Powdery Mildew Resistance Gene Cluster on Chromosome 7AL in Common Wheat J. J. Maxwell1, G. Brown-Guedira2, C. Cowger2, D. Marshall2, and J. P. Murphy1 1Dept. of Crop Science, N. C. State University 2USDA/ARS – Plant Science Research Unit, Raleigh, NC I NTRODUCTION Powdery Mildew caused by Blumeria graminis f.sp. tritici is an economically important disease for wheat (Triticum aestivum) growers in maritime areas Genetic resistance is one of the most consistent and cost effective methods of control Single dominant resistance genes tend to be ephemeral because of the constant virulence shifts in powdery mildew populations Thus, novel resistance genes must be identified to maintain high levels of control To date, two single dominant powdery mildew resistance genes have been identified on chromosome 7AL, Pm1 (Hsam et al., 1998) and Pm 37 (Perugini et al., 2008) in close proximity to each other Objectives To characterize two North Carolina germplasm lines for their resistance to powdery mildew To identify simple sequence repeat (SSR) markers linked to the resistance genes to be used in a marker-assisted selection (MAS) program Table 1: Differential response of germplasm lines and cultivars after inoculation with four different powdery mildew isolates from North Carolina. Each line was scored as resistant (R) or susceptible (S). Linkage Mapping Chromosome specific SSR markers were used to assign the genes in NC-AG12 and NC-AG13 to chromosome 7AL The markers mapped in accordance with the wheat consensus map and also were in close agreement with Perugini et al. (2008) (Figure 2) The NC-AG12 and NC-AG13 genes are postulated to be distal to Pm37 (Figure 2) Cultivar/ Germplasm line Pm gene Bgt Isolate Yuma E3-14 101a2 W72-27 NC-AG12 Unknown R S NC-AG13 NC-AG11 Pm37 Axminster Pm1 Jagger None Saluda Pm 3a Figure 2: Linkage Maps for Chromosome 7AL and relationships between NC-AG12, NC-AG13 and NC-AG11. SSR marker and powdery mildew phenotypic marker names are to the right and Centimorgan estimations to the left of each linkage group. RESULTS: Differential Testing Differential powdery mildew reactions among lines indicated that the resistance genes present in NC-AG12 and NC-AG13 are different from those in NC-AG11 and Axminster Differential tests also confirmed that the genes in NC-AG12 and NC-AG13 were different from each other The genes are also different than those of the recurrent parent Saluda which possesses Pm3a located on chromosome 1AS Xwmc525 NC-AG12 Xgwm332 Xwmc790 Xwmc116 Xwmc273 Pm – AG12 Xwmc809 5 13 22 2 7 Pm – AG13 NC-AG13 1 8 19 3 NC-AG11 Pm37 Xwmc346 9 MATERIALS and METHODS Powdery mildew resistant germplasm lines NC06BGTAG12 and NC06BGTAG13 (Murphy et al., 2007) were crossed to the susceptible cultivar ‘Jagger’. Two F2:3 populations consisting of 130 individuals in each population were developed, which are referred to as populations NC-AG12 and NC-AG13. Both populations, parental lines, and a susceptible check ‘Saluda’ were artificially inoculated with a powdery mildew isolate ‘Yuma’ in the greenhouse. Two pots with five plants each were tested for each F2:3 line. Each test was repeated twice. If all twenty plants over both tests were resistant (showed no signs of infection) they were scored as resistant (R), while if all twenty plants over both tests were susceptible (showed signs of infection) they were scored as susceptible (S). If there was a mixture of both resistant and susceptible plants within the twenty, the line was scored as segregating (H). DNA for linkage analysis was harvested from individual F2 plants by the CTAB method. PCR protocols for linkage analysis with SSR markers were performed as described by Perugini et al. (2008). Figure 1: (A) Greenhouse phenotypic screening, small pots are the individual F2:3 lines and large pots are susceptible spreader Saluda. Differential response of (B) NC-AG12 (C) Jagger. (A) (B) (C) FUTURE WORK Further allelism tests between NC-AG12, NC-AG13, and Axminster are needed to confirm if these genes are linked or allelic to one another. Linkage analyses needs to be conducted between NC-AG11, NC-AG12 and NC-AG13 to determine are linked or allelic to one another. These analyses could be used to determine if there is a Pm gene cluster on 7AL. Phenotypic Screening The powdery mildew isolate Yuma gave a distinct difference in reaction between NC-AG12, NCAG13 and Jagger (Table 1, Figure 1) Greenhouse screening indicated monogenic control of the powdery mildew resistance expressed in both NC-AG12 and NC-AG13 (Table 2) Literature Cited Hsam, S.L.K., X.Q. Huang, F. Ernst, L. Hartl, and F.J. Zeller. 1998. Chromosomal location of genes for resistance to powdery mildew in common wheat (Triticum aestivum L. em Thell.). 5. Alleles at the Pm1 locus. Theor. Appl. Genet. 96:1129-1134. Murphy, J.P., R.A. Navarro, D. Marshall, C. Cowger, T.S. Cox, J.A. Kolmer, S. Leath, and C.S. Gains. 2007. Registration of NC06BGTAG12, and NC06BGTAG13 powdery mildew resistant wheat germplasm. Journal of Plant Registrations 1:75-77. Perugini, L. D., J. P. Murphy, D. S. Marshall, G. Brown-Guedira. 2008. Pm37, a new broadly effective powdery mildew resistance gene from Triticum timopheevii. Theor. Appl. Genet. (Accepted). Table 2: Segregation ratios for NC-AG12 and NC-AG13 populations from an artificial inoculation in the greenhouse. F2:3 Population Number of individuals per class (R:H:S) χ2 P-value for a 1:2:1 (R:H:S) segregation NC-AG12 40 : 65 : 23 0.1029 NC-AG13 23 : 79 : 34 0.0693