How nematodes manipulate plant development pathways for infection

Switching perspectives: The roles of plant cellular reprogramming during nematode parasitism

We propose exploring plant biotrophic parasitism from both a pathogen‐centred and a plant‐centred perspective. This can generate novel research questions and reveal common plant mitigation strategies in response to biotrophic pathogens.

Advances in Migratory Plant Endoparasitic Nematode Effectors

Unlike sedentary plant-parasitic nematodes, migratory plant endoparasitic nematodes (MPENs) are unable to establish permanent feeding sites, and all developmental stages (except eggs) can invade and feed on plant tissues and can be easily overlooked because of the unspecific symptoms. They cause numerous economic losses in agriculture, forestry, and horticulture. In order to understand the pathogenetic mechanism of MPENs, here we describe research on functions and host targets focused on currently identified effectors from six MPENs, namely Radopholus similis, Pratylenchus spp., Ditylenchus destructor, Bursaphelenchus xylophilus, Aphelenchoides besseyi, and Hirschmanniella oryzae. This information will provide valuable insights into understanding MPEN effectors and for future fostering advancements in plant protection.

The nematode effector Mj-NEROSs interacts with Rieske's iron-sulfur protein influencing plastid ROS production to suppress plant immunity

Root-knot nematodes (RKN; Meloidogyne species) are plant pathogens that introduce several effectors in their hosts to facilitate infection. The actual targets and functioning mechanism of these effectors largely remain unexplored. This study illuminates the role and interplay of the Meloidogyne javanica nematode effector ROS suppressor (Mj-NEROSs) within the host plant environment. Mj-NEROSs suppresses INF1-induced cell death as well as flg22-induced callose deposition and reactive oxygen species (ROS) production. A transcriptome analysis highlighted the downregulation of ROS-related genes upon Mj-NEROSs expression. NEROSs interacts with the plant Rieske's iron-sulfur protein (ISP) as shown by yeast-two-hybrid and bimolecular fluorescence complementation. Secreted from the subventral pharyngeal glands into giant cells, Mj-NEROSs localizes in the plastids where it interacts with ISP, subsequently altering electron transport rates and ROS production. Moreover, our results demonstrate that isp Arabidopsis thaliana mutants exhibit increased susceptibility to M. javanica, indicating ISP importance for plant immunity. The interaction of a nematode effector with a plastid protein highlights the possible role of root plastids in plant defense, prompting many questions on the details of this process.

The nematode effector calreticulin competes with the high mobility group protein OsHMGB1 for binding to the rice calmodulin-like protein OsCML31 to enhance rice susceptibility to Meloidogyne graminicola

The root-knot nematode Meloidogyne graminicola secretes effectors into rice tissues to modulate host immunity. Here, we characterised MgCRT1, a calreticulin protein of M. graminicola, and identified its target in the plant. In situ hybridisation showed MgCRT1 mRNA accumulating in the subventral oesophageal gland in J2 nematodes. Immunolocalization indicated MgCRT1 localises in the giant cells during parasitism. Host-induced gene silencing of MgCRT1 reduced the infection ability of M. graminicola, while over-expressing MgCRT1 enhanced rice susceptibility to M. graminicola. A yeast two-hybrid approach identified the calmodulin-like protein OsCML31 as an interactor of MgCRT1. OsCML31 interacts with the high mobility group protein OsHMGB1 which is a conserved DNA binding protein. Knockout of OsCML31 or overexpression of OsHMGB1 in rice results in enhanced susceptibility to M. graminicola. In contrast, overexpression of OsCML31 or knockout of OsHMGB1 in rice decreases susceptibility to M. graminicola. The GST-pulldown and luciferase complementation imaging assay showed that MgCRT1 decreases the interaction of OsCML31 and OsHMGB1 in a competitive manner. In conclusion, when M. graminicola infects rice and secretes MgCRT1 into rice, MgCRT1 interacts with OsCML31 and decreases the association of OsCML31 with OsHMGB1, resulting in the release of OsHMGB1 to enhance rice susceptibility.

Joint Impacts of Meloidogyne incognita and Soil Nutrition on Solanum lycopersicum var. cerasiforme

Strategies for plant nutrient resource allocation under Meloidogyne spp. infection and different soil nutrient conditions are not well established. In response, the objectives of this research are to determine if increased vegetative growth of Solanum lycopersicon var. cerasiforme (cherry tomato) under high nutrition enhances resistance to M. incognita and whether adaptive strategies for growth, reproduction, and nutrient uptake by cherry tomato infected with M. incognita alter nutrient availability. The study was conducted under greenhouse conditions using high, medium, and low soil nutrient regimes. The research results indicate that the total biomass of cherry tomato was less in the presence of M. incognita infection under all three nutrient conditions, compared with plants grown in the absence of this nematode. However, the increase in the root/shoot ratio indicates that cherry tomato allocated more resources to belowground organs. Under the combined impacts of M. incognita infection and low or medium soil nutrition, the nitrogen content in root system tissues and the phosphorus content in shoot system tissues were increased to meet the nutrient requirements of galled root tissue and plant fruit production. It is suggested that plants increase the allocation of reproductive resources to fruits by improving phosphorus transportation to the aboveground reproductive tissues under low and medium nutrient conditions. Overall, the study highlights a significant impact of soil nutrient levels on the growth and resource allocation associated with M. incognita-infected cherry tomato. In response, soil nutrient management is another practice for reducing the impacts of plant-parasitic nematodes on crop production.

Transcription factor WOX11 modulates tolerance to cyst nematodes via adventitious lateral root formation

The transcription factor WUSCHEL-RELATED HOMEOBOX 11 (WOX11) in Arabidopsis (Arabidopsis thaliana) initiates the formation of adventitious lateral roots upon mechanical injury in primary roots. Root-invading nematodes also induce de novo root organogenesis leading to excessive root branching, but it is not known if this symptom of disease involves mediation by WOX11 and if it benefits the plant. Here, we show with targeted transcriptional repression and reporter gene analyses in Arabidopsis that the beet cyst nematode Heterodera schachtii activates WOX11-mediated adventitious lateral rooting from primary roots close to infection sites. The activation of WOX11 in nematode-infected roots occurs downstream of jasmonic acid-dependent damage signaling via ETHYLENE RESPONSE FACTOR109, linking adventitious lateral root formation to nematode damage to host tissues. By measuring different root system components, we found that WOX11-mediated formation of adventitious lateral roots compensates for nematode-induced inhibition of primary root growth. Our observations further demonstrate that WOX11-mediated rooting reduces the impact of nematode infections on aboveground plant development and growth. Altogether, we conclude that the transcriptional regulation by WOX11 modulates root system plasticity under biotic stress, which is one of the key mechanisms underlying the tolerance of Arabidopsis to cyst nematode infections.

Understanding the dynamic interactions of root-knot nematodes and their host: role of plant growth promoting bacteria and abiotic factors

Root-knot nematodes (Meloidogyne spp., RKN) are among the most destructive endoparasitic nematodes worldwide, often leading to a reduction of crop growth and yield. Insights into the dynamics of host-RKN interactions, especially in varied biotic and abiotic environments, could be pivotal in devising novel RKN mitigation measures. Plant growth-promoting bacteria (PGPB) involves different plant growth-enhancing activities such as biofertilization, pathogen suppression, and induction of systemic resistance. We summarized the up-to-date knowledge on the role of PGPB and abiotic factors such as soil pH, texture, structure, moisture, etc. in modulating RKN-host interactions. RKN are directly or indirectly affected by different PGPB, abiotic factors interplay in the interactions, and host responses to RKN infection. We highlighted the tripartite (host-RKN-PGPB) phenomenon with respect to (i) PGPB direct and indirect effect on RKN-host interactions; (ii) host influence in the selection and enrichment of PGPB in the rhizosphere; (iii) how soil microbes enhance RKN parasitism; (iv) influence of host in RKN-PGPB interactions, and (v) the role of abiotic factors in modulating the tripartite interactions. Furthermore, we discussed how different agricultural practices alter the interactions. Finally, we emphasized the importance of incorporating the knowledge of tripartite interactions in the integrated RKN management strategies.

