Engineering hypervirulence in a mycoherbicidal fungus for efficient weed control

Four pillars are required to support a successful biocontrol fungus

Despite biocontrol conceptually being a useful way to control specific pests, there are very few products that are used beyond the glasshouse level, into the field. Only if organisms meet four criteria (four pillars) will they be widely used in the field to replace or augment conventional agrichemicals. (i) The virulence of the biocontrol agent must be enhanced to overcome evolutionary barriers either by mixing with synergistic chemicals or with one or more organisms, and/or by mutagenic or transgenic enhancing of virulence of the biocontrol fungus. (ii) Inoculum production must be cost-effective; many inocula are produced by expensive, labour-intensive solid-phase fermentation. (iii) Inocula must be formulated both to have long shelf life of inocula as well as being formulated to establish on, and control the target pest. Usually spores are formulated, while chopped mycelia from liquid culture are cheaper to produce and are immediately active upon application. (iv) After fulfilling these three criteria, the product must be biosafe: not produce mammalian toxins that affect users and consumers, and have a host range that does not include crops and beneficial organisms, and in most cases that it will not spread from application sites or have environmental residues beyond those needed to control the target pest. © 2023 Society of Chemical Industry.

Ecdysteroid UDP-Glucosyltransferase Expression in Beauveria bassiana Increases Its Pathogenicity against Early Instar Silkworm Larvae

Beauveria bassiana (B. bassiana) is a broad-spectrum entomopathogenic fungus that can control pests in agriculture and forestry. In this study, encoding ecdysteroid uridine diphosphate glucosyltransferase gene (egt) was successfully screened in B. bassiana on the medium containing 500μg/mL G418 sulfate solution through the protoplast transformation method. This enzyme has the function of 20E (20-hydroxyecdysone) inactivation, thus increasing the mortality of the early instar larvae infected with B. bassiana. In this study, we transformed B. bassiana with the egt gene, which deactivates 20-hydroxyecdysone, a key hormone in insect development. The results showed that transgenic B. bassiana killed more silkworms of the 2nd instar larvae than the wild-type with a shorter LT50 time, which was reduced by approximately 20% (day 1 of the 2nd instar silkworm infection of B. bassiana) and 26.4% (day 2 of the 2nd instar silkworm infection of B. bassiana) compared to the wild-type, and also showed a higher mortality number before molting. The transgenic B. bassiana had a higher coverage of the body surface of silkworms compared to the wild type on the 3rd instar. In summary, improving entomopathogenic fungi using biological methods such as genetic engineering is feasible.

Abutilon theophrasti's Resilience against Allelochemical-Based Weed Management in Sustainable Agriculture - Due to Collection of Highly Advantageous Microorganisms?

Abutilon theophrasti Medik. (velvetleaf) is a problematic annual weed in field crops which has invaded many temperate parts of the world. Since the loss of crop yields can be extensive, approaches to manage the weed include not only conventional methods, but also biological methods, for instance by microorganisms releasing phytotoxins and plant-derived allelochemicals. Additionally, benzoxazinoid-rich rye mulches effective in managing common weeds like Amaranthus retroflexus L. have been tested for this purpose. However, recent methods for biological control are still unreliable in terms of intensity and duration. Rye mulches were also ineffective in managing velvetleaf. In this review, we present the attempts to reduce velvetleaf infestation by biological methods and discuss possible reasons for the failure. The resilience of velvetleaf may be due to the extraordinary capacity of the plant to collect, for its own survival, the most suitable microorganisms from a given farming site, genetic and epigenetic adaptations, and a high stress memory. Such properties may have developed together with other advantageous abilities during selection by humans when the plant was used as a crop. Rewilding could be responsible for improving the microbiomes of A. theophrasti.

Functional analysis of the Nep1-like proteins from Plasmopara viticola

Necrosis and ethylene-inducing peptide 1 (Nep1) -like proteins (NLP) are secreted by multiple taxonomically unrelated plant pathogens (bacteria, fungi, and oomycete) and are best known for inducing cell death and immune responses in dicotyledonous plants. A group of putative NLP genes from obligate biotrophic oomycete Plasmopara viticola were predicted by RNA-Seq in our previous study, but their activity has not been established. Therefore, we analyzed the P. viticola NLP (PvNLP) family and identified seven PvNLP genes. They all belong to type 1 NLP genes and form a P. viticola-specific cluster when compared with other pathogen NLP genes. The expression of PvNLPs was induced during early infection process and the expression patterns could be categorized into two groups. Agrobacterium tumefaciens-mediated transient expression assays revealed that only PvNLP7 was cytotoxic and could induce Phytophthora capsici resistance in Nicotiana benthamiana. Functional analysis showed that PvNLP4, PvNLP5, PvNLP7, and PvNLP10 significantly improved disease resistance of Arabidopsis thaliana to Hyaloperonospora arabidopsidis. Moreover, the four genes caused an inhibition of plant growth which is typically associated with enhanced immunity when over-expressed in Arabidopsis. Further research found that PvNLP7 could activate the expression of defense-related genes and its conserved NPP1 domain was critical for cell death- and immunity-inducing activity. This record of NLP genes from P. viticola showed a functional diversification, laying a foundation for further study on pathogenic mechanism of the devastating pathogen.

The Effector Protein CgNLP1 of Colletotrichum gloeosporioides Affects Invasion and Disrupts Nuclear Localization of Necrosis-Induced Transcription Factor HbMYB8-Like to Suppress Plant Defense Signaling

Fungi secrete numerous effectors to modulate host defense systems. Understanding the molecular mechanisms by which fungal effectors regulate plant defense is of great importance for the development of novel strategies for disease control. In this study, we identified necrosis- and ethylene-inducing protein 1 (Nep1)-like protein (NLP) effector gene, CgNLP1, which contributed to conidial germination, appressorium formation, invasive growth, and virulence of Colletotrichum gloeosporioides to the rubber tree. Transient expression of CgNLP1 in the leaves of Nicotiana benthamiana induced ethylene production in plants. Ectopic expression of CgNLP1 in Arabidopsis significantly enhanced the resistance to Botrytis cinerea and Alternaria brassicicola. An R2R3 type transcription factor HbMYB8-like of rubber tree was identified as the target of CgNLP1.HbMYB8-like, localized on the nucleus, and induced cell death in N. benthamiana. CgNLP1 disrupted nuclear accumulation of HbMYB8-like and suppressed HbMYB8-like induced cell death, which is mediated by the salicylic acid (SA) signal pathway. This study suggested a new strategy whereby C. gloeosporioides exploited the CgNLP1 effector to affect invasion and suppress a host defense regulator HbMYB8-like to facilitate infection.

Deciphering the role of plant plasma membrane lipids in response to invasion patterns: how could biology and biophysics help?

