Rice susceptibility to root-knot nematodes is enhanced by the Meloidogyne incognita MSP18 effector gene

Susceptibility and plant immune control-a case of mycorrhizal strategy for plant colonization, symbiosis, and plant immune suppression

Plants and microbes (mycorrhizal fungi to be precise) have evolved together over the past millions of years into an association that is mutualist. The plants supply the fungi with photosynthates and shelter, while the fungi reciprocate by enhancing nutrient and water uptake by the plants as well as, in some cases, control of soil-borne pathogens, but this fungi-plant association is not always beneficial. We argue that mycorrhizal fungi, despite contributing to plant nutrition, equally increase plant susceptibility to pathogens and herbivorous pests' infestation. Understanding of mycorrhizal fungi strategies for suppressing plant immunity, the phytohormones involved and the signaling pathways that aid them will enable the harnessing of tripartite (consisting of three biological systems)-plant-mycorrhizal fungi-microbe interactions for promoting sustainable production of crops.

Cannabis sativa: A look at protozoa, helminths, insect vectors, and pests

Active principles extracted from plants, such as essential oils, have been commonly described in the literature as therapeutic targets for numerous pathological conditions. Cannabis sativa, which has an ancient and peculiar history, has been used for various purposes, from recreational to compounds of pharmacotherapeutic and industrial importance, such as pesticides based on this plant. It is a plant that contains approximately 500 described cannabinoid compounds and is the target of in vitro and in vivo studies at different locations. This review clarifies the role of cannabinoid compounds in parasitic infections caused by helminths and protozoa. In addition, this study briefly presented the use of C. sativa constituents in the formulation of pesticides for vector control, as the latter topic is justified by the economic burden faced by several regions where vector-borne diseases are a troubling reality. Studies involving cannabis compounds with pesticidal potential should be encouraged, especially those that evaluate their effectiveness against the different life cycles of insects, seeking to interrupt vector proliferation after egg laying. Actions aimed at the management and cultivation of plant species with ecologically correct pharmacotherapeutic and pesticide potentials are becoming urgent.

Peat-based hairy root transformation using Rhizobium rhizogenes as a rapid and efficient tool for easily exploring potential genes related to root-knot nematode parasitism and host response

BACKGROUND

Root-knot nematodes (RKNs) pose a worldwide threat to agriculture of many crops including cucumber. Genetic transformation (GT) has emerged as a powerful tool for exploration of plant-RKN interactions and genetic improvement of RKN resistance. However, it is usually difficult to achieve a highly efficient and stable GT protocol for most crops due to the complexity of this process.

RESULTS

Here we firstly applied the hairy root transformation system in exploring root-RKN interactions in cucumber plants and developed a rapid and efficient tool transformation using Rhizobium rhizogenes strain K599. A solid-medium-based hypocotyl-cutting infection (SHI) method, a rockwool-based hypocotyl-cutting infection (RHI) method, and a peat-based cotyledon-node injection (PCI) method was evaluated for their ability to induce transgenic roots in cucumber plants. The PCI method generally outperformed the SHI and RHI methods for stimulating more transgenic roots and evaluating the phenotype of roots during nematode parasitism. Using the PCI method, we generated the CRISPR/Cas9-mediated malate synthase (MS) gene (involved in biotic stress responses) knockout plant and the LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16, a potential host susceptibility gene for RKN) promoter-driven GUS expressing plant. Knockout of MS in hairy roots resulted in effective resistance against RKNs, while nematode infection induced a strong expression of LBD16-driven GUS in root galls. This is the first report of a direct link between these genes and RKN performance in cucumber.

CONCLUSION

Taken together, the present study demonstrates that the PCI method allows fast, easy and efficient in vivo studies of potential genes related to root-knot nematode parasitism and host response.

Gall-specific promoter, an alternative to the constitutive CaMV35S promoter, drives host-derived RNA interference targeting Mi-msp2 gene to confer effective nematode resistance

