Chapter 5 A Major Gene for Powdery Mildew Resistance Transferred to
Common Wheat from Wild Einkorn Wheat**
A major gene for resistance to wheat powdery mildew (Blumeria graminis f. sp. tritici, Bgt = Erysiphe graminis f. sp. tritici) has been successfully transferred into hexaploid common wheat (Triticum aestivum L., 2n=6x=42, AABBDD) from wild einkorn wheat (Triticum monococcum L. ssp. aegilopoides, 2n=2x=14, AA). NC96BGTA5 is a germplasm line with the pedigree: Saluda *3/ PI427662. The response patterns for powdery mildew resistance in NC96BGTA5 were tested with 30 differential isolates of Bgt and the line was resistant to all tested isolates. The analyses of P1, P2, F1, F2, and BC1F1 populations derived from NC96BGTA5 revealed two genes for wheat powdery mildew resistance in the NC96BGTA5. One gene, Pm3a, was from its recurrent parent Saluda, and the second was a new gene introgressed from wild einkorn wheat. The gene was determined to be different from Pm1 to Pm21 by gene for gene and pedigree analyses. The new gene was identified as linked to the Pm3a gene based on the F2 and BC1F1 populations derived from a cross between NC96BGTA5 and a susceptible cultivar NK-Coker 68-15, and the data indicated that the gene was located on chromosome 1A. It is proposed that this new gene be designated Pm25 for wheat powdery mildew resistance in NC96BGTA5. Three RAPD markers, OPX061050, OPAG04950, and OPAI14600, were found to be linked to this new gene.
** Shi, A. N., S. Leath, and J. P. Murphy. 1997. A major gene for powdery mildew resistance transferred to common wheat from wild einkorn wheat. Phytopathology (Submitted)
Twenty seven major alleles have been reported at 21 loci for resistance to wheat powdery mildew (Blumeria graminis (DC.) E.O. Speer f. sp. tritici Em. Marchal = Erysiphe graminis DC. ex Merat f. sp. tritici Em. Marchal), and most of them were derived from the wild relatives of wheat (5,16,20). Because the presence and frequency of virulence genes continuously change in the Bgt pathogen population, and genes for resistance are frequently overcome by new isolates of Bgt (8,13,15). Therefore, it will be useful to identify new resistance genes and to transfer these genes into common wheat if they are present in alien sources.
Molecular markers have been widely used to tag genes for disease resistance in plants (17). Among 21 major gene loci for powdery mildew resistance in wheat, restriction fragment length polymorphism (RFLP) markers linked to Pm1, Pm2, Pm3, Pm4, Pm12, Pm13, and Pm18, and random amplified polymorphic DNA (RAPD) markers linked to Pm18 and Pm21 have been reported (4,6,7,9,14,20).
Wild einkorn (Triticum monococcum L. ssp. aegilopoides (Link) Thell = T. boeoticum) (2n=2x=14, AA) should be a valuable source of genes for diversifying fungal disease resistance in wheat. Valkoun et al. (23) reported on reaction to leaf rust in four accessions of T. monococcum L. ssp. aegilopoides and three of them were resistant. A gene, Sr22, for resistance to wheat stem rust has been successfully transferred into common wheat from T. monococcum L. ssp. Aegilopoides (19). However, genes for wheat powdery mildew resistance have not been identified from this wild wheat relative. We recently released the common wheat germplasm NC96BGTA5 with powdery mildew resistance transferred from the wild einkorn accession PI427662 (18). The objective of this research was to evaluate the inheritance of the resistance and to identify RAPD markers linked to the resistance gene. A preliminary report has been published (21).
MATERIALS AND METHODS
Plant materials. NC96BGTA5 is a new common wheat germplasm with the pedigree Saluda*3/ PI427662 (17). PI427662 is a wild einkorn accession collected in Iraq. Saluda (PI480474) is a soft red winter wheat cultivar released in Virginia (22) which contains the Pm3a gene for resistance to wheat powdery mildew (12). Twenty differential lines (listed in Table 1) with known genes (Pm1 to Pm21, except Pm10, Pm11, Pm14, Pm15, and Pm18), the recurrent parent Saluda, and two susceptible cultivars, Chancellor and NK-Coker 68-15, were used to check response patterns in comparison with NC96BGTA5. Pm10, Pm11, Pm14, and Pm15 are not effective against Bgt (16) and were not considered here; Pm18 was unavailabe.
