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Partnerships for enhanced engagement in research (PEER) SCIENCE
Cycle 2 (2012 Deadline)

Derailing witchweed (Striga) virulence in rice to achieve durable and broad-spectrum resistance

PI: Steven Runo (Kenyatta University)
U.S. Partner: Michael P. Timko (University of Virginia)
Project Dates: August 2013 to July 2015

 Ethiopia Partnership Picture 1
Striga infests two-thirds of arable African land, and constitutes the biggest single biological cause of crop damage in terms of grain yield loss, worth $US 7 billion annually (Photo courtesy Dr. Runo).

Striga spp. are successful parasitic plants that are notoriously difficult to control mainly because the mechanisms of the biological processes underpinning host-parasite compatibility are poorly understood. Striga affects plant growth very quickly after attaching to the host roots. Within 2-4 days of attachment the crop plants are already visibly stunted. Although the mechanism underlying this early negative effect on crop growth is presently unknown, it has implications for control of the parasite, as control measures need to act before or very shortly after attachment of the parasite to the host. It is now emerging that Striga, like other plant pathogens, produces an array of virulence factors (effectors) that may be allowing the parasite to circumvent and subdue the host defense. The long-term goal of the researchers conducting this project is to identify mechanisms controlling release of these virulence factors as a first step toward developing breeding strategies that can be used to build durable resistance to Striga hosts. The specific aims of the current project are designed to provide a comprehensive assessment of the mutations or polymorphisms in Striga effectors as well as their effect in plant cells. The project should result in the identification of various races of Striga for effector genes and their role in virulence, which is of fundamental importance to understanding the molecular nature of the plant-plant resistance interactions.
 
An output of this project will be identification of multiple factors (effectors) that help Striga evade resistance by its host. An additional output will be quantification of how these factors are able to change with time and acquire ability to invade new hosts. It is hoped that the specificity of different Striga virulence races (ecotypes) to different host cultivars will be identified. This knowledge can then be directly applied to breeding of new cultivars resistant to Striga, through gene pyramiding. Because yields of some of the most important crops in Africa, including rice, corn, millet, and sorghum, are being reduced due to the impact of Striga, the results of this project could have a significant impact on agricultural productivity in this region and others where the parasitic plant is a problem.
 
Summary of Recent Activities
 
In the first quarter of 2014, Dr. Runo and his colleagues on the project have been exploring the role of Striga effector proteins in being able to overcome resistance by host plants such as maize, rice, and sorghum. Because of parallels between fungal infections and the way Striga is able to infect crops, the project team tried using known fungal effectors to predict gene orthologs in Striga DNA that could be playing a similar pathogenesis role. The researchers have found three distinct families of genes to study, and designed primers for their amplification. They have induced a polymerase chain reaction (PCR) to clone the genes and have tested two sets of primers.  
 
The next phase of the project is twofold. The project team is looking to determine the host target of effector molecules that are secreted by the Striga weed. This will be done by screening putative host target plants for the Avr1 gene by functional analysis. The team is also looking to determine the diversity of effector molecules; that is, the mechanism by which Striga is able to overcome resistance by several species of plants in the wild and not just one. To this end, the team collected Striga plants from various locations and designed primers targeted at their effector molecules. To continue moving the project forward, Dr. Runo hopes to make a six-month research and training visit to his U.S. partner at the University of Virginia beginning in late May.
 
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