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

Abstract

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

Keywords: effectors; giant cells; oesophageal glands; plant-nematode interaction; root-knot nematode.

Figure 1

Root-knot nematode infection and glands involved in effector synthesis. (A) Root-knot nematode infected roots show a gall-like structure on the root surface. Inside the gall, adult sedentary nematodes reside. These adults secrete a large number of effectors in the plant cells converting them into multinucleated GCs, the feeding sites of nematodes. (B) The anatomy of the anterior structure of pre-parasitic juvenile RKN shows two main oesophageal glands. The oesophageal glands, namely sub-ventral glands (SvGs) and dorsal gland (DG) are the primary sites of effector production in nematodes. The effectors produced by these glands are secreted through the stylet into the plant cells.

Figure 2

Types of effectors secreted by RKNs. Effectors secreted by RKNs are divided into various categories namely cell cycle modulators, PCWDEs, cytoskeleton organizers, plant metabolic re-programmers, plant hormone regulators, and plant defence modulators. Various effectors from these categories work in tight coordination to modify normal plant cells into GCs.

Figure 3

Molecular effects of CWDEs. (A) Pre-parasitic juveniles produce a cocktail of CWDEs in the SvGs. Cellulases act on cellulose, the major backbone of the plant cell wall. Xylanase cleaves hemicellulose. Pectate lyase acts on pectin which weakens the cell wall. (B) The real-time expression profile of CWDEs shows maximum expression during pre-parasitic J2s which reduces with RKN development. The expression data is derived from (Shivakumara et al., 2017). About 400 ng RNA was used to prepare cDNA. The transcript level in all the stages was compared with that in eggs. Expression level was quantified using 2^−ΔΔCt method. 18S rRNA was used as a reference gene. (C) Pre-parasitic juveniles latched to the root surface prior to entering the root. The RNAseq analysis of pre-parasitic J2s at this stage in different host plants could reveal the CWDE isoform expression dynamics. The image is reproduced from (Shivakumara et al., 2017) with permission from the corresponding authors.

Figure 4

Molecular effects of RKN effectors (A) Parasitic juveniles produce effectors in SvGs which modulates plant defence response and cell cycle allowing the formation of GCs. The effectors secreted by parasitic juveniles (shown by red hexagon) target various cellular components and pathways of plants. In the cytoplasm MSP-40 and chorismate mutase suppress the cell death and auxin biosynthesis, respectively. In the plastids, TTL-5 interacts with plant FTRc suppressing the plant ROS response. In the nucleus MSP-16 interacts with plant SCL-6. These interactions result in alteration of mitotic cell division and alternative splicing. (B) The expression dynamics reveals that these effectors are produced pre-parasitic and parasitic J2s. Real-time expression data from various articles is used to generate the illustrative patterns of effector gene expression across the different stages of RKN (Huang et al., 2005b, 2006b; Lin et al., 2016; Niu et al., 2016). The maximum expression for a given effector in a particular life-stage is considered as 10. Accordingly, expression at remaining life-stages is calculated which is less than 10. The bars depict these transformed values. Since, these are transformed values from different data sets, it is not possible to give error bars in this case. (C) Adult sedentary RKNs produce effectors in DG that help in maintaining GCs throughout the parasitism. Eff-1 interacts with multiple plant proteins in the nucleus leading to cell cycle modulation and RNA instability. Similarly, NULG-1 targeted to the nuclei also help in cell cycle modulation. In the cytoplasm, TCTP and Misp-12 suppress the cell death and salicylic acid biosynthesis, respectively. These effectors and various others help in GC maintenance by suppression of cell death, plant defence response, and cell cycle modulation. (D) The expression profile suggests their maximum expression in J3/J4 and adult stages only. Real-time expression data from various articles is used to generate the graph (Xie et al., 2016; Zhuo et al., 2017; Godinho Mendes et al., 2021a,b). The maximum expression for a given effector in a particular life-stage is considered as 10. Accordingly, expression at remaining life-stages is calculated which is less than 10. The bars depict these transformed values. Since, these are transformed values are from different data sets, it is not possible to give error bars in this case.