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Disease resistance. Stem Rust Resistance.

Sr21

Background information

Stem rust resistance gene Sr21 confers resistance to several races of Puccinia graminis f. sp. tritici, including those in the Ug99 group. But contrasting studies reported both susceptible and partial resistant reactions. Sr21 was first identified in diploid wheat Triticum monococcum (1) and later transferred to hexaploid wheat (2), where it was mapped on chromosome arm 2AL, 2.4 cM distal from the centromere. However, this distance might not be precise because chromosomes of T. monococcum and T. aestivum recombine poorly in the presence of the Ph1b gene.

To clarify this point, improve its chromosomal location and determine the value of Sr21 in breeding, Chen et al. (3) developed two large mapping populations of diploid wheat. In addition, since the variations in Sr21 resistance seemed to be affected by environmental variables, they tested if temperature and photoperiod modulated the resistance provided by this gene.

One of the mapping populations was derived from a cross between T. monococcum ssp. monococcum PI 272557 (susceptible) and the spring line DV92, which is supposed to carry Sr21 and Sr35. The other mapping population resulted from a cross between PI 272557 and G3116, a wild winter line of T. monococcum subsp. aegilopoides that was postulated to carry Sr21.

The effect of temperature on Sr21 resistance was studied at 16, 20 and 24 °C and the photoperiod was evaluated at 10, 15 and 20 hours.  The results shown that the recombinant lines carrying Sr21 were susceptible at 16 °C, but resistant at 20 and 24 °C to the Ug99 races tested. In comparison, the effect of photoperiod was smaller and only slight differences in infection type were observed under different photoperiods.

Sr21 can be deployed in breeding programs to provide Ug99 resistance. As usual, it has to be combined with other sources of resistance. This recommendation is especially important in this case due to the environmental effects that affect its effectiveness.

Markers for Sr21

The mapping effort described above produced molecular markers that could be used to monitor the transfer of this gene to hexaploid wheat lines. Two very close CAP markers are FD527726 (0.15 cM, distal) and EX594406 (0.05 cM, proximal). Another useful marker is SSR gwm312, 0.2 cM on the proximal side.

Primer sequences:

gwm312
   WMS312-F 5- ATC GCA TGA TGC ACG TAG AG -3
   WMS312-R 5- ACA TGC ATG CCT ACC TAA TGG -3 

   Annealing temperature 60 °C
FD527726
   FD527726-F 5- CGG CAT CAA TAG GAG AAG A -3  
   FD527726-R 3- TAG GAT ACG TGA CCC AGG A -3
   
   Annealing temperature 53 °C 
EX594406
   EX594406-F 5- TCA ACA ACT TCA ACA AGG C -3 
   EX594406-R 5- AAC AAG AGA ACG AGC ATC G -3 
   
   Annealing temperature 52 °C 
PCR conditions:

FD527726 and EX59446 are CAP markers, requiring an extra step of treatment with a restriction enzymes BsrDI and XmnI, respectively.

PCR reaction mix

Total: 20 µl

Expected products:

PCR products were separated in 6 % non-denaturing acrylamide gels or 1 % agarose gels and stained directly with ethidium bromide. The sizes of the bands associated or not to Sr21 for each marker are:

 

Sr21-

Sr21+

gwm312

180

182

FD527726

806

754

EX594406

420

566


Conditions presented here should be considered only as a starting point of the PCR optimization for individual laboratories.

References

1. Chromosome location of genes conditioning stem rust resistance transferred from diploid to hexaploid wheat. The T. In: Nature New Biology, 1973, 241:256.

2. Cytogenetical studies in wheat. IX. Monosomic analyses, telocentric mapping and linkage relationships of genes Sr21, Pm4 and Mle. McIntosh R, Bennett FG. In: Australian Journal of Biological Sciences, 1979, 32:115-126.

3. Fine mapping and characterization of Sr21, a temperature‑sensitive diploid wheat resistance gene effective against the Puccinia graminis f. sp. tritici Ug99 race group. Knott DR. In: Theoretical and Applied Genetics, 2015, 128:645-656.
DOI: 10.1007/s00122-015-2460-x

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