One of the major obligate plant parasites causing massive economic crop losses belongs to the class of root-knot nematodes (RKNs). Targeting of major nematode parasitism genes via Host Delivered-RNAi (HD-RNAi) to confer silencing is established as one of the most effective approaches to curb nematode infection. Utilizing nematode-responsive root-specific (NRRS) promoters to design a dsRNA molecule targeting approach to hamper nematode parasitism. Here, a previously validated peroxidase gall specific promoter, pAt2g18140, from Arabidopsis was employed to express the dsRNA construct of the nematode effector gene Mi-msp2 from Meloidogyne incognita. Arabidopsis RNAi lines of CaMV35S::Mi-msp2-RNAi and pAt2g18140::Mi-msp2-RNAi were compared with control plants to assess the decrease in plant nematode infection. When subjected to infection, the maximum reductions in the numbers of galls, females and egg masses in the CaMV35S::Mi-msp2-RNAi lines were 61%, 66% and 95%, respectively, whereas for the pAt2g18140::Mi-msp2-RNAi lines, they were 63%, 68% and 100%, respectively. The reduction in transcript level ranged from 79%-82% for CaMV35S::Mi-msp2-RNAi and 72%-79% for the pAt2g18140::Mi-msp2-RNAi lines. Additionally, a reduction in female size and a subsequent reduction in next-generation fecundity demonstrate the efficacy and potential of the gall specific promoter pAt2g18140 for utilization in the development of HD-RNAi constructs against RKN, as an excellent alternative to the CaMV35S promoter.

Host-delivered RNAi-mediated silencing of the root-knot nematode (Meloidogyne incognita) effector genes, Mi-msp10 and Mi-msp23, confers resistance in Arabidopsis and impairs reproductive ability of the root-knot nematode

Mi-msp10 and Mi-msp23 effector genes play a significant role during Meloidogyne incognita parasitism on Arabidopsis roots. The role of these genes was confirmed by demonstrating the decrease of the level of susceptibility of Arabidopsis by the  silencing of Mi-msp10 and Mi-msp23 genes using HD-RNAi technology. Root-knot nematodes (RKNs) are the most damaging pathogens severely affecting global food production. The sustainable options to minimize menace of nematode populations through economically feasible measures are limited. Thus, the development of innovative and target-specific strategies that aid in their management is imperative. RNAi technology has emerged as a sustainable and target-specific alternative to control phytonematodes. Here, we characterized two novel subventral gland and dorsal gland-specific effectors, Mi-msp10 and Mi-msp23, to determine their potential effectiveness in controlling M. incognita. Comparative developmental profiling using qRT-PCR revealed higher expression of both effectors in the adult nematode female. Furthermore, functional evaluation of Mi-msp10 and Mi-msp23 dsRNA cassettes was performed using host-delivered RNAi (HD-RNAi) in Arabidopsis. The transgenic lines were examined against M. incognita, and the phenotypic effect of HD-RNAi was evident with a 61% and 51% reduction in gall formation in the Mi-msp10 and Mi-msp23 RNAi lines, respectively. A significant drop in the nematode adult females by 59% for Mi-msp10 and 49% for Mi-msp23-RNAi lines was observed. Similarly, production in egg masses decreased significantly by 76% (Mi-msp10) and 60% (Mi-msp23) for the RNAi lines, which eventually decreased the reproductive factor by 92% and 75%, respectively. The gene expression analysis showed a significant decrease in the transcript level by up to 72% (Mi-msp10) and 66% (Mi-msp23) in M. incognita females feeding on RNAi lines, providing further evidence of effective gene silencing. Overall, our findings provide useful information and support further development of RNAi-based strategies to control M. incognita.

Recognition and Response in Plant-Nematode Interactions

Plant-parasitic nematodes spend much of their lives inside or in contact with host tissue, and molecular interactions constantly occur and shape the outcome of parasitism. Eggs of these parasites generally hatch in the soil, and the juveniles must locate and infect an appropriate host before their stored energy is exhausted. Components of host exudate are evaluated by the nematode and direct its migration to its infection site. Host plants recognize approaching nematodes before physical contact through molecules released by the nematodes and launch a defense response. In turn, nematodes deploy numerous mechanisms to counteract plant defenses. This review focuses on these early stages of the interaction between plants and nematodes. We discuss how nematodes perceive and find suitable hosts, how plants perceive and mount a defense response against the approaching parasites, and how nematodes fight back against host defenses.

Rooting Out the Mechanisms of Root-Knot Nematode-Plant Interactions

Root-knot nematodes (RKNs; Meloidogyne spp.) engage in complex parasitic interactions with many different host plants around the world, initiating elaborate feeding sites and disrupting host root architecture. Although RKNs have been the focus of research for many decades, new molecular tools have provided useful insights into the biological mechanisms these pests use to infect and manipulate their hosts. From identifying host defense mechanisms underlying resistance to RKNs to characterizing nematode effectors that alter host cellular functions, the past decade of research has significantly expanded our understanding of RKN-plant interactions, and the increasing number of quality parasite and host genomes promises to enhance future research efforts into RKNs. In this review, we have highlighted recent discoveries, summarized the current understanding within the field, and provided links to new and useful resources for researchers. Our goal is to offer insights and tools to support the study of molecular RKN-plant interactions.

