Direct evidence for a new mode of plant defense against insects via a novel polygalacturonase-inhibiting protein expression strategy

Potential of insect endogenous cellulases for lignocellulosic break down deciphered using molecular docking studies

Insects possess cellulolytic system capable of producing variegate enzymes with multifarious specificities to break down complex lignocellulosic products. Astonishingly, endoglucanases, exoglucanases and β-glycosidases act sequentially in a synergistic system to facilitate the breakdown of cellulose to utilisable energy source glucose. In silico docking studies of endo-β-1,4-glucanase from 19 different insects belonging to six different orders identified that it possesses high affinity for all the six substrates, including CMC, cellulose, cellotriose, cellotetraose, cellopentose and cellohexaose. Additionally, β-glucosidase from nearly all the reported insect sources also showed considerable affinity towards cellobiose. Van der Waals, conventional hydrogen bonds and carbon-hydrogen bonds stabilise the interaction between the enzyme and different substrates. Molecular dynamics simulations also held up the stability of various complexes. Efficient breakdown of lignocelluloses-based substrates becoming a major focus of industrial and academic communities worldwide, this study can perhaps complement the propensity of insect cellulases for prospected applications.

Metabolic novelty originating from horizontal gene transfer is essential for leaf beetle survival

Horizontal gene transfer (HGT) provides an evolutionary shortcut for recipient organisms to gain novel functions. Although reports of HGT in higher eukaryotes are rapidly accumulating, in most cases the evolutionary trajectory, metabolic integration, and ecological relevance of acquired genes remain unclear. Plant cell wall degradation by HGT-derived enzymes is widespread in herbivorous insect lineages. Pectin is an abundant polysaccharide in the walls of growing parts of plants. We investigated the significance of horizontally acquired pectin-digesting polygalacturonases (PGs) of the leaf beetle Phaedon cochleariae. Using a CRISPR/Cas9-guided gene knockout approach, we generated a triple knockout and a quadruple PG-null mutant in order to investigate the enzymatic, biological, and ecological effects. We found that pectin-digestion 1) is exclusively linked to the horizontally acquired PGs from fungi, 2) became fixed in the host genome by gene duplication leading to functional redundancy, 3) compensates for nutrient-poor diet by making the nutritious cell contents more accessible, and 4) facilitates the beetles development and survival. Our analysis highlights the selective advantage PGs provide to herbivorous insects and demonstrate the impact of HGT on the evolutionary success of leaf-feeding beetles, major contributors to species diversity.

Tandemly duplicated genes encoding polygalacturonase inhibitors are associated with bruchid (Callosobruchus chinensis) resistance in moth bean (Vigna aconitifolia)

Bruchids are stored-grain insect pests responsible for serious seed loss in legume crops. A previous study using an F₂ population (F2OA) derived from a cross between wild moth-bean (Vigna aconitifolia [Jacq.] Maréchal) accession TN67 (resistant) and cultivated moth-bean accession ICPMO056 (susceptible) revealed that resistance to the azuki bean weevil (Callosobruchus chinensis L.) in TN67 was regulated by a single gene located in the major quantitative trait locus-qVacBrc2.1. In this study, qVacBrc2.1 was finely mapped and candidate genes in this locus were identified using F2OA and another large F₂ population (F2NB) derived from the cross mentioned previously. In contrast to the previous study, segregation analysis in the F2NB population revealed that resistance against this pest was controlled by two genes. Furthermore, the addition of novel markers to qVacBrc2.1 and reanalysis of the QTL in the F2OA population demonstrated that qVacBrc2.1 constituted two linked QTLs-qVacBrc2.1-A and qVacBrc2.1-B. The presence of qVacBrc2.1-B was verified using the population F2NB. Comparative genomics using three Vigna spp. strongly suggested the presence of two tandemly duplicated genes, VacPGIP1 and VacPGIP2, which encoded polygalacturonase inhibitors (polygalacturonase-inhibiting proteins) as the candidates for conferring resistance, but only VacPGIP1 could be successfully cloned and sequenced. The alignment of VacPGIP1 coding sequences of TN67 and ICPMO056 revealed eight single nucleotide polymorphisms, three of which altered the amino-acid sequence of the predicted domains of polygalacturonase inhibitors in ICPMO056. Overall, these findings indicate that VacPGIP1 and VacPGIP2 regulated C. chinensis resistance in TN67.