Plants have to constantly face pathogen attacks. To cope with diseases, they have to detect the invading pathogen as early as possible via the sensing of conserved motifs called invasion patterns. The first step of perception occurs at the plasma membrane. While many invasion patterns are perceived by specific proteinaceous immune receptors, several studies have highlighted the influence of the lipid composition and dynamics of the plasma membrane in the sensing of invasion patterns. In this review, we summarize current knowledge on how some microbial invasion patterns could interact with the lipids of the plasma membrane, leading to a plant immune response. Depending on the invasion pattern, different mechanisms are involved. This review outlines the potential of combining biological with biophysical approaches to decipher how plasma membrane lipids are involved in the perception of microbial invasion patterns.

Two NIS1-like proteins from apple canker pathogen (Valsa mali) play distinct roles in plant recognition and pathogen virulence

Conserved effectors produced by phytopathogens play critical roles in plant-microbe interactions. NIS1-like proteins represent a newly identified family of effectors distributed in multiple fungal species. However, their biological functions in a majority of pathogenic fungi remain largely elusive and require further investigation. In this study, we characterized two NIS1-like proteins VmNIS1 and VmNIS2 from Valsa mali, the causal agent of apple Valsa canker. Both of these two proteins were predicted to be secreted. Using agroinfiltration, we found that VmNIS1 induced intense cell death, whereas VmNIS2 suppressed INF1 elicitin-triggered cell death in Nicotiana benthamiana. Treatment of N. benthamiana with VmNIS1 recombinant protein produced by Escherichia coli activated a series of immune responses and enhanced plant disease resistance against Phytophthora capsici. In contrast, VmNIS2 suppressed plant immune responses and promoted P. capsici infection when transiently expressed in N. benthamiana. Both VmNIS1 and VmNIS2 were shown to be highly induced at late stage of V. mali infection. By individually knocking out of these two genes in V. mali, however, only VmNIS2 was shown to be required for pathogen virulence as well as tolerance to oxidative stress. Notably, we further showed that C-terminal extension of VmNIS1 was essential for plant recognition and VmNIS2 may escape plant detection via sequence truncation. Our data collectively indicate that VmNIS1 and VmNIS2 play distinct roles in plant recognition and pathogen virulence, which provided new insights into the function of NIS1-like proteins in plant-microbe interactions.

Nep1-like Proteins from Valsa mali Differentially Regulate Pathogen Virulence and Response to Abiotic Stresses

Necrosis and ethylene-inducing peptide 1(Nep1)-like protein (NLP) is well known for its cytotoxicity and immunogenicity on dicotyledonous, and it has attracted large attention due to its gene expansion and functional diversification in numerous phytopathogens. Here, two NLP family proteins, VmNLP1 and VmNLP2, were identified in the pathogenic fungus Valsa mali. We showed that VmNLP2 but not VmNLP1 induced cell death when transiently expressed in Nicotiana benthamiana. VmNLP2 was also shown to induce cell death in apple leaves via the treatment of the Escherichia coli-produced recombinant protein. VmNLP1 and VmNLP2 transcripts were drastically induced at the early stage of V. mali infection, whereas only VmNLP2 was shown to be essential for pathogen virulence. We also found that VmNLP1 and VmNLP2 are required for maintaining the integrity of cell membranes, and they differentially contribute to V. mali tolerance to salt- and osmo-stresses. Notably, multiple sequence alignment revealed that the second histidine (H) among the conserved heptapeptide (GHRHDWE) of VmNLP2 is mutated to tyrosine (Y). When this tyrosine (Y) was substituted by histidine (H), the variant displayed enhanced cytotoxicity in N. benthamiana, as well as enhanced virulence on apple leaves, suggesting that the virulence role of VmNLP2 probably correlates to its cytotoxicity activity. We further showed that the peptide among VmNLP2, called nlp25 (VmNLP2), triggered strong immune response in Arabidopsis thaliana. This work demonstrates that NLPs from V. mali involve multiple biological roles, and shed new light on how intricately complex the functions of NLP might be.

Sequence and Gene Expression Analysis of Recently Identified NLP from Plasmopara viticola

Grapevine downy mildew, evoked by the obligate biotrophic oomycete Plasmopara viticola, is one of the most challenging diseases in viticulture. P. viticola establishes an infection by circumvention of plant immunity, which is achieved by the secretion of effector molecules. One family of potential effectors are the necrosis- and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP). NLP are most abundant in plant pathogenic microorganisms and exist in cytotoxic and non-cyctotoxic forms. Cytotoxic NLP often act as virulence factors and are synthesized in necrotrophic or hemibiotrophic pathogens during the transition from biotrophic to necrotrophic growth. In addition to these cytotoxic NLP, many non-cytotoxic NLP have been identified; their function in biotrophic pathogens is still unknown. In 2020, eight different NLP coding genes were identified in P. viticola and named PvNLP1 to PvNLP8 (Plasmopara viticolaNLP 1–8). In the present study, PvNLP4 to PvNLP8 were characterized by using qPCR analysis and transient expression in the model plant Nicotiana benthamiana. Gene expression analysis showed high PvNLP expression during the early stages of infection. Necrosis-inducing activity of PvNLP was not observed in the nonhost N. benthamiana.

Toxicity of Recombinant Necrosis and Ethylene-Inducing Proteins (NLPs) from Neofusicoccum parvum

Neofusicoccum parvum is a fungal pathogen associated with a wide range of plant hosts. Despite being widely studied, the molecular mechanism of infection of N. parvum is still far from being understood. Analysis of N. parvum genome lead to the identification of six putative genes encoding necrosis and ethylene-inducing proteins (NLPs). The sequence of NLPs genes (NprvNep 1-6) were analyzed and four of the six NLP genes were successfully cloned, expressed in E. coli and purified by affinity chromatography. Pure recombinant proteins were characterized according to their phytotoxic and cytotoxic effects to tomato leaves and to mammalian Vero cells, respectively. These assays revealed that all NprvNeps tested are cytotoxic to Vero cells and also induce cell death in tomato leaves. NprvNep2 was the most toxic to Vero cells, followed by NprvNep1 and 3. NprvNep4 induced weaker, but, nevertheless, still significant toxic effects to Vero cells. A similar trend of toxicity was observed in tomato leaves: the most toxic was NprvNep 2 and the least toxic NprvNep 4. This study describes for the first time an overview of the NLP gene family of N. parvum and provides additional insights into its pathogenicity mechanism.

