Transcriptomic analysis of Camellia japonica to scale insects infestation and functional characterization of pectin methylesterase gene CjPME28 and polygalacturonase gene CjPG1.

Key message: This study screened scale insect-resistant camellia resources and revealed the associated transcription factors, metabolic pathways, and the insect resistance mechanisms of the CjPME28 or CjPG1 genes. Camellia japonica has been seriously infested with scale insects in recent years. Pectin participates in the plant's anti-insect. However, the functions of the genes related to pectin have not yet been revealed in C. japonica. The continuous observation for 5 years revealed different C. japonica exhibiting differences in resistance to scale insects. C. japonica with resistance to scale insects were different with susceptibilities in genotype by SCoT molecular marker. Meanwhile, the leaf structure was more compact, and the lignin content, ascorbic acid content, and antioxidant enzyme activity in leaf were more than that of susceptibilities. The scale insects were identified as Pseudaulacaspis cockerelli by the morphological identification. RNA sequencing was performed using C. japonica with resistance versus susceptibility. Transcription factors, such as MYBS3, bHLH3, NAC90, and WRKY40, were significantly upregulated in C. japonica with resistance. The pentose and glutathione metabolism pathway, phenylpropanoid biosynthesis pathway, ascorbate, aldarate metabolism pathway, and glutathione metabolism pathway were the main metabolic pathways emerged as the most significantly enriched for differentially expressed genes. The expression of pectin methylesterase gene (PME) or polygalacturonase gene (PG) was upregulated in the pentose and glutathione metabolism pathway, and their functions were further characterized. Overexpression of CjPME28 or CjPG1 in tobacco plants significantly improved resistance to the scale insects. Enhanced pectinase activity in the CjPME28 or CjPG1 transgenic tobacco lines led to increased lignin synthesis. In conclusion, this study screened a camellia resource with resistance to the scale insects and illustrated the resistance mechanism of scale insects in C. japonica.