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CROP BREEDING, GENETICS & CYTOLOGY

Inheritance of Resistance to Stem Rust in `Triumph 64' Winter Wheat

Dep. of Plant Sciences, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK, Canada S7N 5A8

knott{at}sask.usask.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
The winter wheat (Triticum aestivum L.) cultivar Triumph 64 has resistance to a number of races of stem rust (Puccinia graminis Pers. f. sp. tritici Eriks. & E. Henn) and has been used as a supplementary differential in identifying stem rust races. Triumph 64 was crossed and backcrossed to a susceptible wheat, LMPG, to study the inheritance of its resistance to races MCC and LCB. Four additional backcrosses to LMPG were made to produce near-isogenic lines (NILs) carrying the Triumph 64 genes for resistance to races MCC and LCB. Twenty-two NILs were tested with isolates of 10 stem rust races to determine the number of genes for resistance that were present. The genetic study indicated that Triumph 64 carries two genes conditioning resistance to both races MCC and LCB, and four genes conditioning resistance only to LCB. However, the NILs produced from Triumph 64 appeared to carry at least nine Sr genes, six giving resistance to both races MCC and LCB, and only one giving resistance just to LCB. Furthermore, some of the NILs were resistant to races TMH(15B-1) and TMH(15B-4) to which Triumph 64 is susceptible. One possible explanation for the results is that Triumph 64 carries one or more suppressors that were lost during the backcrossing, allowing the suppressed Sr genes to be expressed. Four of the NILs were resistant to all 10 races of stem rust and should be useful in wheat breeding.

Abbreviations: IT, infection type • NIL, near-isogenic line • R, resistant • S, susceptible

	INTRODUCTION

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THE WINTER WHEAT CULTIVAR Triumph 64 has been used as a supplementary differential genotype in identifying stem rust races (McVey et al., 1996). Roelfs and McVey (1979) reported that virulence on Triumph 64 occurred only in standard races 15 and 56. Williams et al. (1998) studied the inheritance of resistance to stem rust in a spring wheat derivative, `M-T64', from a cross between Triumph 64 and a susceptible spring wheat derived from `McNair 701'. They concluded that M-T64 carries one gene on chromosome arm 4AL closely linked or allelic to Sr7b and a second gene that is possibly on chromosome 2B. The authors did not discuss how M-T64 was produced, but unless a number of races were used, it is unlikely that M-T64 has all of the resistance of Triumph 64.

I decided that it would be useful to study the inheritance of resistance in Triumph 64 and to transfer its genes for resistance to the parent I am using to produce NILs for the Sr genes. Such lines should be more useful than Triumph 64 itself in the identification of stem rust races. This paper reports the results of the work.

	Materials and methods

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Seed of Triumph 64 (CI 13679) was obtained from Dr. A.P. Roelfs, Cereal Disease Laboratory, USDA-ARS, St. Paul, MN. A pot of five plants of Triumph 64 was tested with stem races MCC and LCB and gave infection types (ITs) 2 and 0;, respectively. In all of the later tests with 10 races, there was no evidence of impurities in the cultivar. For a genetic study, the cultivar was crossed and backcrossed to LMPG (`Little Club'//3 x `Prelude'/8 x `Marquis'/3/`Gabo'), a highly rust susceptible, day length insensitive line that was developed for the purpose of producing NILs for the Sr genes (Knott, 1990). The BC₁F₁ plants and BC₁F₂ families were tested with isolates of stem rust races LCB and MCC, designated as described by Roelfs and Martens (1988).

To transfer the genes for stem rust resistance from Triumph 64 to LMPG, four additional backcrosses to LMPG were made. During the backcrossing, plants were selected that had resistance to either or both of races LCB and MCC. After the fifth backcross, homozygous resistant NILs were produced. Since five backcrosses had been made with selection for a specific IT in each line, it was assumed that the NILs carried single Sr genes. After the last backcross, none of the lines appeared to segregate for more than one gene.

The rust tests on the BC₁F₁ plants in the genetic study were carried out in a greenhouse maintained as nearly as possible at 20°C during the day and 15°C at night. Supplementary lighting was used as needed to provide a 16-h day length. Ten seeds from each BC₁F₁ plant were planted in a 15-cm-diam. plastic pot. Seedlings at the two-leaf stage were inoculated with spores of race LCB suspended in a light mineral oil. Eight days later, the plants were inoculated with race MCC. Following inoculation, the inoculated seedlings were kept moist for 18 h using a humidifier. After 14 d when the pustules of the first race were fully developed, the ITs were rated on a scale of 0 to 4, using the system of Stakman et al. (1962), and the infected leaves were removed. After a further 10 d, the second infection was rated. No evidence of cross-protection was ever noted; there were always plants that were resistant to the first race but susceptible to the second, unless the same gene gave resistance to both races. Infection types 0, 1, 2, and X were considered resistant, and 3 and 4 susceptible. Rust tests on the BC₁F₂ plants and the parents for the genetic study and on the 22 NILs were carried out in a growth chamber with a 16-h light period at 20°C and an 8-h dark period at 15°C. Inoculations were done as for the greenhouse tests. The BC₁F₂ plants for the genetic study plus the parents were inoculated with races LCB and MCC 8 d apart.

