Integrated genome and transcriptome analysis reveals pathogenic mechanisms of Calonectria eucalypti in Eucalyptus leaf blight.

Background: Calonectria eucalypti is a destructive fungal pathogen causing Eucalyptus leaf blight in China and Indonesia. Despite its ecological and economic impact, the molecular mechanisms underlying its pathogenicity remain largely unexplored.

Results: We assembled a high-quality 62.1 Mb genome of C. eucalypti comprising 19 contigs and 13,112 predicted genes, including 1,006 carbohydrate-active enzymes (CAZymes), 351 candidate effectors, and 90 secondary metabolite biosynthetic gene clusters (SMGCs). Comparative genomics revealed 60 expanded gene families and eight species-specific clusters in C. eucalypti, predominantly enriched in virulence-associated functions such as transferase activity, sporulation, and host adhesion. Time-course transcriptome profiling during infection of Eucalyptus urophylla × E. tereticornis EC254 at 12, 24, 48, and 72 h post-inoculation (hpi) identified 708 CAZyme genes, 188 putative effectors, and 68 SMGCs as differentially expressed, exhibiting stage-specific patterns related to fungal penetration, host cell wall degradation, immune suppression, and nutrient acquisition. Notably, glycoside hydrolases (GHs) were the most abundantly induced CAZyme family, suggesting a central role in degrading host cell wall components. Prioritized gene candidates, such as GH28 polygalacturonase A08104 and effectors A12658 and A12266, showed strong homology to known virulence factors and high expression during host colonization. Moreover, most SMGCs were located in transposable element (TE)-rich regions, supporting a "two-speed" genome architecture that may facilitates adaptive evolution and pathogenic innovation.

Conclusions: This study provides the first integrative genome and infection-stage transcriptome map of C. eucalypti, revealing a coordinated, stage-specific deployment of pathogenicity-related gene clusters. These findings provide a foundational resource for understanding fungal virulence and developing targeted strategies to manage Eucalyptus leaf blight.