Identification and Characterization of Nep1-Like Proteins From the Grapevine Downy Mildew Pathogen Plasmopara viticola

The obligate biotrophic oomycete Plasmopara viticola causes tremendous problems in viticulture by evoking grapevine downy mildew. P. viticola, like other plant pathogens, achieves infection by suppression of plant innate immunity by secretion of effector molecules into its host plant. An ever-expanding family of proteins with effector-like characteristics is formed by the "Necrosis and Ethylene inducing peptide 1 (Nep1)-like proteins" (NLPs). NLPs can be divided into two groups by their ability to induce necrosis. While cytotoxic NLPs may act as virulence factors for a necrotrophic or hemibiotrophic plant pathogen, the role of non-cytotoxic NLPs is so far unknown. In this study, we identified eight independent NLPs in P. viticola and selected three for functional analysis. While one was identified as a putative pseudo gene, two contain all so far described critical key elements for necrosis formation except for an N-terminal signal peptide. Further characterization revealed that none of the putative necrosis elicitors was able to actually induce necrosis, neither in several susceptible or resistant Vitis species nor in the dicot model plant Nicotiana benthamiana. This inability exists independently of the presence or absence of a signal peptide. However, any possible mechanism for the suppression of the ability to induce necrosis in planta was not detected. Interestingly, expression analysis of the presumed pseudo gene revealed remarkable differences between pure sporangia solution and sporangia in the presence of leaf material. To our knowledge, this is the first report of this kind of regulation that suggests an important function of so far nonfunctional "pseudo" NLP genes during the first hours of infection.

Omics Potential in Herbicide-Resistant Weed Management

The rapid development of omics technologies has drastically altered the way biologists conduct research. Basic plant biology and genomics have incorporated these technologies, while some challenges remain for use in applied biology. Weed science, on the whole, is still learning how to integrate omics technologies into the discipline; however, omics techniques are more frequently being implemented in new and creative ways to address basic questions in weed biology as well as the more practical questions of improving weed management. This has been especially true in the subdiscipline of herbicide resistance where important questions are the evolution and genetic basis of herbicide resistance. This review examines the advantages, challenges, potential solutions, and outlook for omics technologies in the discipline of weed science, with examples of how omics technologies will impact herbicide resistance studies and ultimately improve management of herbicide-resistant populations.

Activity and Phylogenetics of the Broadly Occurring Family of Microbial Nep1-Like Proteins

Necrosis- and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP) have an extremely broad taxonomic distribution; they occur in bacteria, fungi, and oomycetes. NLPs come in two forms, those that are cytotoxic to eudicot plants and those that are noncytotoxic. Cytotoxic NLPs bind to glycosyl inositol phosphoryl ceramide (GIPC) sphingolipids that are abundant in the outer leaflet of plant plasma membranes. Binding allows the NLP to become cytolytic in eudicots but not monocots. The function of noncytotoxic NLPs remains enigmatic, but the expansion of NLP genes in oomycete genomes suggests they are important. Several plant species have evolved the capacity to recognize NLPs as molecular patterns and trigger plant immunity, e.g., Arabidopsis thaliana detects nlp peptides via the receptor-like protein RLP23. In this review, we provide a historical perspective from discovery to understanding of molecular mechanisms and describe the latest developments in the NLP field to shed light on these fascinating microbial proteins.

Identification and functional analysis of the NLP-encoding genes from the phytopathogenic oomycete Phytophthora capsici

Phytophthora capsici is a hemibiotrophic, phytopathogenic oomycete that infects a wide range of crops, resulting in significant economic losses worldwide. By means of a diverse arsenal of secreted effector proteins, hemibiotrophic pathogens may manipulate plant cell death to establish a successful infection and colonization. In this study, we described the analysis of the gene family encoding necrosis- and ethylene-inducing peptide 1 (Nep1)-like proteins (NLPs) in P. capsici, and identified 39 real NLP genes and 26 NLP pseudogenes. Out of the 65 predicted NLP genes, 48 occur in groups with two or more genes, whereas the remainder appears to be singletons distributed randomly among the genome. Phylogenetic analysis of the 39 real NLPs delineated three groups. Key residues/motif important for the effector activities are degenerated in most NLPs, including the nlp24 peptide consisting of the conserved region I (11-aa immunogenic part) and conserved region II (the heptapeptide GHRHDWE motif) that is important for phytotoxic activity. Transcriptional profiling of eight selected NLP genes indicated that they were differentially expressed during the developmental and plant infection phases of P. capsici. Functional analysis of ten cloned NLPs demonstrated that Pc11951, Pc107869, Pc109174 and Pc118548 were capable of inducing cell death in the Solanaceae, including Nicotiana benthamiana and hot pepper. This study provides an overview of the P. capsici NLP gene family, laying a foundation for further elucidating the pathogenicity mechanism of this devastating pathogen.

Inappropriate Expression of an NLP Effector in Colletotrichum orbiculare Impairs Infection on Cucurbitaceae Cultivars via Plant Recognition of the C-Terminal Region

The hemibiotrophic pathogen Colletotrichum orbiculare preferentially expresses a necrosis and ethylene-inducing peptide 1 (Nep1)-like protein named NLP1 during the switch to necrotrophy. Here, we report that the constitutive expression of NLP1 in C. orbiculare blocks pathogen infection in multiple Cucurbitaceae cultivars via their enhanced defense responses. NLP1 has a cytotoxic activity that induces cell death in Nicotiana benthamiana. However, C. orbiculare transgenic lines constitutively expressing a mutant NLP1 lacking the cytotoxic activity still failed to infect cucumber, indicating no clear relationship between cytotoxic activity and the NLP1-dependent enhanced defense. NLP1 also possesses the microbe-associated molecular pattern (MAMP) sequence called nlp24, recognized by Arabidopsis thaliana at its central region, similar to NLPs of other pathogens. Surprisingly, inappropriate expression of a mutant NLP1 lacking the MAMP signature is also effective for blocking pathogen infection, uncoupling the infection block from the corresponding MAMP. Notably, the deletion analyses of NLP1 suggested that the C-terminal region of NLP1 is critical to enhance defense in cucumber. The expression of mCherry fused with the C-terminal 32 amino acids of NLP1 was enough to trigger the defense of cucurbits, revealing that the C-terminal region of the NLP1 protein is recognized by cucurbits and, then, terminates C. orbiculare infection.

Scorpion neurotoxin AaIT-expressing Beauveria bassiana enhances the virulence against Aedes albopictus mosquitoes

To improve the insecticidal efficacy of this entomopathogen Beauveria bassiana, the fungus was genetically modified to express an insect-specific scorpion neurotoxin AaIT. The virulence of the recombinant B. bassiana strain (Bb-AaIT) against Aedes albopictus adults (which occurs via penetration through the cuticle during spore germination or by conidia ingestion), and the larvae (by conidia ingestion) was measured with bioassays. The median lethal concentration (LC50) of Bb-AaIT against A. albopictus larvae was 313.3-fold lower on day 4 and 11.3-fold lower on day 10 than that of the wild type (WT). Through conidia feeding or body contact, Bb-AaIT killed 50% of adult female mosquitoes at 3.9- or 1.9-fold reduced concentrations on day 4 and at 2.1- or 2.4-fold reduced concentrations on day 10. Compared with the results for the WT, the median lethal time (LT50) of Bb-AaIT was reduced by 28.6% at 1 × 107 conidia ml-1 and 34.3% at 1 × 106 conidia ml-1 in the larvae bioassay by conidia ingestion, while it decreased 32.3% at 1 × 107 conidia ml-1 by conidia ingestion and 24.2% at 1 × 108 conidia ml-1 by penetrating through the cuticle in the adult bioassay. All the differences were significant. Our findings indicated that Bb-AaIT had higher virulence and faster action than the WT in killing the larval and adult mosquitoes, and therefore, it is valuable for development as a commercial mosquito pesticide.

