White Rust Genome Exposes Hidden Threat Genes

White rust is a serious problem for oilseed Brassica crops. It can cause major yield loss, and Brassica juncea is one of the most affected crops. The disease is caused by Albugo candida, a biotrophic pathogen. Researchers focused on a virulent Ac2V race that infects B. juncea.

The team used Illumina and Nanopore sequencing to build a high-quality draft genome. The raw sequence data formed a 36.88 Mb genome with 415 scaffolds. The N50 value was 301.91 kb. When this genome was compared with the Ac2VPB reference assembly, variant analysis found 124,974 SNPs. The average density was 3.3 SNPs per kb.

The genome also had a lot of repeated DNA. Repetitive elements made up 24.29% of the assembly, and the team found 1,039 SSRs. In total, 13,715 coding genes were predicted. That equals about 359.03 genes per Mb. Of these genes, 11,556 were annotated through sequence homology.

The study also looked at the pathogen's secreted-protein set. It predicted 355 effectors. These proteins lacked a transmembrane domain but carried an N-terminal signal peptide. Of the 355 effectors, 141 had homologs, while 214 were novel. Conserved motifs were used to classify them. Their role in pathogenesis was tested through qPCR in B. juncea.

Phylogenetic analysis using average nucleotide identity showed that the Canadian and Indian Ac2V isolates were 99.6% similar. Even so, the Indian isolate had 62 new contigs with no sequence similarity to the Canadian isolate. This suggests variation within the race. The work gives researchers genomic resources and a framework for studying the Albugo-Brassica interaction more carefully.