2013 Annual Report
1a.Objectives (from AD-416):
This research involves characterization of host-pathogen interactions and pathogen biology in both fungal and viral pathogen systems in order to facilitate a better understanding of the fundamental processes resulting in resistance, or susceptibility and disease development. Specific objectives: 1. Characterize necrotrophic effectors from Stagonospora nodorum, the causal agent of Stagonospora blotch of wheat, and determine their association with the corresponding wheat susceptibility genes. Apply this information to develop procedures useful in the selection and identification of resistant wheats. 2. Identify and characterize virulence factors of Pyrenophora teres f. teres, and P. teres f. maculata, causal agents of net form net blotch and spot form net blotch of barley, respectively, to elucidate and exploit the mechanism for resistance to these diseases. 3. Identify viral genome sequences and interactions affecting pathogenicity, virulence, and transmissibility of viruses infecting barley, particularly barley stripe mosaic virus and oat blue dwarf virus.
1b.Approach (from AD-416):
Fungal and viral diseases of small grains pose an economic threat to production throughout the U.S. and the world. This project focuses on both fungal and viral pathogens in an effort to solve issues related to pathogenicity, virulence, and host resistance. It is our goal to identify and characterize pathogenicity/virulence factors of Pyrenophora teres (net blotch of barley), Stagonospora nodorum (S. nodorum blotch of wheat), barley stripe mosaic virus (BSMV), and oat blue dwarf virus (OBDV), and evaluate their importance in disease production. Our approach will be to: a) Identify and characterize virulence determinants and host genes important in net form net blotch and spot form net blotch of barley through phenotyping and mapping of segregating host and pathogen populations. Host resistance genes will be mapped and characterized, and pathogen virulence genes will be mapped, characterized, and cloned. Additionally, the newly acquired P. teres f. teres genome sequence will be used to identify necrotrophic effectors (NEs) involved in disease induction. b) Identify and characterize NEs and their corresponding host sensitivity genes important in S. nodorum blotch of wheat. Host-pathogen interactions will be characterized through evaluation of host mapping populations and identification of new NEs through purification and bioinformatic analysis. c) Identify and characterize factors affecting pathogenicity, virulence, and transmissibility of viruses infecting barley through the characterization of BSMV and OBDV model systems. A reverse genetics approach will be used to analyze viral phenotypes and further elucidate genome structure-function relationships. The knowledge gained may ultimately lead to the discovery or development of novel and effective control measures for viral diseases of barley and fungal diseases of both barley and wheat.
Net blotch of barley and Stagonospora nodorum blotch on wheat are two of the most destructive leaf diseases of cereals, both in the US and worldwide. ARS researchers in Fargo, ND have focused on the characterization of pathogen virulence as it relates to host-pathogen interactions in these important diseases.
Net form net blotch (NFNB) of barley. A Pyrenophora teres f. teres population segregating for virulence was previously mapped with SSR and AFLP markers. A genotyping by sequencing approach has now been used to identify single nucleotide polymorphism markers to further saturate this map so that it is more useful in discovery of genomic regions harboring virulence genes.
Spot form net blotch (SFNB) of barley. Highly virulent isolates of the fungus P. teres f. maculata collected during the 2012 growing season were evaluated and crosses were made for the genetic characterization of virulence. Seven populations were developed and phenotyping of these populations will begin soon.
A barley core collection of more than 2000 barley accessions was evaluated with a diverse set of P. teres f. maculata isolates in order to identify lines that are highly resistant to all isolates tested, as well as to identify lines that may be useful in the characterization of virulence. Barley lines identified to be resistant to all isolates tested were used to generate crosses with local varieties. These populations will be used to develop recombinant inbred lines and will be mapped to be used in characterizing the genetics of host resistance.
Stagonospora nodorum blotch (SNB) of wheat. The fungus Stagonospora nodorum, which causes SNB disease, produces several toxins known as necrotrophic effectors that are important pathogen virulence factors. In the past year, the mode of action of one of these effectors (SnTox1) continued to be characterized, and several regions within this protein were identified that need further evaluation for their importance in its function. One of these regions was shown to have homology to several chitin binding domains. In addition, we have shown that SnTox1 physically binds chitin as well as the plant cell wall components cellulose and xylan, indicating a potential secondary role for this protein.
Virus diseases. Oat blue dwarf virus (OBDV) is a phloem-limited virus known to infect plants such as barley, oats, and flax, and is being studied as part of continuing research toward improving our understanding of how viruses function and cause disease. The strategy employed by this virus for expressing its coat proteins was studied through mutational analysis of a cloned copy of the virus. Information gained may have a bearing on the mechanism by which this virus is transmitted by leafhoppers. A closely related virus known as maize rayado fino virus was cloned for the first time and the sequence of its entire genome was determined. The availability of this clone will facilitate further studies regarding host-virus interactions, phloem limitation of some viruses, and transmissibility of viruses by leafhoppers.
Characterization of the role of SnTox1 in disease development. SNB is an important disease of wheat caused by the fungus Stagonospora nodorum. This fungus produces a protein known as SnTox1 that plays a key role in inducing the disease. While investigating the possible function of SnTox1, ARS researchers in Fargo, ND showed that SnTox1 physically binds to chitin in fungal cell walls, as well as to other polysaccharides (cellulose, xylan) found in plant cell walls. Since several antifungal compounds, including chitinases that chew up the fungal cell wall, are produced by plants with SNB, it may be that SnTox1 protects the pathogen from damage during host invasion. This work provides knowledge of how necrotrophic pathogens induce disease and can be used by breeding programs to better select for resistance to this important pathogen of wheat.
Evaluation of the barley core collection for SFNB disease resistance. Spot form net blotch (SFNB) has recently become a major problem for barley growers in the Northern Great Plains region, likely due to a shift in the virulence of the pathogen. Most popular varieties planted in ND, MN, and MT are susceptible. ARS scientists in Fargo, ND evaluated more than 2000 barley lines in the core collection for their reaction to four different isolates of the fungus which causes SFNB. These isolates were collected from different barley growing regions of the world, including Australia, New Zealand, Denmark, and the US. Only a small number of these lines were found to be highly resistant to all isolates tested, and these lines have been crossed with local barley varieties. These barley populations will be useful in characterizing the resistance to SFNB and moving this resistance into locally adapted varieties.
McDonald, M.C., Oliver, R.P., Friesen, T.L., Brunner, P.C., McDonald, B.A. 2013. Global diversity and distribution of three necrotrophic effectors in Phaeosphaeria nodorum and related species. New Phytologist. 199:241-251.
McDonald, M.C., Razavi, M., Friesen, T.L., Brunner, P.C., McDonald, B.A. 2012. Phylogenetic and population genetic analyses of Phaeosphaeria nodorum and its close relatives indicate cryptic species and an origin in the Fertile Crescent. Fungal Genetics and Biology. 49:882-895.
Friesen, T.L., Chu, C., Xu, S.S., Faris, J.D. 2012. SnTox5-Snn5: A novel Stagonospora nodorum effector-wheat gene interaction and its relationship with the SnToxA-Tsn1 and SnTox3-Snn3-B1 interactions. Molecular Plant Pathology. 13(9):1101-1109.
Syme, R.A., Hane, J.K., Friesen, T.L., Oliver, R.P. 2013. Resequencing and comparative genomics of Stagonospora nodorum: Sectional gene absence and effector discovery. Genes, Genomes, Genetics. 3:959-969.