Broad-Spectrum Resistance Genes PI2(T) and PI9(T) are Clustered on Chromosome 6

Abstract

Pi9(t) was introgressed from the wild rice Oryza minuta and Pi2(t) was transferred from a breeding line developed at CIAT. Both Pi2(t) and Pi9(t) were previously mapped on rice chromosome 6 and confer high level of resistance to diverse blast isolates. To further test their resistance spectrum, 43 blast isolates collected from 13 countries were used to inoculate the Pi2(t) and Pi9(t) plants. Pi9(t) was highly resistant to all isolates tested and Pi2(t) was resistant to 36 isolates, confirming the broad-spectrum resistance of these two genes to diverse blast isolates. To understand the molecular basis of broad-spectrum resistance, we conducted fine-mapping of the two blast resistance genes. A highresolution genetic map was established using RAPD markers tightly linked to Pi9(t). Using these markers, a bacterial artificial chromosome (BAC) contig was constructed covering about 100 kb at the Pi9(t) locus. Interestingly, all the Pi9(t) markers closely segregated with the Pi2(t) gene in an F2 mapping population. Whether Pi9(t) and Pi2(t) are tightly linked or allelic is under investigation.