The Australian finger
lime (Citrus australasica) is tolerant to Huanglongbing (HLB; Citrus greening). This species can be utilized to develop HLB tolerant citrus cultivars through conventional breeding and biotechnological approaches. In this report, we conducted a comprehensive analysis of transcriptomic data following a non-choice
infection assay to understand the CaLas tolerance mechanisms in the finger
lime. After filtering 3,768 differentially expressed genes (DEGs), 2,396 were downregulated and 1,372 were upregulated in CaLas-infected finger
lime compared to CaLas-infected HLB-susceptible 'Valencia' sweet orange. Comparative analyses revealed several DEGs belonging to cell wall, β-glucanase, proteolysis, R genes, signaling, redox state,
peroxidases,
glutathione-S-transferase, secondary metabolites, and pathogenesis-related (PR)
proteins categories. Our results indicate that the finger
lime has evolved specific redox control systems to mitigate the
reactive oxygen species and modulate the plant defense response. We also identified candidate genes responsible for the production of Cys-rich secretory
proteins and Pathogenesis-related 1 (PR1-like)
proteins that are highly upregulated in infected finger
lime relative to noninfected and infected 'Valencia' sweet orange. Additionally, the anatomical analysis of phloem and stem tissues in finger
lime and 'Valencia' suggested better regeneration of phloem tissues in finger
lime in response to HLB
infection. Analysis of
callose formation following
infection revealed a significant difference in the production of
callose plugs between the stem phloem of CaLas+ 'Valencia' sweet orange and finger
lime. Understanding the mechanism of resistance will help the scientific community design strategies to protect trees from CaLas
infection and assist citrus breeders in developing durable HLB tolerant citrus varieties.