulgaris, and counter-adaptations in flea beetles mediated by saponins

Three saponins and two sapogenins had differential effects on food consumption in five near-isogenic flea beetle lines, which differ in their ability to utilize a novel host plant, Barbarea vulgaris (Brassicaceae). The ability to live on this plant is controlled by major, dominant R-genes in the flea beetle, Phyllotreta nemorum (Coleoptera: Chrysomelidae: Alticinae). A susceptible genotype (rr) is unable to live on the plant, whereas resistant genotypes (RR and Rr) can utilize the novel host plant. Among compounds isolated from B. vulgaris, hederagenin cellobioside (hederagenin-3-O-(4-O-beta-D-glucopyranosyl)-beta-D-glucopyranoside) inhibited feeding, whereas the effect of oleanolic acid cellobioside was much weaker. The aglycones (sapogenins) were inactive. Although hederagenin cellobioside was active against all flea beetle lines, its effect on food consumption was much stronger on the susceptible genotype (rr) compared to the resistant genotype (Rr). Susceptible and resistant flea beetle genotypes were equally sensitive to a non-host saponin, alpha-hederin (hederagenin-3-O-(2-O-alpha-L-rhamnopyranosyl)-alpha-L-arabinopyranoside). These results suggest that R-alleles in flea beetles might be specific adaptations to defensive saponins in B. vulgaris. A possible mechanism of action of the R-alleles might be to encode for an enzyme (e.g. a glucosidase), which is able to cleave glycosidic bonds in hederagenin cellobioside, but not in alpha-hederin. The potential role of saponins as defensive compounds in B. vulgaris and as targets for counter-adaptations in flea beetles and other insects is discussed.

beta-Glucosidases as detonators of plant chemical defense

Some plant secondary metabolites are classified as phytoanticipins. When plant tissue in which they are present is disrupted, the phytoanticipins are bio-activated by the action of beta-glucosidases. These binary systems--two sets of components that when separated are relatively inert--provide plants with an immediate chemical defense against protruding herbivores and pathogens. This review provides an update on our knowledge of the beta-glucosidases involved in activation of the four major classes of phytoanticipins: cyanogenic glucosides, benzoxazinoid glucosides, avenacosides and glucosinolates. New aspects of the role of specific proteins that either control oligomerization of the beta-glucosidases or modulate their product specificity are discussed in an evolutionary perspective.

Characterization of an unusual cold-active beta-glucosidase belonging to family 3 of the glycoside hydrolases from the psychrophilic isolate Paenibacillus sp. strain C7

We selected for spore-forming psychrophilic bacteria able to use lactose as a carbon source and one isolate, designated Paenibacillus sp. strain C7, that was phylogenetically related to, but distinct from both Paenibacillus macquariensis and Paenibacillus antarcticus. Some Escherichia coli transformants obtained with genomic DNA from this isolate hydrolyzed X-Gal (5-bromo-4-chloro-3-indoyl-beta-D-galactopyranoside) only below 30 degrees C, an indication of cold-active beta-galactosidase activity. Sequencing of the cloned insert revealed an open reading frame encoding a 756-amino acid protein that, rather than belonging to a family typically known for beta-galactosidase activity, belonged to glycoside hydrolase family 3, a family of beta-glucosidases. Because of this unusual placement, the recombinant enzyme (BglY) was purified and characterized. Consistent with its classification, the enzyme had seven times greater activity with the glucoside substrate ONPGlu (o-nitrophenyl-beta-D-glucopyranoside) than with the galactoside substrate ONPGal (o-nitrophenyl-beta-D-galactopyranoside). In addition, the enzyme had, with ONPGlu, a thermal optimum around 30 to 35 degrees C, activity over a broad pH range (5.5 to 10.9), and an especially low Km (<0.003 mM). Further examination of substrate preference showed that the BglY enzyme also hydrolyzed other aryl-beta-glucosides such as helicin, MUG (4-methylumbelliferyl-beta-D-glucopyranoside), esculin, indoxyl-beta-D-glucoside (a natural indigo precursor), and salicin, but had no activity with glucosidic disaccharides or lactose. These characteristics and substrate preferences make the BglY enzyme unique among the family 3 beta-glucosidases. The hydrolysis of a variety of aryl-beta-glucosides suggests that the enzyme may allow the organism to use these substrates in the environment and that its low Km on indoxyl-beta-D-glucoside may make it useful for producing indigo.

