Modern Japanese rice cultivars often carry a nonautonomous retrotransposon-insertion mutation at the pathogenesis-related 1b protein gene locus causing reduced resistance to Pyricularia oryzae.

In Japan, rice cultivars with high eating quality such as Koshihikari are often highly susceptible to the fungus Pyricularia oryzae, which causes rice blast, the most serious disease of rice; however, little is known about the genetic factors leading to this high susceptibility to blast. Here, after our initial inoculations with P. oryzae, the expression of the pathogenesis-related protein 1b (PR1b) gene was not detected in Koshihikari using RT-qPCR, but it was detected in Nipponbare, a moderately resistant cultivar. This unexpected result was due to the insertion of the nonautonomous retrotransposon Dasheng in the coding region of PR1b in Koshihikari. We then showed that blast resistance was higher in transgenic Koshihikari lines that overexpressed PR1b, suggesting that the PR1b mutation was one of the causes of high blast susceptibility of Koshihikari. When we checked for this PR1b mutation in the top 10 most widely grown rice cultivars in Japan and the current leading cultivars in Hokkaido Prefecture as examples, at least the top eight cultivars and all current leading cultivars in Hokkaido Prefecture had this mutation. Thus, the deleterious PR1b mutation seems to be fixed in nearly the entire rice population in Japan. Moreover, our survey of genomic sequences of 36 rice cultivars in public databases showed that Dasheng was inserted into the PR1b gene in two japonica cultivars and an indica cultivar, all bred in China, at a site identical to that in Koshihikari.