In planta RNAi approach targeting three M. incognita effector genes disturbed the process of infection and reduced plant susceptibility

Meloidogyne incognita is the most economically important species of the root-knot nematode complex causing damage to several crops worldwide. During parasitism in host plants, M. incognita secretes several effector proteins to suppress the plant immune system, manipulate the plant cell cycle, and promote parasitism. Several effector proteins have been identified, but their relationship with plant parasitism by M. incognita has not been fully confirmed. Herein, the Minc01696, Minc00344, and Minc00801 putative effector genes were evaluated to assess their importance during soybean and Nicotiana tabacum parasitism by M. incognita. For this study, we used in planta RNAi technology to overexpress dsRNA molecules capable of producing siRNAs that target and downregulate these nematode effector genes. Soybean composite roots and N. tabacum lines were successfully generated, and susceptibility level to M. incognita was evaluated. Consistently, both transgenic soybean roots and transgenic N. tabacum lines carrying the RNAi strategy showed reduced susceptibility to M. incognita. The number of galls per plant and the number of egg masses per plant were reduced by up to 85% in transgenic soybean roots, supported by the downregulation of effector genes in M. incognita during parasitism. Similarly, the number of galls per plant, the number of egg masses per plant, and the nematode reproduction factor were reduced by up to 83% in transgenic N. tabacum lines, which was also supported by the downregulation of the Minc00801 effector gene during parasitism. Therefore, our data indicate that all three effector genes can be a target in the development of new biotechnological tools based on the RNAi strategy in economically important crops for M. incognita control.

Minc03328 effector gene downregulation severely affects Meloidogyne incognita parasitism in transgenic Arabidopsis thaliana

Minc03328 effector gene downregulation triggered by in planta RNAi strategy strongly reduced plant susceptibility to Meloidogyne incognita and suggests that Minc03328 gene is a promising target for the development of genetically engineered crops to improve plant tolerance to M. incognita. Meloidogyne incognita is the most economically important species of root-knot nematodes (RKN) and causes severe damage to crops worldwide. M. incognita secretes several effector proteins to suppress the host plant defense response, and manipulate the plant cell cycle and other plant processes facilitating its parasitism. Different secreted effector proteins have already been identified in M. incognita, but not all have been characterized or have had the confirmation of their involvement in nematode parasitism in their host plants. Herein, we characterized the Minc03328 (Minc3s00020g01299) effector gene, confirmed its higher expression in the early stages of M. incognita parasitism in plants, as well as the accumulation of the Minc03328 effector protein in subventral glands and its secretion. We also discuss the potential for simultaneous downregulation of its paralogue Minc3s00083g03984 gene. Using the in planta RNA interference strategy, Arabidopsis thaliana plants overexpressing double-stranded RNA (dsRNA) were generated to specifically targeting and downregulating the Minc03328 gene during nematode parasitism. Transgenic Minc03328-dsRNA lines that significantly downregulated Minc03328 gene expression during M. incognita parasitism were significantly less susceptible. The number of galls, egg masses, and [galls/egg masses] ratio were reduced in these transgenic lines by up to 85%, 90%, and 87%, respectively. Transgenic Minc03328-dsRNA lines showed the presence of fewer and smaller galls, indicating that parasitism was hindered. Overall, data herein strongly suggest that Minc03328 effector protein is important for M. incognita parasitism establishment. As well, the in planta Minc03328-dsRNA strategy demonstrated high biotechnological potential for developing crop species that could efficiently control RKN in the field.

Effectors of Root-Knot Nematodes: An Arsenal for Successful Parasitism

Root-knot nematodes (RKNs) are notorious plant-parasitic nematodes first recorded in 1855 in cucumber plants. They are microscopic, obligate endoparasites that cause severe losses in agriculture and horticulture. They evade plant immunity, hijack the plant cell cycle, and metabolism to modify healthy cells into giant cells (GCs) - RKN feeding sites. RKNs secrete various effector molecules which suppress the plant defence and tamper with plant cellular and molecular biology. These effectors originate mainly from sub-ventral and dorsal oesophageal glands. Recently, a few non-oesophageal gland secreted effectors have been discovered. Effectors are essential for the entry of RKNs in plants, subsequently formation and maintenance of the GCs during the parasitism. In the past two decades, advanced genomic and post-genomic techniques identified many effectors, out of which only a few are well characterized. In this review, we provide molecular and functional details of RKN effectors secreted during parasitism. We list the known effectors and pinpoint their molecular functions. Moreover, we attempt to provide a comprehensive insight into RKN effectors concerning their implications on overall plant and nematode biology. Since effectors are the primary and prime molecular weapons of RKNs to invade the plant, it is imperative to understand their intriguing and complex functions to design counter-strategies against RKN infection.

