The phytopathogenic fungus Sclerotinia sclerotiorum detoxifies plant glucosinolate hydrolysis products via an isothiocyanate hydrolase

Recent advances in virulence of a broad host range plant pathogen Sclerotinia sclerotiorum: a mini-review

Sclerotinia sclerotiorum is a typical necrotrophic plant pathogenic fungus, which has a wide host range and can cause a variety of diseases, leading to serious loss of agricultural production around the world. It is difficult to control and completely eliminate the characteristics, chemical control methods is not ideal. Therefore, it is very important to know the pathogenic mechanism of S. sclerotiorum for improving host living environment, relieving agricultural pressure and promoting economic development. In this paper, the life cycle of S. sclerotiorum is introduced to understand the whole process of S. sclerotiorum infection. Through the analysis of the pathogenic mechanism, this paper summarized the reported content, mainly focused on the oxalic acid, cell wall degrading enzyme and effector protein in the process of infection and its mechanism. Besides, recent studies reported virulence-related genes in S. sclerotiorum have been summarized in the paper. According to analysis, those genes were related to the growth and development of the hypha and appressorium, the signaling and regulatory factors of S. sclerotiorum and so on, to further influence the ability to infect the host critically. The application of host-induced gene silencing (HIGS)is considered as a potential effective tool to control various fungi in crops, which provides an important reference for the study of pathogenesis and green control of S. sclerotiorum.

Plant glucosinolate biosynthesis and breakdown pathways shape the rhizosphere bacterial/archaeal community

Rhizosphere microbial community assembly results from microbe-microbe-plant interactions mediated by small molecules of plant and microbial origin. Studies with Arabidopsis thaliana have indicated a critical role of glucosinolates in shaping the root and/or rhizosphere microbial community, likely through breakdown products produced by plant or microbial myrosinases inside or outside of the root. Plant nitrile-specifier proteins (NSPs) promote the formation of nitriles at the expense of isothiocyanates upon glucosinolate hydrolysis with unknown consequences for microbial colonisation of roots and rhizosphere. Here, we generated the A. thaliana triple mutant nsp134 devoid of nitrile formation in root homogenates. Using this line and mutants lacking aliphatic or indole glucosinolate biosynthesis pathways or both, we found bacterial/archaeal alpha-diversity of the rhizosphere to be affected only by the ability to produce aliphatic glucosinolates. In contrast, bacterial/archaeal community composition depended on functional root NSPs as well as on pathways of aliphatic and indole glucosinolate biosynthesis. Effects of NSP deficiency were strikingly distinct from those of impaired glucosinolate biosynthesis. Our results demonstrate that rhizosphere microbial community assembly depends on functional pathways of both glucosinolate biosynthesis and breakdown in support of the hypothesis that glucosinolate hydrolysis by myrosinases and NSPs happens before secretion of products to the rhizosphere.

Comparative analysis of infected cassava root transcriptomics reveals candidate genes for root rot disease resistance

Cassava root-rot incited by soil-borne pathogens is one of the major diseases that reduces root yield. Although the use of resistant cultivars is the most effective method of management, the genetic basis for root-rot resistance remains poorly understood. Therefore, our work analyzed the transcriptome of two contrasting genotypes (BRS Kiriris/resistant and BGM-1345/susceptible) using RNA-Seq to understand the molecular response and identify candidate genes for resistance. Cassava seedlings (resistant and susceptible to root-rot) were both planted in infested and sterilized soil and samples from Initial-time and Final-time periods, pooled. Two controls were used: (i) seedlings collected before planting in infested soil (absolute control) and, (ii) plants grown in sterilized soil (mock treatments). For the differentially expressed genes (DEGs) analysis 23.912 were expressed in the resistant genotype, where 10.307 were differentially expressed in the control treatment, 15 DEGs in the Initial Time-period and 366 DEGs in the Final Time-period. Eighteen candidate genes from the resistant genotype were related to plant defense, such as the MLP-like protein 31 and the peroxidase A2-like gene. This is the first model of resistance at the transcriptional level proposed for the cassava × root-rot pathosystem. Gene validation will contribute to screening for resistance of germplasm, segregating populations and/or use in gene editing in the pursuit to develop most promising cassava clones with resistance to root-rot.

Gregarines impact consumption and development but not glucosinolate metabolism in the mustard leaf beetle

Gregarines are usually classified as parasites, but recent studies suggest that they should be viewed on a parasitism-mutualism spectrum and may even be seen as part of the gut microbiota of host insects. As such, they may also impact the consumption of their hosts and/or be involved in the digestion or detoxification of the host's diet. To study such effects of a gregarine species on those traits in its host, the mustard leaf beetle (Phaedon cochleariae) was used. This beetle species feeds on Brassicaceae plants that contain glucosinolates, which form toxic compounds when hydrolyzed by myrosinases. We cleaned host eggs from gametocysts and spores and reinfected half of the larvae with gregarines, to obtain gregarine-free (G-) and gregarine-infected (G+) larvae. Growth and food consumption parameters of these larvae were assessed by rearing individuals on watercress (Nasturtium officinale, Brassicaceae). A potential involvement of gregarines in the glucosinolate metabolism of P. cochleariae larvae was investigated by offering G- and G+ larvae leaf discs of watercress (containing mainly the benzenic 2-phenylethyl glucosinolate and myrosinases) or pea (Pisum sativum, Fabaceae, lacking glucosinolates and myrosinases) treated with the aliphatic 4-pentenyl glucosinolate or the indole 1-methoxy-3-indolylmethyl glucosinolate. Larval and fecal samples were analyzed via UHPLC-QTOF-MS/MS to search for breakdown metabolites. Larval development, body mass, growth rate and efficiency to convert food into body mass were negatively affected by gregarine infection while the pupal mass remained unaffected. The breakdown metabolites of benzenic and aliphatic glucosinolates were conjugated with aspartic acid, while those of the indole glucosinolate were conjugated with glutamic acid. Gregarine infection did not alter the larvae's ability to metabolize glucosinolates and was independent of plant myrosinases. In summary, some negative effects of gregarines on host performance could be shown, indicating parasitism. Future studies may further disentangle this gregarine-host relationship and investigate the microbiome potentially involved in the glucosinolate metabolism.

Identification and characterization of the plasma membrane H+-ATPase genes in Brassica napus and functional analysis of BnHA9 in salt tolerance

As a primary proton pump, plasma membrane (PM) H+-ATPase plays critical roles in regulating plant growth, development, and stress responses. PM H+-ATPases have been well characterized in many plant species. However, no comprehensive study of PM H+-ATPase genes has been performed in Brassica napus (rapeseed). In this study, we identified 32 PM H+-ATPase genes (BnHAs) in the rapeseed genome, and they were distributed on 16 chromosomes. Phylogenetical and gene duplication analyses showed that the BnHA genes were classified into five subfamilies, and the segmental duplication mainly contributed to the expansion of the rapeseed PM H+-ATPase gene family. The conserved domain and subcellular analyses indicated that BnHAs encoded canonical PM H+-ATPase proteins with 14 highly conserved domains and localized on PM. Cis-acting regulatory element and expression pattern analyses indicated that the expression of BnHAs possessed tissue developmental stage specificity. The 25 upstream open reading frames with the canonical initiation codon ATG were predicted in the 5' untranslated regions of 11 BnHA genes and could be used as potential target sites for improving rapeseed traits. Protein interaction analysis showed that BnBRI1.c associated with BnHA2 and BnHA17, indicating that the conserved activity regulation mechanism of BnHAs may be present in rapeseed. BnHA9 overexpression in Arabidopsis enhanced the salt tolerance of the transgenic plants. Thus, our results lay a foundation for further research exploring the biological functions of PM H+-ATPases in rapeseed.

Comparing Fungal Sensitivity to Isothiocyanate Products on Different Botrytis spp

Glucosinolates, the main secondary metabolites accumulated in cruciferous flora, have a major impact on fortifying plant immunity against diverse pathogens. Although Botrytis cinerea exhibits varying sensitivity to these compounds, current research has yet to fully understand the intricate mechanisms governing its response to glucosinolates. Different species of the genus Botrytis were exposed to glucosinolate-derived isothiocyanates, revealing that B. fabae, B. deweyae, and B. convolute, species with the mfsG transporter gene (Bcin06g00026) not detected with PCR, were more sensitive to isothiocyanates than Botrytis species containing that gene, such as B. cinerea, B. pseudocinerea, and B. byssoidea. This finding was further corroborated by the inability of species with the mfsG gene not detected with PCR to infect plants with a high concentration of glucosinolate-derived isothiocyanates. These results challenge established correlations, revealing varying aggressiveness on different plant substrates. An expression analysis highlighted the gene's induction in the presence of isothiocyanate, and a bioinformatic investigation identified homologous genes in other Botrytis species. Our study underscored the importance of advanced biotechnology to help understand these proteins and thus offer innovative solutions for agriculture.