The Nep1-like protein family of Magnaporthe oryzae is dispensable for the infection of rice plants

The necrosis- and ethylene-inducing protein 1 (Nep1)-like proteins (NLPs) are a class of microbe-associated molecular patterns widely distributed across diverse groups of plant-associated microorganisms. In spite of the cytotoxic activity in dicot plants, the role of most NLPs in the virulence of plant pathogens is still largely unknown. We showed that the MoNLP family of rice blast fungus varied very little in amino acid sequence, transient expression of three MoNLPs induced cell death and the production of reactive oxygen species in Nicotiana benthamiana, and the expression of MoNLPs was induced during infection of susceptible rice plants. To further investigate the biological role of the MoNLP family, a marker-free gene replacement vector was developed and used to knock out the whole family in Magnaporthe oryzae. Results showed no significant difference in disease levels caused by wild type and the quadruple ΔMoNLP mutant strains. Likewise, the sporulation and radial growth of the two strains were similar under various unfavorable cultural conditions including malnutrition and abiotic stresses. These observations demonstrated that the MoNLP family is dispensable for the fungal tolerance to the tested adverse cultural conditions, and more importantly, for the virulence of blast fungus on susceptible rice plants.

Persistence and Viability of Lecanicillium lecanii in Chinese Agricultural Soil

The entomopathogenic fungus L. lecanii has been developed as biopesticides and used widely for biological control of several insects in agricultural practice. Due to the lack of isolation/count methods for L. lecanii in soil, the persistence of this fungus in soil appears to have attracted no attention. A selective medium and count method for L. lecanii in soil based on cetyl trimethyl ammonium bromide (CTAB) was developed, and then the persistence and viability of this fungus in soil were investigated under field conditions between 2012 and 2014. The results showed that the rate of recovery for L. lecanii in soil on the selective CTAB medium was satisfactory. The minimum CFUs for L. lecanii on the selective medium (0.5 g/L CTAB) was about 10² conidia/g soil. The L. lecanii density in soil declined quickly in the first month after inoculation with fungal conidia, kept stable for 6 to 10 months, and then decreased gradually until undetectable. L. lecanii could persist for at least 14 months in the agricultural soil of northern China. The colony growth, conidia yield and germination rate on plates, as well as the median lethal concentration or times (LC₅₀ or LT₅₀) to aphids, mycelium growth in aphids and sporulation on aphids of L. lecanii did not change significantly during the persistence in soil. In general, the count method developed here was a very useful tool for monitoring the dynamics of natural or introduced L. lecanii populations in soil, and the data on the persistence of L. lecanii in soil reported here were helpful for biological control and environmental risk assessment.

Simultaneous transcriptome analysis of Colletotrichum gloeosporioides and tomato fruit pathosystem reveals novel fungal pathogenicity and fruit defense strategies

The fungus Colletotrichum gloeosporioides breaches the fruit cuticle but remains quiescent until fruit ripening signals a switch to necrotrophy, culminating in devastating anthracnose disease. There is a need to understand the distinct fungal arms strategy and the simultaneous fruit response. Transcriptome analysis of fungal-fruit interactions was carried out concurrently in the appressoria, quiescent and necrotrophic stages. Conidia germinating on unripe fruit cuticle showed stage-specific transcription that was accompanied by massive fruit defense responses. The subsequent quiescent stage showed the development of dendritic-like structures and swollen hyphae within the fruit epidermis. The quiescent fungal transcriptome was characterized by activation of chromatin remodeling genes and unsuspected environmental alkalization. Fruit response was portrayed by continued highly integrated massive up-regulation of defense genes. During cuticle infection of green or ripe fruit, fungi recapitulate the same developmental stages but with differing quiescent time spans. The necrotrophic stage showed a dramatic shift in fungal metabolism and up-regulation of pathogenicity factors. Fruit response to necrotrophy showed activation of the salicylic acid pathway, climaxing in cell death. Transcriptome analysis of C. gloeosporioides infection of fruit reveals its distinct stage-specific lifestyle and the concurrent changing fruit response, deepening our perception of the unfolding fungal-fruit arms and defenses race.

Optimization of ionic liquid based simultaneous ultrasonic- and microwave-assisted extraction of rutin and quercetin from leaves of velvetleaf (Abutilon theophrasti) by response surface methodology

An ionic liquids based simultaneous ultrasonic and microwave assisted extraction (ILs-UMAE) method has been proposed for the extraction of rutin (RU), quercetin (QU), from velvetleaf leaves. The influential parameters of the ILs-UMAE were optimized by the single factor and the central composite design (CCD) experiments. A 2.00 M 1-butyl-3-methylimidazolium bromide ([C4mim]Br) was used as the experimental ionic liquid, extraction temperature 60°C, extraction time 12 min, liquid-solid ratio 32 mL/g, microwave power of 534 W, and a fixed ultrasonic power of 50 W. Compared to conventional heating reflux extraction (HRE), the RU and QU extraction yields obtained by ILs-UMAE were, respectively, 5.49 mg/g and 0.27 mg/g, which increased, respectively, 2.01-fold and 2.34-fold with the recoveries that were in the range of 97.62–102.36% for RU and 97.33–102.21% for QU with RSDs lower than 3.2% under the optimized UMAE conditions. In addition, the shorter extraction time was used in ILs-UMAE, compared with HRE. Therefore, ILs-UMAE was a rapid and an efficient method for the extraction of RU and QU from the leaves of velvetleaf.

ALY proteins participate in multifaceted Nep1Mo-triggered responses in Nicotiana benthamiana and Arabidopsis thaliana

Previously, it was found that Nep1Mo (a Nep1-like protein from Magnaporthe oryzae) could trigger a variety of plant responses, including stomatal closure, hypersensitive cell death (HCD), and defence-related gene expression, in Nicotiana benthamiana. In this study, it was found that Nep1Mo-induced cell death could be inhibited by the virus-induced gene silencing of NbALY916 in N. benthamiana. NbALY916-silenced plants showed impaired Nep1Mo-induced stomatal closure, decreased Nep1Mo-induced production of hydrogen peroxide (H2O2) and nitric oxide (NO) in guard cells, and reduced Nep1Mo-induced resistance against Phytophthora nicotianae. It also found that the deletion of AtALY4, an orthologue of NbALY916 in Arabidopsis thaliana, impaired Nep1Mo-triggered stomatal closure, HCD, and defence-related gene expression. The compromised stomatal closure observed in the NbALY916-silenced plants and AtALY4 mutants was inhibited by the application of H2O2 and sodium nitroprusside (an NO donor), and both Nep1Mo and H2O2 stimulated guard cell NO synthesis. Conversely, NO-induced stomatal closure was found not to require H2O2 synthesis; and NO treatment did not induce H2O2 production in guard cells. Taken together, these results demonstrate that the NbAlY916/AtAlY4-H2O2-NO pathway mediates multiple Nep1Mo-triggered responses, including stomatal closure, HCD, and defence-related gene expression.

