Genetic inheritance and defense gene dynamics reveal a novel recessive ToLCNDV resistance locus in cucumber

The Tomato Leaf Curl New Delhi Virus (ToLCNDV), a bipartite begomovirus transmitted by Bemisia tabaci, has emerged as a major constraint to cucumber (Cucumis sativus L.) production across tropical and subtropical regions. The deployment of host resistance remains the most sustainable and ecologically sound strategy to mitigate ToLCNDV-associated yield losses. In the present investigation, five cucumber genotypes (DC-773, DC-61, PPC-6, PI-197087, and DC-91) were evaluated under controlled whitefly-mediated inoculation using a characterized ToLCNDV isolate and agro-inoculation. Quantitative PCR-based viral load estimation revealed that DC-61 exhibited markedly reduced viral accumulation, surpassing the previously known resistant donor PI-197087; in contrast, DC-773 and PPC-6 were highly susceptible. To elucidate the molecular basis of resistance, transcript profiling of nine defense-associated genes was conducted in resistant (DC-61, PI-197087) and susceptible (DC-773) genotypes at 8, 12, 24, and 48 h post-agro-inoculation. Notably, ERF008, Kinesin, STY46-like, and CYS/HIS genes were rapidly and highly upregulated in resistant genotypes, indicating early activation of signal transduction and defense pathways. In contrast, the differential expression of Violaxanthin de-epoxidase (VDE) and SGS3 underscored the significance of RNA silencing and reactive oxygen species (ROS) homeostasis in resistance modulation. Segregation analysis in F₁, F₂, and backcross populations revealed that resistance in DC-61 is monogenic and inherited in a recessive manner. The integration of genetic inheritance with temporal gene expression profiling provides compelling evidence that DC-61 harbours a distinct, recessively inherited resistance locus conferring efficient restriction of ToLCNDV replication. These findings establish DC-61 as a novel and robust resistance donor, identifying potential molecular markers and candidate genes for marker-assisted introgression and pyramiding of ToLCNDV resistance in cucumber breeding programs.