Inheritance and allelism tests. F1 hybrid seed was obtained from crosses between NC96BGTA5 and NK-Coker 68-15, NK-Coker 9803, and Saluda (Table 2), and F2 and BC1F1 populations were developed during the 1994-95 and 1995-96 greenhouse seasons .
Powdery mildew evaluation. Powdery mildew evaluations were performed using a detached leaf technique (12). Assessment of reaction was based on a descriptive scale of resistant (0-3), intermediate (4-6), and susceptible (7-9) reaction types (12). Thirty differential isolates of Bgt were used to test response patterns of resistance to powdery mildew in NC96BGTA5. A series of lines with known powdery mildew resistance genes were also included in each test. Five Bgt isolates, well characterized for virulence, were used to test the segregating populations.
DNA extraction and RAPD analysis. Genomic DNA was extracted from fresh leaves of wheat plants (4). Random ten-base primers (kits OP-A through OP-AN) were obtained from Operon Technologies Inc. (Alameda CA). The PCR procedure described by Williams et al. (24) was followed with minor modifications. Each reaction consisted of 2.4 ul reaction buffer mix, 1.2 ul dNTPs (2.5mM), 5 ul primer (4 or 5 ng/ul), 0.2 ul Taq polymerase (5u/ul), 1.2 ul unacetylated bovine serum albumen (BSA), and 5.0 ul (4 or 5 ng/ul) genomic DNA. A total of 41 cycles of PCR amplification were performed using a standard RAPD program with denaturation at 92 C for 1 min, annealing at 35 C for 1 min, and extension at 72 C for 2 min. The reaction products were then visualized by electrophoresis on 1.2-1.5% agarose gels in 1X TBE.
Bulked segregant analysis. A total of seventy-one DNA samples extracted from the BC1F1 individuals derived from the cross of NK-Coker 9803*2/NC96BGTA5 population were pooled into two separate groups: R and S, for bulked segregant analysis. The R group consisted of thirty DNA samples from thirty BC1F1 individuals which were highly resistant to isolate 209a2 of Bgt, and the S group was composed of thirty DNA samples from thirty BC1F1 individuals which were susceptible to isolate 209a2 of Bgt. The other eleven DNA samples from the same BC1F1 population were not pooled into the R or S groups, because the plants were intermediate or variable for resistance to isolate W72-27. A total of 156 ten-base random primers were used to screen for RAPD markers in Saluda, NC96BGTA5, the R and the S groups.
Linkage analysis. The seventy-one DNA samples extracted from the BC1F1 individuals derived from the cross of NK-Coker 9803*2/NC96BGTA5 population were also used to identify RAPD markers linked to the powdery mildew resistance gene in NC96BGTA5. Linkage of loci segregating in the BC1F1 was analyzed based on the maximum likelihood estimator (1). The genetic map was constructed by using of MapMaker Macintosh V2.0 (10 ). The recombination frequency was transformed into cM according to the Kosambi (1944) function (10).
RESULTS AND DISSCUSION
Reaction of resistance. The reactions of twenty-five wheat lines for powdery mildew resistance after inoculation with ten isolates of Bgt were list in Table 1. NC96BGTA5 and the lines carrying Pm12, Pm13, Pm20 and Pm2l were resistant to all tested isolates. It is unlikely that the present gene in NC96BGTA5 is identical to any of these four genes Pm12, Pm13, Pm20 and Pm2l, because they were not derived from the A genome. The resistance in NC96BGTA5 is also unlikely to be controlled by Pm18, because this gene was identified in a common wheat cultivar Weihenstephan M1N, and contained no A genome diploid parent in its pedigree.