Characterization of necrosis-inducing NLP proteins in Phytophthora capsici

BACKGROUND

Effector proteins function not only as toxins to induce plant cell death, but also enable pathogens to suppress or evade plant defense responses. NLP-like proteins are considered to be effector proteins, and they have been isolated from bacteria, fungi, and oomycete plant pathogens. There is increasing evidence that NLPs have the ability to induce cell death and ethylene accumulation in plants.

RESULTS

We evaluated the expression patterns of 11 targeted PcNLP genes by qRT-PCR at different time points after infection by P. capsici. Several PcNLP genes were strongly expressed at the early stages in the infection process, but the expression of other PcNLP genes gradually increased to a maximum at late stages of infection. The genes PcNLP2, PcNLP6 and PcNLP14 showed the highest expression levels during infection by P. capsici. The necrosis-inducing activity of all targeted PcNLP genes was evaluated using heterologous expression by PVX agroinfection of Capsicum annuum and Nicotiana benthamiana and by Western blot analysis. The members of the PcNLP family can induce chlorosis or necrosis during infection of pepper and tobacco leaves, but the chlorotic or necrotic response caused by PcNLP genes was stronger in pepper leaves than in tobacco leaves. Moreover, PcNLP2, PcNLP6, and PcNLP14 caused the largest chlorotic or necrotic areas in both host plants, indicating that these three genes contribute to strong virulence during infection by P. capsici. This was confirmed through functional evaluation of their silenced transformants. In addition, we further verified that four conserved residues are putatively active sites in PcNLP1 by site-directed mutagenesis.

CONCLUSIONS

Each targeted PcNLP gene affects cells or tissues differently depending upon the stage of infection. Most PcNLP genes could trigger necrotic or chlorotic responses when expressed in the host C. annuum and the non-host N. benthamiana. Individual PcNLP genes have different phytotoxic effects, and PcNLP2, PcNLP6, and PcNLP14 may play important roles in symptom development and may be crucial for virulence, necrosis-inducing activity, or cell death during infection by P. capsici.

Horizontal gene transfer in eukaryotic plant pathogens

Gene transfer has been identified as a prevalent and pervasive phenomenon and an important source of genomic innovation in bacteria. The role of gene transfer in microbial eukaryotes seems to be of a reduced magnitude but in some cases can drive important evolutionary innovations, such as new functions that underpin the colonization of different niches. The aim of this review is to summarize published cases that support the hypothesis that horizontal gene transfer (HGT) has played a role in the evolution of phytopathogenic traits in fungi and oomycetes. Our survey of the literature identifies 46 proposed cases of transfer of genes that have a putative or experimentally demonstrable phytopathogenic function. When considering the life-cycle steps through which a pathogen must progress, the majority of the HGTs identified are associated with invading, degrading, and manipulating the host. Taken together, these data suggest HGT has played a role in shaping how fungi and oomycetes colonize plant hosts.

Phytopathogen effectors subverting host immunity: different foes, similar battleground

Phytopathogenic bacteria, fungi, and oomycetes invade and colonize their host plants through distinct routes. These pathogens secrete diverse groups of effector proteins that aid infection and establishment of different parasitic lifestyles. Despite this diversity, a comparison of different plant-pathogen systems has revealed remarkable similarities in the host immune pathways targeted by effectors from distinct pathogen groups. Immune signaling pathways mediated by pattern recognition receptors, phytohormone homeostasis or signaling, defenses associated with host secretory pathways and pathogen penetrations, and plant cell death represent some of the key processes controlling disease resistance against diverse pathogens. These immune pathways are targeted by effectors that carry a wide range of biochemical functions and are secreted by completely different pathogen groups, suggesting that these pathways are a common battleground encountered by many plant pathogens.

The Nicotiana benthamiana mitogen-activated protein kinase cascade and WRKY transcription factor participate in Nep1(Mo)-triggered plant responses

Many bacterial, fungal, and oomycete species secrete necrosis and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP) that trigger programmed cell death (PCD) and innate immune responses in dicotyledonous plants. However, how NLP induce such immune responses is not understood. Here, we show that silencing of the MAPKKKα-MEK2-WIPK mitogen-activated protein kinase (MAPK) cascade through virus-induced gene silencing compromises hydrogen peroxide accumulation and PCD induced by Nep1(Mo) from Magnaporthe oryzae. WIPK interacts with NbWRKY2, a transcription factor in Nicotiana benthamiana, in vitro and in vivo, suggesting an effector pathway that mediates Nep1(Mo)-induced cell death. Unexpectedly, salicylic acid-induced protein kinase (SIPK)- and NbWRKY2-silenced plants showed impaired Nep1(Mo)-induced stomatal closure, decreased Nep1(Mo)-promoted nitric oxide (NO) production in guard cells, and a reduction in Nep1(Mo)-induced resistance against Phytophthora nicotianae. Expression studies by real-time polymerase chain reaction suggested that the MEK2-WIPK-NbWRKY2 pathway regulated Nep1(Mo)triggered NO accumulation could be partly dependent on nitrate reductase, which was implicated in NO synthesis. Taken together, these studies demonstrate that the MAPK cascade is involved in Nep1(Mo)-triggered plant responses and MAPK signaling associated with PCD exhibits shared and distinct components with that for stomatal closure.

Moniliophthora perniciosa necrosis- and ethylene-inducing protein 2 (MpNep2) as a metastable dimer in solution: structural and functional implications

Understanding how Nep-like proteins (NLPs) behave during the cell cycle and disease progression of plant pathogenic oomycetes, fungi and bacteria is crucial in light of compelling evidence that these proteins play a role in Witches` Broom Disease (WBD) of Theobroma cacao, one of the most important phytopathological problems to afflict the Southern Hemisphere. The crystal structure of MpNep2, a member of the NLP family and the causal agent of WBD, revealed the key elements for its activity. This protein has the ability to refold after heating and was believed to act as a monomer in solution, in contrast to the related homologs MpNep1 and NPP from the oomyceteous fungus Phytophthora parasitica. Here, we identify and characterize a metastable MpNep2 dimer upon over-expression in Escherichia coli using different biochemical and structural approaches. We found using ultra-fast liquid chromatography that the MpNep2 dimer can be dissociated by heating but not by dilution, oxidation or high ionic strength. Small-angle X-ray scattering revealed a possible tail-to-tail interaction between monomers, and nuclear magnetic resonance measurements identified perturbed residues involved in the putative interface of interaction. We also explored the ability of the MpNep2 monomer to refold after heating or chemical denaturation. We observed that MpNep2 has a low stability and cooperative fold that could be an explanation for its structure and activity recovery after stress. These results can provide new insights into the mechanism for MpNep2's action in dicot plants during the progression of WBD and may open new avenues for the involvement of NLP- oligomeric species in phytopathological disorders.

