A Mapped Locus on LG A6 of Brassica juncea Line Tumida Conferring Resistance to White Rust Contains a CNL Type R Gene

Abstract

White rust, causal agent oomycete <i>Albugo candida</i>, is a significant disease of the cultivated <i>Brassica</i> species. The Indian gene pool lines of oilseed mustard, <i>Brassica juncea</i>, are highly susceptible to the pathogen. Resistance to <i>A. candida</i> has been reported in the east European gene pool lines of mustard and mapped to LG A4 in line Heera and LG A5 in line Donskaja-IV. A new resistance-conferring locus to <i>A. candida</i> isolate AcB1 has been mapped to LG A6 of <i>B. juncea</i> line Tumida-a Chinese vegetable type mustard using an F₁DH mapping population that has been developed from a Tumida × Varuna (susceptible Indian gene pool line) cross. A molecular map containing 8,303 genic and GBS markers was used to map the resistance trait to an interval of 63.0 cM-70.8 cM on LG A6. Genome assemblies of Tumida and Varuna were used to find the genes present within the flanking markers discerned by genetic mapping. The most likely candidate gene in the mapped interval is <i>BjuA046215</i>, a CC-NBS-LRR (CNL) type R gene that encodes a protein with all the specific subdomains of the proteins encoded by such genes. Alleles of <i>BjuA046215</i> in Varuna and other lines of the Indian and the east European gene pools encode proteins that have truncated LRR domains. Analysis of the syntenic regions in some of the Brassicaceae genomes and phylogenetic analysis of CNL type R genes showed <i>BjuA046215</i> to be closely related to a recently described white rust resistance-conferring R gene <i>BjuWRR1</i> in <i>B. juncea</i> Donskaja-IV, both belonging to the CNL-D group of R genes. Related R genes in <i>Arabidopsis thaliana</i> confer resistance to another oomycete, <i>Peronospora parasitica</i>.