Minc00344 and Mj-NULG1a effectors interact with GmHub10 protein to promote the soybean parasitism by Meloidogyne incognita and M. javanica

Several economically important crops are susceptible to root-knot nematode (RKNs). Meloidogyne incognita and M. javanica are the two most reported species from the RKN complex, causing damage to several crops worldwide. The successful outcome of the Meloidogyne-plant interaction is associated with molecular factors secreted by the nematode to suppress the plant's immune response and promote nematode parasitism. In contrast, several plant factors are associated with defense against nematode infection. In this study, we identified and characterized the specific interaction of Minc00344 and Mj-NULG1a effectors with soybean GmHub10 (Glyma.19G008200) protein in vitro and in vivo. An Arabidopsis thaliana T-DNA mutant of AtHub10 (AT3G27960, an orthologous gene of GmHub10) showed higher susceptibility to M. incognita. Thus, since soybean and A. thaliana Hub10 proteins are involved in pollen tube growth and indirect activation of the defense response, our data suggest that effector-Hub10 interactions could be associated with an increase in plant susceptibility. These findings indicate the potential of these effector proteins to develop new biotechnological tools based on RNA interference and the overexpression of engineered Hub10 proteins for the efficient management of RKN in crops.

Expression interference of a number of Heterodera avenae conserved genes perturbs nematode parasitic success in Triticum aestivum

The cereal cyst nematode, Heterodera avenae is distributed worldwide and causes substantial damage in bread wheat, Triticum aestivum. This nematode is extremely difficult to manage because of its prolonged persistence as unhatched eggs encased in cysts. Due to its sustainable and target-specific nature, RNA interference (RNAi)-based strategy has gained unprecedented importance for pest control. To date, RNAi strategy has not been exploited to manage H. avenae in wheat. In the present study, 40 H. avenae target genes with different molecular function were rationally selected for in vitro soaking analysis in order to assess their susceptibility to RNAi. In contrast to target-specific downregulation of 18 genes, 7 genes were upregulated and 15 genes showed unaltered expression (although combinatorial soaking showed some of these genes are RNAi susceptible), suggesting that a few of the target genes were refractory or recalcitrant to RNAi. However, RNAi of 37 of these genes negatively altered nematode behavior in terms of reduced penetration, development and reproduction in wheat. Subsequently, wheat plants were transformed with seven H. avenae target genes (that showed greatest abrogation of nematode parasitic success) for host-induced gene silencing (HIGS) analysis. Transformed plants were molecularly characterized by PCR, RT-qPCR and Southern hybridization. Production of target gene-specific double- and single-stranded RNA (dsRNA/siRNA) was detected in transformed plants. Transgenic expression of galectin, cathepsin L, vap1, serpin, flp12, RanBPM and chitinase genes conferred 33.24-72.4 % reduction in H. avenae multiplication in T₁ events with single copy ones exhibiting greatest reduction. A similar degree of resistance observed in T₂ plants indicated the consistent HIGS effect in the subsequent generations. Intriguingly, cysts isolated from RNAi plants were of smaller size with translucent cuticle compared to normal size, dark brown control cysts, suggesting H. avenae developmental retardation due to HIGS. Our study reinforces the potential of HIGS to manage nematode problems in crop plant.

Conferring root-knot nematode resistance via host-delivered RNAi-mediated silencing of four Mi-msp genes in Arabidopsis

The root-knot nematode (RKN) Meloidogyne incognita is considered one of the most damaging pests among phytonematodes. The majority of nematode oesophageal gland effector genes are indispensable in facilitating M. incognita parasitization of host plants. We report the effect of host-delivered RNAi (HD-RNAi) silencing of four selected M. incognita effector genes, namely, Mi-msp3, Mi-msp5, Mi-msp18 and Mi-msp24, in Arabidopsis thaliana. Mi-msp5, Mi-msp18 and Mi-msp24, which are dorsal gland genes, were found to be maximally expressed in the adult female stage, whereas Mi-msp3, which is a sub-ventral gland gene, was maximally expressed in an earlier stage. In transgenic plants expressing dsRNA, the reduction in the number of galls on roots was 89 %, 78 %, 86 % and 89 % for the Mi-msp3, Mi-msp5, Mi-msp18 and Mi-msp24 RNAi events, respectively. Moreover, gene transcript abundance was significantly reduced in RKN females feeding on dsRNA-expressing lines by up to 60 %, 84 %, 31 % and 61 % for Mi-msp3, Mi-msp5, Mi-msp18 and Mi-msp24, respectively. Furthermore, the M. incognita reproduction factor was reduced up to 71-, 344-, 107- and 114-fold in Arabidopsis plants expressing Mi-msp3, Mi-msp5, Mi-msp18 and Mi-msp24 dsRNA constructs, respectively. This study provides a set of potential target genes to curb nematode infestation in economically important crops via the HD-RNAi approach.

