S T R I G A :
Search for a Long-Term Solution
Agronomists classify it as the parasitic weed Striga hermonthica. Economists report that it inflicts yield losses ranging from 20 to 80%, and that across sub-Saharan Africa it robs US$ 1 billion in lost productivity from more than 100 million people, most living on subsistence or smallholder homesteads. Maize farmers in Western Kenya simply and distainfully curse it as "witchweed." One thing they all agree on is that this scourge, which exhibits a remarkable resistance to conventional controls, is on the rise—both in terms of area affected and intensity of infestation.
CIMMYT's Applied Biotechnology Center (ABC), together with the Department of Animal and Plant Sciences at the University of Sheffield, is looking for solutions. Funded by the Rockefeller Foundation, scientists from the two institutions are looking for unconventional sources of resistance to Striga. Leading CIMMYT's efforts is molecular geneticist/physiologist Sarah Hearne.
Since 1999, Hearne says, CIMMYT has screened accessions of Tripsacum and teosinte (wild relatives of maize) for resistance to Striga. The screens revealed that apomictic tetraploid Tripsacum is very resistant. Results with teosinte were not as encouraging, although three accessions showed some potential. Jane Ininda, of the Kenya Agricultural Research Institute (KARI), will screen many more teosinte accessions, representing a full range of diversity, later in 2002.
Maize-Tripsacum hybrids, generated by the CIMMYT-Institut de Recherche pour le Développement (IRD) apomixis project, have also stirred a lot of interest (see "Apomixis: Why Is It Taking So Long?). These plants have various combinations of maize and Tripsacum chromosomes. Hearne began screening these hybrids in early 2002, and 60 are being grown and infected with Striga in root observation chambers at Sheffield to evaluate resistance and observe the form it takes. Once resistant lines are identified, Hearne says, it should be possible to determine the chromosomes responsible for Striga resistance by comparing the levels of resistance in the lines and the chromosomes they are known to carry. "If the trait is located on a lot of chromosomes," she says, "it's going to be very difficult to introduce the trait using conventional breeding—it probably won't happen. But if it is located on a single chromosome, we could potentially move the gene or genes controlling the resistance into maize."
Hearne is also trying to identify the basis of Striga resistance in Tripsacum by comparing differences in gene and protein expression between resistant Tripsacum and susceptible maize. The protein research is conducted in collaboration with Christophe Brugidou of IRD. If researchers identify candidate gene(s) responsible for resistance, it may be possible to introduce resistance into maize using a transgenic approach.
Finally, Hearne is looking at the use of genes tagged by Mutator, a so-called transposable element that jumps into genes that control plant functions and turns them off. "For instance, if you had a gene for red pigmentation in the grain and the Mutator jumped into that gene, you would cease to have that pigmentation. We're applying that same principle in the search for Striga resistance, in that there may be a regulatory gene that prevents maize from activating defenses against the weed."
During 199899, about 8,000 Mutator-tagged maize lines were screened under controlled experimental conditions. About 80 showed some level of resistance. Since then, that group has been narrowed to about 20 entries, says Hearne. One looks particularly promising. Once the gene is characterized, researchers will attempt to cross it into adapted material and check its performance.
Although it will be a few years before results of this work reaches farmers' fields, Hearne is optimistic that new knowledge about plant responses to Striga, and some of the research products, will provide long-lasting resistance to the parasitic weed and grant relief to many generations of African farmers.
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For more information:
Sarah Hearne (s.hearne@cgiar.org)
August, 2004