Responses of Heterodera glycines and Meloidogyne incognita Infective Juveniles to Root Tissues, Root Exudates, and Root Extracts from Three Plant Species

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.

Vertical Migration of Second-stage Juveniles of Meloidogyne enterolobii as Influenced by Temperature and Host

Infective second-stage juveniles (J2) of Meloidogyne spp. migrate towards host roots, which depends on several factors, including root exudates and soil temperature. Although Meloidogyne enterolobii is a highly virulent nematode that affects major agricultural crops worldwide, there is limited ecological data about it. The objective of this study was to determine the J2 migration pattern vertically in 14-cm long segmented soil columns towards tomato (Solanum lycopersicum) and marigold (Tagetes patula) roots, each grown at two soil temperatures (20 or 26ºC). Bottomless cups with tomatoes or marigolds were attached to the top of each column; cups with no plants were used as untreated controls. Juveniles (1,000/column) were injected into a hole located 1 cm from the bottom of each column. The apparatuses were placed in growth chambers at 20 or 26ºC, and J2 were allowed to migrate for 3, 6, 9, or 12 days after injection (DAI). At each harvest, J2 were extracted from each ring of the columns and counted to compare their distribution, and root systems were stained to observe root penetration. M. enterolobii migrated over 13 cm vertically 3 DAI regardless of temperature, even without plant stimuli. The vertical migration was greater at 26ºC, where 60% of active J2 were found at distances >13 cm at 12 DAI. Temperature did not affect root penetration. Overall, a greater number of J2 was observed in tomato roots, and root penetration increased over time.

Virus-induced changes in root volatiles attract soil nematode vectors to infected plants

Plant-derived volatiles mediate interactions among plants, pathogenic viruses, and viral vectors. These volatile-dependent mechanisms have not been previously demonstrated belowground, despite their likely significant role in soil ecology and agricultural pest impacts. We investigated how the plant virus, tobacco rattle virus (TRV), attracts soil nematode vectors to infected plants. We infected Nicotiana benthamiana with TRV and compared root growth relative to that of uninfected plants. We tested whether TRV-infected N. benthamiana was more attractive to nematodes 7 d post infection and identified a compound critical to attraction. We also infected N. benthamiana with mutated TRV strains to identify virus genes involved in vector nematode attraction. Virus titre and associated impacts on root morphology were greatest 7 d post infection. Tobacco rattle virus infection enhanced 2-ethyl-1-hexanol production. Nematode chemotaxis and 2-ethyl-1-hexanol production correlated strongly with viral load. Uninfected plants were more attractive to nematodes after the addition of 2-ethyl-1-hexanol than were untreated plants. Mutation of TRV RNA2-encoded genes reduced the production of 2-ethyl-1-hexanol and nematode attraction. For the first time, this demonstrates that virus-driven alterations in root volatile emissions lead to increased chemotaxis of the virus's nematode vector, a finding with implications for sustainable management of both nematodes and viral pathogens in agricultural systems.

Harnessing root exudates for plant microbiome engineering and stress resistance in plants

A wide range of abiotic and biotic stresses adversely affect plant's growth and production. Under stress, one of the main responses of plants is the modulation of exudates excreted in the rhizosphere, which consequently leads to alterations in the resident microbiota. Thus, the exudates discharged into the rhizospheric environment play a preponderant role in the association and formation of plant-microbe interactions. In this review, we aimed to provide a synthesis of the latest and most pertinent literature on the diverse biochemical and structural compositions of plant root exudates. Also, this work investigates into their multifaceted role in microbial nutrition and intricate signaling processes within the rhizosphere, which includes quorum-sensing molecules. Specifically, it explores the contributions of low molecular weight compounds, such as carbohydrates, phenolics, organic acids, amino acids, and secondary metabolites, as well as the significance of high molecular weight compounds, including proteins and polysaccharides. It also discusses the state-of-the-art omics strategies that unveil the vital role of root exudates in plant-microbiome interactions, including defense against pathogens like nematodes and fungi. We propose multiple challenges and perspectives, including exploiting plant root exudates for host-mediated microbiome engineering. In this discourse, root exudates and their derived interactions with the rhizospheric microbiota should receive greater attention due to their positive influence on plant health and stress mitigation.