Antioxidant Responses and Growth Impairment in Cucurbita moschata Infected by Meloidogyne incognita

Pumpkins (Cucurbita moschata), valued for their nutritional, medicinal, and economic significance, face threats from Meloidogyne incognita, a critical plant-parasitic nematode. This study extensively examines the impact of M. incognita on the growth, physiological, and biochemical responses of C. moschata. We demonstrate that M. incognita infection leads to significant growth impairment in C. moschata, evidenced by reduced plant height and biomass, along with the significant development of nematode-induced galls. Concurrently, a pronounced oxidative stress response was observed, characterized by elevated levels of hydrogen peroxide and a significant increase in antioxidant defense mechanisms, including the upregulation of key antioxidative enzymes (superoxide dismutase, glutathione reductase, catalase, and peroxidase) and the accumulation of glutathione. These responses highlight a dynamic interaction between the plant and the nematode, wherein C. moschata activates a robust antioxidant defense to mitigate the oxidative stress induced by nematode infection. Despite these defenses, the persistence of growth impairment underscores the challenge posed by M. incognita to the agricultural production of C. moschata. Our findings contribute to the understanding of plant-nematode interactions, paving the way for the development of strategies aimed at enhancing resistance in Cucurbitaceae crops against nematode pests, thus supporting sustainable agricultural practices.

A critical review on bioaerosols-dispersal of crop pathogenic microorganisms and their impact on crop yield

Bioaerosols are potential sources of pathogenic microorganisms that can cause devastating outbreaks of global crop diseases. Various microorganisms, insects and viroids are known to cause severe crop diseases impeding global agro-economy. Such losses threaten global food security, as it is estimated that almost 821 million people are underfed due to global crisis in food production. It is estimated that global population would reach 10 billion by 2050. Hence, it is imperative to substantially increase global food production to about 60% more than the existing levels. To meet the increasing demand, it is essential to control crop diseases and increase yield. Better understanding of the dispersive nature of bioaerosols, seasonal variations, regional diversity and load would enable in formulating improved strategies to control disease severity, onset and spread. Further, insights on regional and global bioaerosol composition and dissemination would help in predicting and preventing endemic and epidemic outbreaks of crop diseases. Advanced knowledge of the factors influencing disease onset and progress, mechanism of pathogen attachment and penetration, dispersal of pathogens, life cycle and the mode of infection, aid the development and implementation of species-specific and region-specific preventive strategies to control crop diseases. Intriguingly, development of R gene-mediated resistant varieties has shown promising results in controlling crop diseases. Forthcoming studies on the development of an appropriately stacked R gene with a wide range of resistance to crop diseases would enable proper management and yield. The article reviews various aspects of pathogenic bioaerosols, pathogen invasion and infestation, crop diseases and yield.

Syncytium Induced by Plant-Parasitic Nematodes

Plant-parasitic nematodes from the genera Globodera, Heterodera (cyst-forming nematodes), and Meloidogyne (root-knot nematodes) are notorious and serious pests of crops. They cause tremendous economic losses between US $80 and 358 billion a year. Nematodes infect the roots of plants and induce the formation of specialised feeding structures (syncytium and giant cells, respectively) that nourish juveniles and adults of the nematodes. The specialised secretory glands enable nematodes to synthesise and secrete effectors that facilitate migration through root tissues and alter the morphogenetic programme of host cells. The formation of feeding sites is associated with the suppression of plant defence responses and deep reprogramming of the development and metabolism of plant cells.In this chapter, we focus on syncytia induced by the sedentary cyst-forming nematodes and provide an overview of ultrastructural changes that occur in the host roots during syncytium formation in conjunction with the most important molecular changes during compatible and incompatible plant responses to infection with nematodes.

A G-type lectin receptor kinase negatively regulates Arabidopsis immunity against root-knot nematodes

Root-knot nematodes (Meloidogyne spp., RKN) are responsible for extensive crop losses worldwide. During infection, they penetrate plant roots, migrate between plant cells, and establish feeding sites, known as giant cells, near the root vasculature. Previously, we found that nematode perception and early responses in plants were similar to those of microbial pathogens and required the BRI1-ASSOCIATED KINASE1/SOMATIC EMBRYOGENESIS RECEPTOR KINASE3 (BAK1/SERK3) coreceptor in Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum). Here, we implemented a reverse genetic screen for resistance or sensitivity to RKN using Arabidopsis T-DNA alleles of genes encoding transmembrane receptor-like kinases to identify additional receptors involved in this process. This screen identified a pair of allelic mutations with enhanced resistance to RKN in a gene we named ENHANCED RESISTANCE TO NEMATODES1 (ERN1). ERN1 encodes a G-type lectin receptor kinase (G-LecRK) with a single-pass transmembrane domain. Further characterization showed that ern1 mutants displayed stronger activation of MAP kinases, elevated levels of the defense marker MYB51, and enhanced H2O2 accumulation in roots upon RKN elicitor treatments. Elevated MYB51 expression and ROS bursts were also observed in leaves of ern1 mutants upon flg22 treatment. Complementation of ern1.1 with 35S- or native promoter-driven ERN1 rescued the RKN infection and enhanced defense phenotypes. Our results indicate that ERN1 is an important negative regulator of immunity.

Activity of root-knot nematodes associated with composition of a nematode-attached microbiome and the surrounding soil microbiota

We investigated if activity of the pre-infective juveniles (J2s) of root-knot nematodes is linked to the recruitment of a specific microbiome on the nematode surface and/or to the composition of the surrounding microbiota. For this, we determined the J2 activity (active vs. non-motile, which referred to dead and immobile J2s) upon a 3-day incubation in soil suspensions and studied the composition of bacteria, protists, and fungi present on the nematode surface and in the suspensions using amplicon sequencing of the 16S/18S rRNA genes, and ITS region. We also amended suspensions with Pseudomonas protegens strain CHA0 to study its effects on J2 activity and microbial composition. The J2 activity was suppressed in soil suspensions, but increased when suspensions were amended with P. protegens CHA0. The active and non-motile J2s differed in the composition of surface-attached bacteria, which was altered by the presence of P. protegens CHA0 in the soil suspensions. The bacterial genera Algoriphagus, Pedobacter, and Bdellovibrio were enriched on active J2s and may have protected the J2s against antagonists. The incubation time appeared short for attachment of fungi and protists. Altogether, our study is a step forward in disentangling the complex nematode-microbe interactions in soil for more successful nematode control.

Response of Soybean Cultivars to Oxidative Stress caused by Meloidogyne javanica

The present study aimed to investigate the response of soybean cultivars with different susceptibility levels to the root-knot nematode Meloidogyne javanica at varied time intervals by analyzing the initial plant-nematode interaction using antioxidant enzymes as oxidative stress markers. A 4 × 4 × 2 factorial method with 5 repetitions was used to analyze 4 soybean cultivars at 4 different collection times-6, 12, 24, and 48 h-with and without M. javanica inoculation. The parameters evaluated were the activities of antioxidant enzymes phenol peroxidase (POX) and ascorbate peroxidase (APX); the concentrations of hydrogen peroxide (H2O2) and malondialdehyde (MDA); and the number of M. javanica juveniles penetrated into each plant. H2O2 concentration varied among the cultivars with and without inoculation and at different collection times as indicated by MDA concentration and POX and APX activities, demonstrating a rapid response of the host to an infection by M. javanica. Oxidative stress caused by M. javanica did not vary among the soybean cultivars regardless of their susceptibility level; however, the antioxidant enzymes POX and APX responded according to the susceptibility level of the cultivars.