The histidine kinases regulate allyl-isothiocyanate sensitivity in Cochliobolus heterostrophus

BACKGROUD

Two-component histidine kinase (HK) phosphorelay signaling systems play important roles in differentiation, virulence, secondary metabolite production and response to environmental signals. Allyl isothiocyanate (A-ITC) is a hydrolysis product of glucosinolates with excellent antifungal activity. Our previous study indicated that the mycelial growth of Cochliobolus heterostrophus was significantly hindered by A-ITC. However, the function of HK in regulating A-ITC sensitivity was not clear in C. heterostrophus, the causal agent of Southern corn leaf blight.

RESULTS

In this study, the role of HKs was investigated in C. heterostrophus. Deletion of the HK coding gene ChNIK1 resulted in dramatically increased sensitivity of C. heterostrophus to A-ITC. In addition, ΔChnik1 mutant exhibited significantly decreased conidiation and increased sensitivity to NaCl, KCl, tebuconazole and azoxystrobin, but deletion of the other five HK genes did not affect the A-ITC sensitivity of C. heterostrophus. ChSLN1, ChNIK4, ChNIK8 and ChMAK2 are essential for conidiation and response to H2 O2 and sodium dodecyl sulfate. However, deletion of NIKs had on effect on significant virulence.

CONCLUSION

Our findings demonstrate that the HKs play different roles in A-ITC sensitivity in C. heterostrophus. © 2023 Society of Chemical Industry.

Specificity and breadth of plant specialized metabolite-microbe interactions

Plant specialized metabolites shape plant interactions with the environment including plant-microbe interactions. While we often group compounds into generic classes, it is the precise structure of a compound that creates a specific role in plant-microbe or-pathogen interactions. Critically, the structure guides definitive targets in individual interactions, yet single compounds are not limited to singular mechanistic targets allowing them to influence interactions across broad ranges of attackers, from bacteria to fungi to animals. Further, the direction of the effect can be altered by counter evolution within the interacting organism leading to single compounds being both beneficial and detrimental. Thus, the benefit of a single compound to a host needs to be assessed by measuring the net benefit across all interactions while in each specific interaction. Factoring this complexity for single compounds in plant-microbe interactions with the massive expansion in our identification of specialized metabolite pathways means that we need systematic studies to classify the full breadth of activities. Only with this full biological knowledge we can develop mechanistic, ecological, and evolutionary models to understand how plant specialized metabolites fully influence plant-microbe and plant-biotic interactions more broadly.

In vitro inhibition of Sclerotinia sclerotiorum mycelial growth and reduction of sclerotial viability by the volatile bioactive compounds of Brassicaceae crops

AIMS

Sclerotinia sclerotiorum is an important pathogen of a wide range of crops, with current control mostly relying on the use of fungicides. This study assessed the effect of biofumigation on in vitro inhibition of mycelial growth and reduction of sclerotial viability of S. sclerotiorum as an attempt to seek an alternative management strategy.

METHODS AND RESULTS

The effect of different biofumigant crop types to inhibit mycelial growth of ten S. sclerotiorum isolates was investigated, with Brassica juncea 'Caliente 199' being the most effective biofumigant crop. The efficacy of 'Caliente 199' to inhibit mycelial growth and reduce sclerotial viability was influenced by different crop factors. Plant tissue of 'Caliente 199' harvested at 50% or 100% flowering and adjusted to 80% (w/w) moisture resulted in greater mycelial inhibition and a reduction in the sclerotial viability compared with the vegetative tissue with the same plant moisture. Mycelial inhibition and reduction of sclerotial viability were affected by tissue quantity. Whole plant tissue and shoots only resulted in a similar inhibition of mycelial growth, but whole plant tissue resulted in a greater reduction of sclerotial viability. The S. sclerotiorum isolates differed in sensitivity to the volatile bioactive compounds released by the biofumigant plant tissue.

CONCLUSIONS

The volatile bioactive compounds released by 'Caliente 199' resulted in effective mycelial inhibition but did not kill sclerotia completely.

Microbial degradation of plant toxins

Plants produce a variety of secondary metabolites in response to biotic and abiotic stresses. Although they have many functions, a subclass of toxic secondary metabolites mainly serve plants as deterring agents against herbivores, insects, or pathogens. Microorganisms present in divergent ecological niches, such as soil, water, or insect and rumen gut systems have been found capable of detoxifying these metabolites. As a result of detoxification, microbes gain growth nutrients and benefit their herbivory host via detoxifying symbiosis. Here, we review current knowledge on microbial degradation of toxic alkaloids, glucosinolates, terpenes, and polyphenols with an emphasis on the genes and enzymes involved in breakdown pathways. We highlight that the insect-associated microbes might find application in biotechnology and become targets for an alternative microbial pest control strategy.

Crucial Roles of the High-Osmolarity Glycerol Pathway in the Antifungal Activity of Isothiocyanates against Cochliobolus heterostrophus

Isothiocyanates (ITCs) that are found in Brassicaceae exhibited obvious antifungal activity against Cochliobolus heterostrophus, which is the causal agent of southern corn leaf blight. However, the underlying antifungal mechanism of allyl-ITCs (A-ITCs) against C. heterostrophus remains largely unknown. Here, we used transcriptomic analysis to find that the high osmolarity pathway was upregulated significantly when treated with A-ITCs. To investigate the roles of the high osmolarity pathway in adaption to A-ITCs, we constructed Δssk2, Δpbs2, and Δhog1 mutant strains. Deletion of three genes (ChSSK2, ChPBS2, and ChHOG1) involved in the high osmolarity pathway resulted in significantly increased sensitivity of C. heterostrophus to ITCs. In addition, the phosphorylation level of ChHog1 was induced by A-ITC and was dependent on the presence of ChSsk2 and ChPbs2. Moreover, Δssk2, Δpbs2, and Δhog1 mutants exhibited a dramatically decreased virulence on maize leaves. Our findings demonstrated that the high osmolarity pathway played a positive role in ITC tolerance and virulence, which may provide novel insights into developing ITCs as a new fungicide against C. heterostrophus.

Metabolism of plant-derived toxins from its insect host increases the success of the entomopathogenic fungus Beauveria bassiana

Beauveria bassiana is a soil fungus that parasitizes a large number of arthropod species, including numerous crop pests, causing white muscardine disease and is therefore used as a biological insecticide. However, some insects, such as the cabbage aphid (Brevicoryne brassicae), defend themselves chemically by sequestering dietary pro-toxins (glucosinolates) from their Brassicales host plants. Glucosinolates are accumulated by cabbage aphids and activated to form toxic isothiocyanates when under attack. While isothiocyanate formation protects aphids against most attackers, B. bassiana is still able to infect the cabbage aphid under natural conditions. We therefore investigated how this fungus is able to circumvent the chemical defense system of the cabbage aphid. Here, we describe how B. bassiana infection activates the cabbage aphid defense system, but the resulting toxins are metabolized by B. bassiana via the mercapturic acid pathway, of which the first step is catalyzed by glutathione-S-transferases of low substrate specificity. This detoxification pathway enhances B. bassiana growth when isothiocyanates are present in natural concentrations, and so appears to be an important factor in fungal parasitization of these chemically defended aphids.

Spectral Reflectance Indexes Reveal Differences in the Physiological Status of Brassica oleracea with Contrasting Glucosinolate Content under Biotic Stress

Brassica species produce glucosinolates, a specific group of secondary metabolites present in the Brassicaceae family with antibacterial and antifungal properties. The employment of improved varieties for specific glucosinolates would reduce the production losses caused by pathogen attack. However, the consequences of the increment in these secondary metabolites in the plant are unknown. In this work, we utilized reflectance indexes to test how the physiological status of Brasica oleracea plants changes depending on their constitutive content of glucosinolates under nonstressful conditions and under the attack of the bacteria Xanthomonas campestris pv. campestris and the fungus Sclerotinia sclerotiorum. The modification in the content of glucosinolates had consequences in the resistance to both necrotrophic pathogens, and in several physiological aspects of the plants. By increasing the content in sinigrin and glucobrassicin, plants decrease photosynthesis efficiency (PR531, F_vF_m), biomass production (CHL-NDVI, SR), pigment content (SIPI, NPQI, RE), and senescence (YI) and increase their water content (WI900). These variables may have a negative impact in the productivity of crops in an agricultural environment. However, when plants are subjected to the attack of both necrotrophic pathogens, an increment of sinigrin and glucobrassicin confers an adaptative advantage to the plants, which compensates for the decay of physiological parameters.

