Guest Entry: Les J. Szabo on the sequencing of the wheat stem rust fungal genome

John Bakum
Tuesday, July 19, 2011

Les Szabo
Les Szabo
In a paper titled, "Obligate biotrophy features unraveled by the genomic analysis of rust fungi", published in the May 31, 2011 issue of Proceedings of the National Academy of SciencesUSA(PNAS), Duplessis et al. describe the characterization of the genome and transcriptome of two rust fungi, Puccinia graminis f. sp. tritici (Pgt, wheat stem rust pathogen) and Melampsora larici-populina (Mlp, leaf rust of poplar). This represents the first genomes of any rust fungi to be sequenced, and provides the first detail insight into the how these obligate biotrophic plant pathogens are able to cause disease and rapidly evolve to overcome host resistance.

Besides the importance of this work to basic understanding of rust fungi, these findings provide a treasure chest of information that can be used to in a practical manner to control wheat stem rust. The Pgt genome sequence has already provided essential tools for developing DNA-based resources that enabled the development of a rapid molecular diagnostic assays for Ug99 and related races. In addition, molecular markers (SSRs and SNPs) are being used to genetically characterize isolates of Pgt to better understand the evolution of new races. Furthermore, these tools will likely provide a more accurate means to track long distant movement of this pathogen and provide a basis for the development of an early warning system.

Host-parasite interaction, including the perception by the host to the presence of the wheat stem rust pathogen, likely involves small secreted-proteins or effectors produced by the fungus. The analysis of the Pgt genome identified of a large number of small-secreted proteins (1,108). Work in several laboratories has already begun to identify the specific effectors that trigger host resistance controlled by stem rust resistance (R) genes. Characterization of these effector genes will not only allow better understanding of the molecular mechanism of host resistance, but also provide new tools to screen a wide range of germplasm for new resistance genes without having to use the rust pathogen. Furthermore, these candidate effector genes open the door for the development of non-R gene host resistance, such as, using host-induced gene silencing (RNAi) to target key fungal pathways involved in pathogenicity and nutrient uptake.

Duplessis S, et al. (2011) Obligate biotrophy features unraveled by the genomic analysis of rust fungiProc Natl Acad Sci USA 108:9166-9171.

Les Szabo is a research geneticist at the Cereal Disease Laboratory in St. Paul, MN