Molecular characterization and functional analysis of a necrosis- and ethylene-inducing, protein-encoding gene family from Verticillium dahliae

Verticillium dahliae Kleb. is a hemibiotrophic, phytopathogenic fungus that causes wilt disease in a wide range of crops, including cotton. Successful host colonization by hemibiotrophic pathogens requires the induction of plant cell death to provide the saprophytic nutrition for the transition from the biotrophic to the necrotrophic stage. In this study, we identified a necrosis-inducing Phytophthora protein (NPP1) domain-containing protein family containing nine genes in a virulent, defoliating isolate of V. dahliae (V592), named the VdNLP genes. Functional analysis demonstrated that only two of these VdNLP genes, VdNLP1 and VdNLP2, encoded proteins that were capable of inducing necrotic lesions and triggering defense responses in Nicotiana benthamiana, Arabidopsis, and cotton plants. Both VdNLP1 and VdNLP2 induced the wilting of cotton seedling cotyledons. However, gene-deletion mutants targeted by VdNLP1, VdNLP2, or both did not affect the pathogenicity of V. dahliae V592 in cotton infection. Similar expression and induction patterns were found for seven of the nine VdNLP transcripts. Through a comparison of the conserved amino acid residues of VdNLP with different necrosis-inducing activities, combined with mutagenesis-based analyses, we identified several novel conserved amino acid residues, in addition to the known conserved heptapeptide GHRHDWE motif and the cysteine residues of the NPP domain-containing protein, that are indispensable for the necrosis-inducing activity of the VdNLP2 protein.

The NLP toxin family in Phytophthora sojae includes rapidly evolving groups that lack necrosis-inducing activity

Necrosis- and ethylene-inducing-like proteins (NLP) are widely distributed in eukaryotic and prokaryotic plant pathogens and are considered to be important virulence factors. We identified, in total, 70 potential Phytophthora sojae NLP genes but 37 were designated as pseudogenes. Sequence alignment of the remaining 33 NLP delineated six groups. Three of these groups include proteins with an intact heptapeptide (Gly-His-Arg-His-Asp-Trp-Glu) motif, which is important for necrosis-inducing activity, whereas the motif is not conserved in the other groups. In total, 19 representative NLP genes were assessed for necrosis-inducing activity by heterologous expression in Nicotiana benthamiana. Surprisingly, only eight genes triggered cell death. The expression of the NLP genes in P. sojae was examined, distinguishing 20 expressed and 13 nonexpressed NLP genes. Real-time reverse-transcriptase polymerase chain reaction results indicate that most NLP are highly expressed during cyst germination and infection stages. Amino acid substitution ratios (Ka/Ks) of 33 NLP sequences from four different P. sojae strains resulted in identification of positive selection sites in a distinct NLP group. Overall, our study indicates that expansion and pseudogenization of the P. sojae NLP family results from an ongoing birth-and-death process, and that varying patterns of expression, necrosis-inducing activity, and positive selection suggest that NLP have diversified in function.

The MAP kinase Bbslt2 controls growth, conidiation, cell wall integrity, and virulence in the insect pathogenic fungus Beauveria bassiana

Entomopathogenic fungi, such as Beauveria bassiana, are key environmental pathogens of insects that have been exploited for biological control of insect pests. Mitogen-activated protein (MAP) kinases play crucial roles in regulating fungal development, growth, and pathogenicity, mediating responses to the environment. Bbslt2, encoding for an Slt2 family MAPK, was isolated and characterized from B. bassiana. Gene disruption of Bbslt2 affected growth, caused a significant reduction in conidial production and viability, and increased sensitivity to Congo Red and fungal cell wall degrading enzymes. ΔBbslt2 mutants were altered in cell wall structure and composition, which included temperature dependent chitin accumulation, reductions in conidial and hyphal hydrophobicity, and alterations in cell surface carbohydrate epitopes. The ΔBbslt2 strain also showed hypersensitivity to heat shock and altered trehalose accumulation, which could only be partially attributed to changes in the expression of trehalase (ntl1). Insect bioassays revealed decreased virulence in the ΔBbslt2 strain using both topical and intrahemoceol injection assays. These results indicate that Bbslt2 plays an important role in conidiation, viability, cell wall integrity and virulence in B. bassiana. Our findings are discussed within the context of the two previous MAP kinases characterized from B. bassiana.

Nontoxic Nep1-like proteins of the downy mildew pathogen Hyaloperonospora arabidopsidis: repression of necrosis-inducing activity by a surface-exposed region

The genome of the downy mildew pathogen Hyaloperonospora arabidopsidis encodes necrosis and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP). Although NLP are widely distributed in eukaryotic and prokaryotic plant pathogens, it was surprising to find these proteins in the obligate biotrophic oomycete H. arabidopsidis. Therefore, we analyzed the H. arabidopsidis NLP (HaNLP) family and identified 12 HaNLP genes and 15 pseudogenes. Most of the 27 genes form an H. arabidopsidis-specific cluster when compared with other oomycete NLP genes, suggesting this class of effectors has recently expanded in H. arabidopsidis. HaNLP transcripts were mainly detected during early infection stages. Agrobacterium tumefaciens-mediated transient expression and infiltration of recombinant NLP into tobacco and Arabidopsis leaves revealed that all HaNLP tested are noncytotoxic proteins. Even HaNLP3, which is most similar to necrosis-inducing NLP proteins of other oomycetes and which contains all amino acids that are critical for necrosis-inducing activity, did not induce necrosis. Chimeras constructed between HaNLP3 and the necrosis-inducing PsojNIP protein demonstrated that most of the HaNLP3 protein is functionally equivalent to PsojNIP, except for an exposed domain that prevents necrosis induction. The early expression and species-specific expansion of the HaNLP genes is suggestive of an alternative function of noncytolytic NLP proteins during biotrophic infection of plants.

Have biopesticides come of age?

Biopesticides based on living microbes and their bioactive compounds have been researched and promoted as replacements for synthetic pesticides for many years. However, lack of efficacy, inconsistent field performance and high cost have generally relegated them to niche products. Recently, technological advances and major changes in the external environment have positively altered the outlook for biopesticides. Significant increases in market penetration have been made, but biopesticides still only make up a small percentage of pest control products. Progress in the areas of activity spectra, delivery options, persistence of effect and implementation have contributed to the increasing use of biopesticides, but technologies that are truly transformational and result in significant uptake are still lacking.