Variable Tandem Glycine-Rich Repeats Contribute to Cell Death-Inducing Activity of a Glycosylphosphatidylinositol-Anchored Cell Wall Protein That Is Associated with the Pathogenicity of Sclerotinia sclerotiorum

Glycosylphosphatidylinositol (GPI) anchoring of proteins is a conserved posttranslational modification in eukaryotes. GPI-anchored proteins are widely distributed in fungal plant pathogens, but the specific roles of the GPI-anchored proteins in the pathogenicity of Sclerotinia sclerotiorum, a devastating necrotrophic plant pathogen with a worldwide distribution, remain largely unknown. This research addresses SsGSR1, which encodes an S. sclerotiorum glycine- and serine-rich protein named SsGsr1 with an N-terminal secretory signal and a C-terminal GPI-anchor signal. SsGsr1 is located at the cell wall of hyphae, and deletion of SsGSR1 leads to abnormal cell wall architecture and impaired cell wall integrity of hyphae. The transcription levels of SsGSR1 were maximal in the initial stage of infection, and SsGSR1-deletion strains showed impaired virulence in multiple hosts, indicating that SsGSR1 is critical for the pathogenicity. Interestingly, SsGsr1 targeted the apoplast of host plants to induce cell death that relies on the glycine-rich 11-amino-acid repeats arranged in tandem. The homologs of SsGsr1 in Sclerotinia, Botrytis, and Monilinia species contain fewer repeat units and have lost their cell death activity. Moreover, allelic variants of SsGSR1 exist in field isolates of S. sclerotiorum from rapeseed, and one of the variants lacking one repeat unit results in a protein that exhibits loss of function relative to the cell death-inducing activity and the virulence of S. sclerotiorum. Taken together, our results demonstrate that a variation in tandem repeats provides the functional diversity of GPI-anchored cell wall protein that, in S. sclerotiorum and other necrotrophic pathogens, allows successful colonization of the host plants. IMPORTANCE Sclerotinia sclerotiorum is an economically important necrotrophic plant pathogen and mainly applies cell wall-degrading enzymes and oxalic acid to kill plant cells before colonization. In this research, we characterized a glycosylphosphatidylinositol (GPI)-anchored cell wall protein named SsGsr1, which is critical for the cell wall architecture and the pathogenicity of S. sclerotiorum. Additionally, SsGsr1 induces rapid cell death of host plants that is dependent on glycine-rich tandem repeats. Interestingly, the number of repeat units varies among homologs and alleles of SsGsr1, and such a variation creates alterations in the cell death-inducing activity and the role in pathogenicity. This work advances our understanding of the variation of tandem repeats in accelerating the evolution of a GPI-anchored cell wall protein associated with the pathogenicity of necrotrophic fungal pathogens and prepares the way toward a fuller understanding of the interaction between S. sclerotiorum and host plants.

Botrytis cinerea tolerates phytoalexins produced by Solanaceae and Fabaceae plants through an efflux transporter BcatrB and metabolizing enzymes

Botrytis cinerea, a plant pathogenic fungus with a wide host range, has reduced sensitivity to fungicides as well as phytoalexins, threatening cultivation of economically important fruits and vegetable crops worldwide. B. cinerea tolerates a wide array of phytoalexins, through efflux and/or enzymatic detoxification. Previously, we provided evidence that a distinctive set of genes were induced in B. cinerea when treated with different phytoalexins such as rishitin (produced by tomato and potato), capsidiol (tobacco and bell pepper) and resveratrol (grape and blueberry). In this study, we focused on the functional analyses of B. cinerea genes implicated in rishitin tolerance. LC/MS profiling revealed that B. cinerea can metabolize/detoxify rishitin into at least 4 oxidized forms. Heterologous expression of Bcin08g04910 and Bcin16g01490, two B. cinerea oxidoreductases upregulated by rishitin, in a plant symbiotic fungus Epichloë festucae revealed that these rishitin-induced enzymes are involved in the oxidation of rishitin. Expression of BcatrB, encoding an exporter of structurally unrelated phytoalexins and fungicides, was significantly upregulated by rishitin but not by capsidiol and was thus expected to be involved in the rishitin tolerance. Conidia of BcatrB KO (ΔbcatrB) showed enhanced sensitivity to rishitin, but not to capsidiol, despite their structural similarity. ΔbcatrB showed reduced virulence on tomato, but maintained full virulence on bell pepper, indicating that B. cinerea activates BcatrB by recognizing appropriate phytoalexins to utilize it in tolerance. Surveying 26 plant species across 13 families revealed that the BcatrB promoter is mainly activated during the infection of B. cinerea in plants belonging to the Solanaceae, Fabaceae and Brassicaceae. The BcatrB promoter was also activated by in vitro treatments of phytoalexins produced by members of these plant families, namely rishitin (Solanaceae), medicarpin and glyceollin (Fabaceae), as well as camalexin and brassinin (Brassicaceae). Consistently, ΔbcatrB showed reduced virulence on red clover, which produces medicarpin. These results suggest that B. cinerea distinguishes phytoalexins and induces differential expression of appropriate genes during the infection. Likewise, BcatrB plays a critical role in the strategy employed by B. cinerea to bypass the plant innate immune responses in a wide variety of important crops belonging to the Solanaceae, Brassicaceae and Fabaceae.

Identification and in vitro enzymatic activity analysis of the AOP2 gene family associated with glucosinolate biosynthesis in Tumorous stem mustard (Brassica juncea var. tumida)

The major enzyme encoded by the glucosinolate biosynthetic gene AOP2 is involved in catalyzing the conversion of glucoiberin (GIB) into sinigrin (SIN) in Brassicaceae crops. The AOP2 proteins have previously been identified in several Brassicaceae species, but not in Tumorous stem mustard. As per this research, the five identified members of the AOP2 family from the whole genome of Brassica juncea named BjuAOP2.1-BjuAOP2.5 were found to be evenly distributed on five chromosomes. The subcellular localization results implied that BjuAOP2 proteins were mainly concentrated in the cytoplasm. Phylogenetic analysis of the AOP2 proteins from the sequenced Brassicaceae species in BRAD showed that BjuAOP2 genes were more closely linked to Brassica carinata and Brassica rapa than Arabidopsis. In comparison with other Brassicaceae plants, the BjuAOP2 members were conserved in terms of gene structures, protein sequences, and motifs. The light response and hormone response elements were included in the BjuAOP2 genes' cis-regulatory elements. The expression pattern of BjuAOP2 genes was influenced by the different stages of development and the type of tissue being examined. The BjuAOP2 proteins were used to perform the heterologous expression experiment. The results showed that all the five BjuAOP2 proteins can catalyze the conversion of GIB to SIN with different catalytic activity. These results provide the basis for further investigation of the functional study of BjuAOP2 in Tumorous stem mustard glucosinolate biosynthesis.

Characterization of a Fungal Virus Representing a Novel Genus in the Family Alphaflexiviridae

Sclerotinia sclerotiorum is an ascomycetous fungus and hosts various mycoviruses. In this study, a novel fungal alphaflexivirus with a special genomic structure, named Sclerotinia sclerotiorum alphaflexivirus 1 (SsAFV1), was cloned from a hypovirulent strain, AHS31. Strain AHS31 was also co-infected with two botourmiaviruses and two mitoviruses. The complete genome of SsAFV1 comprised 6939 bases with four open reading frames (ORFs), a conserved 5'-untranslated region (UTR), and a poly(A) tail in the 3' terminal; the ORF1 and ORF3 encoded a replicase and a coat protein (CP), respectively, while the function of the proteins encoded by ORF2 and ORF4 was unknown. The virion of SsAFV1 was flexuous filamentous 480-510 nm in length and 9-10 nm in diameter. The results of the alignment and the phylogenetic analysis showed that SsAFV1 is related to allexivirus and botrexvirus, such as Garlic virus X of the genus Allexivirus and Botrytis virus X of the genus Botrevirus, both with 44% amino-acid (aa) identity of replicase. Thus, SsAFV1 is a novel virus and a new genus, Sclerotexvirus, is proposed to accommodate this novel alphaflexivirus.

Natural rubber reduces herbivory and alters the microbiome below ground

Laticifers are hypothesized to mediate both plant-herbivore and plant-microbe interactions. However, there is little evidence for this dual function. We investigated whether the major constituent of natural rubber, cis-1,4-polyisoprene, a phylogenetically widespread and economically important latex polymer, alters plant resistance and the root microbiome of the Russian dandelion (Taraxacum koksaghyz) under attack of a root herbivore, the larva of the May cockchafer (Melolontha melolontha). Rubber-depleted transgenic plants lost more shoot and root biomass upon herbivory than normal rubber content near-isogenic lines. Melolontha melolontha preferred to feed on artificial diet supplemented with rubber-depleted rather than normal rubber content latex. Likewise, adding purified cis-1,4-polyisoprene in ecologically relevant concentrations to diet deterred larval feeding and reduced larval weight gain. Metagenomics and metabarcoding revealed that abolishing biosynthesis of natural rubber alters the structure but not the diversity of the rhizosphere and root microbiota (ecto- and endophytes) and that these changes depended on M. melolontha damage. However, the assumption that rubber reduces microbial colonization or pathogen load is contradicted by four lines of evidence. Taken together, our data demonstrate that natural rubber biosynthesis reduces herbivory and alters the plant microbiota, which highlights the role of plant-specialized metabolites and secretory structures in shaping multitrophic interactions.