A novel saponin hydrolase from Neocosmospora vasinfecta var. vasinfecta

We isolated a soybean saponin hydrolase from Neocosmospora vasinfecta var. vasinfecta PF1225, a filamentous fungus that can degrade soybean saponin and generate soyasapogenol B. This enzyme was found to be a monomer with a molecular mass of about 77 kDa and a glycoprotein. Nucleotide sequence analysis of the corresponding gene (sdn1) indicated that this enzyme consisted of 612 amino acids and had a molecular mass of 65,724 Da, in close agreement with that of the apoenzyme after the removal of carbohydrates. The sdn1 gene was successfully expressed in Trichoderma viride under the control of the cellobiohydrolase I gene promoter. The molecular mass of the recombinant enzyme, about 69 kDa, was smaller than that of the native enzyme due to fewer carbohydrate modifications. Examination of the degradation products obtained by treatment of soyasaponin I with the recombinant enzyme showed that the enzyme hydrolyzed soyasaponin I to soyasapogenol B and triose [alpha-L-rhamnopyranosyl (1-->2)-beta-D-galactopyranosyl (1-->2)-D-glucuronopyranoside]. Also, when soyasaponin II and soyasaponin V, which are different from soyasaponin I only in constituent saccharides, were treated with the enzyme, the ratio of the reaction velocities for soyasaponin I, soyasaponin II, and soyasaponin V was 2,680:886:1. These results indicate that this enzyme recognizes the fine structure of the carbohydrate moiety of soyasaponin in its catalytic reaction. The amino acid sequence of this enzyme predicted from the DNA sequence shows no clear homology with those of any of the enzymes involved in the hydrolysis of carbohydrates.

A novel beta-glucosidase in Uromyces fabae: feast or fight?

Efficient nutrient mobilization is a key element for biotrophic plant parasites such as the rust fungi. In the course of a cDNA library screen for elements involved in sugar utilization in Uromyces fabae, we identified a sequence with homology to beta-glucosidases. Full-length genomic and cDNA clones of the gene, termed BGL1, were isolated and sequenced. The BGL1 gene comprises 3,372 nucleotides, including nine introns. The open reading frame encompasses 2,532 bases and codes for a polypeptide of 843 amino acids with an apparent molecular mass of 92.4 kDa. Analysis of the polypeptide revealed a potential secretion signal, indicating an extracellular localization of mature BGL1p (89.8 kDa). BGL1 seems to be expressed in all stages of growth, including haustoria, the feeding structures of rust fungi. In the course of immunolocalization studies, the gene product BGL1p was localized in the periphery of intercellular hyphae and haustoria. On the basis of sequence homology, the BGL1 gene was identified as a fungal beta-glucosidase.

Constitutive plant toxins and their role in defense against herbivores and pathogens

Most recent investigations have focused on induced, rather than constitutive, plant defenses. Yet significant research has helped to illuminate some of the principal characteristics of constitutive defenses, including mechanisms of action and synergistic effects, as well as strategies used by herbivores and pathogens to circumvent them.

Pathogenicity genes of phytopathogenic fungi

Summary Recently many fungal genes have been identified that, when disrupted, result in strains with a reduction or total loss of disease symptoms. Such pathogenicity genes are the subject of this review. The large number of pathogenicity genes identified is due to the application of tagged mutagenesis techniques (random or targeted). Genes have been identified with roles in the formation of infection structures, cell wall degradation, overcoming or avoiding plant defences, responding to the host environment, production of toxins, and in signal cascades. Additionally, genes with no database matches and with 'novel' functions have also been found. Improved technologies for mutation analysis and for sequencing and analysing fungal genomes hold promise for identifying many more pathogenicity genes.