Expression of an oxalate decarboxylase impairs the necrotic effect induced by Nep1-like protein (NLP) of Moniliophthora perniciosa in transgenic tobacco

Oxalic acid (OA) and Nep1-like proteins (NLP) are recognized as elicitors of programmed cell death (PCD) in plants, which is crucial for the pathogenic success of necrotrophic plant pathogens and involves reactive oxygen species (ROS). To determine the importance of oxalate as a source of ROS for OA- and NLP-induced cell death, a full-length cDNA coding for an oxalate decarboxylase (FvOXDC) from the basidiomycete Flammulina velutipes, which converts OA into CO(2) and formate, was overexpressed in tobacco plants. The transgenic plants contained less OA and more formic acid compared with the control plants and showed enhanced resistance to cell death induced by exogenous OA and MpNEP2, an NLP of the hemibiotrophic fungus Moniliophthora perniciosa. This resistance was correlated with the inhibition of ROS formation in the transgenic plants inoculated with OA, MpNEP2, or a combination of both PCD elicitors. Taken together, these results have established a pivotal function for oxalate as a source of ROS required for the PCD-inducing activity of OA and NLP. The results also indicate that FvOXDC represents a potentially novel source of resistance against OA- and NLP-producing pathogens such as M. perniciosa, the causal agent of witches' broom disease of cacao (Theobroma cacao L.).

Expression and regulation of Sclerotinia sclerotiorum necrosis and ethylene-inducing peptides (NEPs)

Successful host colonization by necrotrophic plant pathogens requires the induction of plant cell death to provide the nutrients needed for infection establishment and progression. We have cloned two genes encoding necrosis and ethylene-inducing peptides from Sclerotinia sclerotiorum, which we named SsNep1 and SsNep2. The peptides encoded by these genes induce necrosis when expressed transiently in tobacco leaves. SsNep1 is expressed at a very low level relative to SsNep2 during infection. The expression of SsNep2 was induced by contact with solid surfaces and occurred in both the necrotic zone and at the leading margin of the infection. SsNep2 expression was dependent on calcium and cyclic adenosine monophosphate signalling, as compounds affecting these pathways reduced or abolished SsNep2 expression coincident with a partial or total loss of virulence.

The potential of genetically enhanced plants to address food insecurity

Food insecurity is one of the most important social issues faced today, with 840 million individuals enduring chronic hunger and three billion individuals suffering from nutrient deficiencies. Most of these individuals are poverty stricken and live in developing countries. Strategies to address food insecurity must aim to increase agricultural productivity in the developing world in order to tackle poverty, and must provide long-term improvements in crop yields to keep up with demand as the world's population grows. Genetically enhanced plants provide one route to sustainable higher yields, either by increasing the intrinsic yield capability of crop plants or by protecting them from biotic and abiotic constraints. The present paper discusses a range of transgenic approaches that could increase agricultural productivity if applied on a large scale, including the introduction of genes that confer resistance to pests and diseases, or tolerance of harsh environments, and genes that help to lift the intrinsic yield capacity by increasing metabolic flux towards storage carbohydrates, proteins and oils. The paper also explores how the nutritional value of plants can be improved by genetic engineering. Transgenic plants, as a component of integrated strategies to relieve poverty and deliver sustainable agriculture to subsistence farmers in developing countries, could have a significant impact on food security now and in the future.

Gene silencing of mannose 6-phosphate reductase in the parasitic weed Orobanche aegyptiaca through the production of homologous dsRNA sequences in the host plant

Orobanche spp. (broomrape) are parasitic plants which subsist on the roots of a wide range of hosts, including tomato, causing severe losses in yield quality and quantity. Large amounts of mannitol accumulate in this parasitic weed during development. Mannose 6-phosphate reductase (M6PR) is a key enzyme in mannitol biosynthesis, and it has been suggested that mannitol accumulation may be very important for Orobanche development. Therefore, the Orobanche M6PR gene is a potential target for efforts to control this parasite. Transgenic tomato plants were produced bearing a gene construct containing a specific 277-bp fragment from Orobanche aegyptiaca M6PR-mRNA, in an inverted-repeat configuration. M6PR-siRNA was detected in three independent transgenic tomato lines in the R1 generation, but was not detected in the parasite. Quantitative RT-PCR analysis showed that the amount of endogenous M6PR mRNA in the tubercles and underground shoots of O. aegyptiaca grown on transgenic host plants was reduced by 60%-80%. Concomitant with M6PR mRNA suppression, there was a significant decrease in mannitol level and a significant increase in the percentage of dead O. aegyptiaca tubercles on the transgenic host plants. The detection of mir390, which is involved with cytoplasmic dsRNA processing, is the first indication of the existence of gene-silencing mechanisms in Orobanche spp. Gene silencing mechanisms are probably involved with the production of decreased levels of M6PR mRNA in the parasites grown on the transformed tomato lines.

Molecular characterization and functional analysis of MgNLP, the sole NPP1 domain-containing protein, from the fungal wheat leaf pathogen Mycosphaerella graminicola

Analysis of the fully sequenced genome of the wheat leaf-specific fungal pathogen Mycosphaerella graminicola identified only a single gene encoding a member of the necrosis- and ethylene-inducing peptide 1 (Nep1)-like protein family (NLP). NLP proteins have frequently been shown to trigger cell death and the activation of defense signaling reactions in dicotyledonous plants. However, complete loss-of-function reverse genetics analyses for their importance in the virulence of eukaryotic plant pathogens are generally lacking. Real-time quantitative polymerase chain reaction on MgNLP demonstrated the gene to be specifically expressed in planta. Peak expression was observed during the immediate presymptomatic phase of colonization of a susceptible host genotype. This was followed by a dramatic decrease during disease lesion formation which, in this system, exhibits characteristics of host programmed cell death (PCD). No comparable peak in transcript levels was seen during an incompatible interaction with a host genotype exhibiting gene-for-gene-based disease resistance. Heterologously expressed MgNLP protein induced necrotic cell death and the activation of defense-related genes when infiltrated into Arabidopsis leaves but not in leaves of a susceptible wheat genotype. MgNLP infiltration also failed to stimulate wheat mitogen-activated protein kinase activities. Finally, targeted deletion of M. graminicola MgNLP caused no detectable reduction in plant pathogenicity or virulence, suggesting that this protein is not a major virulence determinant during fungal infection of its host plant. To our knowledge, this represents the first complete loss-of-function analysis of NLP in a eukaryotic plant pathogen and we discuss our findings in the context of possible functions for NLP in pathogens which only infect monocotyledonous plants.

Transforming a NEP1 toxin gene into two Fusarium spp. to enhance mycoherbicide activity on Orobanche--failure and success

BACKGROUND

The NEP1 gene encoding a fungal toxin that successfully conferred hypervirulence when transformed into Colletotrichum coccodes (Wallr.) Hughes attacking Abutilon theophrasti (L.) Medic. was tested to ascertain if it would enhance pathogenicity of Fusarium species to Orobanche aegyptiaca Pers. parasitising crops.

