Asian soybean rust (ASR) is caused by Phakopsora pachyrhizi, which is an obligate biotrophic fungus. The primary source of inoculum is urediniospores produced on infected soybeans or on alternative legume hosts. This disease has been particularly destructive in Brazil, where in the second year the disease was present, it caused an estimated US$2 billion in economic losses, and it threatens soybean production in the United States. So far, seven major SBR Resistance (R) loci (named Rpp1-Rpp1b - Rpp7) have been identified. Their effectiveness is limited by virulent ASR isolates that are able to overcome them. For this reason, the use of genetic resistance has not yet been successful, and the major control method is the timely application of fungicides. Rpp1 confers an immune phenotype. The other R genes confer limited fungal growth and the formation of reddish-brown lesions. Compatible interactions are typically characterized by tan-colored lesions with fully sporulating uredinia.
To gain better insight into the molecular basis of the interactions of soybean and ASR, our group, in collaboration with researchers at Embrapa soja and the USDA, has carried out extensive expression profiling experiments involving the Rpp2, 3, and 4 resistance genes. These experiments have shown that ASR causes a strong defense-like response within the first 12 hours after inoculation in both resistant and susceptible interactions followed by a period where the fungus appears to grow in the plant almost undetected. Defense-like responses occur again in both resistant and susceptible plants, but they happen earlier and with a higher magnitude in resistant genotypes. Among the genes showing this profile are a number of transcription factors and kinases that are excellent candidates for regulators of soybean defense networks. These candidate genes will be tested in the VIGS system for their contributions to soybean defenses to ASR as well as other important pathogens.
We also have an interest in identifying the effector molecules that are secreted by the fungus that trigger the soybean responses. These effector proteins may be secreted from a specialized feeding structure called the haustorium. The characterization of the messenger RNA transcripts that are produced in the haustorium of ASR is expected to lead to the discovery of the proteins that are secreted from the fungus into the plant, where they modify the functions of the plant cell to create an environment favorable for infection and nutrient acquisition.
Source : iastate.edu