A Secreted Chorismate Mutase from Xanthomonas arboricola pv. juglandis Attenuates Virulence and Walnut Blight Symptoms

Walnut blight is a significant above-ground disease of walnuts caused by Xanthomonas arboricola pv. juglandis (Xaj). The secreted form of chorismate mutase (CM), a key enzyme of the shikimate pathway regulating plant immunity, is highly conserved between plant-associated beta and gamma proteobacteria including phytopathogens belonging to the Xanthomonadaceae family. To define its role in walnut blight disease, a dysfunctional mutant of chorismate mutase was created in a copper resistant strain Xaj417 (XajCM). Infections of immature walnut Juglans regia (Jr) fruit with XajCM were hypervirulent compared with infections with the wildtype Xaj417 strain. The in vitro growth rate, size and cellular morphology were similar between the wild-type and XajCM mutant strains, however the quantification of bacterial cells by dPCR within walnut hull tissues showed a 27% increase in XajCM seven days post-infection. To define the mechanism of hypervirulence, proteome analysis was conducted to compare walnut hull tissues inoculated with the wild type to those inoculated with the XajCM mutant strain. Proteome analysis revealed 3296 Jr proteins (five decreased and ten increased with FDR ≤ 0.05) and 676 Xaj417 proteins (235 increased in XajCM with FDR ≤ 0.05). Interestingly, the most abundant protein in Xaj was a polygalacturonase, while in Jr it was a polygalacturonase inhibitor. These results suggest that this secreted chorismate mutase may be an important virulence suppressor gene that regulates Xaj417 virulence response, allowing for improved bacterial survival in the plant tissues.

Constitutive expression and discovery of antimicrobial peptides in Zygogramma bicolorata (Coleoptera: Chrysomelidae)

The expression, identification, and discovery of less toxic antimicrobial peptides (AMPs) are significant in managing infectious pathogens. AMPs triggered in response to the immune system have evolved to defend against pathogens and wounding. The protein composition of Zygogramma bicolorata hemolymph is of diagnostic importance as the open circulatory systems of the insects involve signaling through hemolymph. They have conserved many ancestral vertebrate genes that may help better understand the evolution of innate immunity. The present work describes the isolation, purification, identification, and bioinformatics analysis of AMPs from the immunized hemolymph of Z. bicolorata. Thirty-nine peptides were isolated from reverse-phase high-performance liquid chromatography and sequenced via mass spectrometry analysis. The immunization process recorded a threefold higher protein concentration in immunized hemolymph when compared with nonimmunized one. For the first time, the proteomic study on Z. bicolorata hemolymph unveils the three novel proteins in the family Chrysomelidae with no homology in the database, indicating its novelty and the expression of the rest of 36 well-known proteins, including heat-shock, immune, structural, signaling proteins, and others speak for its method validity. Combining the expression of novel AMPs, detoxifying enzymes, hemolytic, and cytotoxic assays, and this work can elucidate new pathways to immune response mechanisms. Its molecular basis also holds the potential applicability in the future drug development process against pathogenic fungi such as Aspergillus niger and Candida albicans.

Changes in the expression level of genes encoding transcription factors and cell wall-related proteins during Meloidogyne arenaria infection of maize (Zea mays)

BACKGROUND

Meloidogyne arenaria is an economically important root-knot nematode (RKN) species whose hosts include maize (Zea mays). The plant response to RKN infection activates many cellular mechanisms, among others, changes in the expression level of genes encoding transcription and elongation factors as well as proteins related to cell wall organization.

METHODS AND RESULTS

This study is aimed at characterization of expression of selected transcription and elongation factors encoding the genes WRKY53, EF1a, and EF1b as well as the ones encoding two proteins associated with cell wall functioning (glycine-rich RNA-binding protein, GRP and polygalacturonase, PG) during the maize response to M. arenaria infection. The changes in the relative level of expression of genes encoding these proteins were assessed using the reverse transcription-quantitative real-time PCR. The material studied were leaves and root samples collected from four maize varieties showing different susceptibilities toward M. arenaria infection, harvested at three different time points. Significant changes in the expression level of GRP between susceptible and tolerant varieties were observed.

CONCLUSIONS

Results obtained in the study suggest pronounced involvement of glycine-rich RNA-binding protein and EF1b in the maize response and resistance to RKN.