Metabolic variations in root tissues and rhizosphere soils of weak host plants potently lead to distinct host status and chemotaxis regulation of Meloidogyne incognita in intercropping

Root-knot nematodes (RKNs) inflict extensive damage to global agricultural production. Intercropping has been identified as a viable agricultural tool for combating RKNs, but the mechanisms by which intercropped plants modulate RKN parasitism are still not well understood. Here, we focus on the cucumber-amaranth intercropping system. We used a range of approaches, including the attraction assay, in vitro RNA interference (RNAi), untargeted metabolomics, and hairy root transformation, to unveil the mechanisms by which weak host plants regulate Meloidogyne incognita chemotaxis towards host plants and control infection. Amaranth roots showed a direct repellence to M. incognita through disrupting its chemotaxis. The in vitro RNAi assay demonstrated that the Mi-flp-1 and Mi-flp-18 genes (encoding FMRFamide-like peptides) regulated M. incognita chemotaxis towards cucumber and controlled infection. Moreover, M. incognita infection stimulated cucumber and amaranth to accumulate distinct metabolites in both root tissues and rhizosphere soils. In particular, naringenin and salicin, enriched specifically in amaranth rhizosphere soils, inhibited the expression of Mi-flp-1 and Mi-flp-18. In addition, overexpression of genes involved in the biosynthesis of pantothenic acid and phloretin, both of which were enriched specifically in amaranth root tissues, delayed M. incognita development in cucumber hairy roots. Together, our results reveal that both the distinct host status and disruption of chemotaxis contribute to M. incognita inhibition in intercropping.

Involvement of cyclic nucleotide-gated channels in soybean cyst nematode chemotaxis and thermotaxis

The soybean cyst nematode (SCN) is one of the most damaging pests affecting soybean production. SCN displays important host recognition behaviors, such as hatching and infection, by recognizing several compounds produced by the host. Therefore, controlling SCN behaviors such as chemotaxis and thermotaxis is an attractive pest control strategy. In this study, we found that cyclic nucleotide-gated channels (CNG channels) regulate SCN chemotaxis and thermotaxis and Hg-tax-2, a gene encoding a CNG channel, is an important regulator of SCN behavior. Gene silencing of Hg-tax-2 and treatment with a CNG channel inhibitor reduced the attraction of second-stage juveniles to nitrate, an attractant with a different recognition mechanism from the host-derived chemoattractant(s), and to host soybean roots, as well as their avoidance behavior toward high temperatures. Co-treatment of ds Hg-tax-2 with the CNG channel inhibitor indicated that Hg-tax-2 is a major regulator of SCN chemotaxis and thermotaxis. These results suggest new avenues for research on control of SCN.

Diagnosis and Characterization of Ditylenchus destructor Isolated from Mazus japonicus in China

The potato rot nematode (Ditylenchus destructor) is one of the most destructive pests in the production of tuber crops, resulting in severely decreased yields and inferior product quality. In 2021, a great number of nematodes were detected in the roots of Mazus japonicus, a weed that is harmful to crop growth, in Qingdao, Shandong Province, China. The present study was undertaken to characterize and identify the nematodes isolated from M. japonicus through morphological identification and molecular approaches. Their morphological characteristics were highly consistent with the descriptions of D. destructor Thorne, 1945. The nematodes collected from M. japonicus were identified as D. destructor haplotype B using D1/D2 and sequence characterized amplified region (SCAR) primers. PCR-ITS-RFLP analysis was conducted to monitor intraspecific variations. In addition, the phylogenetic analysis of the internal transcribed spacer (ITS) demonstrated that this D. destructor population was clustered in haplotype B, supported by a 100% bootstrap value. Another assay, in which M. japonicus was inoculated with a mixture of the life stages of D. destructor, was performed. This assay showed that M. japonicus exhibited a high susceptibility to D. destructor in pots. This is the first record of D. destructor parasitizing M. japonicus in China, and it is of great importance because M. japonicus could be a potential reservoir for D. destructor in the field.

