Submitted to: Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2002
Publication Date: 9/1/2002
Citation: Interpretive Summary: Leaf rust is an economically important pathogen on corn crops, especially sweetcorn. In the corn plant, resistance to leaf rust is conferred by a handful of disease resistance genes, commonly occurring in clusters and/or families of closely related genes. Prior to this work, only one other resistance gene, rp1, had been cloned, sequenced and analyzed. This paper describes how a second resistance gene family, rp3, was isolated and characterized. Fourteen full-length rp3 genes were cloned, sequenced, and characterized. At least five of these genes appear to produce functional proteins and it could not be determined which, if any, was the gene responsible for conferring resistance to leaf rust. The genes were very closely related, being between 93% to 99% identical in their DNA sequence. The rp3 gene family appear genetically and structurally similar to rp1 genes, but only ~25% similar in amino acid sequence, and . Comparison of rp3 and rp1 clearly suggested similar mechanisms by which these genes evolve to try to stay one step ahead of a changing pathogen population. This phenomenon is called unequal crossing over and appears to occur after different resistance gene family members exchanging different DNA sequence segments to result in unique new resistance gene family members in the plant.
Technical Abstract: In maize, the Rp3 gene confers resistance to common rust caused by Puccinia sorghi. Flanking marker analysis of rust-susceptible rp3 variants suggested that most of them arose via unequal crossing over, indicating that rp3 is a complex locus like rp1. The PIC13 probe identifies a nucleotide binding site-leucine-rich repeat (NBS-LRR) gene family that maps to the complex. Rp3 variants show losses of PIC13 family members relative to the resistant parents when probed with PIC13, indicating that the Rp3 gene is a member of this family. Gel blots and sequence analysis suggest that at least 9 family members are at the locus in most Rp3-carrying lines and that at least 5 of these are transcribed in the Rp3-A haplotype. The coding regions of 14 family members, isolated from three different Rp3-carrying haplotypes, had DNA sequence identities from 93 to 99%. Partial sequencing of clones of a BAC contig spanning the rp3 locus in the maize inbred line B73 identified five different PIC13 paralogues in a region of ~140 kb