Twenty-two NILs were tested with isolates of 10 races of stem rust: LCB(111), MCC(56), TMH(15B-1), TMH(15B-4), TMB(15B-1L), RCH(11), HFC(29-1), HFC(48A), HDG (C15), and QCL(C65) (the old race number is given in parentheses since two pairs of races that differed under the former system of nomenclature had the same designations under the new system). For the NILs, six seeds were planted in a 12.5-cm-diam. pot. Susceptible checks, LMPG in all tests and Prelude/8 x Marquis in most tests, were included. Each set of NILs was inoculated with only one race. Under the controlled growth chamber conditions, the ITs are very reproducible from one test to the next. As far as possible, pairs of NILs that were closely related in pedigree and had shown similar ITs to race LCB and MCC during their production were tested to provide a degree of replication.

The number of segregating and susceptible BC₁F₂ families was tested against the ratios expected in a backcross for varying numbers of genes, using a chi-square goodness-of-fit test. To determine the expected ratio of resistant and susceptible plants in segregating backcross families, the frequency of each type of segregating family was determined and then multiplied by the expected ratio for that type. For example, if one dominant and one recessive gene are segregating, one-third of the families will segregate 3 resistant (R):1 susceptible (S), one-third 13R:3S, and one-third 1R:3S (a combined ratio of 29R:19S). With five dominant and one recessive genes (as proposed in the Results), there are 64 types of families and the expected ratio for the 63 types of segregating families combined is 240269R:17779S.

	Results

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Genetic Study
One hundred forty-two BC₁F₂ families from the backcross, Triumph 64/2 x LMPG, were tested with isolates of two races, MCC and LCB. With race MCC, 104 families segregated for resistance and 38 were homozygous susceptible (Table 1) , a good fit to the 3:1 ratio expected for two genes . All 104 families that segregated with MCC also segregated with LCB, indicating that each gene gave resistance to both races. With race MCC, the 104 segregating families clearly fell into two types (not shown in Table 1). One group of 26 families segregated for a 2^-2 IT and the gene responsible was recessive. In these families, the combined segregation was 55R:191S, a good fit to a 1:3 ratio . A second group of 78 families had more R than S plants and segregated for either a dominant gene, which typically conditioned a 1⁺2^- IT, or for both the dominant gene and the recessive gene. Within this second group of segregating families, the segregation was 600R:163S plants, an excellent fit to the 25:7 ratio expected if one-half the families segregated 3:1 and one-half 13:3 . The number of families that segregated only for the recessive gene is probably somewhat lower than it should have been since only 10 plants per family were tested. A few families that were truly segregating would fail to include a resistant plant just by chance and would be misclassified and included in the 38 homozygous susceptible families. The probability of getting 10 susceptible plants in a family segregating 1R:3S is 0.056. Nevertheless, despite this bias in the results, the 78:26 segregation does not differ significantly from the expected 2:1 ratio . The BC₁F₂ results confirmed that one gene was recessive. Some susceptible BC₁F₁ plants gave rise to segregating BC₁F₂ families.

The combined data for the two races are a good fit to the 48:15:0:1 ratio expected with six genes and will also fit the 24:7:0:1 ratio expected with five genes (Table 1).

Near-Isogenic Lines
Twenty-two NILs derived from five backcrosses of Triumph 64 to LMPG were tested with isolates of 10 races of stem rust. They appeared to fall into 14 different types, although in some cases different types seemed to be quite similar (Table 2) .

The first NIL, Triumph 64/6 x LMPG-1, gave resistance only to race LCB with an IT 0;1. The pattern of resistance and IT are typical of the gene Sr18, which is found frequently in hexaploid wheat (Baker et al., 1970) and may be present in Triumph 64.

NIL-2 gave resistance to only two races, HFC(48A) and LCB, with the ITs being 2^-2⁺ and , respectively. This pattern of resistance and ITs was found in one line of the 252 tested, a NIL in the LMPG background derived from an SrKt'2' line obtained from Dr. A. P. Roelfs (Roelfs and McVey, 1979). The gene SrKt'2' is derived from `Kota', which is not a parent of Triumph 64. A test has not yet been made to determine whether NIL-2 and the SrKt'2' line carry the same gene.