Large-scale data mining pipeline for identifying novel soybean genes involved in resistance against the soybean cyst nematode

The soybean cyst nematode (SCN) [Heterodera glycines Ichinohe] is a devastating pathogen of soybean [Glycine max (L.) Merr.] that is rapidly becoming a global economic issue. Two loci conferring SCN resistance have been identified in soybean, Rhg1 and Rhg4; however, they offer declining protection. Therefore, it is imperative that we identify additional mechanisms for SCN resistance. In this paper, we develop a bioinformatics pipeline to identify protein-protein interactions related to SCN resistance by data mining massive-scale datasets. The pipeline combines two leading sequence-based protein-protein interaction predictors, the Protein-protein Interaction Prediction Engine (PIPE), PIPE4, and Scoring PRotein INTeractions (SPRINT) to predict high-confidence interactomes. First, we predicted the top soy interacting protein partners of the Rhg1 and Rhg4 proteins. Both PIPE4 and SPRINT overlap in their predictions with 58 soybean interacting partners, 19 of which had GO terms related to defense. Beginning with the top predicted interactors of Rhg1 and Rhg4, we implement a "guilt by association" in silico proteome-wide approach to identify novel soybean genes that may be involved in SCN resistance. This pipeline identified 1,082 candidate genes whose local interactomes overlap significantly with the Rhg1 and Rhg4 interactomes. Using GO enrichment tools, we highlighted many important genes including five genes with GO terms related to response to the nematode (GO:0009624), namely, Glyma.18G029000, Glyma.11G228300, Glyma.08G120500, Glyma.17G152300, and Glyma.08G265700. This study is the first of its kind to predict interacting partners of known resistance proteins Rhg1 and Rhg4, forming an analysis pipeline that enables researchers to focus their search on high-confidence targets to identify novel SCN resistance genes in soybean.

Intraspecific variation for host immune activation by the spider mite Tetranychus evansi

Many parasites can interfere with their host's defences to maximize their fitness. Here, we investigated if there is heritable variation in the spider mite Tetranychus evansi for traits associated with how they interact with their host plant. We also determined if this variation correlates with mite fecundity. Tetranychus evansi can interfere with jasmonate (JA) defences which are the main determinant of anti-herbivore immunity in plants. We investigated (i) variation in fecundity in the presence and absence of JA defences, making use of a wild-type tomato cultivar and a JA-deficient mutant (defenseless-1), and (ii) variation in the induction of JA defences, in four T. evansi field populations and 59 inbred lines created from an outbred population originating from controlled crosses of the four field populations. We observed a strong positive genetic correlation between fecundity in the presence (on wild-type) and the absence of JA defences (on defenseless-1). However, fecundity did not correlate with the magnitude of induced JA defences in wild-type plants. Our results suggest that the performance of the specialist T. evansi is not related to their ability to manipulate plant defences, either because all lines can adequately reduce levels of defences, or because they are resistant to them.

Soybean transporter AAT Rhg1 abundance increases along the nematode migration path and impacts vesiculation and ROS

Rhg1 (Resistance to Heterodera glycines 1) mediates soybean (Glycine max) resistance to soybean cyst nematode (SCN; H. glycines). Rhg1 is a 4-gene, ∼30-kb block that exhibits copy number variation, and the common PI 88788-type rhg1-b haplotype carries 9 to 10 tandem Rhg1 repeats. Glyma.18G022400 (Rhg1-GmAAT), 1 of 3 resistance-conferring genes at the complex Rhg1 locus, encodes the putative amino acid transporter AATRhg1 whose mode of action is largely unknown. We discovered that AATRhg1 protein abundance increases 7- to 15-fold throughout root cells along the migration path of SCN. These root cells develop an increased abundance of vesicles and large vesicle-like bodies (VLB) as well as multivesicular and paramural bodies. AATRhg1 protein is often present in these structures. AATRhg1 abundance remained low in syncytia (plant cells reprogrammed by SCN for feeding), unlike the Rhg1 α-SNAP protein, whose abundance has previously been shown to increase in syncytia. In Nicotiana benthamiana, if soybean AATRhg1 was present, oxidative stress promoted the formation of large VLB, many of which contained AATRhg1. AATRhg1 interacted with the soybean NADPH oxidase GmRBOHG, the ortholog of Arabidopsis thaliana RBOHD previously found to exhibit upregulated expression upon SCN infection. AATRhg1 stimulated reactive oxygen species (ROS) generation when AATRhg1 and GmRBOHG were co-expressed. These findings suggest that AATRhg1 contributes to SCN resistance along the migration path as SCN invades the plant and does so, at least in part, by increasing ROS production. In light of previous findings about α-SNAPRhg1, this study also shows that different Rhg1 resistance proteins function via at least 2 spatially and temporally separate modes of action.

Characterization of a Root-Knot Nematode Infecting Aconitum carmichaelii in Southwest China

Growth of the Chinese herbal medicine industry has resulted in several new pests and diseases. China is one of the world largest producers of monkshood (Aconitum carmichaelii Debx.), but an unidentified root-knot nematode has become a significant pest in the southwestern provinces of Yunnan and Sichuan. Morphological characteristics and the ribosomal DNA-internal transcribed spacer and D2-D3 region of the 28S ribosomal RNA gene sequences were used to identify the nematode as Meloidogyne hapla. Through investigation, this is the first report of M. hapla infecting monkshood in Yunnan and Sichuan Provinces.

Root architecture plasticity in response to endoparasitic cyst nematodes is mediated by damage signaling

Plant root architecture plasticity in response to biotic stresses has not been thoroughly investigated. Infection by endoparasitic cyst nematodes induces root architectural changes that involve the formation of secondary roots at infection sites. However, the molecular mechanisms regulating secondary root formation in response to cyst nematode infection remain largely unknown. We first assessed whether secondary roots form in a nematode density-dependent manner by challenging wild-type Arabidopsis plants with increasing numbers of cyst nematodes (Heterodera schachtii). Next, using jasmonate-related reporter lines and knockout mutants, we tested whether tissue damage by nematodes triggers jasmonate-dependent secondary root formation. Finally, we verified whether damage-induced secondary root formation depends on local auxin biosynthesis at nematode infection sites. Intracellular host invasion by H. schachtii triggers a transient local increase in jasmonates, which activates the expression of ERF109 in a COI1-dependent manner. Knockout mutations in COI1 and ERF109 disrupt the nematode density-dependent increase in secondary roots observed in wild-type plants. Furthermore, ERF109 regulates secondary root formation upon H. schachtii infection via local auxin biosynthesis. Host invasion by H. schachtii triggers secondary root formation via the damage-induced jasmonate-dependent ERF109 pathway. This points at a novel mechanism underlying plant root plasticity in response to biotic stress.

Host-delivered RNAi-mediated silencing using fusion cassettes of different functional groups of genes precludes Meloidogyne incognita multiplication in Nicotiana tabacum

This study demonstrates multi-gene silencing approach for simultaneous silencing of several functional genes through a fusion gene strategy for protecting plants against root-knot nematode, Meloidogyne incognita. The ability of root-knot nematode (RKN), Meloidogyne incognita, to cause extensive yield decline in a wide range of cultivated crops is well-documented. Due to the inadequacies of current management approaches, the alternatively employed contemporary RNA interference (RNAi)-based host-delivered gene silencing (HD-RNAi) strategy targeting different functional effectors/genes has shown substantial potential to combat RKNs. In this direction, we have explored the possibility of simultaneous silencing of four esophageal gland genes, six plant cell-wall modifying enzymes (PCWMEs) and a serine protease gene of M. incognita using the fusion approach. In vitro RNAi showed that combinatorial gene silencing is the most effective in affecting nematode behavior in terms of reduced attraction, penetration, development, and reproduction in tomato and adzuki beans. In addition, qRT-PCR analysis of M. incognita J2s soaked in fusion-dsRNA showed perturbed expression of all the genes comprising the fusion construct confirming successful dsRNA processing which is also supported by increased mRNA abundance of five key-RNAi pathway genes. In addition, hairpin RNA expressing constructs of multi-gene fusion cassettes were developed and used for generation of Nicotiana tabacum transgenic plants. The integration of gene constructs and expression of siRNAs in transgenic events were confirmed by Southern and Northern blot analyses. Besides, bio-efficacy analyses of transgenic events, conferred up to 87% reduction in M. incognita multiplication. Correspondingly, reduced transcript accumulation of the target genes in the M. incognita females extracted from transgenic events confirmed successful gene silencing.

