Transcriptome analysis revealed the molecular mechanism of the response to leaf spot disease in Camellia sasanqua

• Pathogen Identification: Four fungal pathogens ( Fusarium sambucinum, Alternaria alternata , Phyllosticta capitalensis, and Diaporthe amygdali) were isolated from diseased Camellia sasanqua leaves, potentially collaborating to cause leaf spot disease. • Transcriptomic Insights: RNA-seq revealed 8139 differentially expressed genes (DEGs) (4544 up-/3595 down-regulated), uncovering a complex defense network involving SA/ABA signaling, MYB/AP2/ERF/WRKY transcription factors, and phenylpropanoid biosynthesis. • Phenylpropanoid Pathway Activation: Key genes ( CAD, COMT ) in lignin synthesis were significantly up-regulated, suggesting their role in strengthening cell walls against pathogens. • Functional Validation: Overexpression of CsCAD5 and CsCOMT1 in tobacco enhanced lignin accumulation and enhanced resistance against pathogens, demonstrating their potential in fortifying physical barriers to pathogen invasion. The pathogens on the leaves of the Camellia sasanqua seriously affected the ornamental value. Illuminating the underlying molecular mechanisms is of extraordinary importance in C. sasanqua to improve resistance variety. We isolated the pathogens of Fusarium sambucinum, Alternaria alternata, Phyllosticta capitalensis , and Diaporthe amygdali from the diseased leaves of C. sasanqua. These four pathogens might cause the leaf spot of C. sasanqua by working together. The transcriptomic analysis detected differences between healthy and diseased leaf samples of C. sasanqua , and 8139 DEGs were identified, including 4544 up-regulated genes and 3595 down-regulated genes. KEGG enrichment analysis of the canonical defensive pathways and the genes with high expression levels indicated that the response of C. sasanqua to pathogens was a complex signal network, including signal recognition and transmission, plant hormones including SA and ABA, activation of transcription factors of MYB, AP2/ERF, WRKY, and secondary metabolic accumulation. The genes of the phenylpropanoid biosynthesis pathway were expressed significantly in the response processes. We further cloned the genes of CAD (Cinnamic alcohol dehydrogenase) and COMT (Caffeic acid 3-O-methyltransferase), respectively, in the phenylpropanoid biosynthesis pathway, and then characterized their functions in tobacco. The results showed that overexpressing CsCAD5 and CsCOMT1 could influence the accumulation of lignins. We speculated that the increased lignin content in plants might be achieved by thickening and lignifying the cell wall. In summary, our findings discussed the complexities and interactions of C. sasanqua responses to leaf spot, identifying potential resistance genes and molecular mechanisms for preventing and controlling plant diseases.