Disparate genetic variants associated with distinct components of cowpea resistance to the seed beetle Callosobruchus maculatus

Polygenic genome-wide association mapping identified two regions of the cowpea genome associated with different components of resistance to its major post-harvest pest, the seed beetle Callosobruchus maculatus. Cowpea (Vigna unguiculata) is an important grain and fodder crop in arid and semi-arid regions of Africa, Asia, and South America, where the cowpea seed beetle, Callosobruchus maculatus, is a serious post-harvest pest. Development of cultivars resistant to C. maculatus population growth in storage could increase grain yield and quality and reduce reliance on insecticides. Here, we use a MAGIC (multi-parent, advanced-generation intercross) population of cowpea consisting of 305 recombinant inbred lines (RILs) to identify genetic variants associated with resistance to seed beetles. Because inferences regarding the genetic basis of resistance may depend on the source of the pest or the assay protocol, we used two divergent geographic populations of C. maculatus and two complementary assays to measure several aspects of resistance. Using polygenic genome-wide association mapping models, we found that the cowpea RILs harbor substantial additive-genetic variation for most resistance measures. Variation in several components of resistance, including larval development time and survival, was largely explained by one or several linked loci on chromosome 5. A second region on chromosome 8 explained increased seed resistance via the induction of early-exiting larvae. Neither of these regions contained genes previously associated with resistance to insects that infest grain legumes. We found some evidence of gene-gene interactions affecting resistance, but epistasis did not contribute substantially to resistance variation in this mapping population. The combination of mostly high heritabilities and a relatively consistent and simple genetic architecture increases the feasibility of breeding for enhanced resistance to C. maculatus.

Lytic Polysaccharide Monooxygenases as Chitin-Specific Virulence Factors in Crayfish Plague

The oomycete pathogen Aphanomyces astaci, also known as “crayfish plague”, is an obligate fungal-like parasite of freshwater crustaceans and is considered responsible for the ongoing decline of native European crayfish populations. A. astaci is thought to secrete a wide array of effectors and enzymes that facilitate infection, however their molecular mechanisms have been poorly characterized. Here, we report the identification of AA15 lytic polysaccharide monooxygenases (LPMOs) as a new group of secreted virulence factors in A. astaci. We show that this enzyme family has greatly expanded in A. astaci compared to all other oomycetes, and that it may facilitate infection through oxidative degradation of crystalline chitin, the most abundant polysaccharide found in the crustacean exoskeleton. These findings reveal new roles for LPMOs in animal–pathogen interactions, and could help inform future strategies for the protection of farmed and endangered species.

Exploring the potential of engineering polygalacturonase-inhibiting protein as an ecological, friendly, and nontoxic pest control agent

In plants, polygalacturonase-inhibiting proteins (PGIPs) play critical roles for resistance to fungal disease by inhibiting the pectin-depolymerizing activity of endopolygalacturonases (PGs), one type of enzyme secreted by pathogens that compromises plant cell walls and leaves the plant susceptible to disease. Here, the interactions between PGIPs from Phaseolus vulgaris (PvPGIP1 and PvPGIP2) and PGs from Aspergillus niger (AnPG2), Botrytis cinerea (BcPG1 and BcPG2), and Fusarium moniliforme (FmPG3) were reconstituted through a yeast two hybrid (Y2H) system to investigate the inhibition efficiency of various PvPGIP1 and 2 truncations and mutants. We found that tPvPGIP2_5-8, which contains LRR5 to LRR8 and is only one-third the size of the full length peptide, exhibits the same level of interactions with AnPG and BcPGs as the full length PvPGIP2 via Y2H. The inhibitory activities of tPvPGIP2_5-8 on the growth of A. niger and B. cinerea were then examined and confirmed on pectin agar. On pectin assays, application of both full length PvPGIP2 and tPvPGIP2_5-8 clearly slows down the growth of A. niger and B. cinerea. Investigation on the sequence-function relationships of PGIP utilizing a combination of site directed mutagenesis and a variety of peptide truncations suggests that LRR5 could have the most essential structural feature for the inhibitory activities, and may be a possible target for the future engineering of PGIP with enhanced activity. This study highlights the potential of plant-derived PGIPs as a candidate for future in planta evaluation as a pest control agent.

Two polygalacturonase-inhibiting proteins (VrPGIP) of Vigna radiata confer resistance to bruchids (Callosobruchus spp.)