Segregation and inheritance. All four F1 progenies, NK-Coker 68-15/NC96BGTA5, NC96BGTA5/NK-Coker 68-15, Saluda/NC96BGTA5, and NK-Coker 9803/NC96BGTA5, were rated as resistant or moderately resistant to isolates 209a2, W72-27, # 8, E325, and Wkin91 which indicated either complete or partial dominance of resistance (Table 2).
The F2 segregation ratios for resistance in three populations to Bgt isolates 209a2 and W72-27 was in accordance with a 3R:1S ratio expected for segregation of a single dominant gene (Table 3). The BC1F1 segregation ratios conformed to a 1R:1S ratio to isolates 209a2 and W72-27 when the susceptible cultivars were used as recurrent parents (Table 4). The dominant action of the resistance gene was further observed where all BC1F1 plants were resistant to isolates 209a2 and W72-27, when NC96BGTA5 was used as the recurrent parent (Table 4).
Allelism tests revealed the presence of Pm3a in NC96BGTA5. All individuals were resistant to isolates # 8 and E325 in the Saluda/NC96BGTA5 F2 population, and the Saluda*2/ NC96BGTA5 population (Table 3 and 4). These results indicated one common allele for resistance to powdery mildew in Saluda and NC96BGTA5. Because Saluda contained Pm3a, which confers resistance to isolates # 8 and E325, NC96BGTA5 should also carry the resistance allele Pm3a. Therefore, resistance to powdery mildew in NC96BGTA5 is controlled by two dominant genes. One gene, Pm3a, was from its recurrent parent Saluda, and the second was a new gene from wild einkorn wheat. It is proposed that this new gene be designated Pm25 for wheat powdery mildew resistance in NC96BGTA5.
The resistance gene in NC96BGTA5 was found to be linked to Pm3a based on the F2 and BC1F1 populations derived from the crosses between NC96BGTA5 and a susceptible cultivar NK-Coker 68-15. The cosegregations of Pm3a and Pm25 for resistance to # 8 and E325 did not fit 15R:1S or 3R:1S ratios expected for independent segregation of two dominant genes in F2 and BC1F1 populations, respectively (Tables 3 and 4). The recombination frequency between the two genes was approximately 0.21 based on the pooled F2 and BC1F1 data and calculated from the formula [(1-r)/2]2 + (1-r)/2 = 42/(65+42) + 26/(138+26) (Table 3 and 4). Pm3a is located on chromosome 1A (2), while Pm18, the only A genome differential we were unable to evaluate, is located on chromosome 7A (7).
Marker analysis. Three RAPD markers, OPX061050, OPAG04950, and OPAI14600, revealed clear polymorphisms (Fig. 1) and were found to be linked to gene Pm25. The frequencies of recombination between the gene Pm25 and the three markers were 11.3 " 3.76%, 15.5 " 4.29%, and 19.7 " 4.72%, respectively (Table 5), i.e. the linkage distance between them was 12.8 " 3.96 cM, 17.2 " 4.48 cM and 21.6 " 4.88 cM, respectively. The frequencies of recombination between the RAPD markers were 4.2 " 2.38%, 8.5 " 3.31% and 4.2 " 2.38%, respectively (Table 5), i.e. the linkage distance between them was 4.4 " 2.43 cM, 12.8 " 3.36 cM and 4.4 " 2.43 cM, respectively. A genetic map of the region carrying the Pm25 gene was constructed based on the data from the NK-Coker 9803*2/NC96BGTA5 BC1F1 population (Fig. 2). The three markers were not detected in twenty-five wheat lines which contained other Pm genes. They may be useful for distinguishing Pm25 from other Pm genes and for genetic mapping of Chromosome 1A. However, marker assisted selection for Pm25 by use of the three markers is difficult due to the distance between markers and Pm25.
We wish to thank Dr. Martin L. Carson, Dr. Ben-Hui Liu, and Dr. Rebeca C. Rufty for their valuable discussions and constructive criticism during the course of this study. We also thank Dr. Charlie Opperman and Dr. Paul H. Sisco for their critical review of this manuscript. Special gratitude and thanks are also expressed to Amy S. Niewoehner, Scott Walker, and Lynda C. Whitcher for their excellent technical assistance.
