Metabolic responses of plants to Meloidogyne species parasitism: A review on molecular events and functions

2.1 RKNs interaction provokes morphological reorganization of feeding cell

RKNs enter host cells by needle-like stylet and secrete the various cell wall degrading enzymes like endoglucanases, endoxylanases, and expansin-like proteins pectate lyases that dissolve the soft middle lamella of root cells (Wieczorek et al., 2014; Mitsumasu et al., 2015). Juveniles (J2s) of RKNs enter the vascular cylinder and migrate intracellularly to the plant cortex, where they target specific host cells and convert them into feeding sites composed of 5 to 7 enlarged cells, called giant cells that are multi-nuclei and hypertrophied. These cells are visible as galls or knots on the plant root (Bozbuga et al., 2018). These cells have a dense cytoplasm that is packed with a large number of organelles (Plastids, mitochondria, endoplasmic reticulum (ER), ribosomes, Golgi bodies) and serve as a nutrient source for developing RKN (Favery et al., 2016).They also have a high level of metabolic activity. RKNs secrete a specific molecule called an effector produced in the oesophageal glands that manipulate the host cells and induce the transformation of host cells into feeding cells (Table 1). These effectors help RKNs successfully parasitize by influencing plant metabolism, suppressing host defence mechanisms, and ensuring successful infestation (Vieira and Gleason, 2019). The transcription activator MYB3R4 was indicated to be necessary for gall formation in the host plant (Suzuki et al., 2021). Mejias et al. (2021) reported that M. incognita secretes an effector protein (MiEFF18) during the plant-nematode interaction that is localized by immunolocalization. The MiEFF18 causes the root cells of the host plant to change into feeding sites with many nuclei and modifies the gene expression of giant cell ontogenesis by modulation of SmD1 (plant core spliceosomal protein), which is essential for successful nematode development. SmD1 is necessary for the spliceosome in pre-mRNA splicing and alternative splicing. M. incognita is the most studied RKN species that comprises more than 100 effector proteins. In another study, Zhao et al. (2020) characterized an effector protein MiPDI1 (Meloidogyne incognita protein disulfide isomerase) in Solanaceae and Arabidopsis as a pathogenic factor promoting nematode parasitism, and upregulation of MiPDI1 protein interacts with a stress-associated protein (SAP), causing diseases development (Fig. 2).

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Fig. 2

Schematic overview of plant and Meloidogyne species interaction and molecular response. Nematode elicitors are recognized by the plant's innate immune system, which triggers a defence response. At the site of nematode infection, the plant may develop a hypersensitive reaction, referred to as a localized cell death reaction. Elicitors can interfere with the plant's signalling pathways. PR- Pathogenesis-related protein, ROS- Reactive Oxygen Species, NBS- Nucleotide binding sequence.

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RKN interaction with the plants involves producing protein molecules that interfere with the plant-associated defence molecule for the nematode infection. Mendes et al. (2021) reported the interaction between M. incognita effectors and soybean proteins through in vitro and in vivo studies. Their findings revealed that Mi-EFF1/Minc17998 effector interferes with the soybean protein GmHub6 (Glyma.17G099100; TCP14), disrupting the transcription activity factor GmHub6 responsible for the cell cycle, and suppresses the host response to RKNs infection. Fitoussi et al. (2021) studied a comprehensive oxylipin profiling of tomato plants and identified differentially expressed genes that exhibited a fundamental role in the developmental process of plants. Oxylipin is a lipid molecule involved in the plant defence mechanism. These lipid molecules also regulate the nematode transcriptome and facilitate nematode parasitism, acting as communication signals between the tomato and the nematode Meloidogyne javanica. They are known as a mediator in host nematode relationships by interfering with signalling pathways. However, the work of effectors in parasitism is still not well known, but more evidence describes nematode effectors as being able to regulate the various signalling cascades that suppress cell death and promote nematode parasitism.

Some studies revealed the role of miRNAs, non-coding RNAs that induce host plant gene expression (Díaz-Manzanoet al., 2016; Jaubert-Possamai et al., 2019). During the plant nematode interaction, RKN suppresses the host RNA silencing through various activities necessary for successful nematode infestation (Walsh et al., 2017). Some evidence proved the role of miRNA in nematode parasitism. A study by Zhao et al. (2015) identified the jasmonic acid-associated micro RNAs and characterized the functions of the miR319/TCP4 in tomatoes with RKN interaction. The involvement of different phytohormones in the feeding site formation is well documented, and various excellent reports suggested the role of plant hormones in the plant nematode interaction (Table 1). Dowd et al. (2017) demonstrated that cytokinins are crucial in developing nematode feeding sites and induced giant cells in the host. Rutter et al. (2014) found a CLE motif in RKN called a MAP (Meloidogyne A virulence Protein), which is needed to form the nematode feeding site. Kim et al. (2018) reported a MiIDL1 gene of RKN, M. incognita, produces a functional IDA (Inflorescence Deficient in Abscission) mimic that exhibits a crucial role in the successful gall development of Arabidopsis.

