Insights into the molecular basis of beet curly top resistance in sugar beet through a transcriptomic approach at the early stage of symptom development

Curly top disease caused by <i>Beet curly top virus</i> (BCTV) is a limiting factor for sugar beet production. The most economical and sustainable control of BCTV in sugar beet would be via the growth of resistant cultivars, although most commercial cultivars possess only low-to-moderate quantitative resistance. A double haploid line (KDH13) showed a high level of resistance to BCTV infection. However, the mechanism of resistance and response of this line to BCTV infection is unknown. Here, we tested the response of this line to both local and systemic BCTV infections. The virus replicated at a high level in locally infected tissue but lower than in susceptible KDH19 plants. Resistant KDH13 plants systemically infected with BCTV showed only mild enation without leaf curling after 30 days. In contrast, severe leaf curling appeared after 12 days in susceptible plants with higher virus accumulation. Transcriptome analysis of the BCTV-infected KDH13 plants at the early stage of symptom development showed only 132 genes that were exclusively deregulated compared to the regulation of a large number of genes (1018 genes) in KDH19 plants. Pathway enrichment analysis showed that differentially expressed genes were predominantly involved in hormone metabolism, DNA methylation, immune response, cell cycle, biotic stress and oxidative stress. The auxin level in both resistant and susceptible plants increased in response to BCTV infection. Remarkably, exogenous application of auxin caused leaf curling phenotype in the absence of the virus. This study demonstrates the response of resistant and susceptible plants to BCTV infection at both local and systemic infections and highlights the defence-related genes and metabolic pathways including auxin for their contribution towards BCTV symptom development and resistance in sugar beet.