Transcriptomic and Biochemical Analysis of Pummelo x Finger Lime Hybrids in Response to Huanglongbing (HLB)

AbstractHuanglongbing (HLB) is a devastating bacterial disease caused by the bacterium Candidatus Liberibacter asiaticus (CaLas) that affects the citrus industry worldwide. This study investigated the response of two pummelo x finger lime hybrid siblings to natural infection with CaLas. The hybrids were identified primarily using leaf morphology and molecular marker assessments and were selected for further studies on the basis of the CaLas titers in leaf petioles. HLB-infected budwoods from the selected hybrids (PFL 2-61 and PFL 1-11), in addition to that of the two parental plants, were propagated by grafting onto Swingle citrumelo rootstocks for further evaluation. Samples were collected two years after grafting. Leaves of PFL2-61 exhibited decreased CaLas titers compared with those of PFL 1-11. Additionally, compared to the other hybrid and parent plants, we recorded increased chlorophyll content, total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity in PFL 2-61 compared to PFL 1-11 and the parents. We subsequently conducted a detailed investigation of leaf samples of these two hybrid siblings via transcriptome analysis. Among the 20,675 differentially expressed genes (DEGs) identified, 1,416 were downregulated in PFL 2-61 compared with PFL 1-11, whereas 326 were upregulated. Transcriptome analysis revealed that the DEGs were associated with the cell wall structure, redox homeostasis, and biotic stress responses, Moreover, key genes related to the biosynthesis of secondary metabolites and phytohormones, including PAL1, jasmonate-related genes, and WRKY transcription factors, were upregulated in the tolerant hybrid (PFL 2-61). In contrast, three transcripts associated with the Sieve Element Occlusion N-Terminus (SEO_N) domain were downregulated in the tolerant hybrid (PFL 2-61). Our findings provide valuable insights into the molecular mechanisms of tolerance and susceptibility to HLB in finger lime, highlighting the potential of these citrus relatives for developing disease-tolerant varieties.