Comparisons of constitutive resistances to soybean cyst nematode between PI 88788- and Peking-type sources of resistance in soybean by transcriptomic and metabolomic profilings

Soybean cyst nematode (SCN) is a serious damaging disease in soybean worldwide. Peking- and PI 88788-type sources of resistance are two most important germplasm used in breeding resistant soybean cultivars against this disease. However, until now, no comparisons of constitutive resistances to soybean cyst nematode between these two types of sources had been conducted, probably due to the influences of different backgrounds. In this study, we used pooled-sample analysis strategy to minimize the influence of different backgrounds and directly compared the molecular mechanisms underlying constitutive resistance to soybean cyst nematode between these two types of sources via transcriptomic and metabolomic profilings. Six resistant soybean accessions that have identical haplotypes as Peking at Rgh1 and Rhg4 loci were pooled to represent Peking-type sources. The PI88788-type and control pools were also constructed in a same way. Through transcriptomic and metabolomics anaylses, differentially expressed genes and metabolites were identified. The molecular pathways involved in the metabolism of toxic metabolites were predicted to play important roles in conferring soybean cyst nematode resistance to soybean. Functions of two resistant candidate genes were confirmed by hairy roots transformation methods in soybean. Our studies can be helpful for soybean scientists to further learn about the molecular mechanism of resistance to soybean cyst nematode in soybean.

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.

SlWRKY45 interacts with jasmonate-ZIM domain proteins to negatively regulate defense against the root-knot nematode Meloidogyne incognita in tomato

Parasitic root-knot nematodes (RKNs) cause a severe reduction in crop yield and seriously threaten agricultural production. The phytohormones jasmonates (JAs) are important signals regulating resistance to multiple biotic and abiotic stresses. However, the molecular mechanism for JAs-regulated defense against RKNs in tomato remains largely unclear. In this study, we found that the transcription factor SlWRKY45 interacted with most JA-ZIM domain family proteins (JAZs), key repressors of the JA signaling. After infection by the RKN Meloidogyne incognita, the slwrky45 mutants exhibited lower gall numbers and egg numbers per gram of roots than wild type, whereas overexpression of SlWRKY45 attenuated resistance to Meloidogyne incognita. Under M. incognita infection, the contents of jasmonic acid (JA) and JA-isoleucine (JA-Ile) in roots were repressed by SlWRKY45-overexpression. Furthermore, SlWRKY45 bound to and inhibited the promoter of the JA biosynthesis gene ALLENE OXIDE CYCLASE (AOC), and repressed its expression. Overall, our findings revealed that the SlJAZ-interaction protein SlWRKY45 attenuated RKN-regulated JA biosynthesis and repressed defense against the RKN M. incognita in tomato.

Peptide Effectors in Phytonematode Parasitism and Beyond

Peptide signaling is an emerging paradigm in molecular plant-microbe interactions with vast implications for our understanding of plant-nematode interactions and beyond. Plant-like peptide hormones, first discovered in cyst nematodes, are now recognized as an important class of peptide effectors mediating several different types of pathogenic and symbiotic interactions. Here, we summarize what has been learned about nematode-secreted CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) peptide effectors since the last comprehensive review on this topic a decade ago. We also highlight new discoveries of a diverse array of peptide effectors that go beyond the CLE peptide effector family in not only phytonematodes but in organisms beyond the phylum Nematoda.

Application Potential of Bacterial Volatile Organic Compounds in the Control of Root-Knot Nematodes

Plant-parasitic nematodes (PPNs) constitute the most damaging group of plant pathogens. Plant infections by root-knot nematodes (RKNs) alone could cause approximately 5% of global crop loss. Conventionally, chemical-based methods are used to control PPNs at the expense of the environment and human health. Accordingly, the development of eco-friendly and safer methods has been urged to supplement or replace chemical-based methods for the control of RKNs. Using microorganisms or their metabolites as biological control agents (BCAs) is a promising approach to controlling RKNs. Among the metabolites, volatile organic compounds (VOCs) have gained increasing attention because of their potential in the control of not only RKNs but also other plant pathogens, such as insects, fungi, and bacteria. This review discusses the biology of RKNs as well as the status of various control strategies. The discovery of VOCs emitted by bacteria from various environmental sources and their application potential as BCAs in controlling RKNs are specifically addressed.

Water Stress Differentially Modulates the Expression of Tomato Cell Wall Metabolism-Related Genes in Meloidogyne incognita Feeding Sites

Microscopic observations and transcriptomic RNA-Seq analyses were applied to investigate the effect of water stress during the formation of tomato galls formation 1 and 2 weeks after inoculation with the root-knot nematode Meloidogyne incognita. Water stress affected root growth and the nematode ability to mount an efficient parasitism. The effects of water stress on the feeding site development were already observed at 1 week after nematode inoculation, with smaller giant cells, delayed development, and thinner cell walls. These features suggested changes in the expression levels of genes involved in the feeding site formation and maintenance. Gene Ontology (GO) enrichment and expression patterns were used to characterize differentially expressed genes. Water stress modified the expression profile of genes involved in the synthesis, degradation, and remodeling of the cell wall during the development of nematode feeding site. A comparison of gene expression with unstressed galls revealed that water stress intensified the up or downregulation of most genes. However, it particularly influenced the expression pattern of expansin A11 (Solyc04g081870.4.1), expansin-like B1(Solyc08g077910.3.1), a pectin acetylesterase (Solyc08g005800.4.1), and the pectin methylesterase pmeu1 (Solyc03g123630.4.1) which were upregulated in unstressed galls and repressed by water stress, at both sampling times. The expression of most members of the genes involved in cell wall metabolism, i.e., those coding for Csl, fasciclin, and COBRA proteins, were negatively influenced. Interestingly, alteration in the expression profiles of most dirigent protein genes (DIRs) and upregulation of five gene coding for Casparian strip domain protein (CASP)-like proteins were found. Gene expression analysis of galls from water stressed plants allowed us to better understand the molecular basis of M. incognita parasitism in tomato. Specific genes, including those involved in regulation of cellulose synthesis and lignification process, require further study to develop defense strategies against root-knot nematodes.

Identification of the Complex Interplay Between Nematode-Related lncRNAs and Their Target Genes in Glycine max L

Soybean (Glycine max) is a major plant protein source and oilseed crop. However, plant-parasitic nematodes (PPNs) affect its annual yield. In the current study, in order to better understand the regulation of defense mechanism against PPNs in soybean, we investigated the role of long non-coding RNAs (lncRNAs) in response to two nematode species, Heterodera glycines (SCN: soybean cyst nematode) and Rotylenchulus reniformis (reniform). To this end, two publicly available RNA-seq data sets (SCN data set and RAD: reniform-associated data set) were employed to discover the lncRNAome profile of soybean under SCN and reniform infection, respectively. Upon identification of unannotated transcripts in these data sets, a seven-step pipeline was utilized to sieve these transcripts, which ended up in 384 and 283 potential lncRNAs in SCN data set and RAD, respectively. These transcripts were then used to predict cis and trans nematode-related targets in soybean genome. Computational prediction of target genes function, some of which were also among differentially expressed genes, revealed the involvement of putative nematode-responsive genes as well as enrichment of multiple stress responses in both data sets. Finally, 15 and six lncRNAs were proposed to be involved in microRNA-mediated regulation of gene expression in soybean in response to SNC and reniform infection, respectively. Collectively, this study provides a novel insight into the signaling and regulatory network of soybean-pathogen interactions and opens a new window for further research.