2.2 Plant defence-related compounds regulate host-nematode interactions

In response to a nematode attack, the plant's first barrier cell wall is activated to accumulate various chemical compounds that are toxic to the nematode. Local defence response includes the phytoalexins biosynthesis and deposition of lignin (lignification) and induction of suberin as a physical barrier. In addition, the plant immune inducers beta-aminobutyric acid (BABA), thiamine, and sclareol, all contribute to the buildup of lignin in feeding sites.

Huang et al. (2016) reported that thiamine-induced defence activates the multifaceted defence response against the RKN (Meloidogyne graminicola) on rice (Oryza sativa cv. Nipponbare). The results showed that phenylalanine ammonia-lyase (PAL), the major enzyme in the phenylpropanoid pathway, is expressed at a higher rate after being treated with thiamine, leading to increased production of hydrogen peroxide and lignin. This inhibits root-knot invading and hampers nematode infection. Holbein et al. (2019) investigated the role of suberin and lignin-based Casparian strips in inhibiting nematode penetration and acting as a defence barrier against RKN. These are the apoplastic barriers or structural barriers to parasites that prevent invasion. Various reports suggested that the plant produces phytochemicals such as caffeic acid and chlorogenic acid, which also regulate the plant nematode interaction (D’Addabbo et al., 2013). The host plant develops complex molecular strategies to avoid RKNs invasion. In this interaction, the elicitor is the component of a specific signal recognized by the host and induces the gene expression and corresponding proteins involved in the defence response. The plant response can develop two pathway systems against the nematode invasion; the first recognition system is pathogen-associated molecular patterns (PAMPs) and NAMPs (nematode-associated molecular patterns) that trigger the PTI (pattern-triggered immunity), and the second system is effector-triggered immunity (ETI). In the host cell, pattern recognition receptors (PRRs) induce a network of cellular signalling processes by activating pathogen-associated molecular patterns (PAMPs).

Interaction between PAMP and pattern recognition receptors leads to the activation of various complex signalling cascades such as mitogen-activated protein kinase (MAPK) signalling, ion fluxes pathway, Ca²⁺ cascade, production of defence hormones, and reactive oxygen species (Zhou and Zhang, 2020; Biere and Goverse, 2016). Mitogen-activated protein kinases (MAPK) phosphatases are involved in the phosphorylation of proteins and play a crucial key role in the signal transduction process during nematode parasitism by activation of complex signalling of defence-related compounds, hormones like salicylic acid (SA), jasmonic acid (JA), and ethylene (ET).

Brassinosteroids (BRs) are not able to induce the defence response by triggering jasmonic acid, salicylic acid, ethylene, or ABA signalling cascades, but they induce accumulation of ROS via respiratory burst oxidase homolog 1 (RBOH 1) and whitefly-induced 1 (WFI 1), mitogen-activated protein kinase-MPK1, MPK2, and MPK3 that BR-induced resistance to RKN is mediated via RBOH-dependent MPK activation (Song et al., 2018).

Sung et al. (2019) investigated the transcriptomic-based analysis of the sweet potato peroxidase gene in response to RKN, the susceptible cultivar Yulmi, and the resistant cultivar Juhwangmi. The study's findings identified the expression profiles of peroxidase genes and determined their crucial roles in protecting plants against the RKN, M. incognita infestation. In addition, plant defence response also triggered the nematode-associated molecular patterns (NAMPs) produced by nematodes known asascaroside, an evolutionarily conserved family of nematode pheromones. Manosalva et al. (2015) found that Ascr#18 are specific Ascarosides that pathogens stimulate mitogen-activated protein kinases, which in turn induce the expression of immunity-marker genes in plants' leaves and roots, and ascarosides play a key role in these processes including the SA- and JA-mediated signalling pathways. In the case of effector-triggered immunity (ETI), there are R genes or resistance proteins present in the host cell that recognize the effectors produced by nematode leading to the activation of transcription factors, including a range of pathogenesis-related genes. The resistant plant can recognize effector molecules by the intracellular nucleotide-binding site (NBS), and the leucine-rich (LRR) repeat domain and induces ETI that acceleration is faster than Pathogen triggered immunity (PTI). Pathogenesis-related (PR) proteins also induce a variety of nematodes, and their higher accumulation is associated with the long-distance immune response called systemic acquired resistance (SAR).