Specialized metabolites as versatile tools in shaping plant-microbe associations

Plants are rich repository of a large number of chemical compounds collectively referred to as specialized metabolites. These compounds are of importance for adaptive processes including responses against changing abiotic conditions and interactions with various co-existing organisms. One of the strikingly affirmed functions of these specialized metabolites is their involvement in plants' life-long interactions with complex multi-kingdom microbiomes including both beneficial and harmful microorganisms. Recent developments in genomic and molecular biology tools not only help to generate well-curated information about regulatory and structural components of biosynthetic pathways of plant specialized metabolites but also to create and screen mutant lines defective in their synthesis. In this review, we have comprehensively surveyed the function of these specialized metabolites and discussed recent research findings demonstrating the responses of various microbes on tested mutant lines having defective biosynthesis of particular metabolites. In addition, we attempt to provide key clues about the impact of these metabolites on the assembly of the plant microbiome by summarizing the major findings of recent comparative metagenomic analyses of available mutant lines under customized and natural microbial niches. Subsequently, we delineate benchmark initiatives that aim to engineer or manipulate the biosynthetic pathways to produce specialized metabolites in heterologous systems but also to diversify their immune function. While denoting the function of these metabolites, we also discuss the critical bottlenecks associated with understanding and exploiting their function in improving plant adaptation to the environment.

Antifungal mechanism of isothiocyanates against Cochliobolus heterostrophus

BACKGROUND

Isothiocyanates (ITCs) generated from the 'glucosinolates-myrosinase' defense system in the Brassicaceae exhibit broad antagonistic activity to various fungal pathogens. Nevertheless, the antifungal activity of ITCs to non-adapted fungi of Brassicaceae plants were seldom determined. The inhibitory effects of ITCs on Cochliobolus heterostrophus were evaluated and the antagonistic mechanism was explored.

RESULTS

The mycelium growth of C. heterostrophus was hindered significantly by allyl, 4-(methylthio)-butyl, and phenyethyl ITCs, 4MTB-ITC exhibited the highest inhibitory effect on mycelium growth with an IC₅₀ value of 53.4 μmol L^-1 . In addition, ITCs exhibited obvious inhibitory effect on conidia germination and pathogenicity of C. heterostrophus. Proteomic analysis indicated that the inhibition of C. heterostrophus by A-ITC downregulated the expression of genes related to energy metabolism, oxidoreductase activity, melanin biosynthesis, and cell wall-degrading enzymes. Furthermore, mutants ΔChtrx2 and ΔChnox1 showed increased sensitivity to ITCs, and melanin biosynthesis was inhibited significantly in C. heterostrophus in response to A-ITC. Interestingly, unlike other pathogens that infected Brassicaceae plants, the SaxA in C. heterostrophus displayed no function in ITC degradation. In addition, the ITCs also exhibited obvious inhibitory effect on mycelium growth of Setosphaeria turcica, Fusarium graminearum, and Magnaporthe oryzae.

CONCLUSION

This study indicated that non-Brassicaceae-adapted pathogens are more sensitive to ITCs, and ITCs could have applications in protecting non-Brassicaceae crops in future. In addition, loss of ChNOX1 and ChTRX2 increased the sensitivity of C. heterostrophus to ITCs. Our results provided potential utilization of ITCs to control diseases caused by non-Brassicaceae pathogenic fungi. © 2022 Society of Chemical Industry.

Untargeted metabolomics screening reveals unique secondary metabolite production from Alternaria section Alternaria

Alternaria section Alternaria is comprised of many species that infect a broad diversity of important crop plants and cause post-harvest spoilage. Alternaria section Alternaria species, such as A. alternata and A. arborescens, are prolific producers of secondary metabolites that act as virulence factors of disease and are mycotoxins that accumulate in infected tissues-metabolites that can vary in their spectrum of production between individuals from the same fungal species. Untargeted metabolomics profiling of secondary metabolite production using mass spectrometry is an effective means to detect phenotypic anomalies in secondary metabolism within a species. Secondary metabolite phenotypes from 36 Alternaria section Alternaria isolates were constructed to observe frequency of production patterns. A clear and unique mass feature pattern was observed for three of the strains that were linked with the production of the dehydrocurvularin family of toxins and associated detoxification products. Examination of corresponding genomes revealed the presence of the dehydrocurvularin biosynthesis gene cluster associated with a sub-telomeric accessory region. A comparison of sequence similarity and occurrences of the dehydrocurvularin biosynthetic gene cluster within Pleosporalean fungi is presented and discussed.

Inhibitor of Glucosinolate Sulfatases as a Potential Friendly Insecticide to Control Plutella xylostella

The glucosinolate-myrosinase system is a two-component defense system characteristic of cruciferous plants. To evade the glucosinolate-myrosinase system, the crucifer specialist insect, Plutella xylostella, promptly desulfates the glucosinolates into harmless compounds by glucosinolate sulfatases (GSSs) in the gut. In this study, we identified an effective inhibitor of GSSs by virtual screening, molecular docking analysis, and in vitro enzyme inhibition assay. The combined effect of the GSS inhibitor with the plant glucosinolate-myrosinase system was assessed by the bioassay of P. xylostella. We show that irosustat is a GSS inhibitor and the inhibition of GSSs impairs the ability of P. xylostella to detoxify the glucosinolate-myrosinase system, leading to the systematic accumulation of toxic isothiocyanates in larvae, thereby severely affecting feeding, growth, survival, and reproduction of P. xylostella. While fed on the Arabidopsis mutants deficient in myrosinase or glucosinolates, irosustat had no significant negative effect on P. xylostella. These findings reveal that the GSS inhibitor is a novel friendly insecticide to control P. xylostella utilizing the plant glucosinolate-myrosinase system and promote the development of insecticide-plant chemical defense combination strategies.

Endophytic fungi from kale (Brassica oleracea var. acephala) modify roots-glucosinolate profile and promote plant growth in cultivated Brassica species. First description of Pyrenophora gallaeciana

Endophytic fungi of crops can promote plant growth through various mechanisms of action (i.e., improve nutrient uptake and nutrient use efficiency, and produce and modulate plant hormones). The genus Brassica includes important horticultural crops, which have been little studied in their interaction with endophytic fungi. Previously, four endophytic fungi were isolated from kale roots (Brassica oleracea var. acephala), with different benefits for their host, including plant growth promotion, cold tolerance, and induction of resistance to pathogens (Xanthomonas campestris) and pests (Mamestra brassicae). In the present work, the molecular and morphological identification of the four different isolates were carried out, describing them as the species Acrocalymma vagum, Setophoma terrestris, Fusarium oxysporum, and the new species Pyrenophora gallaeciana. In addition, using a representative crop of each Brassica U’s triangle species and various in vitro biochemical tests, the ability of these fungi to promote plant growth was described. In this sense, the four fungi used promoted the growth of B. rapa, B. napus, B. nigra, B. juncea, and B. carinata, possibly due to the production of auxins, siderophores, P solubilization or cellulase, xylanase or amylase activity. Finally, the differences in root colonization between the four endophytic fungi and two pathogens (Leptosphaeria maculans and Sclerotinia sclerotiorum) and the root glucosinolate profile were studied, at different times. In this way, how the presence of progoitrin in the roots reduces their colonization by endophytic and pathogenic fungi was determined, while the possible hydrolysis of sinigrin to fungicidal products controls the colonization of endophytic fungi, but not of pathogens.

High Sclerotinia sclerotiorum resistance in rapeseed plant has been achieved by OsPGIP6

Sclerotinia sclerotiorum, a worldwide distributed fungal pathogen, causes serious adverse effects on the yield and seed quality of rapeseed. Polygalacturonase-inhibiting proteins (PGIPs) can protect the cell wall from degradation by pathogen-secreted polygalacturonases (PGs). The present study found several PGIPs from Oryza sativa, especially OsPGIP6 and 3 have much higher inhibitory activities to SsPGs than BnPGIP2 from Brassica napus. Among them, OsPGIP1, 4, 6 can significantly elevate the resistance of transgenic Arabidopsis to S. sclerotiorum. Subsequently, OsPGIP1, 3, 4, 6 were subjected to SSR resistance assay in transgenic rapeseed plants. Among which, OsPGIP6 showed the highest resistance to S. sclerotiorum. At 48 h after detached leaves inoculation, the lesion area of OE-OsPGIP6 rapeseed plants is only 17.93% of the non-transgenic line, and 22.17, 21.32, 52.78, 56.47%, compared to OE-BnPGIP2, OE-OsPGIP1, OE-OsPGIP2, OE-OsPGIP4, respectively. Furthermore, the lesion area of OE-OsPGIP6 reached 10.11% compared to WT at 72 hpi. Also, the lesion length on the stem of OE-OsPGIP6 plants was reduced by 36.83% compared to WT. These results reveal that OsPGIP family, especially OsPGIP6, has a great potential in rapeseed S. sclerotiorum-resistance breeding.