NIL-3 gave resistance to four races, a 1 IT to race LCB and ITs in the 1⁺2 range to HFC(29-1), HFC(48A), and HDG. This pattern of resistance is not present in any of the lines previously tested so the NIL must carry a new gene derived from Triumph 64.

Two NILs, NIL-4 and NIL-5, conditioned resistance to four races, HFC(48A), MCC, LCB, and QCL. However, their ITs with race LCB (0; and 1⁺ respectively) and MCC (1^- and 2^-2, respectively) were sufficiently distinct to suggest that they carry different genes. Alternatively, one line may carry a gene or genes that modify its reaction to LCB and MCC. Again the pattern of resistance is not present in any of the lines previously tested and must result from a new gene or genes derived from Triumph 64.

NIL-6 and NIL-7 were resistant to five races, HFC(29-1), HFC(48A), QCL, MCC, and LCB. However, their ITs were distinctly different (00; vs. 11⁺) with races HFC(48A), LCB, and QCL, suggesting that different genes are involved. Interestingly, NIL-6 is the more resistant to two of the races, but NIL-7 is more resistant to the third. This pattern of resistance is also not present in any of the lines tested previously.

NIL-8 appeared to be resistant to seven races. It is probably immune (0 IT) to race QCL and its resistance to race RCH was borderline (2⁺3 IT). In two tests with QCL, no pustules appeared, but in both tests the infection on other lines was light. Regardless of its IT with QCL, the pattern of resistance of NIL-8 is different than that of any of the other lines tested, and it must carry a new gene derived from Triumph 64.

NIL-9 also was resistant to seven races, but to a different set than NIL-8. The pattern of resistance was different from that of any of the lines tested earlier. Thus, it must carry a new gene from Triumph 64.

NIL-10 was resistant to eight races and susceptible only to races TMB and HDG. Again this pattern of resistance is new and the NIL must carry a new gene from Triumph 64.

The last four NILs were each resistant to all 10 races. However, there were sufficient differences in their ITs with particular races to suggest that they all carry different genes. In all but one case, the differences involve one NIL giving a 0 or 0; IT to a race and the second NIL giving a 1⁺ or 2. If the four NILs all carry the same gene for resistance, they must carry other genes that modify their resistance to specific races. Of the 252 lines and parents tested with 9, 10, or 13 races as described earlier, 33 NILs in the genetic background of LMPG (not including the four Triumph NILs) were resistant to all races with which they were tested. Eighteen of the 33 are clearly more resistant than any of the four Triumph NILs. None of the remaining 14 had ITs that appeared to match exactly the ITs of the Triumph NILs. For example, none of them gave 0 or 0; ITs only to races HFC(29-1), HFC(48A), and LCB as NIL-10 does. It would be very surprising for Triumph 64 to carry four different genes, each conditioning resistance to all of the races with which it was tested. The alternative is that the four NILs carry the same gene for resistance but have different modifying genes even after five backcrosses to LMPG. This would be more likely to occur if some modifiers were linked to the gene for resistance and were lost in some NILs but not in others.

	Discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
A surprising feature of the data is the number of possible genes for resistance carried by Triumph 64. Even if the possible duplicates (NILs-4 and -5, NILs-6 and -7, and NILs -11, -12, -13, and -14) and the possible outcross (NIL-2) are eliminated, Triumph 64 appears to carry nine genes. The possibility that outcrossing or the presence of stray seed accounted for the unexpected number of genes is minimal. Only one of the nine genes results in a pattern of resistance that matches that of any of the 230 other NILs and parents from previous genetic studies, which are the likely source of outcrossing or contamination. The 230 lines carry most of the identified Sr genes. Although Triumph 64 appears to carry an unexpectedly large number of Sr genes, several of them condition resistance to only a few older races. This undoubtedly resulted from the fact that two older races were used in the selection process and LCB is particularly avirulent.

Another possibility is that one or more of the more resistant lines carry two of the genes that give resistance to fewer races. In combination, the two genes would provide all of the resistance of the more resistant line. There is no evidence of this in Table 2. There is no case where the resistance of a higher-ranked NIL could have resulted from a combination of the genes in two of the lower ranked NILs.

The BC₁F₂ genetic analysis indicated that Triumph 64 carries two genes that give resistance to both races MCC and LCB and four additional genes that give resistance only to LCB. However, of the minimum of nine genes carried by Triumph 64 and present in the NILs, six give resistance to both MCC and LCB, and only one to just LCB. The three additional genes that condition resistance to LCB but not to MCC and were detected in the genetic study could easily have been lost during the backcrosses to LMPG. The ITs they control are probably not distinct from those of other genes and they would not be selected. It is more difficult to explain why four of the six genes conditioning resistance to both races were not detected in the genetic study. One possibility is that Triumph 64 carries suppressors that prevent the expression of some of its genes for resistance. If a suppressor were fairly closely linked to the gene for resistance that it suppresses, the gene for resistance would not be detected in the genetic study. The occasional crossover that might occur would increase the frequency of segregating families slightly but probably not enough to upset the backcross ratio. However, during the five backcrosses used to produce the NILs, there would be a greater opportunity for the linkage to be broken and the resistance to be uncovered and selected for.