Plant Genotype Shapes the Soil Nematode Community in the Rhizosphere of Tomatoes with Different Resistance to Meloidognye incognita

Soil nematodes are considered indicators of soil quality due to their immediate responses to changes in the soil environment and plants. However, little is known about the effects of plant genotypes on the soil nematode community. To elucidate this, high-throughput sequencing and gas chromatography/mass spectrometry analysis was conducted to analyze the soil nematode community and the structure of root exudates in the rhizosphere of tomatoes with different resistance to Meloidognye incognita. The dominant soil nematode group in the soil of resistant tomatoes was Acrobeloides, while the soil nematode group in the rhizosphere of the susceptible and tolerant tomatoes was Meloidognye. Hierarchical clustering analysis and non-metric multidimensional scaling showed that the three soil nematode communities were clustered into three groups according to the resistance level of the tomato cultivars. The soil nematode community of the resistant tomatoes had a higher maturity index and a low plant-parasite index, Wasilewska index and disease index compared to the values of the susceptible and tolerant tomatoes. Redundancy analysis revealed that the disease index and root exudates were strongly related to the soil nematode community of three tomato cultivars. Taken together, the resistance of the tomato cultivars and root exudates jointly shapes the soil nematode community. This study provided a valuable contribution to understanding the mechanism of plant genotypes shaping the soil nematode community.

Rhamnogalacturonan-I as a nematode chemoattractant from Lotus corniculatus L. super-growing root culture

INTRODUCTION

The soil houses a tremendous amount of micro-organisms, many of which are plant parasites and pathogens by feeding off plant roots for sustenance. Such root pathogens and parasites often rely on plant-secreted signaling molecules in the rhizosphere as host guidance cues. Here we describe the isolation and characterization of a chemoattractant of plant-parasitic root-knot nematodes (Meloidogyne incognita, RKN).

METHODS

The Super-growing Root (SR) culture, consisting of excised roots from the legume species Lotus corniculatus L., was found to strongly attract infective RKN juveniles and actively secrete chemoattractants into the liquid culture media. The chemo-attractant in the culture media supernatant was purified using hydrophobicity and anion exchange chromatography, and found to be enriched in carbohydrates.

RESULTS

Monosaccharide analyses suggest the chemo-attractant contains a wide array of sugars, but is enriched in arabinose, galactose and galacturonic acid. This purified chemoattractant was shown to contain pectin, specifically anti-rhamnogalacturonan-I and anti-arabinogalactan protein epitopes but not anti-homogalacturonan epitopes. More importantly, the arabinose and galactose sidechain groups were found to be essential for RKN-attracting activities. This chemo-attractant appears to be specific to M. incognita, as it wasn't effective in attracting other Meloidogyne species nor Caenorhabditis elegans.

DISCUSSION

This is the first report to identify the nematode attractant purified from root exudate of L corniculatus L. Our findings re-enforce pectic carbohydrates as important chemicals mediating micro-organism chemotaxis in the soil, and also highlight the unexpected utilities of the SR culture system in root pathogen research.

