ARS | Publication request: Defense-related gene expression in soybean leaves and seeds inoculated with Cercospora kikuchii and Diaporthe phaseolorum var. meriodinales

Submitted to: Physiological and Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 31, 2010
Publication Date: September 15, 2010
Citation: Upchurch, R.G. and Ramirez, M.E. 2010. Defense-related gene expression in soybean leaves and seeds inoculated with Cercospora kikuchii and Diaporthe phaseolorum var. meriodinales. Physiological and Molecular Plant Pathology. 75:64-70.

Interpretive Summary: Cercospora kikuchii is the causal agent of Cercospora leaf blight (CLB) and purple seed stain (PSS) in soybean. Diaporthe phaseolorum var. meriodinales is the causal agent of southern stem canker (SSC) and occasionally, Phomopsis seed decay (PSD) in soybean. Even though resistance to CLB, PSS, SSC, and PSD is known and in some situations deployed, this resistance is largely uncharacterized. Given the periodic reemergence of these diseases in U.S. soybean growing areas, concern remains for the durability of this resistance. Thus, research to molecularly characterize soybean resistance to these pathogens may assist breeders in identifying and incorporating improved resistance into adapted germplasm. We compared defense gene expression and pathogen growth in pathogen inoculated soybean leaves and seeds. In leaves inoculated with C. kikuchii or Diaporthe phaseolorum, defense genes PR3 and PR10 were activated by both pathogens and pathogen growth declined during the 48 hour post inoculation period (hpi). In seeds inoculated with either pathogen, other defense genes were activated but not PR3 and PR10, and pathogen growth increased over the 48 hpi period. Results suggest that simultaneous activation of PR3 and PR10 is associated with growth inhibition and basal resistance to C. kikuchii and D. phaseolorum in leaves, but this did not occur in seeds and resistance was not expressed in the cultivar used.

Technical Abstract: We characterized basal resistance in soybean inoculated with Cercospora kikuchii and Diaporthe phaseolorum var. meriodinales by comparing defense-related gene expression and pathogen growth in leaves and detached seeds. In leaves inoculated with C. kikuchii, 8/13 defense-related genes were upregulated; five were pathogenicity-related (PR) genes, one a phenylpropanoid biosynthesis gene, and two oxylipin biosynthesis genes. In leaves inoculated with D. phaseolorum, 3/13 defense-related genes were upregulated and all were PRs. Two genes, PR3 and PR10 were upregulated by both pathogens. In leaves, pathogen growth declined during the 48 h-post inoculation period. In seeds inoculated with C. kikuchii, 7/13 genes were upregulated; three were PRs, two phenylpropanoid biosynthesis, and two oxylipin biosynthesis genes. In seeds inoculated by D. phaseolorum, 6/13 genes were upregulated; three were PRs, two phenylpropanoid biosynthesis, and one an oxylipin biosynthesis gene. In seeds five genes, PR1, matrix metalloproteinase (MMP2), chalcone synthase (CHS), phenylalanine ammonia-lyase (PAL), and allene oxide synthase (AOS) were upregulated by both pathogens; however, neither pathogen simultaneously upregulated PR3 and PR10. Despite the upregulation of these five genes, including a remarkably high upregulation of MMP2, growth of the pathogens increased in seeds over the 48 hpi period. Results suggest that simultaneous upregulation of PR3 and PR10 is associated with growth inhibition and basal resistance to C. kikuchii and D. phaseolorum in leaves, but this did not occur at a sufficient level for the expression of resistance in seeds.