Parallel analysis of Asian soybean rust variability, candidate soybean resistance genes and soybean rust effectors
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Obligate basdiomycete Phakopsora pachyrhizi, is the causal agent of Asian soybean rust (ASR) and is one of the most destructive foliar soybean diseases. It is an exotic pathogen that first arrived in the United States mainland in 2004, and has the potential to cause severe yield loss. ASR is currently managed by fungicides. However, fungicide-resistant P. pachyrhizi are inevitable and the economic cost of continued fungicide use is not practical. Host resistance is therefore, considered to be the most economical and practical method for controlling ASR. To date, traditional breeding strategies generate soybean cultivars that are resistant to particular races of ASR. Once deployed, resistance is soon overcome by the ever changing pathogen. It is therefore necessary to characterize ASR pathogen variability and to define the genetics of resistance to ASR in soybean. In this study, eight P. pachyrhizi Georgia field isolates were characterized and grouped into three different isolates based on phenotype when inoculated on to a set of soybean differentials. Population genetic analysis of each ASR field isolate was also performed and demonstrated that the Georgia field isolates were diverse and were a mixture of genotypes from different countries and states boarding Georgia. One predominate isolate was identified in the Georgia fields isolates and one resistant Japanese cultivar was resistant to all of the Georgia field isolates. RNA sequence analysis was performed on the predominate ASR isolate and a resistant cultivar to simultaneously identify soybean defense-related genes and ASR effector genes expressed during an incompatible interaction in the host and the pathogen. Nineteen NBS-LRR resistance genes, 24 GmWRKY genes, and 11 MYB genes were differentially expressed (> 2 fold change) in the compatible and incompatible interactions. A total of 1,786 putative fungal effector genes were identified in the pathogen. Twenty-six of these genes were similar to known secreted protein genes in Melampsora Larici-populina. These findings will have major implications for the characterization of resistance mechanisms that may be more durable and provide new sources of resistance to assist with the development of novel strategies to control ASR.
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