A new source of bacterial myrosinase isolated from endophytic Bacillus sp. NGB-B10, and its relevance in biological control activity

Plant metabolism interacts strongly with the plant microbiome. Glucosinolates, secondary metabolites synthesized by Brassica plants, are hydrolyzed by myrosinase into bioactive compounds of great importance in human health and plant protection. Compared with myrosinase from plant sources, myrosinase enzymes of microbial origin have not been extensively investigated. Therefore, seven endophytic strains corresponding to Bacillus sp. were isolated from Eruca vesicaria ssp. sativa plants that could hydrolyse glucosinolates (sinigrin) in the culture medium and showed myrosinase activity (0.08–19.92 U mL⁻¹). The bglA myrosinase-related gene encoding the 6-phospho-β-glucosidase (GH 1) from Bacillus sp. NGB-B10, the most active myrosinase-producing bacterium, was successfully identified. Response surface methodology (RSM) was applied to statistically optimize culture conditions for myrosinase production from Bacillus sp. strain NGB-B10. The Plackett–Burman design indicated that nitrogen concentration, incubation period, and agitation speed were the significant parameters in myrosinase production. The application of the Box–Behnken design of RSM resulted in a 10.03-fold increase in enzyme activity as compared to the non-optimized culture conditions. The myrosinase was partially purified by 40% fractionation followed by SDS-PAGE analysis which yielded two subunits that had a molecular weight of 38.6 and 35.0 KDa. The purified enzyme was stable under a broad range of pH (5.5–10) and temperatures (10–65 °C). The hydrolysis products released by bacterial myrosinase from some glucosinolate extracts had higher and/or equivalent in vitro antagonistic activity against several phytopathogenic fungi compared to the nystatin (a broad-spectrum antifungal agent). This study provides original information about a new source of bacterial myrosinase and affords an optimized method to enhance myrosinase production.

The isothiocyanate sulforaphane induces respiratory burst oxidase homologue D-dependent reactive oxygen species production and regulates expression of stress response genes

Sulforaphane (SFN) is an isothiocyanate-type phytomolecule present in crucifers, which is mainly synthesized in response to biotic stress. In animals, SFN incorporated in the diet has anticancer properties among others. The mechanism of action and signaling are well described in animals; however, little is known in plants. The goal in the present study is to elucidate components of the SFN signaling pathway, particularly the production of reactive oxygen species (ROS), and its effect on the transcriptome. Our results showed that in Arabidopsis, SFN causes ROS production exclusively through the action of the NADPH oxidase RBOH isoform D that requires calcium as a signaling component for the ROS production. To add to this, we also analyzed the effect of SFN on the transcriptome by RNAseq. We observed the highest expression increase for heat shock proteins (HSP) genes and also for genes associated with the response to oxidative stress. The upregulation of several genes linked to the biotic stress response confirms the interplay between SFN and this stress. In addition, SFN increases the levels of transcripts related to the response to abiotic stress, as well as phytohormones. Taken together, these results indicate that SFN induces an oxidative burst leading to signaling events. This oxidative burst may cause the increase of the expression of genes such as heat shock proteins to restore cellular homeostasis and genes that codify possible components of the signaling pathway and putative effectors.

Interactions of fungi with non-isothiocyanate products of the plant glucosinolate pathway: A review on product formation, antifungal activity, mode of action and biotransformation

The glucosinolate pathway, which is present in the order Brassicales, is one of the most researched defensive natural product biosynthesis pathways. Its core molecules, the glucosinolates are broken down upon pathogen challenge or tissue damage to yield an array of natural products that may help plants defend against the stressor. Though the most widely known glucosinolate decomposition products are the antimicrobial isothiocyanates, there is a wide range of other volatile and non-volatile natural products that arise from this biosynthetic pathway. This review summarizes our current knowledge on the interaction of these much less examined, non-isothiocyanate products with fungi. It deals with compounds including (1) glucosinolates and their biosynthesis precursors; (2) glucosinolate-derived nitriles (e.g. derivatives of 1H-indole-3-acetonitrile), thiocyanates, epithionitriles and oxazolidine-2-thiones; (3) putative isothiocyanate downstream products such as raphanusamic acid, 1H-indole-3-methanol (= indole-3-carbinol) and its oligomers, 1H-indol-3-ylmethanamine and ascorbigen; (4) 1H-indole-3-acetonitrile downstream products such as 1H-indole-3-carbaldehyde (indole-3-carboxaldehyde), 1H-indole-3-carboxylic acid and their derivatives; and (5) indole phytoalexins including brassinin, cyclobrassinin and brassilexin. Herein, a literature review on the following aspects is provided: their direct antifungal activity and the proposed mechanisms of antifungal action, increased biosynthesis after fungal challenge, as well as data on their biotransformation/detoxification by fungi, including but not limited to fungal myrosinase activity.

The evolutionary and molecular features of the broad-host-range plant pathogen Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a pathogenic fungus that infects hundreds of plant species, including many of the world's most important crops. Key features of S. sclerotiorum include its extraordinary host range, preference for dicotyledonous plants, relatively slow evolution, and production of protein effectors that are active in multiple host species. Plant resistance to this pathogen is highly complex, typically involving numerous polymorphisms with infinitesimally small effects, which makes resistance breeding a major challenge. Due to its economic significance, S. sclerotiorum has been subjected to a large amount of molecular and evolutionary research. In this updated pathogen profile, we review the evolutionary and molecular features of S. sclerotiorum and discuss avenues for future research into this important species.

Polyphasic Characterization of Four Aspergillus Species as Potential Biocontrol Agents for White Mold Disease of Bean

The genus Aspergillus comprises several species that play pivotal roles in agriculture. Herein, we morphologically and physiologically characterized four genetically distinct Aspergillus spp., namely A. japonicus, A. niger, A. flavus, and A. pseudoelegans, and examined their ability to suppress the white mold disease of bean caused by Sclerotinia sclerotiorum in vitro and under greenhouse conditions. Seriation type of Aspergillus spp. correlates with conidiospores discharge as detected on the Petri glass lid. Members of Nigri section cover their conidial heads with hard shells after prolonged incubation. In addition, sporulation of the tested Aspergillus isolates is temperature sensitive as it becomes inhibited at low temperatures and the colonies become white. Examined Aspergillus spp. were neither infectious to legumes nor aflatoxigenic as confirmed by HPLC except for A. flavus and A. pseudoelegans which, secreted 5 and 1 ppm of aflatoxin B1, respectively. Co-inoculations of Sclerotinia’s mycelium or sclerotia with a spore suspension of Aspergillus spp. inhibited their germination on PDA at 18 °C and 28 °C, and halted disease onset on detached common bean and soybean leaves. Similarly, plants treated with A. japonicus and A. niger showed the highest survival rates compared to untreated plants. In conclusion, black Aspergillus spp. are efficient biocides and safe alternatives for the management of plant diseases, particularly in organic farms.

Draft genome sequence of Bacillus sp. strain FSQ1, a biological control agent against white mold in common bean (Phaseolus vulgaris L.)

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Bacillus sp. FSQ1 was isolated from the rhizosphere of the common bean (Phaseolus vulgaris L.).

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Bacillus sp. FSQ1 can inhibit the growth of Sclerotinia sclerotiorum.

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The draft genome of strain Bacillus sp. FSQ1 consists of 3598,499 bp; 925,913 bp N50; 2 L50; 33 contigs; 97 RNAs and 3908 predicted CDS.

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The GC content in the draft genome of Bacillus sp. FSQ1 is 43.0%.

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Based on genome mining, potential metabolites involved in the biocontrol activity of strain Bacillus sp. FSQ1 are rhizocticin A and bacillibactin.

Bacillus sp. strain FSQ1 was isolated from the common bean (Phaseolus vulgaris L.). The genome of this strain presented 3,598,499 bp; 43.0% G + C content; 925,913 bp N50; 2 L50; 33 contigs; 97 RNAs and 3,908 predicted coding DNA sequences (CDS) distributed in 315 subsystems. Based on genome mining, the biological control activity of strains FSQ1 could be associated with the biosynthesis of rhizocticin A and bacillibactin. Thus, this strain is a promising active ingredient for the formulation of biopesticides.