RESULTS

None of the Fusarium oxysporum (#CNCM I-1622) NEP1 transformants was hypervirulent. NEP1 transformants of a new but unnamed Fusarium sp. (#CNCM I-1621--previously identified as F. arthrosporioides) killed Orobanche more rapidly than the wild type. Transformed lines of both species were NEP1 PCR positive, as was the wild type of F. oxysporum #CNCM I-1622 and five other formae speciales of F. oxysporum. All six wild-type formae speciales of F. oxysporum tested excrete minute amounts of immunologically and bioassay-detectable NEP1-like protein. NEP1 expression of most F. oxysporum transformants was suppressed, suggesting that the native gene and the transgene silence each other. The sequence of the putative NEP1 gene in Fusarium oxysporum #CNCM I-1622 differs from the sequence in the toxin-overproducing strain of F. oxysporum f. sp. erythroxyli in four or five amino acids in the first exon.

CONCLUSION

Wild-type Fusarium sp. #CNCM I-1621 does not contain a NEP1-like gene, explaining why it seemed amenable to transformation with high expression, and its virulence was probably enhanced by not cosuppressing the endogenous gene as occurred with Fusarium oxysporum #CNCM I-1622.

Fungal LysM effectors: extinguishers of host immunity?

Lysin motifs (LysMs) have been recognized in prokaryotes and plants as carbohydrate-binding protein modules. Recently, a novel virulence factor with LysMs was characterized from the plant pathogenic fungus Cladosporium fulvum. Here, we present a survey of public sequence data of 70 fungal species to demonstrate that putatively secreted LysM-containing proteins are widespread in the fungal kingdom, as they are found in mammalian and plant pathogenic species, in addition to saprophytes. We propose that these putative LysM effectors might have a role in sequestration of chitin oligosaccharides - breakdown products of fungal cell walls that are released during invasion and act as triggers of host immunity - to dampen host defence.

Purification, crystallization and preliminary X-ray diffraction analysis of an oomycete-derived Nep1-like protein

The elicitor protein Nep1-like protein from the plant pathogen Pythium aphanidermatum was purified and crystallized using the hanging-drop vapour-diffusion method. A native data set was collected to 1.35 A resolution at 100 K using synchrotron radiation. Since selenomethionine-labelled protein did not crystallize under the original conditions, a second crystal form was identified that yielded crystals that diffracted to 2.1 A resolution. A multiple-wavelength anomalous dispersion (MAD) experiment was performed at 100 K and all four selenium sites were identified, which allowed solution of the structure.

A scorpion neurotoxin increases the potency of a fungal insecticide

The low virulence of the insecticidal fungus Metarhizium anisopliae has stymied its widespread use in controlling insect pests. We show that high-level expression of an insect-specific neurotoxin from the scorpion Androctonus australis in hemolymph by M. anisopliae increases fungal toxicity 22-fold against tobacco hornworm (Manduca sexta) caterpillars and ninefold against adult yellow fever mosquitoes (Aedes aegypti) without compromising host specificity. Prelethal effects include reduced mobility and feeding of the insects targeted.

Functional analysis of NLP genes from Botrytis elliptica

SUMMARY We functionally analysed two Nep1-like protein (NLP) genes from Botrytis elliptica (a specialized pathogen of lily), encoding proteins homologous to the necrosis and ethylene-inducing protein (NEP1) from Fusarium oxysporum. Single gene replacement mutants were made for BeNEP1 and BeNEP2, providing the first example of transformation and successful targeted mutagenesis in this fungus. The virulence of both mutants on lily leaves was not affected. BeNEP1 and BeNEP2 were individually expressed in the yeast Pichia pastoris, and the necrosis-inducing activity was tested by infiltration of both proteins into leaves of several monocots and eudicots. Necrotic symptoms developed on the eudicots tobacco, Nicotiana benthamiana and Arabidopsis thaliana, and cell death was induced in tomato cell suspensions. No necrotic symptoms developed on leaves of the monocots rice, maize and lily. These results support the hypothesis that the necrosis-inducing activity of NLPs is limited to eudicots. We conclude that NLPs are not essential virulence factors and they do not function as host-selective toxins for B. elliptica.

Bcl-2 proteins link programmed cell death with growth and morphogenetic adaptations in the fungal plant pathogen Colletotrichum gloeosporioides

Proteins belonging to the Bcl-2 family regulate apoptosis in mammals by controlling mitochondria efflux of cytochrome c and other apoptosis-related proteins. Homologues of human Bcl-2 proteins are found in different metazoan organisms where they play a similar role, while they seem to be absent in plants and fungi. Nonetheless, Bcl-2 protein members can induce or prevent yeast cell death, suggesting that enough functional conservation exists between apoptotic machineries of mammals and fungi. Here we show that induction or prevention of apoptosis by Bcl-2 proteins in the fungal plant pathogen Colletotrichum gloeosporioides is tightly linked with growth and morphogenetic adaptation that occur throughout the entire fungal life cycle. Isolates expressing the pro-apoptotic Bax protein underwent cell death with apoptotic characteristics, and showed alterations in conidial germination that are associated with pathogenic and non-pathogenic life styles. Isolates expressing the anti-apoptotic Bcl-2 protein had prolonged longevity, were protected from Bax-induced cell death, and exhibited enhanced stress resistance. These isolates also had enhanced mycelium and conidia production, and were hyper virulent to host plants. Our findings show that apoptotic-associated machinery regulates morphogenetic switches, which are critical for proper responses and adaptation fungi to different environments.

Phytotoxicity and innate immune responses induced by Nep1-like proteins

We show that oomycete-derived Nep1 (for necrosis and ethylene-inducing peptide1)-like proteins (NLPs) trigger a comprehensive immune response in Arabidopsis thaliana, comprising posttranslational activation of mitogen-activated protein kinase activity, deposition of callose, production of nitric oxide, reactive oxygen intermediates, ethylene, and the phytoalexin camalexin, as well as cell death. Transcript profiling experiments revealed that NLPs trigger extensive reprogramming of the Arabidopsis transcriptome closely resembling that evoked by bacteria-derived flagellin. NLP-induced cell death is an active, light-dependent process requiring HSP90 but not caspase activity, salicylic acid, jasmonic acid, ethylene, or functional SGT1a/SGT1b. Studies on animal, yeast, moss, and plant cells revealed that sensitivity to NLPs is not a general characteristic of phospholipid bilayer systems but appears to be restricted to dicot plants. NLP-induced cell death does not require an intact plant cell wall, and ectopic expression of NLP in dicot plants resulted in cell death only when the protein was delivered to the apoplast. Our findings strongly suggest that NLP-induced necrosis requires interaction with a target site that is unique to the extracytoplasmic side of dicot plant plasma membranes. We propose that NLPs play dual roles in plant pathogen interactions as toxin-like virulence factors and as triggers of plant innate immune responses.