Responses of infective juveniles of the soybean cyst nematode (Heterodera glycines) and the root-knot nematodes (Meloidogyne hapla, M. incognita) to amino acids

Plant-parasitic nematode infective juveniles (J2) use phytochemical signals released into the rhizosphere to locate host roots. Amino acids are the second most abundant metabolites of root exudates, but it is unknown if they are associated with J2 chemotaxis. In this study, J2 chemotaxis and mortality of the soybean cyst nematode (Heterodera glycines) and root-knot nematodes (Meloidogyne incognita and M. hapla) were examined in response to 15 amino acids and the corresponding pH values for tested amino acid solutions were measured. Responses varied by amino acid and among the species. Significant attraction, determined by J2 count within amino acid solution dispensers after 24 h exposure, occurred with 19 out of 45 J2-amino acid combinations. Heterodera glycines, M. hapla and M. incognita were attracted to nine, three and seven amino acids, respectively. Strongest attractions were to acidic polar amino acids aspartate and glutamate (H. glycines, M. hapla) and basic polar arginine (M. hapla), as previously reported, acid and basic pH attracting nematodes, thereby indicating that pH might be one of the attraction factors for these amino acids. All three nematodes exhibited clustering behaviours, such as halo or balling formations, just outside amino acid solution dispensers, with H. glycines, M. hapla and M. incognita responding to four, 12 and two amino acids, respectively. Six of 15 amino acid solutions, representing a range of pH values, caused increased mortality. Certain aspartate and glutamate affected both H. glycines and M. hapla; arginine, aspartate, cysteine, lysine, methionine affected M. incognita; and cysteine caused complete mortality in M. hapla. All the results suggest that amino acids affect nematode attraction and mortality.

Genetic disruption of Arabidopsis secondary metabolite synthesis leads to microbiome-mediated modulation of nematode invasion

In-depth understanding of metabolite-mediated plant-nematode interactions can guide us towards novel nematode management strategies. To improve our understanding of the effects of secondary metabolites on soil nematode communities, we grew Arabidopsis thaliana genetically altered in glucosinolate, camalexin, or flavonoid synthesis pathways, and analyzed their root-associated nematode communities using metabarcoding. To test for any modulating effects of the associated microbiota on the nematode responses, we characterized the bacterial and fungal communities. Finally, as a proxy of microbiome-modulating effects on nematode invasion, we isolated the root-associated microbiomes from the mutants and tested their effect on the ability of the plant parasitic nematode Meloidogyne incognita to penetrate tomato roots. Most mutants had altered relative abundances of several nematode taxa with stronger effects on the plant parasitic Meloidogyne hapla than on other root feeding taxa. This probably reflects that M. hapla invades and remains embedded within root tissues and is thus intimately associated with the host. When transferred to tomato, microbiomes from the flavonoid over-producing pap1-D enhanced M. incognita root-invasion, whereas microbiomes from flavonoid-deficient mutants reduced invasion. This suggests microbiome-mediated effect of flavonoids on Meloidogyne infectivity plausibly mediated by the alteration of the abundances of specific microbial taxa in the transferred microbiomes, although we could not conclusively pinpoint such causative microbial taxa.

Temperature: a driving factor for Meloidogyne floridensis migration toward different hosts

The peach root-knot nematode, Meloidogyne floridensis, is an emerging species and may become a threat to peach growers if contamination and spread are not avoided. The influence of temperature and two plants – tomato (Solanum lycopersicum) and French marigold (Tagete patula) – on the vertical migration of second-stage juveniles (J2) of M. floridensis was studied using 14-cm long segmented soil columns. Plants were transplanted into cups attached to the top of each column. Nylon meshes were placed between cups and columns to prevent downward root growth. About 1,000 freshly hatched J2 were injected into the base of the columns and then the columns were transferred to growth chambers at 20 and 26°C under a completely randomized block design with four replicates. The number of J2 in each ring of the columns as well as inside tomato or marigold roots was recorded at 3, 6, 9, and 12 days after injection (DAI). Nematode data were subjected to a repeated measures MANOVA. The presence of plants did not improve J2 migration as compared to control. M. floridensis migration was best at 20°C at first, with J2 migrating more than 13 cm as soon as 3 DAI, while it took 9 DAI for J2 to migrate long distances at 26°C. The distribution of J2 along the columns was similar at both temperatures at 12 DAI. Temperature had no influence on J2 penetration. French marigold did not hinder J2 migration, but fewer J2 penetrated its roots.