Mechanisms of Isothiocyanate Detoxification in Larvae of Two Belowground Herbivores, Delia radicum and D. floralis (Diptera: Anthomyiidae)

Like aboveground herbivores, belowground herbivores are confronted with multiple plant defense mechanisms including complex chemical cocktails in plant tissue. Roots and shoots of Brassicaceae plants contain the two-component glucosinolate (GSL)-myrosinase defense system. Upon cell damage, for example by herbivore feeding, toxic and pungent isothiocyanates (ITCs) can be formed. Several aboveground-feeding herbivores have developed biochemical adaptation strategies to overcome the GSL-ITC defenses of their host plant. Whether belowground herbivores feeding on Brassica roots possess similar mechanisms has received little attention. Here, we analyze how two related belowground specialist herbivores detoxify the GSL-ITC defenses of their host plants. The larvae of the fly species Delia radicum and D. floralis are common pests and specialized herbivores on the roots of Brassicaceae. We used chemical analyses (HPLC-MS/MS and HPLC-UV) to examine how the GSL-ITC defense system is metabolized by these congeneric larvae. In addition, we screened for candidate genes involved in the detoxification process using RNAseq and qPCR. The chemical analyses yielded glutathione conjugates and amines. This indicates that both species detoxify ITCs using potentially the general mercapturic acid pathway, which is also found in aboveground herbivores, and an ITC-specific hydrolytic pathway previously characterized in microbes. Performance assays confirmed that ITCs negatively affect the survival of both species, in spite of their known specialization to ITC-producing plants and tissues, whereas ITC breakdown products are less toxic. Interestingly, the RNAseq analyses showed that the two congeneric species activate different sets of genes upon ITC exposure, which was supported by qPCR data. Based on our findings, we conclude that these specialist larvae use combinations of general and compound-specific detoxification mechanisms with differing efficacies and substrate preferences. This indicates that combining detoxification mechanisms can be an evolutionarily successful strategy to handle plant defenses in herbivores.

Rapid and Selective Absorption of Plant Defense Compounds From the Gut of a Sequestering Insect

Many herbivorous insects exploit defense compounds produced by their host plants for protection against predators. Ingested plant defense compounds are absorbed via the gut epithelium and stored in the body, a physiological process that is currently not well understood. Here, we investigated the absorption of plant defense compounds from the gut in the horseradish flea beetle, Phyllotreta armoraciae, a specialist herbivore known to selectively sequester glucosinolates from its brassicaceous host plants. Feeding experiments using a mixture of glucosinolates and other glucosides not found in the host plants showed a rapid and selective uptake of glucosinolates in adult beetles. In addition, we provide evidence that this uptake mainly takes place in the foregut, whereas the endodermal midgut is the normal region of absorption. Absorption via the foregut epithelium is surprising as the apical membrane is covered by a chitinous intima. However, we could show that this cuticular layer differs in its structure and overall thickness between P. armoraciae and a non-sequestering leaf beetle. In P. armoraciae, we observed a thinner cuticle with a less dense chitinous matrix, which might facilitate glucosinolate absorption. Our results show that a selective and rapid uptake of glucosinolates from the anterior region of the gut contributes to the selective sequestration of glucosinolates in P. armoraciae.

How do plants defend themselves against pathogens-Biochemical mechanisms and genetic interventions

In agro-ecosystem, plant pathogens hamper food quality, crop yield, and global food security. Manipulation of naturally occurring defense mechanisms in host plants is an effective and sustainable approach for plant disease management. Various natural compounds, ranging from cell wall components to metabolic enzymes have been reported to protect plants from infection by pathogens and hence provide specific resistance to hosts against pathogens, termed as induced resistance. It involves various biochemical components, that play an important role in molecular and cellular signaling events occurring either before (elicitation) or after pathogen infection. The induction of reactive oxygen species, activation of defensive machinery of plants comprising of enzymatic and non-enzymatic antioxidative components, secondary metabolites, pathogenesis-related protein expression (e.g. chitinases and glucanases), phytoalexin production, modification in cell wall composition, melatonin production, carotenoids accumulation, and altered activity of polyamines are major induced changes in host plants during pathogen infection. Hence, the altered concentration of biochemical components in host plants restricts disease development. Such biochemical or metabolic markers can be harnessed for the development of "pathogen-proof" plants. Effective utilization of the key metabolites-based metabolic markers can pave the path for candidate gene identification. This present review discusses the valuable information for understanding the biochemical response mechanism of plants to cope with pathogens and genomics-metabolomics-based sustainable development of pathogen proof cultivars along with knowledge gaps and future perspectives to enhance sustainable agricultural production.

Transcriptional response to host chemical cues underpins the expansion of host range in a fungal plant pathogen lineage

The host range of parasites is an important factor in assessing the dynamics of disease epidemics. The evolution of pathogens to accommodate new hosts may lead to host range expansion, a process the molecular bases of which are largely enigmatic. The fungus Sclerotinia sclerotiorum has been reported to parasitize more than 400 plant species from diverse eudicot families while its close relative, S. trifoliorum, is restricted to plants from the Fabaceae family. We analyzed S. sclerotiorum global transcriptome reprogramming on hosts from six botanical families and reveal a flexible, host-specific transcriptional program. We generated a chromosome-level genome assembly for S. trifoliorum and found near-complete gene space conservation in two representative strains of broad and narrow host range Sclerotinia species. However, S. trifoliorum showed increased sensitivity to the Brassicaceae defense compound camalexin. Comparative analyses revealed a lack of transcriptional response to camalexin in the S. trifoliorum strain and suggest that regulatory variation in detoxification and effector genes at the population level may associate with the genetic accommodation of Brassicaceae in the Sclerotinia host range. Our work proposes transcriptional plasticity and the co-existence of signatures for generalist and polyspecialist adaptive strategies in the genome of a plant pathogen.

Identification of Two Myrosinases from a Leclercia adecarboxylata Strain and Investigation of Its Tolerance Mechanism to Glucosinolate Hydrolysate

Glucosinolates (GSLs), secondary metabolites synthesized by cruciferous plants, can be hydrolyzed by myrosinase into compounds, such as isothiocyanates (ITCs), with various bioactivities. Thus, myrosinase plays an important role in the utilization of GSLs. We isolated a bacterial strain, which was identified as Leclercia adecarboxylata, from the rhizosphere soil of rape seedlings and identified two myrosinase genes and an ITC hydrolase gene. Both myrosinases are intracellular and have 658 amino acid residues. Via molecular docking and chemical modification assays investigating the active sites of the myrosinases, arginine was found to be essential for their catalytic activity. Transcriptomic analysis of the response to sinigrin revealed significant up-regulation of some genes involved in allyl-ITC detoxification, with metallo-β-lactamase 3836 having the highest fold change. Thus, we discovered two myrosinases from L. adecarboxylata and demonstrated that the mechanism of tolerance of the bacterium to allyl-ITC likely involved metallo-β-lactamase activity.

Exploring the Symbiotic Mechanism of a Virus-Mediated Endophytic Fungus in Its Host by Dual Unique Molecular Identifier-RNA Sequencing

The symbiosis of endophytes and plants is universal in nature. However, how endophytes grow in plants is not entirely clear. Previously, we reported that a virus-infected fungal pathogen could grow in plants as an endophyte. In this study, we utilized Sclerotinia sclerotiorum strain DT-8, a virus-mediated endophyte, to investigate the mechanism of symbiosis with rapeseed by dual unique molecular identifier-RNA sequencing (dual-UMI RNA-seq). We found that the expressions of genes encoding S. sclerotiorum amylase/glucoamylase, glucose transporters, and rapeseed sugars will eventually be exported transporter 11 (SWEET11) were upregulated. It suggested that strain DT-8 might utilize plant starch as a nutrient. The defense systems of rapeseed were also activated, such as production of reactive oxygen species, phenylpropanoids, and brassinin, to control the growth of strain DT-8, while strain DT-8 counteracted host suppression by producing effector-like proteins, detoxification enzymes, and antioxidant components. Moreover, rapeseed also upregulated pectate lyase and pectinesterase genes to facilitate the colonization by strain DT-8. Our findings provide novel insights into the interaction of virus-mediated endophytes and their hosts that warrant further study. IMPORTANCE Although endophytes are widespread in nature, the interactions between endophytes and their hosts are still not fully understood. Members of a unique class of endophytes, the virus-mediated endophytic fungi, are continuously being discovered and have received wide attention. In this study, we investigated the interaction between a mycovirus-mediated endophytic fungus and its host rapeseed by using dual-UMI RNA-seq. According to the dual-UMI RNA-seq results, an aerial view of symbiotic mechanism under balanced regulation was suggested. This research expands our understanding of the symbiotic mechanisms of virus-fungus-plant interactions and could establish a foundation for the further development of practical application with virus-mediated hypovirulent fungi.

