Huanglongbing (HLB), also known as citrus greening, is a devastating disease affecting the citrus industry worldwide. This study aimed to investigate the transcriptional responses of two Rutaceae species, Ponkan Mandarin (susceptible) and Punctate Wampee (resistant), to HLB infection. Comparative transcriptome analysis was conducted to identify differentially expressed genes (DEGs) and pathways involved in defense mechanisms. The transcriptome data showed that in the susceptible Ponkan Mandarin, there were 1519 upregulated genes and 700 downregulated genes, while in the resistant Punctate Wampee variety, there were 1611 upregulated genes and 1727 downregulated genes. Upon infection, 297 genes were upregulated in both varieties, while 211 genes were downregulated in both. These genes included transcription factors from different families such as WRKY, ERF, and MYB. Ponkan Mandarin primarily relies on pathways like lignin synthesis and cell wall modification to defend against HLB, whereas Punctate Wampee mainly resists HLB by regulating cellular homeostasis and metabolism. Weighted Gene Co-expression Network Analysis (WGCNA) identified ten potential key resistance genes in the resistant Punctate Wampee variety, including genes involved in lignin biosynthesis and genes related to cellular signaling pathways. These findings not only enhance our understanding of the distinct defense mechanisms employed by citrus species against HLB infection but also offer novel perspectives for developing effective prevention and management strategies against this disease.

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