2.3 Changes in gene expression and metabolic pathways under nematode attack

After the attack, the host cell synthesizes various defence genes and enzymes that activate a new physiological status. Here, the important question is how the physiological pathway changes and metabolic activity are induced within the host cells. Kihika et al. (2020) investigated the influence of M. javanica in tomatoes through chemical analysis. The different compounds such as monoterpenes beta-pinene, alpha-phellandrene, and beta-phellandrene, (+)-(2)-carene and methyl salicylate and (Z)-methyl dihydro jasmonate are released after the nematode attack. This interfered with the chemoreception of the nematode and subsequently reduced the nematode response. In the past decades, the metabolic changes during the formation of giant cells have not been well understood, but lately, various genomic and transcriptomic analyses have been applied to realize host-nematode interactions better. Among molecular studies, metabolic profiling is a well-established method to measure host cell's metabolite level and physiological status after nematode infestation.

Martins et al. (2020) compared the resistant and susceptible peanut cultivars and identified a new candidate gene, AsMLP34, for Meloidogyne arenaria resistance in wild Arachis. The research found that some of the genes found to be differentially abundant (DAPs) encoded proteins associated with plant defence responses. These proteins include MLP-like protein 34 (MLP34), alcohol dehydrogenase (ADH), eukaryotic translation initiation factor 5A (eIF5A), enolase (ENO) and cinnamoyl-CoA reductase 1 (CCR1). Postnikova et al. (2015) reported the transcriptome analysis of resistant (cv. Moapa 69) and susceptible (cv. Lahontan) Alfalfa cultivars infected with RKN, M. incognita. Bioinformatic analysis of these cultivars revealed several differentially expressed genes in the susceptible cultivar and showed a more complex defence pathway to nematode infection. The development of gall in the vascular tissues is one of the RKN-specific features. Many shifts occur due to Populus and M. incognita interacting with one another. Genes encoding transcription factors involved in hormone signalling, production of salicylic acid, and secondary cell wall deposition were identified using comparative transcript profiling of poplar root (Baldacci-Cresp et al., 2020). Xing et al. (2017) employed the root transcriptome analysis of resistant (Yuyan 12) and susceptible (Changbohuang) tobacco (Nicotiana tabacum) varieties infected with RKN, M. incognita infection. The RNA-seq study identified a significant number of genes that had varied levels of expression (DEGs), in which six genes encoded pectinesterase/pectinesterase inhibitor (PMEU1), endoglucanase (GUN9), beta-glucosidase (BGL), polygalacturonase (PGLR4), glucan endo-1, 3-beta-glucosidase (E1312), and leucine-rich repeat extensin-like protein (LRX4) are up-regulated in the susceptible varieties of tobacco. These genes encode defence enzymes related to cell wall modification, defence-related hormone (Auxin), ROS scavenging route, and transcription factor in response to nematode infection (Fig. 3).

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Fig. 3

Diagram representation shows molecular changes in response to the root-knot nematode within plant.

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Eloh et al. (2016) used the GC−MS metabolomics analysis for tomato plants infected with M. incognita to identify characteristic metabolites and biomarkers. The study indicated that metabolomics is sensitive and a specific approach can recognize an infected plant from controls. Their findings suggested that local changes in plant metabolome are responsible for the induction of metabolites and defence compounds in tomato leaves. β-alanine, phenylalanine, and melibiose in response to RKN attack and the case of the stem, fructose, glucose ribose and sucrose are up-regulated. This modification of metabolites is involved in the nematode response pathways. In the last decades, focusing on the fusion gene construct improved performance, enhanced the plant’s resistance against nematode invasion, and negatively reduced attraction. Another recent study by Hada et al. (2021) first reported the multiple effectors silencing in M. incognita using two fusion cassettes Mi-msp1, Mi-msp16, Mi-msp20, and two FMRF amide-like peptides, Mi-fp14, Mi-fp18, and Mi-msp20 in Nicotiana tabacum alter the nematode behaviour. They alter reproduction and root invasion that show better function than the one gene knockdown, leading to the development of the highly resistant plant. Nowadays, it is well known and recognized that host epigenome could change host genotypes and parasitic interaction and determine the outcome of host-nematode interaction. Beesa et al. (2022) demonstrated that phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO) and peroxidase (POD) activity in resistant cultivars decreased the nematode activity that contributes to the formation of defence barriers in rice. However, research on the host root metabolic response on RKN is still progressing, and their result is very conflicting.

Recent years have seen tremendous advancements in the study of host-nematode interactions. Proteomics and metabolomics have shed light on the secretions and host responses, while transcriptome and genome studies have revealed the genes and regulatory components involved. Functional genomics methods such as CRISPR-Cas9 and RNA interference have made it possible to manipulate genes in nematodes and hosts precisely, which has aided in the research of certain genes. The discovery of immune cells and signalling pathways and our understanding of host immunological responses have advanced. It is now well-known which effector proteins nematodes use to manipulate their hosts, and studies on the influence of the microbiota have only started. These developments have an impact on a variety of fields, including immunology, genetics, and evolutionary biology, and present prospective paths for the efficient management of nematodes in agriculture and medicine.