Root infection by the nematode Meloidogyne incognita modulates leaf antiherbivore defenses and plant resistance to Spodoptera exigua

Studies on plant-mediated interactions between root parasitic nematodes and aboveground herbivores are rapidly increasing. However, outcomes for the interacting organisms vary, and the mechanisms involved remain ambiguous. We hypothesized that the impact of root infection by the root-knot nematode Meloidogyne incognita on the performance of the aboveground caterpillar Spodoptera exigua is modulated by the nematode's infection cycle. We challenged root-knot nematode-infected tomato plants with caterpillars when the nematode's infection cycle was at the invasion, galling, and reproduction stages. We found that M. incognita root infection enhanced S. exigua performance during the galling stage, while it did not affect the caterpillar's performance at the invasion and reproduction stages. Molecular and chemical analyses performed at the different stages of the nematode infection cycle revealed that M. incognita root infection systemically affected the jasmonic acid-, salicylic acid-, and abscisic acid-related responses, as well as the changes in the leaf metabolome triggered during S. exigua feeding. The M. incognita-induced leaf responses varied over the nematode's root infection cycle. These findings suggest that specific leaf responses triggered systemically by the nematode at its different life-cycle stages underlie the differential impact of M. incognita on plant resistance against the caterpillar S. exigua.

Host-mediated RNAi for simultaneous silencing of different functional groups of genes in Meloidogyne incognita using fusion cassettes in Nicotiana tabacum

KEY MESSAGE

This study establishes possibility of combinatorial silencing of more than one functional gene for their efficacy against root-knot nematode, M. incognita. Root-knot nematodes (RKN) of the genus Meloidogyne are the key important plant parasitic nematodes (PPNs) in agricultural and horticultural crops worldwide. Among RKNs, M. incognita is the most notorious that demand exploration of novel strategies for their management. Due to its sustainable and target-specific nature, RNA interference (RNAi) has gained unprecedented importance to combat RKNs. However, based on the available genomic information and interaction studies, it can be presumed that RKNs are dynamic and not dependent on single genes for accomplishing a particular function. Therefore, it becomes extremely important to consider silencing of more than one gene to establish any synergistic or additive effect on nematode parasitism. In this direction, we have combined three effectors specific to subventral gland cells of M. incognita, Mi-msp1, Mi-msp16, Mi-msp20 as fusion cassettes-1 and two FMRFamide-like peptides, Mi-flp14, Mi-flp18, and Mi-msp20 as fusion cassettes-2 to establish their possible utility for M. incognita management. In vitro RNAi assay in tomato and adzuki bean using these two fusion gene negatively altered nematode behavior in terms of reduced attraction, invasion, development, and reproduction. Subsequently, Nicotiana tabacum plants were transformed with these two fusion gene hairpin RNA-expressing vectors (hpRNA), and characterized via PCR, qRT-PCR, and Southern blot hybridization. Production of siRNAs specific to Mi-flp18 and Mi-msp1 was also confirmed by Northern hybridization. Further, transgenic events expressing single copy insertions of hpRNA constructs of fusion 1 and fusion-2 conferred up to 85% reduction in M. incognita multiplication. Besides, expression quantification revealed a significant reduction in mRNA abundance of target genes (up to 1.8-fold) in M. incognita females extracted from transgenic plants, and provided additional evidence for successful gene silencing.

Soybean Cyst Nematode Resistance Quantitative Trait Locus cqSCN-006 Alters the Expression of a γ-SNAP Protein

Soybean cyst nematode (SCN) is the most economically damaging pathogen of soybean and host resistance is a core management strategy. The SCN resistance quantitative trait locus cqSCN-006, introgressed from the wild relative Glycine soja, provides intermediate resistance against nematode populations, including those with increased virulence on the heavily used rhg1-b resistance locus. cqSCN-006 was previously fine-mapped to a genome interval on chromosome 15. The present study determined that Glyma.15G191200 at cqSCN-006, encoding a γ-SNAP, contributes to SCN resistance. CRISPR/Cas9-mediated disruption of the cqSCN-006 allele reduced SCN resistance in transgenic roots. There are no encoded amino acid polymorphisms between resistant and susceptible alleles. However, other cqSCN-006-specific DNA polymorphisms in the Glyma.15G191200 promoter and gene body were identified, and we observed differing induction of γ-SNAP protein abundance at SCN infection sites between resistant and susceptible roots. We identified alternative RNA splice forms transcribed from the Glyma.15G191200 γ-SNAP gene and observed differential expression of the splice forms 2 days after SCN infection. Heterologous overexpression of γ-SNAPs in plant leaves caused moderate necrosis, suggesting that careful regulation of this protein is required for cellular homeostasis. Apparently, certain G. soja evolved quantitative SCN resistance through altered regulation of γ-SNAP. Previous work has demonstrated SCN resistance impacts of the soybean α-SNAP proteins encoded by Glyma.18G022500 (Rhg1) and Glyma.11G234500. The present study shows that a different type of SNAP protein can also impact SCN resistance. Little is known about γ-SNAPs in any system, but the present work suggests a role for γ-SNAPs during susceptible responses to cyst nematodes.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Meloidogyne graminicola-A Threat to Rice Production: Review Update on Distribution, Biology, Identification, and Management

Simple Summary

New risks to plant health are constantly emerging. Such is the case of the rice root knot nematode Meloidogyne graminicola, adapted to flooded conditions and representing a risk to all types of rice agro-systems. It has been recently detected in Italy and added to the European and Mediterranean Plant Protection Organization (EPPO) Alert List. The presence of this nematode in Europe poses a threat to rice production, as there is a high probability to spread, due to trade activities and climate changes. In view of its importance, an extensive updated review was carried out.

Abstract

Rice (Oryza sativa L.) is one of the main cultivated crops worldwide and represents a staple food for more than half of the world population. Root-knot nematodes (RKNs), Meloidogyne spp., and particularly M. graminicola, are serious pests of rice, being, probably, the most economically important plant-parasitic nematode in this crop. M. graminicola is an obligate sedentary endoparasite adapted to flooded conditions. Until recently, M. graminicola was present mainly in irrigated rice fields in Asia, parts of the Americas, and South Africa. However, in July 2016, it was found in northern Italy in the Piedmont region and in May 2018 in the Lombardy region in the province of Pavia. Following the first detection in the EPPO region, this pest was included in the EPPO Alert List as its wide host range and ability to survive during long periods in environments with low oxygen content, represent a threat for rice production in the European Union. Considering the impact of this nematode on agriculture, a literature review focusing on M. graminicola distribution, biology, identification, and management was conducted.

Leaf herbivory counteracts nematode-triggered repression of jasmonate-related defenses in tomato roots

Shoot herbivores may influence the communities of herbivores associated with the roots via inducible defenses. However, the molecular mechanisms and hormonal signaling underpinning the systemic impact of leaf herbivory on root-induced responses against nematodes remain poorly understood. By using tomato (Solanum lycopersicum) as a model plant, we explored the impact of leaf herbivory by Manduca sexta on the performance of the root knot nematode Meloidogyne incognita. By performing glasshouse bioassays, we found that leaf herbivory reduced M. incognita performance in the roots. By analyzing the root expression profile of a set of oxylipin-related marker genes and jasmonate root content, we show that leaf herbivory systemically activates the 13-Lipoxigenase (LOX) and 9-LOX branches of the oxylipin pathway in roots and counteracts the M. incognita-triggered repression of the 13-LOX branch. By using untargeted metabolomics, we also found that leaf herbivory counteracts the M. incognita-mediated repression of putative root chemical defenses. To explore the signaling involved in this shoot-to-root interaction, we performed glasshouse bioassays with grafted plants compromised in jasmonate synthesis or perception, specifically in their shoots. We demonstrated the importance of an intact shoot jasmonate perception, whereas having an intact jasmonate biosynthesis pathway was not essential for this shoot-to-root interaction. Our results highlight the impact of leaf herbivory on the ability of M. incognita to manipulate root defenses and point to an important role for the jasmonate signaling pathway in shoot-to-root signaling.