Extra-large G-proteins influence plant response to Sclerotinia sclerotiorum by regulating glucosinolate metabolism in Brassica juncea

Heterotrimeric G-proteins are one of the highly conserved signal transducers across phyla. Despite the obvious importance of G-proteins in controlling various plant growth and environmental responses, there is no information describing the regulatory complexity of G-protein networks during pathogen response in a polyploid crop. Here, we investigated the role of extra-large G-proteins (XLGs) in the oilseed crop Brassica juncea, which has inherent susceptibility to the necrotrophic fungal pathogen Sclerotinia sclerotiorum. The allotetraploid B. juncea genome contains multiple homologs of three XLG genes (two BjuXLG1, five BjuXLG2, and three BjuXLG3), sharing a high level of sequence identity, gene structure organization, and phylogenetic relationship with the progenitors' orthologs. Quantitative reverse transcription PCR analysis revealed that BjuXLGs have retained distinct expression patterns across plant developmental stages and on S. sclerotiorum infection. To determine the role of BjuXLG genes in the B. juncea defence response against S. sclerotiorum, RNAi-based suppression was performed. Disease progression analysis showed more rapid lesion expansion and fungal accumulation in BjuXLG-RNAi lines compared to the vector control plants, wherein suppression of BjuXLG3 homologs displayed more compromised defence response at the later time point. Knocking down BjuXLGs caused impairment of the host resistance mechanism to S. sclerotiorum, as indicated by reduced expression of defence marker genes PDF1.2 and WRKY33 on pathogen infection. Furthermore, BjuXLG-RNAi lines showed reduced accumulation of leaf glucosinolates on S. sclerotiorum infection, wherein aliphatic glucosinolates were significantly compromised. Overall, our data suggest that B. juncea XLG genes are important signalling nodes modulating the host defence pathways in response to this necrotrophic pathogen.

Effects of BrMYC2/3/4 on Plant Development, Glucosinolate Metabolism, and Sclerotinia sclerotiorum Resistance in Transgenic Arabidopsis thaliana

MYC2/3/4, known as a basic helix-loop-helix (bHLH) transcription factor, directly activate the genes involved in diverse plant development and secondary metabolites biosynthesis. In this study, we identified and cloned five MYC paralogs (BrMYC2/3-1/3-2/4-1/4-2) from Chinese cabbage (Brassica rapa ssp. pekinensis). In-silico analyses for the physicochemical properties suggested that BrMYC2/3-1/3-2/4-2/4-3 are unstable hydrophobic and acidic proteins, while BrMYC4-1 is an unstable hydrophobic and basic protein. BrMYC2/3/4 belong to the bHLH superfamily and are closely related to AthMYC2/3/4 orthologs that mediate the regulation of various secondary metabolites. It was demonstrated that BrMYC2/3/4-GFP fusion protein localized in the nucleus and expression levels of five BrMYC2/3/4 homologous genes all elevated relative to control (Ctrl). When expressed in Arabidopsis under the control of 35S promoter, each of the BrMYC2/3-1/3-2/4-1/4-2 transgenes differentially influenced root and shoot elongation, vegetative phase change, flowering time, plant height and tiller number after flowering, and seed production. Despite the variation of phenotypes between the transgenic lines, all the lines except for BrMYC4-2 exhibited shorter seed length, less seed weight, higher accumulation of glucosinolates (GSs), and resistance to Sclerotinia sclerotiorum than Ctrl. Notably, BrMYC2 overexpression (OE) line significantly reduced the lengths of root and hypocotyl, seed length, and weight, along with faster bolting time and strikingly higher accumulation of total GSs. Accumulation of GSs at the highest levels in the BrMYC2 ^OE line conferred the highest resistance to S. sclerotiorum. Unlike BrMYC3 ^OE and BrMYC4 ^OE , BrMYC2 ^OE stimulated the growth of plant height after fluorescence. The results of this study point to the BrMYC2 overexpression that may provide a beneficial effect on plant growth and development via plant resistance to the fungal pathogen.

Sclerotinia sclerotiorum Response to Long Exposure to Glucosinolate Hydrolysis Products by Transcriptomic Approach

White mold disease, caused by the necrotrophic fungus Sclerotinia sclerotiorum, affects Brassica crops. Brassica crops produce a broad array of compounds, such as glucosinolates, which contribute to the defense against pathogens. From their hydrolysis, several products arise that have antimicrobial activity (GHPs) whose toxicity is structure dependent. S. sclerotiorum may overcome the toxic effect of moderate GHP concentrations after prolonged exposure to their action. Our objective was to identify the molecular mechanism underlying S. sclerotiorum response to long exposure to two chemically diverse GHPs: aliphatic GHP allyl-isothiocyanate (AITC) and indole GHP indol-3-carbinol (I3C). We found that the transcriptomic response is dependent on the type of GHP and on their initial target, involving cell membranes in the case of AITC or DNA in the case of I3C. Response mechanisms include the reorganization of chromatin, mediated by histone chaperones hip4 and cia1, ribosome synthesis controlled by the kinase-phosphatase pair aps1-ppn1, catabolism of proteins, ergosterol synthesis, and induction of detoxification systems. These mechanisms probably help S. sclerotiorum to grow and survive in an environment where GHPs are constantly produced by Brassica plants upon glucosinolate breakdown. IMPORTANCEBrassica species, including important vegetable crops, such as cabbage, cauliflower, or broccoli, or oil crops, such as rapeseed, produce specific chemical compounds useful to protect them against pests and pathogens. One of the most destructive Brassica diseases in temperate areas around the world is Sclerotinia stem rot, caused by the fungus Sclerotinia sclerotiorum. This is a generalist pathogen that causes disease over more than 400 plant species, being a serious threat to economically important crops worldwide, including potato, bean, soybean, and sunflower, among many others. Understanding the mechanisms utilized by pathogens to overcome specific plant defensive compounds can be useful to increase plant resistance. Our study demonstrated that Sclerotinia shows different adaptation mechanisms, including detoxification systems, to grow and survive when plant protective compounds are present.

SnRK1.1-mediated resistance of Arabidopsis thaliana to clubroot disease is inhibited by the novel Plasmodiophora brassicae effector PBZF1

Plants have evolved a series of strategies to combat pathogen infection. Plant SnRK1 is probably involved in shifting carbon and energy use from growth-associated processes to survival and defence upon pathogen attack, enhancing the resistance to many plant pathogens. The present study demonstrated that SnRK1.1 enhanced the resistance of Arabidopsis thaliana to clubroot disease caused by the plant-pathogenic protozoan Plasmodiophora brassicae. Through a yeast two-hybrid assay, glutathione S-transferase pull-down assay, and bimolecular fluorescence complementation assay, a P. brassicae RxLR effector, PBZF1, was shown to interact with SnRK1.1. Further expression level analysis of SnRK1.1-regulated genes showed that PBZF1 inhibited the biological function of SnRK1.1 as indicated by the disequilibration of the expression level of SnRK1.1-regulated genes in heterogeneous PBZF1-expressing A. thaliana. Moreover, heterogeneous expression of PBZF1 in A. thaliana promoted plant susceptibility to clubroot disease. In addition, PBZF1 was found to be P. brassicae-specific and conserved. This gene was significantly highly expressed in resting spores. Taken together, our results provide new insights into how the plant-pathogenic protist P. brassicae employs an effector to overcome plant resistance, and they offer new insights into the genetic improvement of plant resistance against clubroot disease.

Improvement of glucosinolates by metabolic engineering in Brassica crops

Glucosinolates (GSLs) are a class of sulfur- and nitrogen-containing, and amino acid-derived important secondary metabolites, which mainly present in plants of Brassicaceae family, including Brassica crops, such as broccoli, cabbage, and oilseed rape. The bioactive GSL metabolites confer benefits to plant defense, human health, and the unique flavor of some Brassica crops. However, certain GSL profiles have adverse effects and are known as anti-nutritional factors. This has attracted mounting attempts to increase beneficial GSLs and reduce detrimental ones in the most commonly consumed Brassica crops. We provide a comprehensive overview of metabolic engineering applied in Brassica crops to achieve this purpose, including modulation of GSL biosynthesis, ablation of GSL hydrolysis, inhibition of GSL transport processes, and redirection of metabolic flux to GSL. Moreover, advances in omics approaches, i.e., genomics, transcriptome, and metabolome, applied in the elucidation of GSL metabolism in Brassica crops, as well as promising and potential genome-editing technologies are also discussed.

Effects of Glucosinolate-Derived Isothiocyanates on Fungi: A Comprehensive Review on Direct Effects, Mechanisms, Structure-Activity Relationship Data and Possible Agricultural Applications

Plants heavily rely on chemical defense systems against a variety of stressors. The glucosinolates in the Brassicaceae and some allies are the core molecules of one of the most researched such pathways. These natural products are enzymatically converted into isothiocyanates (ITCs) and occasionally other defensive volatile organic constituents (VOCs) upon fungal challenge or tissue disruption to protect the host against the stressor. The current review provides a comprehensive insight on the effects of the isothiocyanates on fungi, including, but not limited to mycorrhizal fungi and pathogens of Brassicaceae. In the review, our current knowledge on the following topics are summarized: direct antifungal activity and the proposed mechanisms of antifungal action, QSAR (quantitative structure-activity relationships), synergistic activity of ITCs with other agents, effects of ITCs on soil microbial composition and allelopathic activity. A detailed insight into the possible applications is also provided: the literature of biofumigation studies, inhibition of post-harvest pathogenesis and protection of various products including grains and fruits is also reviewed herein.