Analyses of the Root-Knot Nematode (Meloidogyne graminicola) Transcriptome during Host Infection Highlight Specific Gene Expression Profiling in Resistant Rice Plants

The plant-parasitic nematode Meloidogyne graminicola causes considerable damages to rice (Oryza sativa) culture. Resistance to M. graminicola in the related species Oryza glaberrima reduces root penetration by juveniles and stops further nematode development. M. graminicola genes expressed during O. sativa infection were previously characterized but no information is available about the molecular dialogue established with a resistant plant. We compared the M. graminicola transcriptomes of stage-two juveniles (J2s) before and during infection of susceptible or resistant rice. Among 36,121 M. graminicola genes surveyed, 367 were differentially expressed during infection of resistant or susceptible plants. Genes encoding cell wall-degrading enzymes, peptidases and neuropeptides were expressed for a longer time in resistant plants compared to susceptible plants. Conversely, genes related to nematode development were not activated in the resistant host. The majority of M. graminicola effector genes had similar expression patterns, whatever the host genotype. However, two venom allergen-like protein (VAP)-encoding genes were specifically induced in resistant plants and Mg-VAP1 silencing in J2s reduced their ability to colonize roots. This study highlighted that M. graminicola adapts its gene expression to the host susceptibility. Further investigation is required to assess the role of Mg-VAPs in the rice-nematode interaction.

The impact of Spodoptera exigua herbivory on Meloidogyne incognita-induced root responses depends on the nematodes' life cycle stages

Induced responses to above-ground and below-ground herbivores may interact via systemic signalling in plants. We investigated whether the impact of above-ground herbivory on root-knot nematode-induced responses depends on the nematode's life cycle stages. Tomato plants were infected with the nematode (Meloidogyne incognita) for 5, 15 or 30 days before receiving Spodoptera exigua caterpillars above-ground. We collected root materials after 24 h of caterpillar feeding. We investigated phytohormones and α-tomatine levels, and the expression of defence and glycoalkaloid metabolism (GAME) marker genes in tomato roots. Nematode infection alone increased the endogenous root levels of jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA), α-tomatine and the expression of the GLYCOALKALOID METABOLISM 1 (GAME1) gene mostly at 30 days post-nematode inoculation. Caterpillar feeding alone upregulated Lipoxygenase D and downregulated Basic-β-1-glucanase and GAME1 expression in roots. On nematode-infected plants, caterpillar feeding decreased JA levels, but it increased the expression of Leucine aminopeptidase A. The induction patterns of ABA and SA suggest that caterpillars cause cross-talk between the JA-signalling pathway and the SA and ABA pathways. Our results show that caterpillar feeding attenuated the induction of the JA pathway triggered by nematodes, mostly in the nematodes' reproduction stage. These results generate a better understanding of the molecular and chemical mechanisms underlying frequent nematode-plant-caterpillar interactions in natural and agricultural ecosystems.

Development of nematode resistance in Arabidopsis by HD-RNAi-mediated silencing of the effector gene Mi-msp2

Root-knot nematodes (RKNs) are devastating parasites that infect thousands of plants. As RKN infection is facilitated by oesophageal gland effector genes, one such effector gene, Mi-msp2, was selected for a detailed characterization. Based on domain analysis, the Mi-MSP2 protein contains an ShKT domain, which is likely involved in blocking K⁺ channels and may help in evading the plant defence response. Expression of the Mi-msp2 gene was higher in juveniles (parasitic stage of RKNs) than in eggs and adults. Stable homozygous transgenic Arabidopsis lines expressing Mi-msp2 dsRNA were generated, and the numbers of galls, females and egg masses were reduced by 52-54%, 60-66% and 84-95%, respectively, in two independent RNAi lines compared with control plants. Furthermore, expression analysis revealed a significant reduction in Mi-msp2 mRNA abundance (up to 88%) in female nematodes feeding on transgenic plants expressing dsRNA, and northern blot analysis confirmed expression of the Mi-msp2 siRNA in the transgenic plants. Interestingly, a significant reduction in the reproduction factor was observed (nearly 40-fold). These data suggest that the Mi-msp2 gene can be used as a potential target for RKN management in crops of economic importance.