VII Biotic Stress Response
Stem Rust Resistance: Two Approaches
Abstract Stem rust, caused by Puccinia graminis f.sp. tritici (Pgt), is a destructive disease of wheat that has historically caused significant yield losses in much of the global wheat production area. Over the past 50 years, stem rust has been effectively controlled by deploying cultivars carrying stem rust resistance (Sr) genes. With the emergence of new Pgt races, namely Ug99 and its variants, there has been a reinvestment in stem rust research. This includes discovery, characterization, genetic mapping, and cloning of Sr genes. Here we investigated two such examples of genetic characterization and mapping of stem rust resistance. In the first example, a region on chromosome 6DS harbouring resistance to Ug99 was examined in several populations and from several sources. In the second example, a less typical genetic model of resistance was studied in which seedling resistance was activated by an independent locus exhibiting an apparent “nonsuppressing” effect. The knowledge gained by these and other lines of research will contribute to the goal of durable resistance to stem rust.
Stem rust, caused by Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn. (Pgt), is a disease of wheat that has the potential to cause devastating losses in grain yield. The last epidemic of stem rust in North America occurred from 1953 to 1955 (Peturson 1958). Since then, resistant cultivars have successfully controlled the disease.
However, the appearance of Pgt race TTKSK, also known as Ug99, rendered most of the globally grown wheat cultivars vulnerable to stem rust (Singh et al. 2011; Fetch et al. 2012). The distinguishing characteristic of this race is its virulence to Sr31, a broadly deployed stem rust resistance (Sr) gene (Pretorius et al. 2000). Furthermore, Ug99 has continued to evolve and accumulate different virulence combinations (Jin et al. 2009; Park et al. 2011). This has led to renewed efforts to find and deploy new Sr genes (e.g. Njau et al. 2010). More generally, this threat from Pgt has stimulated investment in several aspects of wheat, Pgt and their interactions. This has also underscored the need to employ multiple strategies to achieving resistance and utilise diverse resistance. To meet these needs it is critical to understand the resistance present in breeding material and cultivars, understand the relationship between different sources of resistance (e.g. allelism, linkage, breadth of resistance, target in the pathogen etc.), and develop tools to implement responsible breeding strategies.
It is in the context of renewed and diverse investigation into the wheat stem rust host-pathogen system that this paper was presented. Two lines of research were presented: (1) comparative mapping and fine-mapping of a chromosome region conferring qualitative resistance and (2) genetic interactions that enhance stem rust resistance at the seedling stage and in field conditions across multiple environments. Here, the pertinent literature for these studies is reviewed and our preliminary findings are discussed.