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Published on April 6, 2007; 10.1104/pp.107.097865

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Received February 12, 2007
Accepted March 28, 2007

RNAi Silencing of Genes for Elicitation or Biosynthesis of 5-deoxyisoflavonoids Suppresses Race-specific Resistance and HR Cell Death in Phytophthora sojae infected tissues

Terrence L. Graham *, Madge Y. Graham , Senthil Subramanian , and Oliver Yu

Department of Plant Pathology and Plant Molecular Biology and Biotechnology Program, Ohio State University, Columbus, Ohio, USA 43210; Donald Danforth Plant Science Center, St. Louis, MO 63132

* Corresponding author; email: graham.1{at}osu.edu.

Isoflavonoids are thought to play an important role in soybean (Glycine max) resistance to Phytophthora sojae. This was addressed by silencing two genes for their biosynthesis and a third gene controlling their elicitation. Silencing of genes for isoflavone synthase (IFS) or chalcone reductase (CHR) was achieved in soybean roots through an Agrobacterium rhizogenes mediated RNAi approach. Effectiveness of silencing was followed both by qRT-PCR and HPLC analyses. Silencing either IFS or CHR led to a breakdown of Rps-mediated resistance to race 1 of Phytophthora sojae in cultivars W79 (Rps 1c) or W82 (Rps 1k). Loss of resistance was accompanied by suppression of hypersensitive (HR) cell death in both cultivars and suppression of cell-death-associated activation of hydrogen peroxide and peroxidase. The various results suggest that the 5-deoxyisoflavonoids play a critical role in the establishment of cell death and race-specific resistance. The P. sojae cell wall glucan elicitor (WGE), a potent elicitor of 5-deoxyisoflavonoids, triggered a cell death response in roots that was also suppressed by silencing either CHR or IFS. Furthermore, silencing of the elicitor-releasing endoglucanase, PR-2, led to a loss of HR cell death and race-specific resistance to P. sojae and also to a loss of isoflavone and cell death responses to WGE. Taken together, these results suggest that in situ release of active fragments from a general resistance elicitor (PAMP) is necessary for HR cell death in soybean roots carrying resistance genes at the Rps 1 locus, and that this cell death response is mediated through accumulations of the 5-deoxyisoflavones.




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