Location: Foreign Disease-weed Science Research2011 Annual Report
1a. Objectives (from AD-416)
Identify genomic and phenotypic elements to characterize emerging and foreign fungal plant pathogens. Develop an understanding of the biology, genetics and epidemiology of emerging and foreign fungal plant pathogens. Screen germplasm for resistant sources to emerging and foreign fungal plant pathogens. Per PDRAM for Research on Emerging Fungal Plant Pathogens at Frederick, MD #R07 Objective 4: Develop rapid and reliable diagnostics for new and emerging foreign fungal plant pathogens that can be transferred to regulatory agencies and the private sector.
1b. Approach (from AD-416)
Utilize the Bio-Safety Level 3 Plant Pathogen Containment facilities to investigate and characterize virulence, genetic variability, epidemiology, host range, and survival of foreign and emerging fungal plant pathogens considered to be a threat to the U.S. Establish pathogen collections, compare exotic and endemic isolates for morphology, virulence, and genomics using a variety of scientific methods. Develop detection techniques using PCR and immunological methodologies. Investigate pathogen genomics and host plant resistance using classical and molecular approaches.
3. Progress Report
A proteomic analysis of appressoria, specialized structures required for penetration of soybean leaves by Phakopsora pachryhizi, was completed. A study to determine the relationship of plant and leaf age to susceptibility to soybean rust is in progress. Plants ranging in age from 24 to 78 days were inoculated, placed over night in dew chambers, and then transferred to the greenhouse for disease development. Numbers of lesions produced on each leaf were counted. To date, no significant differences in susceptibility have been detected in plants at ages ranging from 24 to 78 days. Likewise, four different age leaves on 50-day-old soybean plants did not show any significant differences in the number of lesions per leaf or per cm2 leaf area following inoculation with P. pachyrhizi. Studies to determine the effect of dew period, temperature and condial concentration of Magnaporthe oryzae on wheat infection are ongoing. In collaboration with ARS scientists at Stoneville, MS, rust resistant soybean germplasm identified in field trials in Paraguay was evaluated further in seedling inoculations with purified isolates of P. pachyrhizi at the FDWSRU. Using a virus-induced gene silencing (VIGS) approach, over 150 genes have been silenced in soybean plants that are resistant to specific isolates of P. pachyrizi that are mediated by the Rpp2 resistance gene. To date, eleven genes that contribute to Rpp2-mediated resistance have been identified, including genes that encode transcriptional regulatory proteins and biosynthetic enzymes. Downstream gene networks and metabolic pathways will be targeted next for VIGS. In collaboration with scientists at the University of Georgia, rust resistance in the soybean cultivar Hyuuga was further characterized by inoculating with P. pachyrhizi isolates at the FDWSRU. Results indicate that Hyuuga carries the Rpp3 gene and an additional gene located at the same locus where Rpp5 has been mapped. Vectors for silencing candidate genes at both the Rpp3 and Rpp5 loci are now being constructed. Reactions of 23 ancestral soybean lines were evaluated by 2 week-old soybean plants with a suspension of ground mycelium and sclerotia of Phoma glycinicola. Ancestral lines were compared with the susceptible cultivar Williams 82. Replicated data obtained for 14 of the ancestral lines indicated that no resistance to P. glycinicola was present. The 29 wheat cultivars identified in 2010 as having less than 10% M. grisea infection under greenhouse inoculation conditions were retested in 2011. Only 12 of the wheat cultivars consistently showed less than 10% M. grisea infection. These cultivars will be vernalized and field-tested at two sites in Bolivia and at two sites in Paraguay this fall. New isolates of M. oryzae have been obtained from collaborators in Bolivia. Winter wheat cultivars previous identified in this project as resistant to the M. oryzae isolate T-25 isolated in 1988 in Brazil, have been inoculated with one of the isolates acquired from Bolivia in 2011.
1. Diagnostic reagents for early detection of soybean rust. Phakopsora pachyrhizi, the causal agent of Asian soybean rust, has spread from Asia to Africa, South America and finally North America. Since arriving in the U.S. in 2004, the need to use fungicide for control has increased production costs to soybean producers in the Southeast. ARS researchers at Ft. Detrick, Maryland, have developed sensitive, specific antibody-based diagnostic reagents that can identify and diagnose the disease in soybean leaves before symptoms occur. The diagnostic antibodies have been licensed for production of kits for field detection of the pathogen. Early detection of soybean rust will allow soybean producers to reduce fungicide input costs leading to increased yield and profits.
2. Gene expression patterns in rust resistant soybeans. Soybean rust is a serious foliar disease caused by the pathogen Phakopsora pachyrhizi that can drastically reduce yields due to premature leaf drop and lower seed set and weight. ARS researchers at Ft. Detrick, MD, in collaboration with scientists at Iowa State University and the ARS Corn Insect and Crop Genetics Research Unit at Ames, Iowa, monitored the expression of approximately 37,000 genes in a soybean line containing the rust resistance gene Rpp3. Results showed a dramatic increase in gene expression at 12 and 48 hours after infection with P. pachyrhizi that correlated with the formation of specialized infection structures. The genes identified in this study are candidates to investigate for a functional role in resistance and will useful to government, academic and private sector researchers and breeders developing rust resistant soybean cultivars.
Kendrick, M.D., Harris, D.K., Ha, B., Hyten, D.L., Cregan, P.B., Frederick, R.D., Boema, H.R., Pedley, K.F. 2011. Identification of a second Asian soybean rust resistance gene in Hyuuga soybean. Phytopathology. 101:535-543.