Control of Meloidogyne incognita in Three-Dimensional Model Systems and Pot Experiments by the Attract-and-Kill Effect of Furfural Acetone

Meloidogyne incognita causes large-scale losses of agricultural crops worldwide. The natural metabolite furfural acetone has been reported to attract and kill M. incognita, but whether the attractant and nematicidal activities of furfural acetone on M. incognita function simultaneously in the same system, especially in three-dimensional spaces or in soil, is still unknown. Here, we used 23% Pluronic F-127 gel and a soil simulation device to demonstrate that furfural acetone has a significant attract-and-kill effect on M. incognita in both three-dimensional model systems. At 24 h, the chemotaxis index and the corrected mortality of nematodes exposed to 60 mg/ml of furfural acetone in 23% Pluronic F-127 gel were as high as 0.82 and 74.44%, respectively. Soil simulation experiments in moist sand showed that at 48 h, the chemotaxis index and the corrected mortality of the nematode toward furfural acetone reached 0.63 and 82.12%, respectively, and the effect persisted in the presence of tomato plants. In choice experiments, nematodes selected furfural acetone over plant roots and were subsequently killed. In pot studies, furfural acetone had a control rate of 82.80% against M. incognita. Collectively, these results provide compelling evidence for further investigation of furfural acetone as a novel nematode control agent.

Slight induction and strong inhibition of Heterodera glycines hatching by short-chain molecules released by different plant species

New management tools are necessary to reduce the damage caused by the soybean cyst nematode (SCN), Heterodera glycines. Identification of molecules that can stimulate second-stage juveniles (J2) hatching in an environment without food may contribute to that. In in vitro experiments, we evaluate the effect of volatile organic compounds (VOCs) released by soybean (Glycine max), bean (Phaseolus vulgaris), ryegrass (Lolium multiflorum), and alfalfa (Medicago sativa) on H. glycines egg hatching. VOCs released by all plant species significantly (p < 0.05) increased egg hatching. Short-chain molecules released by leaves and roots of soybean and bean increased the hatching up to 71.4%. The analysis of the volatilome done by gas chromatography coupled with mass spectrometry revealed 44 compounds in the plant emissions. Four of them, namely 3-octanol, 1-hexanol, hexanal and linalool were tested individually as hatching inductors. Under concentrations of 200, 600, and 1,000 µg/ml there was no hatching induction of H. glycines J2 by these compounds. On the other hand, in these concentrations, the compounds 3-octanol and 1-hexanol caused hatching reduction with values similar to the commercial nematicide carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methyl carbamate). In subsequent tests, the compounds 1-hexanol and 3-octanol showed lethal concentration values required to kill 50% of thenematode population (LC50) of 210 and 228 µg/ml, respectively, in the first experiment and, 230 and 124 µg/mlin the second one. Although we have not identified any molecules acting as hatching factor (HF), here we present a list (44 candidate molecules) that can be explored in future studies to find an efficient HF.

Cyst nematode bio-communication with plants: implications for novel management approaches

Bio-communication occurs when living organisms interact with each other, facilitated by the exchange of signals including visual, auditory, tactile and chemical. The most common form of bio-communication between organisms is mediated by chemical signals, commonly referred to as 'semiochemicals', and it involves an emitter releasing the chemical signal that is detected by a receiver leading to a phenotypic response in the latter organism. The quality and quantity of the chemical signal released may be influenced by abiotic and biotic factors. Bio-communication has been reported to occur in both above- and below-ground interactions and it can be exploited for the management of pests, such as cyst nematodes, which are pervasive soil-borne pests that cause significant crop production losses worldwide. Cyst nematode hatching and successful infection of hosts are biological processes that are largely influenced by semiochemicals including hatching stimulators, hatching inhibitors, attractants and repellents. These semiochemicals can be used to disrupt interactions between host plants and cyst nematodes. Advances in RNAi techniques such as host-induced gene silencing to interfere with cyst nematode hatching and host location can also be exploited for development of synthetic resistant host cultivars. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Constituents Leached by Tomato Seeds Regulate the Behavior of Root-Knot Nematodes and Their Antifungal Effects against Seed-Borne Fungi