A Qa-SNARE complex contributes to soybean cyst nematode resistance via regulation of mitochondria-mediated cell death

The resistance to Heterodera glycines 1 (Rhg1) locus is widely used by soybean breeders to reduce yield loss caused by soybean cyst nematode (SCN). α-SNAP (α-soluble NSF attachment protein) within Rhg1 locus contributes to SCN resistance by modulation of cell status at the SCN feeding site; however, the underlying mechanism is largely unclear. Here, we identified an α-SNAP-interacting protein, GmSYP31A, a Qa-SNARE (soluble NSF attachment protein receptor) protein from soybean. Expression of GmSYP31A significantly induced cell death in Nicotiana benthamiana leaves, and co-expression of α-SNAP and GmSYP31A could accelerate cell death. Overexpression of GmSYP31A increased SCN resistance, while silencing or overexpression of a dominant-negative form of GmSYP31A increased SCN sensitivity. GmSYP31A expression also disrupted endoplasmic reticulum-Golgi trafficking, and the exocytosis pathway. Moreover, α-SNAP was also found to interact with GmVDAC1D (voltage-dependent anion channel). The cytotoxicity induced by the expression of GmSYP31A could be relieved either with the addition of an inhibitor of VDAC protein, or by silencing the VDAC gene. Taken together, our data not only demonstrate that α-SNAP works together with GmSYP31A to increase SCN resistance through triggering cell death, but also highlight the unexplored link between the mitochondrial apoptosis pathway and vesicle trafficking.

Constraints and Prospects of Improving Cowpea Productivity to Ensure Food, Nutritional Security and Environmental Sustainability

Providing safe and secure food for an increasing number of people globally is challenging. Coping with such a human population by merely applying the conventional agricultural production system has not proved to be agro-ecologically friendly; nor is it sustainable. Cowpea (Vigna unguiculata (L) Walp) is a multi-purpose legume. It consists of high-quality protein for human consumption, and it is rich in protein for livestock fodder. It enriches the soil in that it recycles nutrients through the fixation of nitrogen in association with nodulating bacteria. However, the productivity of this multi-functional, indigenous legume that is of great value to African smallholder farmers and the rural populace, and also to urban consumers and entrepreneurs, is limited. Because cowpea is of strategic importance in Africa, there is a need to improve on its productivity. Such endeavors in Africa are wrought with challenges that include drought, salinity, the excessive demand among farmers for synthetic chemicals, the repercussions of climate change, declining soil nutrients, microbial infestations, pest issues, and so forth. Nevertheless, giant strides have already been made and there have already been improvements in adopting sustainable and smart biotechnological approaches that are favorably influencing the production costs of cowpea and its availability. As such, the prospects for a leap in cowpea productivity in Africa and in the enhancement of its genetic gain are good. Potential and viable means for overcoming some of the above-mentioned production constraints would be to focus on the key cowpea producer nations in Africa and to encourage them to embrace biotechnological techniques in an integrated approach to enhance for sustainable productivity. This review highlights the spectrum of constraints that limit the cowpea yield, but looks ahead of the constraints and seeks a way forward to improve cowpea productivity in Africa. More importantly, this review investigates applications and insights concerning mechanisms of action for implementing eco-friendly biotechnological techniques, such as the deployment of bio inoculants, applying climate-smart agricultural (CSA) practices, agricultural conservation techniques, and multi-omics smart technology in the spheres of genomics, transcriptomics, proteomics, and metabolomics, for improving cowpea yields and productivity to achieve sustainable agro-ecosystems, and ensuring their stability.

The Meloidogyne javanica effector Mj2G02 interferes with jasmonic acid signalling to suppress cell death and promote parasitism in Arabidopsis

Plant-parasitic nematodes can cause devastating damage to crops. These nematodes secrete effectors that suppress the host immune responses to enhance their survival. In this study, Mj2G02, an effector from Meloidogyne javanica, is described. In situ hybridization and transcriptional analysis showed that Mj2G02 was highly expressed in the early infection stages and exclusively expressed in the nematode subventral oesophageal gland cells. In planta RNA interference targeting Mj2G02 impaired M. javanica parasitism, and Mj2G02-transgenic Arabidopsis lines displayed more susceptibility to M. javanica. Using an Agrobacterium-mediated transient expression system and plant immune response assays, we demonstrated that Mj2G02 localized in the plant cell nuclei and could suppress Gpa2/RBP-1-induced cell death. Moreover, by RNA-Seq and quantitative reverse transcription PCR analyses, we showed that Mj2G02 was capable of interfering with the host jasmonic acid (JA) signalling pathway. Multiple jasmonate ZIM-domain (JAZ) genes were significantly upregulated, whereas the JAR1 gene and four JA-responsive genes, MYC3, UPI, THI2.1, and WRKY75, were significantly downregulated. In addition, HPLC analysis showed that the endogenous jasmonoyl-isoleucine (JA-Ile) level in Mj2G02-transgenic Arabidopsis lines was significantly decreased compared to that in wildtype plants. Our results indicate that the M. javanica effector Mj2G02 suppresses the plant immune response, therefore facilitating nematode parasitism. This process is probably mediated by a JA-Ile reduction and JAZ enhancement to repress JA-responsive genes.

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.

Impact of Meloidogyne incognita (nematode) infection on root tissues and cell wall composition of okra (Abelmoschus esculentus L. Moench, Malvaceae)

Root-knot nematodes are endoparasites whose mature females lodge and grow inside the root of some cultivated plants, leading to losses in productivity. Herein, we investigated if the infection of okra, Abelmoschus esculentus (Malvaceae), promoted by the root-knot nematode Meloidogyne incognita (Meloidogynidae) changes some agronomic traits of the host plant, as well as the cell wall composition of the root tissues. The okra Santa Cruz 47® cultivar was infected with a suspension of 5000 M. incognita juveniles. The inoculated and non-inoculated okra plants were then submitted to morphological analysis at the end of experiment, as well as histological (at 4, 11, 18, 39, ad 66 days after inoculation) and immunocytochemical analysis (control and 66 days after inoculation). Root-knot nematode infection reduced the dry weight of the stem system but, unexpectedly, the number and weight of fruits increased. At 11 days after inoculation, we detected the presence of giant cells that increased in number and size until the end of the experiment, at 66 days after inoculation. These cells came from the xylem parenchyma and showed intense and moderate labeling for epitopes recognized by JIM5 and JIM7. The presence of homogalacturonans (HGs) with different degrees of methyl esterification seems to be related to the injuries caused by the nematode feeding activity and to the processes of giant cell hypertrophy. In addition, the presence of HGs with high methyl-esterified groups can increase the cell wall porosity and facilitate the flux of nutrients for the root-knot nematode.

Can microorganisms assist the survival and parasitism of plant-parasitic nematodes?

Plant-parasitic nematodes (PPNs) remain a hardly treatable problem in many crops worldwide. Low efficacy of many biocontrol agents may be due to negligence of the native microbiota that is naturally associated with nematodes in soil, and which may protect nematodes against microbial antagonists. This phenomenon is more extensively studied for other nematode parasites, so we compiled these studies and drew parallels to the existing knowledge on PPN. We describe how microbial-mediated modulation of host immune responses facilitate nematode parasitism and discuss the role of Caenorhabditis elegans-protective microbiota to get an insight into the microbial protection of PPNs in soil. Molecular mechanisms of PPN-microbial interactions are also discussed. An understanding of microbial-aided PPN performance is thus pivotal for efficient management of PPNs.

