Molecular Identification and Functional Characterization of the Fatty Acid- and Retinoid-Binding Protein Gene Rs-far-1 in the Burrowing Nematode Radopholus similis (Tylenchida: Pratylenchidae)

Fatty acid- and retinoid-binding protein (FAR) is a nematode-specific protein expressed in the nematode hypodermis. It is involved in nematode development, reproduction, and infection and can disrupt the plant defense reaction. In this study, we obtained the full-length sequence of the far gene from Radopholus similis (Rs-far-1), which is 828 bp long and includes a 558 bp ORF encoding 186 amino acids. A protein homology analysis revealed that Rs-FAR-1 is 75% similar to Mj-FAR-1 from Meloidogyne javanica. A neighbor-joining phylogenetic tree was inferred and showed that Rs-FAR-1 is most similar to Pv-FAR-1 from Pratylenchus vulnus. A fluorescence-based ligand-binding analysis confirmed that Rs-FAR-1 can combine with fatty acids and retinol. qPCR was used to assess Rs-far-1 expression levels at different developmental stages in different R. similis populations, and its expression was 2.5 times greater in the highly pathogenic Rs-C population than in the less pathogenic Rs-P population. The highest expression was found in females, followed by eggs, juveniles and males. When R. similis was treated with Rs-far-1 dsRNA for 36 h, the reproduction and pathogenicity decreased significantly. In situ hybridization revealed Rs-far-1 transcripts in the R. similis hypodermis. Additionally, R. similis treated with Rs-far-1 dsRNA or water were inoculated into Arabidopsis thaliana. Allene oxide synthase (AOS) expression in A. thaliana was upregulated during early infection in both treatments and then returned to the expression levels of the control plant. Compared with the control plant, AOS expression significantly decreased in A. thaliana inoculated with water-treated R. similis but significantly increased in A. thaliana inoculated with Rs-far-1 dsRNA-treated R. similis. This finding indicates that Rs-far-1 regulates AOS expression in A. thaliana. Rs-FAR-1 plays a critical role in R. similis development, reproduction, and infection and can disturb the plant defense reaction. Therefore, Rs-far-1 is an important target gene to control R. similis.