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United States Department of Agriculture

Agricultural Research Service

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Location: Foreign Disease-weed Science Research

2009 Annual Report

1a. Objectives (from AD-416)
The objective of the proposed research that will be conducted at the FDWSRU is to utilize high throughput virus-induced gene silencing (VIGS) assays to investigate the role of genes necessary for resistance towards Asian Soybean Rust (ASR).

1b. Approach (from AD-416)
Genes targeted for silencing will be selected based upon information garnered from soybean gene expression studies, resistance gene mapping studies, and soybean orthologs of key defense regulators in Arabidopsis. Using the VIGS vectors these candidate genes will be silenced, or "turned off", in soybean plants that display resistance towards select isolates of ASR. Silenced plants will then be evaluated for a breakdown of resistance by challenging the plants with the pathogen and scoring for the development of disease.

3. Progress Report
Asian Soybean Rust (ASR) is an aggressive pathogen that occurs in all major soybean-producing countries and has caused major yield losses in severely affected areas, resulting in billions of dollars lost. In 2004, the disease was detected in the continental U.S., and is considered to be a major threat to U.S. agriculture and trade. Although soybean is the second most economically important crop species in the U.S., very little information is available on the specific genes that regulate soybean defenses. Five resistance genes (Rpp1, Rpp2, Rpp3, Rpp4, and Rpp5) have been described that recognize specific isolates of the soybean rust pathogen, Phakopsora pachyrhizi. The soybean cultivars PI200492 and PI230970 contain either Rpp1 or Rpp2, respectively, and both confer resistance towards P. pachyrhizi isolate LA04-1. Rpp1-mediated resistance is characterized by a lack of visible symptoms whereas the Rpp2-mediated resistance response results in visible reddish-brown lesions on the surface of the leaves when challenged with the pathogen. The goal of this research is to identify soybean genes that contribute towards resistance in plants that harbor either the Rpp1 or Rpp2 resistance genes using Virus-Induced Gene Silencing (VIGS). This system, based upon the Bean Pod Mottle Virus (BPMV), allows for the silencing (suppression) of individual genes. The approach involves infecting young plants with a BPMV derivative that contains a short sequence of the targeted gene. As the plant grows, BPMV spreads and natural defense mechanisms of the plant directed at suppressing the replication of the virus also result in the specific degradation of the plant mRNAs of the targeted gene. Genes that have a demonstrated role in defense towards pathogens in other plant systems will be targeted for silencing in plants containing either Rpp1 or Rpp2. The silenced plants will then be screened for a loss of resistance using P. pachyrhizi isolate LA04-1 and observing visible ASR symptoms. To date, approximately 150 individual soybean genes have been tested and several have resulted in a loss of resistance. This project was monitored by meeting with collaborators semiannually, and by numerous e-mails and telephone calls to plan activities and discuss data.

4. Accomplishments

Last Modified: 06/22/2017
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