Interaction of Symbiotic Rhizobia and Parasitic Root-Knot Nematodes in Legume Roots: From Molecular Regulation to Field Application

Legumes form two types of root organs in response to signals from microbes, namely, nodules and root galls. In the field, these interactions occur concurrently and often interact with each other. The outcomes of these interactions vary and can depend on natural variation in rhizobia and nematode populations in the soil as well as abiotic conditions. While rhizobia are symbionts that contribute fixed nitrogen to their hosts, parasitic root-knot nematodes (RKN) cause galls as feeding structures that consume plant resources without a contribution to the plant. Yet, the two interactions share similarities, including rhizosphere signaling, repression of host defense responses, activation of host cell division, and differentiation, nutrient exchange, and alteration of root architecture. Rhizobia activate changes in defense and development through Nod factor signaling, with additional functions of effector proteins and exopolysaccharides. RKN inject large numbers of protein effectors into plant cells that directly suppress immune signaling and manipulate developmental pathways. This review examines the molecular control of legume interactions with rhizobia and RKN to elucidate shared and distinct mechanisms of these root-microbe interactions. Many of the molecular pathways targeted by both organisms overlap, yet recent discoveries have singled out differences in the spatial control of expression of developmental regulators that may have enabled activation of cortical cell division during nodulation in legumes. The interaction of legumes with symbionts and parasites highlights the importance of a comprehensive view of root-microbe interactions for future crop management and breeding strategies.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Induced Local and Systemic Defense Responses in Tomato Underlying Interactions Between the Root-Knot Nematode Meloidogyne incognita and the Potato Aphid Macrosiphum euphorbiae

Plants mediate interactions between different herbivores that attack simultaneously or sequentially aboveground (AG) and belowground (BG) organs. The local and systemic activation of hormonal signaling pathways and the concomitant accumulation of defense metabolites underlie such AG-BG interactions. The main plant-mediated mechanisms regulating these reciprocal interactions via local and systemic induced responses remain poorly understood. We investigated the impact of root infection by the root-knot nematode (RKN) Meloidogyne incognita at different stages of its infection cycle, on tomato leaf defense responses triggered by the potato aphid Macrosiphum euphorbiae. In addition, we analyzed the reverse impact of aphid leaf feeding on the root responses triggered by the RKN. We focused specifically on the signaling pathways regulated by the phytohormones jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA), and indole-3-acetic acid (IAA) as well as steroidal glycoalkaloids as induced defense compounds. We found that aphid feeding did not induce AG hormonal signaling, but it repressed steroidal glycoalkaloids related responses in leaves, specifically when feeding on plants in the vegetative stage. Root infection by the RKN impeded the aphid-triggered repression of the steroidal glycoalkaloids-related response AG. In roots, the RKN triggered the SA pathway during the entire infection cycle and the ABA pathway specifically during its reproduction stage. RKN infection also elicited the steroidal glycoalkaloids related gene expression, specifically when it was in the galling stage. Aphid feeding did not systemically alter the RKN-induced defense responses in roots. Our results point to an asymmetrical interaction between M. incognita and Ma. euphorbiae when co-occurring in tomato plants. Moreover, the RKN seems to determine the root defense response regardless of a later occurring attack by the potato aphid AG.

Plants-nematodes-microbes crosstalk within soil: A trade-off among friends or foes

Plants interact with enormous biotic and abiotic components within ecosystem. For instance, microbes, insects, herbivores, animals, nematodes etc. In general, these interactions are studied independently with plants, that condenses only specific information about the interaction. However, the limitation to study the cross-interactions masks the collaborative role of organisms within ecosystem. Beneficial microbes are most prominent organisms that are needed to be studied due to their bidirectional nature towards plants. Fascinatingly, Plant-Parasitic Nematodes (PPNs) have been profoundly observed to cause mass destruction of agricultural crops worldwide. The huge demand for agriculture for present-day population requires optimization of production potential by curbing the damage caused by PPNs. Chemical nematicides combats their proliferation, but their extended usage has abruptly affected flora, fauna and human populations. Because of consistent pressing issues in regard to environment, the use of biocontrol agents are most favourable alternatives for managing agriculture. However, this association is somehow, tug of war, and understanding of plant-nematode-microbial relation would enable the agriculturists to monitor the overall development of plants along with limiting the use of agrochemicals. Soil microbes are contemporary bio-nematicides emerging in the market, that stimulates the plant growth and impedes PPNs populations. They form natural enemies and trap nematodes, henceforth, it is crucial to understand these interactions for ecological and biotechnological perspectives for commercial use. Moreover, acquiring the diversity of their relationship and molecular-based mechanisms, outlines their cascade of signaling events to serve as biotechnological ecosystem engineers. The omics based mechanisms encompassing hormone gene regulatory pathways and elicitors released by microbes are able to modulate pathogenesis-related (PR) genes within plants. This is achieved via Induced Systemic Resistance (ISR) or acquired systemic channels. Taking into account all these validations, the present review mainly advocates the relationship among microbes and nematodes in plants. It is believed that this review will boost zest and zeal within researchers to effectively understand the plant-nematodes-microbes relations and their ecological perspectives.

Identification and Expression Analysis of the CsMYB Gene Family in Root Knot Nematode-Resistant and Susceptible Cucumbers

MYB (myeloblastosis) transcription factors (TFs) play important roles in controlling various physiological processes in plants, such as responses to biotic and abiotic stress, metabolism, and defense. A previous study identified a gene, Csa6G410090, encoding a plant lipid transfer protein (LTP), as a possible regulator in cucumber (Cucumis sativus L.) of the resistance response to root-knot nematode (RKN) [Meloidogyne incognita Kofoid and White (Chitwood)]. Myb-type DNA-binding TFs were presumed to regulate downstream genes expression, including LTPs, however, the regulation mechanism remained unclear. To elucidate whether and which MYB TFs may be involved in regulation of the resistance response, this study identified 112 genes as candidate members of the CsMYB gene family by combining CDD and SMART databases, using the Hidden Markov Model (HMM) and manual calibration. Within this group, ten phylogenetic subgroups were resolved according to sequence-based classification, consistent with results from comprehensive investigation of gene structure, conserved motifs, chromosome locations, and cis-element analysis. Distribution and collinearity analysis indicated that amplification of the CsMYB gene family in cucumber has occurred mainly through tandem repeat events. Spatial gene expression analysis showed that 8 CsMYB genes were highly expressed at differing levels in ten different tissues or organs. The roots of RKN-resistant and susceptible cucumbers were inoculated with M. incognita, finding that CsMYB (Csa6G538700, Csa1G021940, and Csa5G641610) genes showed up-regulation coincident with upregulation of the "hub" gene LTP (Csa6G410090) previously implicated as a major gene in the resistance response to RKN in cucumber. Results of this study suggest hypotheses regarding the elements and regulation of the resistant response as well as possible RKN resistance-enhancing strategies in cucumber and perhaps more broadly in plants.

The Genomic Impact of Selection for Virulence against Resistance in the Potato Cyst Nematode, Globodera pallida

Although the use of natural resistance is the most effective management approach against the potato cyst nematode (PCN) Globodera pallida, the existence of pathotypes with different virulence characteristics constitutes a constraint towards this goal. Two resistance sources, GpaV (from Solanum vernei) and H3 from S. tuberosum ssp. andigena CPC2802 (from the Commonwealth Potato Collection) are widely used in potato breeding programmes in European potato industry. However, the use of resistant cultivars may drive strong selection towards virulence, which allows the increase in frequency of virulent alleles in the population and therefore, the emergence of highly virulent nematode lineages. This study aimed to identify Avirulence (Avr) genes in G. pallida populations selected for virulence on the above resistance sources, and the genomic impact of selection processes on the nematode. The selection drive in the populations was found to be specific to their genetic background. At the genomic level, 11 genes were found that represent candidate Avr genes. Most of the variant calls determining selection were associated with H3-selected populations, while many of them seem to be organised in genomic islands facilitating selection evolution. These phenotypic and genomic findings combined with histological studies performed revealed potential mechanisms underlying selection in G. pallida.

