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

The effector MgCRT1 of Meloidogyne graminicola targets the pathogenesis-related (PR) protein OsPR1#101 to facilitate nematodes parasitism in rice

BACKGROUND

The root-knot nematodes Meloidogyne graminicola (M. graminicola) are an important pathogenic nematode that harms rice. During infection, plant parasitic nematodes use needles to penetrate the plant cell wall, and secrete effectors into cells, thus destroying the root system's ability to absorb water and nutrients. Rice root-knot nematode disease is difficult to control once established, with traditional chemical methods prone to pollution and slow efficacy. Exploring the pathogenic mechanism of effector proteins of plant parasitic nematodes will provide a solid theoretical basis for the utilization of plant innate immunity and genetic engineering or molecular breeding of nematode resistance.

RESULTS

In this study, we characterized a pathogenesis-related protein OsPR1#101, which was targeted by the effector MgCRT1. We found that OsPR1#101 interacted directly with MgCRT1 in vitro and in vivo. The expression of OsPR1#101 was significantly induced in rice at the early stage of M. graminicola infection, but it was decreased in the late stage. Meanwhile, the expression of OsPR1#101 was significantly suppressed in the MgCRT1-overexpression line. Knocking out OsPR1#101 resulted in enhanced susceptibility to M. graminicola. However, overexpression of OsPR1#101 did not affect rice resistance against M. graminicola.

CONCLUSION

In conclusion, when M. graminicola nematodes secrete MgCRT1 into rice, MgCRT1 interacts with OsPR1#101 to interfere with rice defense to promote nematode parasitism. This finding is significant as it deepens our understanding of the molecular mechanism of nematode-plant interaction, which can potentially lead to the development of new strategies for nematode control in rice. © 2025 Society of Chemical Industry.

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.

The many faces of parasite calreticulin

Calreticulin from parasites and its vertebrate hosts share ~50% identity and many of its functions are equally conserved. However, the existing amino acid differences can affect its biological performance. Calreticulin plays an important role in Ca2+ homeostasis and as a chaperone involved in the correct folding of proteins within the endoplasmic reticulum. Outside the endoplasmic reticulum, calreticulin is involved in several immunological functions such as complement inhibition, enhancement of efferocytosis, and immune upregulation or inhibition. Several parasite calreticulins have been shown to limit immune responses and promote infectivity, while others are strong immunogens and have been used for the development of potential vaccines that limit parasite growth. Furthermore, calreticulin is essential in the dialogue between parasites and hosts, inducing Th1, Th2 or regulatory responses in a species-specific manner. In addition, calreticulin participates as initiator of endoplasmic reticulum stress in tumor cells and promotion of immunogenic cell death and removal by macrophages. Direct anti-tumoral activity has also been reported. The highly immunogenic and pleiotropic nature of parasite calreticulins, either as positive or negative regulators of the immune response, render these proteins as valuable tools to modulate immunopathologies and autoimmune disorders, as well as a potential treatment of neoplasms. Moreover, the disparities in the amino acid composition of parasite calreticulins might provide subtle variations in the mechanisms of action that could provide advantages as therapeutic tools. Here, we review the immunological roles of parasite calreticulins and discuss possible beneficial applications.