Stem Rust Resistance Gene Mapped

Puccinia graminis f. sp. tritici, the causal agent of wheat stem rust, produced large epidemics in North America and other parts of the world in the early 1950s. These epidemics were controlled by the deployment of stem rust resistance genes that have provided adequate resistance for the last several decades.

Unfortunately, these genes are no longer effective against a new race of stem rust, Ug99, which was identified in Uganda in 1999. Ug99 and its close derivatives have an unusually broad virulence spectrum and pose a serious threat to global wheat production. If new wheat varieties with appropriate resistances genes are not developed and deployed in time, the spread of Ug99 could result in food shortages.

Therefore, the identification and mapping of new resistance genes effective against Ug99 have a high priority. Scientists from the University of California in Davis, Kansas State University, and the USDA Cereal Disease Laboratory in Minnesota have mapped and characterized the stem rust resistance gene Sr35. This gene is effective against Ug99 and its derivatives as well as another broadly virulent race from Yemen (known as
TRTTF), providing a useful tool to fight this new stem rust epidemic. Results from the research are published in the November–December 2010 issue of Crop Science.

The researchers identified molecular markers closely flanking the Sr35 gene on the long arm of chromosome 3A and then used comparative genomics to identify a small set of candidate genes among the collinear genes in rice and the model grass species Brachypodium. The candidate genes and closely linked molecular markers found in this study can be used to accelerate the deployment of Sr35 in wheat breeding programs using marker-assisted selection.

Since Sr35 is not effective against all known races of stem rust, it needs to be deployed in combination with other stem rust resistance genes. The presence of multiple resistance genes is expected to extend the durability of resistance, since the probability of simultaneous mutations in the pathogen to overcome multiple resistance mechanisms is much lower than the probability to overcome individual mutations.
The Sr35 gene was originally identified in diploid wheat and then transferred to common wheat by crosses and recombination.

Dr. Wenjun Zhang and collaborators identified common wheat lines that have recombination events close to the Sr35 gene. These lines are important to reduce the length of the large chromosome segment introduced from diploid wheat.

The reduction of the alien chromosome segment, which recombines poorly with the wheat chromosomes, is important to minimize the potential negative impact of other genes from the diploid wheat on the agronomic and quality characteristics of the commercial wheat varieties.

According to the authors, the precise map of Sr35 presented in this study is the first step towards the positional cloning of this stem rust resistance gene. They say the cloning of Sr35 will provide a perfect diagnostic marker for this gene and a tool for the understanding of the resistance mechanisms against Ug99.

Adapted from Zhang, W., E. Olson,
C. Saintenac, M. Rouse, Z. Abate, Y.
Jin, E. Akhunov, M. Pumphrey, and J.
Dubcovsky. 2010. Genetic maps of stem
rust resistance gene Sr35 in diploid and
hexaploid wheat. Crop Sci. 50:2464–2474.
View the full article online at www.crops.org/publications/cs/tocs/50/6

Photo by the authors