HaCRT1 of Heterodera avenae Is Required for the Pathogenicity of the Cereal Cyst Nematode

Cereal cyst nematodes are sedentary biotrophic endoparasites that secrete effector proteins into plant tissues to transit normal cells into specialized feeding sites and suppress plant defenses. To understand the function of nematode effectors in Heterodera avenae, here, we identified a calreticulin protein HaCRT1, which could suppress the cell death induced by Bax when expressed in Nicotiana benthamiana. HaCRT1 is synthetized in the subventral gland cells of pre-parasitic second-stage nematodes. Real-time PCR assays indicated that the expression of HaCRT1 was highest in parasitic second-stage juveniles. The expression of an HaCRT1-RFP fusion in N. benthamiana revealed that it was localized in the endoplasmic reticulum of the plant cell. The ability of H. avenae infecting plants was significantly reduced when HaCRT1 was knocked down by RNA interference in vitro. Arabidopsis thaliana plants expressing HaCRT1 were more susceptible than wild-type plants to Pseudomonas syringae. The induction of defense-related genes, PAD4, WRKY33, FRK1, and WRKY29, after treatment with flg22 was suppressed in HaCRT1-transgenic plants. Also, the ROS accumulation induced by flg22 was reduced in the HaCRT1-transgenic plants compared to wild-type plants. HaCRT1 overexpression increased the cytosolic Ca²⁺ concentration in A. thaliana. These data suggested that HaCRT1 may contribute to the pathogenicity of H. avenae by suppressing host basal defense.

Comparative Genomics Reveals Novel Target Genes towards Specific Control of Plant-Parasitic Nematodes

Plant-parasitic nematodes cause extensive annual yield losses to worldwide agricultural production. Most cultivated plants have no known resistance against nematodes and the few bearing a resistance gene can be overcome by certain species. Chemical methods that have been deployed to control nematodes have largely been banned from use due to their poor specificity and high toxicity. Hence, there is an urgent need for the development of cleaner and more specific control methods. Recent advances in nematode genomics, including in phytoparasitic species, provide an unprecedented opportunity to identify genes and functions specific to these pests. Using phylogenomics, we compared 61 nematode genomes, including 16 for plant-parasitic species and identified more than 24,000 protein families specific to these parasites. In the genome of Meloidogyne incognita, one of the most devastating plant parasites, we found ca. 10,000 proteins with orthologs restricted only to phytoparasitic species and no further homology in protein databases. Among these phytoparasite-specific proteins, ca. 1000 shared the same properties as known secreted effectors involved in essential parasitic functions. Of these, 68 were novel and showed strong expression during the endophytic phase of the nematode life cycle, based on both RNA-seq and RT-qPCR analyses. Besides effector candidates, transcription-related and neuro-perception functions were enriched in phytoparasite-specific proteins, revealing interesting targets for nematode control methods. This phylogenomics analysis constitutes a unique resource for the further understanding of the genetic basis of nematode adaptation to phytoparasitism and for the development of more efficient control methods.

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.

Synthesis and nematicidal activities of 1,2,3-benzotriazin-4-one derivatives containing benzo[d][1,2,3]thiadiazole against Meloidogyne incognita

Based on the characteristic of benzo[d][1,2,3]thiadiazole to induce the systemic acquired resistance and improve the immunity of plants, benzo[d][1,2,3]thiadiazole was introduced into 1,2,3-benzotriazin-4-one, thirty-one novel 1,2,3-benzotriazin-4-one derivatives containing benzo[d][1,2,3]thiadiazole were designed and synthesized. Nematicidal activity showed that most of the synthesized compounds exhibited great inhibitory activity in vivo against Meloidogyne incognita at 20 mg/L. Among 31 tested compounds, A2 and A3 showed an excellent nematicidal activity with the inhibition rate of 50.4% and 53.1% at the concentration of 1.0 mg/L, respectively. The influence of substituent type and position was investigated. The relationship between structure and activity was also preliminary analyzed.

CCS52 and DEL1 function in root-knot nematode giant cell development in Xinjiang wild myrobalan plum (Prunus sogdiana Vassilcz)

Root-knot nematodes (RKNs) are highly invasive plant parasites that establish permanent feeding sites within the roots of the host plant. Successful establishment of the feeding site is essential for the survival of RKN. The formation and development of the feeding cell, also called giant cell, involve both cell division and endoreduplication. Here, we examined giant cell development and endoreduplication in Prunus sogdiana infected with the RKN. We found that feeding sites were established 3-5 days post inoculation (dpi) and matured at 21-28 dpi. The giant cells began to form 5 dpi and continued to increase in size from 7 to 21 dpi. The large numbers of dividing nuclei were observed in giant cells from 7 to 14 dpi. However, nuclear division was rarely observed after 28 days. RT-PCR and in situ hybridization analyses revealed that PsoCCS52A was abundantly expressed at 7-21 dpi and the PsoCCS52A signal observed in giant cell nucleus at 7-14 dpi. The PsoCCS52B is highly expressed at 14 dpi, and the hybridization signal was mainly in the cytoplasm of giant cells. The PsoDEL1 expression was lowest 7-21 dip, with negligible transcript detected in the giant cells. This indicates that the PsoCCS52A plays a role in the process of cell division, while the CCS52B plays a role in the development of giant cells. The PsoDEL1 plays a negative regulatory role in megakaryocyte nuclear replication. These data suggest that an increased expression of PsoCCS52A promotes nuclear division and produces a large number of polyploid nuclei, the area of giant cells and feeding sites increase, ultimately leading to the formation of galls in Prunus sogdiana.

Compounds Associated with Infection by the Root-Knot Nematode, Meloidogyne javanica, Influence the Ability of Infective Juveniles to Recognize Host Plants

Plant root chemistry is altered by the parasitism of plant-parasitic nematodes (PPN). Here, we investigated the influence of the infective stage juveniles (J2) of Meloidogyne javanica in inducing tomato (Solanum lycopersicum) root volatiles and chemotactic effect on conspecifics. In olfactometer assays, J2 avoided the roots of 2-day infected plants but preferred 7-day-infected tomato compared to healthy plants. Chemical analysis showed a 2-7-fold increase in the amounts of monoterpenes emitted from tomato roots infected with M. javanica relative to healthy roots. In further bioassays, the monoterpenes β-pinene, (+)-(2)-carene, α-phellandrene, and β-phellandrene differentially attracted (51-87%) J2 relative to control. Concurrent reduction and increase in the levels of methyl salicylate and (Z)-methyl dihydrojasmonate, respectively, in the root volatiles reduced J2 responses. These results demonstrate that the host plant can alter its root volatile composition to inhibit PPN attack. The observed plant-produced inhibition of J2 warrants further investigation as a potential management tool for growers.

Gall-Inducing Parasites: Convergent and Conserved Strategies of Plant Manipulation by Insects and Nematodes

Gall-inducing insects and nematodes engage in sophisticated interactions with their host plants. These parasites can induce major morphological and physiological changes in host roots, leaves, and other tissues. Sedentary endoparasitic nematodes, root-knot and cyst nematodes in particular, as well as gall-inducing and leaf-mining insects, manipulate plant development to form unique organs that provide them with food from feeding cells. Sometimes, infected tissues may undergo a developmental switch resulting in the formation of aberrant and spectacular structures (clubs or galls). We describe here the complex interactions between these plant-reprogramming sedentary endoparasites and their infected hosts, focusing on similarities between strategies of plant manipulation. We highlight progress in our understanding of the host plant response to infection and focus on the nematode and insect molecules secreted in planta. We suggest thatlooking at similarities may identify convergent and conserved strategies and shed light on the promise they hold for the development of new management strategies in agriculture and forestry.

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.