Survey of the response of 82 domestic landraces of Zea mays to cucumber mosaic virus (CMV) reveals geographical region-related resistance to CMV in Japan

Zea mays has been historically imported to Japan via two independent geographical routes: into southern Japan by trading with Europe in the 16th century and into northern Japan by import from North America in the 19th century. Breeding to genetically improve on quality traits and high yields has led to the current domestic landraces in each region. In a survey of 82 domestic landraces, nine out of 38 landraces originating from southern Japan showed complete immunity to cucumber mosaic virus yellow strain (CMV(Y)) without the formation of necrotic local lesions (NLLs). In contrast, three out of 44 landraces originating from northern Japan developed NLLs, but revealed no systemic spread of the virus. Due to the absence of good documentation on NLL formation in Z. mays, the response of domestic landraces Aso-1 and Aso-3, originating from Ibaraki in northern Japan, to a challenge with CMV(Y) and CMV(Ma-1) was further analysed. Aso-3 only formed NLL in response to CMV(Y) but not to CMV(Ma-1). Moreover, in CMV(Y)-inoculated Aso-3, virus spread was restricted to the primary infection site and the expression of defence-related genes was up-regulated, whereas Aso-1 became systemically infected with either CMV(Y) or CMV(Ma-1). The response of Aso-3 to CMV(Y) was inherited as a single dominant trait. Together, these results pointed towards the induction of hypersensitive response (HR)-mediated resistance to CMV(Y) in Aso-3. Although HR-mediated resistance to viruses has been studied mainly in dicots, the pathosystem CMV-Z. mays may provide a model to investigate HR-mediated resistance to viruses in monocot plants.