The Mdm1 Locus and Maize Resistance to Maize dwarf mosaic virus

Maize dwarf mosaic and sugarcane mosaic, the diseases incited by Maize dwarf mosaic virus (MDMV) and Sugarcane mosaic virus (SCMV), are among the most important maize viral diseases worldwide (28). The two viruses are related members of the genus Potyvirus that are nonpersistently transmitted by aphids (16) and can be mechanically transmitted experimentally. They are differentiated from one another by host range, geographic distribution, and genome sequence. Sources of resistance have been identified in tropical (1), European (13), and U.S. corn belt (8) germplasm. MDMV resistance was linked with a locus on the short arm of chromosome 6 in studies using reciprocal translocations (4,7,24,25), morphological markers (22), and restriction fragment length polymorphism (RFLP) analysis (11,12). The genetically dominant locus identified in maize inbred line Pa405 was named Mdm1 (11). However, these early genetic analyses showed variation in both the numbers of genes involved in resistance to MDMV and their modes of action. This variation was attributed to the use of different resistance sources, disease escapes, mixtures of viruses and virus strains, different classification systems for resistant and susceptible plants, and the type of analysis used (6). In addition, McMullen and Louie (11) suggested that other genes modify the activity of Mdm1. SCMV resistance was associated with a dominant gene, Scm1, on the short arm of chromosome 6 in maize inbred line D32 (3,13). In contrast to MDMV, a second dominant locus for SCMV resistance, Scm2, was found near the centromere of chromosome 3 (2,3). Although allelism tests indicated the presence of a common dominant gene for SCMV resistance in European inbreds and Pa405 (30), it is not known whether Mdm1 and Scm1 are tightly linked genes or the same gene. The goal of this study was to determine the responses of a widely based collection of germplasm to inoculation with MDMV and SCMV, and to test the hypothesis that Mdm1 is associated with resistance to MDMV in this diverse germplasm. MATERIALS AND METHODS MDMV and SCMV isolates. The MDMV and SCMV isolates used were originally collected in Ohio and have been maintained at Wooster, OH by serial passage on susceptible maize (Zea mays L.; 6). Virus identity was verified at each passage by bioassay on sorghum (Sorghum bicolor L. Monench) cv. Atlas and Sart, wheat (Triticum aestivum L.) cv. Monon, and the susceptible maize inbred line Oh28 (6). Atlas is resistant to MDMV and Wheat streak mosaic virus (WSMV), but susceptible to SCMV. Sart is susceptible to MDMV and SCMV, but resistant to WSMV. Wheat is susceptible only to WSMV, and Oh28 is susceptible to all three viruses. Enzyme-linked immunosorbent assay (9) was used to confirm virus identity prior to these experiments using antisera raised against MDMV strain A and SCMV (MDMV strain B) (10). Plant growth, inoculation, and symtom evaluation. A collection of 115 maize inbred lines was screened for response to MDMV and SCMV inoculation in 1999 to 2000. The collection consisted of 83 openpedigree inbred lines with tropical backgrounds (1) obtained from J. L. Brewbaker (Department of Tropical Plant and Soil Sciences, University of Hawaii at Manoa, Honolulu). In previous analyses, most of these lines were resistant or partially resistant to MDMV and/or SCMV (1). In addition, 32 temperate inbred lines previously reported to have resistance to MDMV or other viruses (8) maintained at the Ohio Agricultural Research Center, Wooster were used. Seed of all lines was increased in Wooster prior to these experiments. Experiments to monitor germplasm response to virus inoculation were conducted under greenhouse conditions. Natural light was supplemented with 400-W highpressure sodium lights (P. L. Light Systems, Beamsville, ON, Canada) between October and April to attain photoperiods of 12 h of light and 12 h of darkness. Temperature was maintained at 25 and 18oC (day and night). Twelve seed of each inbred line were planted into one of four rows in perforated plastic trays (30 by 46 by 7 cm) containing autoclaved greenhouse soil. At 14 days after planting, leaves were rub inoculated four times at 2day intervals as previously described (6). Plants were scored for the presence of mosaic, chlorotic streak, and limited delayed chlorotic streak symptoms 11, 14, 19, 21, and 24 days post inoculation (dpi) ABSTRACT Jones, M. W., Redinbaugh, M. G., and Louie, R. 2007. The Mdm1 locus and maize resistance to Maize dwarf mosaic virus. Plant Dis. 91:185-190.