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Table 1. Reactions of twenty-five wheat lines for powdery mildew resistance after inoculation with ten isolates of Blumeria graminis f. sp .tritici.
Isolates of Blumeria graminis f. sp. tritici
Line Gene Chromosome E314 E325 Wkin91 209a2 W72-27 127 144 85063 137a1 #8 ___________________________________________________________________________________________________________
NK-Coker 68-15 None S S S S S S S S S S
Chancellor (Cc) None S S S S S S S S S S
Axminster/8*Cc Pm1 7AL R S S R S R R S R S
Ulka/8*Cc Pm2 5DS R R S R R,I S R R S R
Asosan/8*Cc Pm3a 1AS R R S S S R S,I R S R
Chul/8*Cc Pm3b 1AS R R R R R S,I S R I R
Sonora/8*Cc Pm3c 1AS R S S R I S S R S S
Michgan Amber/8*Cc Pm3f 1AS I S S - S S S R S S
Khapli/8*Cc Pm4a 2AL S S R S S R S R S S
Ronos Pm4b 2AL S S R R R R R R,I R S
Kormoran Pm5 7BL S S S S S S S I S S
C747 Pm6 2BL R I S S S I S I S S,I
Transec Pm7 4B/5R S S S S S S S S S S
Kavkaz Pm8 1BL-1RS S S R R R R R S R S
Normandie Pm9+1+2 7AL+5DS R R,I I R R,I S R R R R
Line #31 Pm12 6BS-6SS.6SL R R R R R R R R R R
Chinese Spring Pm13 3B+3D R R,I R R R R R R R R,I
BRG 3N/76 Pm16 4A R R S,I R R R R R R R,I
Amigo Pm17 1AL-1RS S I R R I R R S,I I S
D85350 Pm19 7D R,I I S,I I S I S,I R S,I S,I
TAM 104/Thatcher Pm20 6BS-6RL R R R R,I R,I R R R R,I R
PM941181 Pm21 6AL-6VS R R R R R R R R R R
NK-Coker 9803 ? R S S S S R S R S S
Saluda Pm3a 1AS R R S S S R S,I R S R
NC96BGTA5 Pm25+3a 1A R R R,I R,I R R,I R R R R
aR=resistant, I = intermediate, and S = susceptible.
Table 2. Reactions to five Bgt isolates in four wheat F1 progenies and their parents.
209a2 W72-27 #8 E325 Wkin91
Line or F1 n 0 " Sn 0 " Sn 0 " Sn 0 " Sn 0 " Sn
NC96BGTA5 10 0.3 " 0.46 0.6 " 0.66 0.2 " 0.40 0.8 " 1.17 0.9 " 1.22
NK-Coker 68-15 10 8.9 " 0.30 8.9 " 0.30 8.4 " 0.80 8.6 " 0.66 8.7 " 0.64
NK-Coker 9803 10 8.5 " 0.67 8.0 " 0.89 8.6 " 0.66 8.5 " 0.67 8.8 " 0.40
Saluda 10 8.4 " 0.80 8.9 " 0.30 0.6 " 0.49 0.4 " 0.66 9.0 " 0.00
NK-Coker 68-15/NC96BGTA5 30 0.4 " 0.80 0.8 " 1.07 4.0 " 0.93 3.7 " 1.39 4.0 " 0.85
NC96BGTA5/NK-Coker 68-15 13 0.4 " 0.49 1.2 " 1.31 2.5 " 1.74 3.0 " 1.52 3.8 " 1.31
Saluda/NC96BGTA5 8 0.1 " 0.33 1.4 " 1.87 0.4 " 0.48 0.0 " 0.00 3.0 " 1.32
NK-Coker 9803/NC96BGTA5 16 0.6 " 0.93 0.7 " 0.75 1.9 " 1.78 1.8 " 1.95 3.3 " 1.30
an = the number of samples, 0 = mean, and Sn = standard deviation.
bscale of resistant type 0-3 = resistant, 4-6 = intermediate, and 7-9 = susceptible.
Table 3. Segregation of resistance to Bgt in three F2 populations.
