Transgressive segregation reveals mechanisms of<i>Arabidopsis</i>immunity to<i>Brassica</i>-infecting races of white rust (<i>Albugo candida</i>)

Abstract

<i>Arabidopsis thaliana</i> accessions are universally resistant at the adult leaf stage to white rust (<i>Albugo candida</i>) races that infect the crop species <i>Brassica juncea</i> and <i>Brassica oleracea</i> We used transgressive segregation in recombinant inbred lines to test if this apparent species-wide (nonhost) resistance in <i>A. thaliana</i> is due to natural pyramiding of multiple <i>Resistance</i> (<i>R</i>) genes. We screened 593 inbred lines from an <i>Arabidopsis</i> multiparent advanced generation intercross (MAGIC) mapping population, derived from 19 resistant parental accessions, and identified two transgressive segregants that are susceptible to the pathogen. These were crossed to each MAGIC parent, and analysis of resulting F₂ progeny followed by positional cloning showed that resistance to an isolate of <i>A. candida</i> race 2 (Ac2V) can be explained in each accession by at least one of four genes encoding nucleotide-binding, leucine-rich repeat (NLR) immune receptors. An additional gene was identified that confers resistance to an isolate of <i>A. candida</i> race 9 (AcBoT) that infects <i>B. oleracea</i> Thus, effector-triggered immunity conferred by distinct NLR-encoding genes in multiple <i>A. thaliana</i> accessions provides species-wide resistance to these crop pathogens.