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Functional Analysis of the Asian Soybean Rust Resistance Pathway Mediated by Rpp2

¹Foreign Disease-Weed Science Research Unit, United States Department of Agriculture–Agricultural Research Service (USDA-ARS), 1301 Ditto Avenue, Ft. Detrick, MD 21702, U.S.A.; ²Department of Plant Pathology, Iowa State University, Ames 50011, U.S.A.; ³Corn Insects and Crop Genetics Research Unit, USDA-ARS, Ames, IA 50011, U.S.A.; ⁴Department of Agronomy and ⁵Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames

Asian soybean rust is an aggressive foliar disease caused by the obligate biotrophic fungus Phakopsora pachyrhizi. On susceptible plants, the pathogen penetrates and colonizes leaf tissue, resulting in the formation of necrotic lesions and the development of numerous uredinia. The soybean Rpp2 gene confers resistance to specific isolates of P. pachyrhizi. Rpp2-mediated resistance limits the growth of the pathogen and is characterized by the formation of reddish-brown lesions and few uredinia. Using virus-induced gene silencing, we screened 140 candidate genes to identify those that play a role in Rpp2 resistance toward P. pachyrhizi. Candidate genes included putative orthologs to known defense-signaling genes, transcription factors, and genes previously found to be upregulated during the Rpp2 resistance response. We identified 11 genes that compromised Rpp2-mediated resistance when silenced, including GmEDS1, GmNPR1, GmPAD4, GmPAL1, five predicted transcription factors, an O-methyl transferase, and a cytochrome P450 monooxygenase. Together, our results provide new insight into the signaling and biochemical pathways required for resistance against P. pachyrhizi.