Metabolic and biotransformation effects on dietary glucosinolates, their bioavailability, catabolism and biological effects in different organisms

Glucosinolate-producing plants have long been recognized for both their distinctive benefits to human nutrition and their resistance traits against pathogens and herbivores. Despite the accumulation of glucosinolates (GLS) in plants is associated with their resistance to various biotic and abiotic stresses, the defensive and biological activities of GLS are commonly conveyed by their metabolic products. In view of this, metabolism is considered the driving factor upon the interactions of GLS-producing plants with other organisms, also influenced by plant and plant attacking or digesting organism characteristics. Several microbial pathogens and insects have evolved the capacity to detoxify GLS-hydrolysis products or inhibit their formation via different means, highlighting the relevance of their metabolic abilities for the plants' defense system activation and target organism detoxification. Strikingly, some bacteria, fungi and insects can likewise produce their own myrosinase (MYR)-like enzymes in one of the most important adaptation strategies against the GLS-MYR plant defense system. Knowledge of GLS metabolic pathways in herbivores and pathogens can impact plant protection efforts and may be harnessed upon for genetically modified plants that are more resistant to predators. In humans, the interest in the implementation of GLS in diets for the prevention of chronic diseases has grown substantially. However, the efficiency of such approaches is dependent on GLS bioavailability and metabolism, which largely involves the human gut microbiome. Among GLS-hydrolytic products, isothiocyanates (ITC) have shown exceptional properties as chemical plant defense agents against herbivores and pathogens, along with their health-promoting benefits in humans, at least if consumed in reasonable amounts. Deciphering GLS metabolic pathways provides critical information for catalyzing all types of GLS towards the generation of ITCs as the biologically most active metabolites. This review provides an overview on contrasting metabolic pathways in plants, bacteria, fungi, insects and humans towards GLS activation or detoxification. Further, suggestions for the preparation of GLS containing plants with improved health benefits are presented.

Comparison of glucosinolate diversity in the crucifer tribe Cardamineae and the remaining order Brassicales highlights repetitive evolutionary loss and gain of biosynthetic steps

We review glucosinolate (GSL) diversity and analyze phylogeny in the crucifer tribe Cardamineae as well as selected species from Brassicaceae (tribe Brassiceae) and Resedaceae. Some GSLs occur widely, while there is a scattered distribution of many less common GSLs, tentatively sorted into three classes: ancient, intermediate and more recently evolved. The number of conclusively identified GSLs in the tribe (53 GSLs) constitute 60% of all GSLs known with certainty from any plant (89 GSLs) and apparently unique GSLs in the tribe constitute 10 of those GSLs conclusively identified (19%). Intraspecific, qualitative GSL polymorphism is known from at least four species in the tribe. The most ancient GSL biosynthesis in Brassicales probably involved biosynthesis from Phe, Val, Leu, Ile and possibly Trp, and hydroxylation at the β-position. From a broad comparison of families in Brassicales and tribes in Brassicaceae, we estimate that a common ancestor of the tribe Cardamineae and the family Brassicaceae exhibited GSL biosynthesis from Phe, Val, Ile, Leu, possibly Tyr, Trp and homoPhe (ancient GSLs), as well as homologs of Met and possibly homoIle (intermediate age GSLs). From the comparison of phylogeny and GSL diversity, we also suggest that hydroxylation and subsequent methylation of indole GSLs and usual modifications of Met-derived GSLs (formation of sulfinyls, sulfonyls and alkenyls) occur due to conserved biochemical mechanisms and was present in a common ancestor of the family. Apparent loss of homologs of Met as biosynthetic precursors was deduced in the entire genus Barbarea and was frequent in Cardamine (e.g. C. pratensis, C. diphylla, C. concatenata, possibly C. amara). The loss was often associated with appearance of significant levels of unique or rare GSLs as well as recapitulation of ancient types of GSLs. Biosynthetic traits interpreted as de novo evolution included hydroxylation at rare positions, acylation at the thioglucose and use of dihomoIle and possibly homoIle as biosynthetic precursors. Biochemical aspects of the deduced evolution are discussed and testable hypotheses proposed. Biosyntheses from Val, Leu, Ile, Phe, Trp, homoPhe and homologs of Met are increasingly well understood, while GSL biosynthesis from mono- and dihomoIle is poorly understood. Overall, interpretation of known diversity suggests that evolution of GSL biosynthesis often seems to recapitulate ancient biosynthesis. In contrast, unprecedented GSL biosynthetic innovation seems to be rare.

Sclerotinia sclerotiorum Infection Triggers Changes in Primary and Secondary Metabolism in Arabidopsis thaliana

Sclerotinia sclerotiorum is a devastating plant pathogen that causes substantial losses in various agricultural crops. Although plants have developed some well-known defense mechanisms against invasive fungi, much remains to be learned about plant responses to fungal pathogens. In this study, we investigated how S. sclerotiorum infection affects plant primary and secondary metabolism in the model plant Arabidopsis thaliana. Our results showed that soluble sugar and amino acid content changed significantly in A. thaliana leaves upon fungal colonization, with a decrease in sucrose and an increase in mannitol, attributed to fungal biosynthesis. Furthermore, the jasmonate signaling pathway was rapidly activated by S. sclerotiorum infection, and there was a striking accumulation of antifungal metabolites such as camalexin, p-coumaroyl agmatine, feruloyl agmatine, and N^δ-acetylornithine. On the other hand, the characteristic defense compounds of the Brassicaceae, the glucosinolates, were not induced in A. thaliana infected by S. sclerotiorum. Our study provides a better understanding of how A. thaliana primary and secondary metabolism is modified during infection by a fungal pathogen like S. sclerotiorum that has both hemibiotrophic and necrotrophic stages.

Evolutionary changes in the glucosinolate biosynthetic capacity in species representing Capsella, Camelina and Neslia genera

Glucosinolates are unique thioglucosides that evolved in the order Brassicales. These compounds function in plant adaptation to the environment, including combating plant pathogens, herbivore deterrence and abiotic stress tolerance. In line with their defensive functions glucosinolates usually accumulate constitutively in relatively high amounts in all tissues of Brassicaceae plants. Here we performed glucosinolate analysis in different organs of selected species representing Capsella, Camelina and Neslia genera, which similarly as the model plant Arabidopsis thaliana belong to the Camelineae tribe. We also identified orthologs of A. thaliana glucosinolate biosynthetic genes in the published genomes of some of the investigated species. Subsequent gene expression and phylogenetic analyses enabled us an insight into the evolutionary changes in the transcription of these genes and in the sequences of respective proteins that occurred within the Camelineae tribe. Our results indicated that glucosinolates are highly abundant in siliques and roots of the investigated species but hardly, if at all, produced in leaves. In addition to this unusual tissular distribution we revealed reduced structural diversity of methionine-derived aliphatic glucosinolates (AGs) with elevated accumulation of rare long chain AGs. This preference seems to correlate with evolutionary changes in genes encoding methylthioalkylmalate synthases that are responsible for the elongation of AG side chains. Finally, our results indicate that the biosynthetic pathway for tryptophan-derived indolic glucosinolates likely lost its main functions in immunity and resistance towards sucking insects and is on its evolutionary route to be shut off in the investigated species.

Genome-wide alternative splicing profiling in the fungal plant pathogen Sclerotinia sclerotiorum during the colonization of diverse host families

Sclerotinia sclerotiorum is a notorious generalist plant pathogen that threatens more than 600 host plants, including wild and cultivated species. The molecular bases underlying the broad compatibility of S. sclerotiorum with its hosts is not fully elucidated. In contrast to higher plants and animals, alternative splicing (AS) is not well studied in plant-pathogenic fungi. AS is a common regulated cellular process that increases cell protein and RNA diversity. In this study, we annotated spliceosome genes in the genome of S. sclerotiorum and characterized their expression in vitro and during the colonization of six host species. Several spliceosome genes were differentially expressed in planta, suggesting that AS was altered during infection. Using stringent parameters, we identified 1,487 S. sclerotiorum genes differentially expressed in planta and exhibiting alternative transcripts. The most common AS events during the colonization of all plants were retained introns and the alternative 3' receiver site. We identified S. sclerotiorum genes expressed in planta for which (a) the relative accumulation of alternative transcripts varies according to the host being colonized and (b) alternative transcripts harbour distinct protein domains. This notably included 42 genes encoding predicted secreted proteins showing high-confidence AS events. This study indicates that AS events are taking place in the plant pathogenic fungus S. sclerotiorum during the colonization of host plants and could generate functional diversity in the repertoire of proteins secreted by S. sclerotiorum during infection.