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 January 2016
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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
During the third quarter, the team undertook the following activities:
Identification of candidate Striga virulence genes: They used two approaches to home in on Striga genes that may be helping Striga overcome resistance from host: One was a bioinformatics pipeline to identify a suite of candidate virulence genes. They then used genomics to profile a subset of genes that were differentially expressed upon infection to susceptible and resistant host. They identified 5 candidate genes that were involved in cell wall modification of the host (degradation).
Identification of differentially expressed genes for Striga resisistance in sorghum: In their genomic profiling of candidate Striga resistance genes in host sorghum, they observed that a large number of genes were differentially expressed in resistant sorghum compared to susceptible sorghum. Of interest were 3 genes (Chitinases) that they think degrade the cell wall of the parasite.
Determination of pre-germination resistance of sorghum: They also obtained data on pre-germination resistance of 7 varieties of sorghum. The data mirrors their now completed post-germination resistance. They observed that wild soghum cultivars produce less Striga germination stimulant compared to cultivated sorghum.
Completion of sorghum resistance screening: Their Striga resistance screening of the various cultivars of sorghum is complete. They are currently analyzing the data.