Targeted mapping of rice ESTs to the LmPi1 locus for grey leaf spot resistance in Italian ryegrass

Epidemics of blast disease, also called grey leaf spot (GLS), in ryegrass species have recently become a serious problem in Japan and the USA. GLS causes serial growth inhibition and the abnormal morphology of withering in ryegrass seedlings. If the lesions expand they can completely destroy the plant at several growth stages. We attempted to construct a partial genetic linkage map around the LmPi1 locus, which confers resistance to GLS and is located on linkage group (LG) 5 of Italian ryegrass, by using synteny with rice. Because LG5 of Italian ryegrass is syntenic with rice chromosome (Chr) 9, we designed intron-scanning primers with alignment between the rice genome sequence and expressed sequence tag (EST) information on rice Chr 9. These primers were used to genotype an F-1 mapping population, which had been used for detection of the LmPi1 locus, by detecting single-strand conformation polymorphisms (SSCPs). We successfully constructed a partial genetic linkage map around the LmPi1 locus, spanning 66.3 cM. A comparative genomic analysis between the partial genetic linkage map and the rice Chr 9 genetic linkage map revealed that the LmPi1 locus was a candidate for orthology to the Pi5 locus or Pi15 locus, both of which are co-located in a resistance gene cluster on rice Chr 9 and are associated with broad-spectrum resistance to rice blast.