Other evidence indicates that Triumph 64 carries one or more suppressors. It is susceptible to TMH(15B-1) and probably also to TMH(15B-4). However, NILs-10, -11, -12, -13, and -14 are resistant to both races. Thus, the resistance must have been suppressed in Triumph 64 and the suppressor(s) lost during the backcrossing to LMPG. In addition, six NILs were more resistant (0 IT) to one or both of races HFC(29-1) and HFC(48A) than is Triumph 64 (2^-2 IT). This may mean that one gene for resistance to these races is suppressed in Triumph 64 and the suppressor was lost during backcrossing. Similarly, one or more NILs appear more resistant than Triumph 64 to races RCH, TMB, MCC, and QCL.

Another possibility is that Triumph 64 is heterogeneous and plants with different genotypes were used as parents in the genetic study and the backcrosses to produce the NILs, although this seems unlikely. In the tests of Triumph 64 with 10 stem rust races, no evidence of heterogeneity was found.

Suppressors of stem rust resistance in wheat have been reported by several authors (e.g., Kerber and Green, 1980; Bai and Knott, 1992; Williams et al., 1992). In an example that is similar to the present case, Knott (2000) transferred genes for stem rust resistance from Medea durum wheat to LMPG and obtained resistance that evidently had been suppressed in Medea.

The present study demonstrates the complexity of rust resistance in Triumph 64. The process of backcrossing a parent to LMPG and selecting for resistance to one or more races, particularly relatively avirulent races, appears to be particularly useful in detecting Sr genes carried by the parent, and also the presence of suppressors.

It is not possible to determine the relationship of the two genes identified by Williams et al. (1998) in a Triumph 64 derivative with the genes identified in this study because different races of stem rust were used.

If they prove to carry Sr genes not previously identified, NILs-11, -12, -13, and -14 should be useful in wheat breeding.

	ACKNOWLEDGMENTS

 
The author is pleased to acknowledge the financial support of the Saskatchewan Agricultural Development Fund.

Received for publication July 7, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 

• Bai D., Knott D.R. Suppression of rust resistance in bread wheat (Triticum aestivum L.) by D-genome chromosomes. Genetics 1992;35:276-282.
• Baker E.P., Sanghi A.K., McIntosh R.A., Luig N.H. Cytogenetical studies in wheat III. Studies of a gene conditioning resistance to stem rust strains with unusual genes for avirulence. Aust. J. Biol. Sci. 1970;23:369-375.
• Kerber E.R., Green G.J. Suppression of stem rust resistance in hexaploid wheat cv. Canthatch by chromosome 7DL. Can. J. Bot. 1980;58:1347-1350.
• Knott D.R. Near-isogenic lines of wheat carrying genes for stem rust resistance. Crop Sci. 1990;30:901-905.[Abstract/Free Full Text]
• Knott D.R. Inheritance of resistance to stem rust in Medea durum wheat and the role of suppressors. Crop Sci. 2000;40:98-102.[Abstract/Free Full Text]
• McVey D.V., Long D.L., Roberts J.J. Races of Puccinia graminis in the United States during 1994. Plant Dis. 1996;80:85-89.
• Roelfs A.P., Martens J.W. An international system of nomenclature for Puccinia graminis f. sp. tritici. Phytopathology 1988;78:526-533.[ISI]
• Roelfs A.P., McVey D.V. Low infection types produced by Puccinia graminis f. sp. tritici and wheat lines with designated genes for resistance. Phytopathology 1979;69:722-730.
• Stakman, E.C., D.M. Stewart, and W.Q. Loegering. 1962. Identification of physiologic races of Puccinia graminis var. tritici.USDA-ARS Rep. E617. U.S. Gov. Print. Office, Washington, DC.
• Williams N.D., Miller J.D., Klindworth D.L. Induced mutations of a genetic suppressor of resistance to stem rust. Crop Sci. 1992;32:612-616.[Abstract/Free Full Text]
• Williams, N.D., J.D. Miller, D.L. Klindworth, and L.R. Joppa. 1998. Genes for wheat stem rust resistance from Triumph 64. p. 342–344. In A.E. Slinkard (ed.) Proc. 9th Intern. Wheat Genet. Symp. Vol. 3. Saskatoon, SK. 2–7 Aug. 1998. Univ. of Saskatchewan, Extension Press, Saskatoon, SK, Canada.

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