Bruchids (Callosobruchus spp.) are destructive storage pests of mung beans (Vigna radiata). Bruchids infest mature seeds during storage and in the field causing heavy losses. Bruchid resistance in mung bean has been characterized as a dominant trait controlled by a single gene. Several independent mapping studies showed that the Br locus on chromosome 5 was a key quantitative trait loci (QTL) involved in bruchid resistance. Two polygalacturonase-inhibitor protein (PGIP) family genes, VrPGIP1 and VrPGIP2, located in the Br locus may be the primary genes responsible for bruchid resistance in mung bean but no experimental proof is available. We isolated the VrPGIP1 and VrPGIP2 genes from bruchid resistant mung bean cultivar V2802 and purified the proteins by prokaryotic expression. Both VrPGIP1 and VrPGIP2 had polygalacturonase inhibitor activity and both of the PGIP proteins conferred resistance to bruchids in an artificial seed test system. VrPGIPs can inhibit the enzyme activity of polygalacturonase present in males, females and fourth instar larvae of C. maculatus. These results demonstrated that VrPGIP1 and VrPGIP2 play a critical role in bruchid resistance probably through inhibiting polygalacturonase activity.

Host Cell Wall Damage during Pathogen Infection: Mechanisms of Perception and Role in Plant-Pathogen Interactions

The plant cell wall (CW) is a complex structure that acts as a mechanical barrier, restricting the access to most microbes. Phytopathogenic microorganisms can deploy an arsenal of CW-degrading enzymes (CWDEs) that are required for virulence. In turn, plants have evolved proteins able to inhibit the activity of specific microbial CWDEs, reducing CW damage and favoring the accumulation of CW-derived fragments that act as damage-associated molecular patterns (DAMPs) and trigger an immune response in the host. CW-derived DAMPs might be a component of the complex system of surveillance of CW integrity (CWI), that plants have evolved to detect changes in CW properties. Microbial CWDEs can activate the plant CWI maintenance system and induce compensatory responses to reinforce CWs during infection. Recent evidence indicates that the CWI surveillance system interacts in a complex way with the innate immune system to fine-tune downstream responses and strike a balance between defense and growth.

New Players in the Interaction Between Beetle Polygalacturonases and Plant Polygalacturonase-Inhibiting Proteins: Insights From Proteomics and Gene Expression Analyses

Plants possess various defense strategies to counter attacks from microorganisms or herbivores. For example, plants reduce the cell-wall-macerating activity of pathogen- or insect-derived polygalacturonases (PGs) by expressing PG-inhibiting proteins (PGIPs). PGs and PGIPs belong to multi-gene families believed to have been shaped by an evolutionary arms race. The mustard leaf beetle Phaedon cochleariae expresses both active PGs and catalytically inactive PG pseudoenzymes. Previous studies demonstrated that (i) PGIPs target beetle PGs and (ii) the role of PG pseudoenzymes remains elusive, despite having been linked to the pectin degradation pathway. For further insight into the interaction between plant PGIPs and beetle PG family members, we combined affinity purification with proteomics and gene expression analyses, and identified novel inhibitors of beetle PGs from Chinese cabbage (Brassica rapa ssp. pekinensis). A beetle PG pseudoenzyme was not targeted by PGIPs, but instead interacted with PGIP-like proteins. Phylogenetic analysis revealed that PGIP-like proteins clustered apart from "classical" PGIPs but together with proteins, which have been involved in developmental processes. Our results indicate that PGIP-like proteins represent not only interesting novel PG inhibitor candidates in addition to "classical" PGIPs, but also fascinating new players in the arms race between herbivorous beetles and plant defenses.

Dampening the DAMPs: How Plants Maintain the Homeostasis of Cell Wall Molecular Patterns and Avoid Hyper-Immunity

Several oligosaccharide fragments derived from plant cell walls activate plant immunity and behave as typical damage-associated molecular patterns (DAMPs). Some of them also behave as negative regulators of growth and development, and due to their antithetic effect on immunity and growth, their concentrations, activity, time of formation, and localization is critical for the so-called "growth-defense trade-off." Moreover, like in animals, over accumulation of DAMPs in plants provokes deleterious physiological effects and may cause hyper-immunity if the cellular mechanisms controlling their homeostasis fail. Recently, a mechanism has been discovered that controls the activity of two well-known plant DAMPs, oligogalacturonides (OGs), released upon hydrolysis of homogalacturonan (HG), and cellodextrins (CDs), products of cellulose breakdown. The potential homeostatic mechanism involves specific oxidases belonging to the family of berberine bridge enzyme-like (BBE-like) proteins. Oxidation of OGs and CDs not only inactivates their DAMP activity, but also makes them a significantly less desirable food source for microbial pathogens. The evidence that oxidation and inactivation of OGs and CDs may be a general strategy of plants for controlling the homeostasis of DAMPs is discussed. The possibility exists of discovering additional oxidative and/or inactivating enzymes targeting other DAMP molecules both in the plant and in animal kingdoms.