Derailing Witchweed (Striga) Virulence to Achieve Durable and Broad-Spectrum Resistance
Steven RunoDepartment of Biochemistry and Biotechnology
Kenyatta [email protected]
1 AbstractCrop losses caused by parasitic plants of the genusStriga pose great danger to livelihoods of millions ofsmallholder farmers in Africa. The parasite attachesto host crops and siphon nutrients leading to severeretardation and crop death. Striga control is difficultbecause of the parasite’s ability to produce largeamounts of seeds that can stay dormant in soil fordecades only germinating in response to chemicalcues from the host. In recent years, breeding cropsfor host-based resistance has been prioritized.However, such programs have not taken into accountthe changes in Striga’s aggressiveness and severity– described as virulence. This is critical because aparasite’s virulence is an important determinant ofits potential to overcome host resistance andthereby expand its host and natural range. In thisproject, we firstly identified new sources of Strigaresistance from wild sorghum accessions. We thenused genomics tools – RNA sequencing to i) identifyStriga resistance genes in wild sorghum and ii)Striga virulence genes. Striga resistant wild sorghumgenotypes identified in the project are now availableintegrating Striga resistance in Kenya’s cerealfarming systems.
Fig 1. Striga hermonthica on maize. Striga infects allcultivated cereal crops including maize, millet,sorghum and rice.
2 Research plan2.1 HypothesisDomestication—the process of transforming wild species into elite cultivars—inevitably leads to decreased genetic diversity in the selectedcrops. In some cases, the lost genetic diversity may represent the organism's capacity to adopt changes, such as pathogen resistance.Logically, pathogen virulence exhibits itself differently in resistant and susceptible hosts.
3 Results 4 Impact
4.2 Genomic aided selection of Striga resistant cereals
6 Acknowledgement5 Dissemination
2.2 ApproachWe explored the resistance interactions between wildsorghum accessions and the parasitic plant Strigahermonthica at the parasite's center of origin innortheastern Africa. We showedremarkable Striga resistance in wild sorghumaccession – WSA1, WSA2, and WSE1races (Fig 2).Identified Striga resistance in wild sorghum makes itpossible to expand the genetic basis of cultivatedsorghum using genomics tools. We therefore useddualRNA sequencing (simultaneous sequencing of hostand parasite tissue) to compare differential geneexpression profile between wild sorghum (resistant)and cultivated (susceptible) sorghum. We furthercompared differential gene expression profile of Strigainfecting wild sorghum with that of Striga infectingcultivated sorghum . Finally, we developed a pipeline topredict molecules that potentially aid Striga toovercome host immune response (effectors).
3.1 Host resistance genesWe found that Striga induced a larger number of differentiallyexpressed genes in wild sorghum in comparison to cultivatedsorghum (Fig 3a, 3b and3c). The number of differentially expressedgenes in different hosts correlated with resistance level. Forexample, WSE1, WSA1 and WSA2 were the most resistant sorghumaccessions. These accessions also had the highest number ofdifferentially expressed genes (Fig 3c). This group of gensincluded: transporters, cell wall fortifying genes, as well as typicalresistance genes that act to protect host cells by degrading cell wallcomponents of pathogens (Fig 3d).
3.2 Striga virulence genesWe also observed that a larger number of genes were differentiallyexpressed in Striga infecting wild sorghum compared to Striga infectingcultivated sorghum. When we subjected the differentially expressedgenes in Striga to an analysis pipeline (Fig 3e, 3f and 3g) that is usedto predict molecules that aid pathogens to overcome host immuneresponse (effectors) i.e. sequences containing a signal peptide, acleavage site within the first 40 amino acids, with no trans-membranedomain outside of cleaved region and not targeted to the mitochondria,we obtained a rapporteur of candidate effectors. This list included cellwall degrading enzymes as well as some homologues of pathogenicavirulence/virulence genes (Fig 3h).
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To demonstrate and influence policy onconservation and utilization of wild sorghumgenetic resources, we set up a fielddemonstration site for Striga resistant wildsorghum at Alupe in Western Kenya (Fig 4).This field site was set up in a Striga endemicarea and shows the diversity of wild sorghumrelatives and their potential to elevate bioticand abiotic constrains of sorghumproduction.
Fig 4. Diverse sorghum genotypes that canact as reservoirs for disease resistancegenes in a demonstration field site in Alupe,Western Kenya. The field site also serves asa genebank for conservation.
4.1 Conservation and sustainable utilization of wild sorghum
We have described a platform for developing resistance in sorghum against Striga based on marker assisted selection of resistance from wildsorghum. This process will expand the genetic basis of cultivated sorghum to cope with evolving Striga virulence. Fig 5.
1. Runo S, Kuria, E. Habits of a Highly Successful Cereal Killer, Striga. PLoS Pathogens, 2018. 14(1): e1006731.
2. Mbuvi, D. A., Masiga, C. W., Kuria, E., Masanga, J., Wamalwa, M., Mohamed, A., Timko, MP, Runo, S. Novel Sources of Witchweed (Striga)Resistance from Wild Sorghum Accessions. Frontiers in Plant Science, 2017 8, 271.
The National Academies of Science (NAS) supported this research project under the Partnerships Enhanced Engagement in Research(PEER) program (contract number NAS Sub-Grant Award Letter Agreement Number PGA-2000003439). The Sub-Grant Agreement wasfunded under Prime Agreement Number AID-OAA-A-11-00012, which was entered into by and between the NAS and the United StatesAgency for International Development (USAID).
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Fig 2. Resistance response of wild sorghum to 3 ecotypes of S. hermonthica(Kibos, Alupe and Mbita). Resistant sorghum genotypes have significantly lessattachments while virulent parasite ecotypes cause more attachments.
Fig 3a to 3h. Gene expression profiling of Striga resistance genes(from wild sorghum) and virulence genes (from Striga)