Germinating seeds can release diverse phytochemicals that repel, inhibit, or kill pathogens such as root-knot nematodes and seed-borne fungi. However, little is known about the composition of these phytochemicals and their effects on pathogens. In this study, we demonstrated that tomato seed exudates can attract the nematode Meloidogyne incognita using a dual-choice assay. Eighteen compounds were then isolated and identified from the exudates. Of these, esters (1-3), fatty acids (4-6), and phenolic acids (10-12) were proven to be the signaling molecules that facilitated the host-seeking process of second-stage juveniles (J2s) of nematodes, while alkaloids (17 and 18) disrupted J2s in locating their host. Furthermore, some phenolic acids and alkaloids showed antifungal effects against seed-borne fungi. In particular, ferulic acid (12) showed obvious activity against Aspergillus flavus (minimum inhibitory concentration (MIC), 32 μg/mL), while dihydrocapsaicin (17) showed noticeable activity against Fusarium oxysporum (MIC, 16 μg/mL). Overall, this study presents the first evidence that M. incognita can be attracted to or deterred by various compounds in seed exudates through identification of the structures of the compounds in the exudates and analysis of their effects on nematodes. Furthermore, some antifungal compounds were also found. The findings of this work suggest that seed exudates are new source for finding insights into the development of plant protective substances with nematocidal and antifungal effects.

Chemotactic Host-Finding Strategies of Plant Endoparasites and Endophytes

Plants interact with microorganisms in the environment during all stages of their development and in most of their organs. These interactions can be either beneficial or detrimental for the plant and may be transient or long-term. In extreme cases, microorganisms become endoparastic or endophytic and permanently reside within a plant, while the host plant undergoes developmental reprogramming and produces new tissues or organs as a response to the invasion. Events at the cellular and molecular level following infection have been extensively described, however the mechanisms of how these microorganisms locate their plant hosts via chemotaxis remain largely unknown. In this review, we summarize recent findings concerning the signalling molecules that regulate chemotaxis of endoparasitic/endophytic bacteria, fungi, and nematodes. In particular, we will focus on the molecules secreted by plants that are most likely to act as guidance cues for microorganisms. These compounds are found in a wide range of plant species and show a variety of secondary effects. Interestingly, these compounds show different attraction potencies depending on the species of the invading organism, suggesting that cues perceived in the soil may be more complex than anticipated. However, what the cognate receptors are for these attractants, as well as the mechanism of how these attractants influence these organisms, remain important outstanding questions. Host-targeting marks the first step of plant-microorganism interactions, therefore understanding the signalling molecules involved in this step plays a key role in understanding these interactions as a whole.

Impacts of Root Metabolites on Soil Nematodes

Plant parasitic nematodes cause significant crop damage globally. Currently, many nematicides have been banned or are being phased out in Europe and other parts of the world because of environmental and human health concerns. Therefore, we need to focus on sustainable and alternative methods of nematode control to protect crops. Plant roots contain and release a wide range of bioactive secondary metabolites, many of which are known defense compounds. Hence, profound understanding of the root mediated interactions between plants and plant parasitic nematodes may contribute to efficient control and management of pest nematodes. In this review, we have compiled literature that documents effects of root metabolites on plant parasitic nematodes. These chemical compounds act as either nematode attractants, repellents, hatching stimulants or inhibitors. We have summarized the few studies that describe how root metabolites regulate the expression of nematode genes. As non-herbivorous nematodes contribute to decomposition, nutrient mineralization, microbial community structuring and control of herbivorous insect larvae, we also review the impact of plant metabolites on these non-target organisms.