Comparative transcriptome profiling uncovers a Lilium regale NAC transcription factor, LrNAC35, contributing to defence response against cucumber mosaic virus and tobacco mosaic virus.

Cucumber mosaic virus (CMV) is a highly prevalent viral pathogen causing substantial damage to the bulb and cut-flower production of Lilium spp. Here, we performed an Illumina RNA sequencing (RNA-Seq) study on the leaf tissues of a virus-resistant species Lilium regale inoculated with mock control and CMV. A total of 1346 differentially expressed genes (DEGs) were identified in the leaves of L. regale upon CMV inoculation, which contained 34 up-regulated and 40 down-regulated DEGs that encode putative transcription factors (TFs). One up-regulated TF, LrNAC35, belonging to the NAM/ATAF/CUC (NAC) superfamily, was selected for further functional characterization. Aside from CMV, lily mottle virus and lily symptomless virus infections provoked a striking increase in LrNAC35 transcripts in both resistant and susceptible Lilium species. The treatments with low temperature and several stress-related hormones activated LrNAC35 expression, contrary to its reduced expression under salt stress. Ectopic overexpression of LrNAC35 in petunia (Petunia hybrida) resulted in reduced susceptibility to CMV and Tobacco mosaic virus infections, and enhanced accumulation of lignin in the cell walls. Four lignin biosynthetic genes, including PhC4H, Ph4CL, PhHCT and PhCCR, were found to be up-regulated in CMV-infected petunia lines overexpressing LrNAC35. In vivo promoter-binding tests showed that LrNAC35 specifically regulated the expression of Ph4CL. Taken together, our results suggest a positive role of transcriptome-derived LrNAC35 in transcriptional modulation of host defence against viral attack.