## 3 Discussion  
Fatty acid and retinol binding proteins, which are nematode-specific proteins, are mainly responsible for assisting nematodes in obtaining essential fatty acids and retinols. Many studies have also revealed their potential role in nematodes' host infection and pathogenicity, acquired resistance, and immune regulation [30,34]. In previous studies, eight _Ab-far_ genes were identified in RWTN, but their functions vary. Among these, _Ab-far-1_ has been shown to probably be involved in the reproduction and pathogenic processes of nematodes [26,39,47]. However, the detailed functions and mechanisms of _Ab-far-1_ in the parasitism and pathogenicity process of nematodes have not been studied. In this study, the role and mechanism of _Ab-far-1_ in RWTN parasitism were studied in the model plant _A. thaliana_. We discovered a novel mechanism of Ab-FAR-1, which interacts with _A. thaliana_ AtADF3 to alter and regulate the state and turnover process of cellular actin filament networks, thereby promoting nematode parasitism and pathogenicity. Proteins secreted by the oesophageal glands and body walls of parasitic nematodes are considered the key factors that regulate the interaction of the nematodes with their hosts and thus contribute to their parasitism and pathogenesis [11,21,48]. Protein Ab-FAR-1 has a secretory signal peptide at the \(N\)-terminal, and its mRNA is present in the hypodermis of juvenile and adult nematodes and the gonads of adult nematodes [26]. The current study further confirmed that the Ab-FAR-1\({}^{\text{2 SP}}\) protein was mainly located in the body wall of nematodes and in the cytoplasm and nucleus of host cells by using immunofluorescence and subcellular localisation. The cytoplasm and nucleus are the main functional sites of many nematode effector proteins [18]. Therefore, during parasitism by RWTN, the Ab-FAR-1 protein is secreted into the host through the body wall of the nematodes and might act on the cytoplasm and nucleus of the host cells, which in turn, might play important regulatory roles in the host defence response. However, how RWTN-secreted Ab-FAR-1 crosses the cell membrane and enters the cells of the host may require further study. To test the role of Ab-FAR-1 in RWTN parasitism, transgenic _A. thaliana_ lines overexpressing Ab-FAR-1\({}^{\text{Asp}}\) were generated. Overexpression of Ab-FAR-1\({}^{\text{Asp}}\) interfered with the growth and development of _A. thaliana_ and caused thinner plants and shorter roots than those of WT _A. thaliana_. Previous studies have reported that overexpression of some nematode effector proteins in plants can cause phenotypic changes. For example, the lengths of the primary roots are elongated in rice overexpressing _M. gramniticola_ MgMO237 [18], and overexpression of _Meloalogyne enterolobii_ MeCTP causes reduction of lateral roots in _A. thaliana_[49]. It is speculated that the changes in the root phenotype may be due to the effector proteins secreted by nematodes causing metabolic disorders within the plant host cells. To further validate the virulence function in nematode parasitism, _Ab-far-1_ was silenced using _in planta_ RNAi. Compared with WT _A. thaliana_, transgenic lines overexpressing Ab-FAR-1\({}^{\text{Asp}}\) were more susceptible to nematode infection, whereas _Ab-far-1_ RNAi lines showed the opposite trend. Therefore, Ab-FAR-1 may play a role in the parasitism and pathogenicity of RWTN. In recent years, the discovery of host proteins that interact with nematode-secreted effectors has led to the increasingly specific elucidation of the pathogenic mechanisms of parasitic nematodes. _M. gramniticola_ MgMO289 was recently discovered and demonstrated to promote nematode parasitism by utilising the host O\({}_{2}\)\({}^{\bullet}\)- scavenging system to eliminate O\({}_{2}\)\({}^{\bullet}\)- and suppress plant immunity [50]. _M. incognita_ MiMIF-2 can interact with the two annexins of _A. thaliana_ and damage Ca\({}^{2+}\) signal transduction to interfere with plant immunity [51].