Comprehensive Transcriptomic Profiling of <i>Citrus australasica</i> Unveils Antimicrobial Peptides and Immune Pathways for Huanglongbing Tolerance

Huanglongbing (HLB) is a destructive disease that threatens the citrus industry worldwide. It is associated with "<i>Candidatus</i> Liberibacter asiaticus" (<i>C</i>Las) and vectored by <i>Diaphorina citri</i>, known as the Asian citrus psyllid (ACP). Australian finger lime (<i>Citrus australasica</i>), a promising candidate for HLB tolerance, was examined in response to <i>C</i>Las-infected ACPs through comprehensive transcriptomic profiling to identify key antimicrobial peptides (AMPs) and immune-related pathways. GO, KEGG, and genome enrichment analysis found that most DEGs were enriched in metabolic pathways, biosynthesis of secondary metabolites, and hormone signal transduction. Furthermore, immune pathways associated with pathogen recognition and defense mechanisms were characterized. DEGs were screened and confirmed through quantitative real-time qPCR (RT-qPCR). By analyzing gene expression in <i>C. australasica</i>, we identified several potential AMPs with significant antimicrobial properties. CaAMP1, CaAMP2, CaAMP4, and CaShepharin exhibited strong inhibitory effects against multiple bacterial suspensions with minimum EC50 values. Similarly, CaAMP4 also caused a significant reduction in <i>C</i>Las titer through Agrobacterium-mediated transient expression. This study provides a novel perspective for understanding the transcriptomic information on <i>C. australasica</i> in response to <i>C</i>Las-infected ACPs. These findings offer valuable insights into the development of AMP-based, environmentally sustainable strategies for managing HLB and enhancing citrus plant resilience.