No. of individuals
Cross Isolate Resistant Susceptible Expected ratio 2 value
Saluda/NC96BGTA5 209a2 189 77 3:1 2.211
W72-27 189 77 3:1 2.211
#8 266 0 1:0 0.000
E325 266 0 1:0 0.000
NK-Coker 9803/NC96BGTA5 209a2 147 42 3:1 0.778
W72-27 102 22 3:1 3.484
NK-Coker 68-15/NC96BGTA5 209a2 129 35 3:1 1.171
W72-27 129 35 3:1 1.171
#8 138 26 3:1 7.317** a, b
E325 138 26 3:1 7.317**
a** is significant at P = 0.01.
bThe frequency of recombination between Pm3a and Pm25, r = 0.204 (r was calculated
based on the formula [(1-r)/2]2 = 26/(138+26).
Table 4. Segregation of resistance to Bgt in four BC1F1 populations.
No. of individuals
Cross Isolate Resistant Susceptible Expected ratio 2 value
Saluda*2/NC96BGTA5 209a2 35 31 1:1 0.242
W72-27 35 31 1:1 0.242
#8 66 0 1:0 0.000
E325 66 0 1:0 0.000
NK-Coker 9803*2/ NC96BGTA5 209a2 39 36 1:1 0.120
W72-27 44 31 1:1 2.253
NK-Coker 9803/NC96BGTA5*2 209a2 46 0 1:0 0.000
W72-27 46 0 1:0 0.000
NK-Coker 68-15*2/NC96BGTA5 209a2 55 52 1:1 0.084
W72-27 60 47 1:1 1.579
#8 65 42 1:1 4.944* a, b
3:1 11.592** a
E325 65 42 1:1 4.944*
a* and ** are significant at P = 0.05 and 0.01, respectively.
bThe frequency of recombination between Pm3a and Pm25, r = 0.215 (r was calculated
based on the formula (1-r)/2 = 42/(65+42)).
Table 5. Cosegregation of resistance gene Pm25 and RAPD markers in the NK-Coker 9803*2/NC96BGTA5 BC1F1 population.
Locus Phenotype 2A 2B 2AB Recombination
A B RM Rm SM Sm (1:1) (1:1) (1:1:1:1) (%)
Pm25 OPAG04950 32 6 2 31 0.352 0.127 43.085** 11.3 " 3.76
Pm25 OPX061050 31 7 4 29 0.352 0.014 34.183** 15.5 " 4.29
Pm25 OPAI14600 28 10 4 29 0.352 0.690 27.084** 19.7 " 4.72
Locus Genotype 2A 2B 2AB Recombination
A B MaMb Mamb maMb mamb (1:1) (1:1) (1:1:1:1) (%)
OPAG04950 OPX061050 33 1 2 35 0.127 0.014 59.648** 4.2 " 2.38
OPAG04950 OPAI14600 30 4 2 35 0.127 0.690 49.845** 8.5 " 3.31
OPX061050 OPAI14600 32 3 0 36 0.014 0.690 60.211** 4.2 " 2.38
a ** is significant at P = 0.01.
b R = resistant, S = susceptible, M = the marker present, and m = the marker absent.
Phenotype: RM = powdery mildew resistant and presence of the RAPD marker, Rm = powdery mildew resistant and absence of the marker, SM = powdery mildew
susceptible and presence of the marker, and Sm = powdery mildew susceptible and absence of the marker.
Genotype MaMb signifies presence of the two RAPD markers, Mamb signifies presence of the marker A and absence of the marker B, maMb signifies absence of the marker A and presence of the marker B, and mamb signifies absence of the two markers.
Fig. 1. Amplification pattern of DNA detecting OPX061050, OPAG04950, and OPAI14600 RAPD fragments in the NK-Coker 9803*2/NC96BGTA5 BC1F1 population. Lane 1 to 5 from susceptible plants and lane 6 to 11 from resistant plants with lane 1 and 7 indicating
Rec Dist Marker
Frac cM id Name
( 4.2% ) 4.4
(4.2 % ) 4.4
(11.3 %) 12.8
Fig. 2 A genetic map of the region carrying Pm25 constructed
from the NK-Coker 9803*2/NC96BGTA5 BC1F1 population.