Using barley stripe mosaic virus–virus-induced gene silencing (BSMV-VIGS) to identify genes functioning in Lr21-mediated resistance to leaf rust. All plants were infected with the indicated BSMV constructs 7 d after germination and then spray-inoculated with the leaf rust fungus Puccinia triticina 8 d after viral infection. The photographs were taken 10 d after inoculation with the leaf rust fungus. R, WGRC7 genotype expressing Lr21-mediated resistance; S, the susceptible genotype Wichita, which lacks Lr21 Left column photographs: Infection with control constructs does not alter resistance or susceptibility. Right column photographs: BSMV constructs silencing Lr21, RAR1, SGT1, and HSP90 abrogate Lr21-mediated resistance. (Scofield et al., 2005).