Comprehensive Identification of CrRLK1Ls Family Genes in Pinus tabulaeformis and Functional Characterization of PtTHESEUS1 in Response to Bursaphelenchus xylophilus Infestation

Pine wilt disease (PWD), caused by pine wood nematode (PWN, Bursaphelenchus xylophilus), severely threatens global pine forests. The Catharanthus roseus receptor-like kinase 1 L (CrRLK1L) family plays a critical role in plant defense against pathogen invasion. While recent studies have begun to elucidate the molecular mechanism of interaction between host trees and PWN, the function of CrRLK1L family members in gymnosperms remain poorly understood. Here, we analyzed the transcriptomic response of Pinus tabulaeformis to PWN infection and identified 58 CrRLK1L genes. Among them, PtTHESEUS1 (PtTHE1), a homolog of Arabidopsis THESEUS1, exhibited sustained transcriptional induction during PWN infection, as validated by RT-qPCR. Functional analyses showed that Arabidopsis theseus1 mutant was more susceptible to PWN, suggesting the involvement of PtTHE1 in host defense. The extracellular domain of PtTHE1 interact with BxEF1, a PWN secreted protein. BxEF1 was strongly expressed during early infection stage and predominantly localized in the dorsal gland of the PWN. Silencing of BxEF1 significantly impaired PWN feeding speed and reduced pathogenicity. In summary, BxEF1, a parasitism-promoting protein secreted by PWN, is perceived by PtTHE1, ultimately triggering host immunity. Our findings reveal a novel molecular mechanism of pine resistance to PWN and provide functional insights into CrRLK1L-mediated immunity in gymnosperms.