Reference Genome Assembly for Australian Ascochyta rabiei Isolate ArME14

Area of Science:

• Genomics
• Plant Pathology
• Mycology

Background:

• Ascochyta rabiei causes ascochyta blight, a significant disease affecting chickpea crops globally.
• High-quality genome assemblies are crucial for understanding pathogen biology and developing control strategies.

Purpose of the Study:

• To generate a high-quality, gapless genome assembly for the Australian Ascochyta rabiei isolate ArME14 using PacBio sequencing.
• To compare this assembly with a previously generated Illumina assembly of the Indian isolate ArD2.
• To analyze the genomic structure, gene content, and identify key functional elements within the ArME14 genome.

Main Methods:

• PacBio HiFi sequencing technology was employed for genome assembly.
• Bioinformatic tools including OcculterCut were used for genome segmentation and analysis.
• Gene prediction pipelines were utilized to identify protein-coding genes, transcription factors, secreted proteins, carbohydrate-active enzymes (CAZymes), secondary metabolite clusters, and putative protein effectors.

Main Results:

• A gapless genome assembly for ArME14 was achieved, spanning nine chromosomes with telomeric sequences.
• The ArME14 assembly (40.9 Mb) was larger than the ArD2 assembly (34.6 Mb), revealing significant genomic differences.
• Analysis identified 21% AT-rich isochores, 15% repetitive DNA, and predicted 11,257 protein-coding genes in ArME14, compared to 10,596 in ArD2.
• Similar complements of transcription factors and secreted proteins were found, but ArME14 showed higher numbers of CAZymes (450-650), effectors (39), and secondary metabolite clusters (26).

Conclusions:

• The PacBio assembly provides a more contiguous and comprehensive genome sequence for Ascochyta rabiei.
• Genomic differences, particularly in gene-sparse regions, may contribute to variations between isolates.
• The identified functional elements offer targets for understanding pathogenicity and developing novel disease management strategies for chickpea blight.