The prospects for rice self-sufficiency in sub-Saharan Africa are being subverted by abiotic and biotic factors, including plant diseases. Several important rice diseases have emerged recently in SSA. The most notorious of these has been Rice yellow mottle virus (RYMV), which is restricted to only Africa. In this review, we present an update of RYMV genetics in relation to virus-rice interactions and discuss some interventions focusing on natural genetic resistance. In addition, we highlight the implications of using known genetic resistance in breeding for durable resistance to the virus in SSA.
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Cereal crops account for more than 60% of food intake and will continue to contribute significantly to global food security. This contribution is reflected in projected increases in the consumption of wheat, corn, and rice by 2050. Of these three main crops, rice is the most important food crop for people in low- and lower-middle-income countries. In sub-Saharan Africa (SSA), where most of the population falls within the category of low incomes, the demand for rice has increased considerably since 1982.
This increased demand is due to improving economies, rising household incomes, and growing population and urbanization that have led to increased consumption of rice as a major staple. To meet the increased demand, the region is continuously pushing for coordinated efforts to increase rice self-sufficiency in several rice-producing countries.
However, the prospects for rice self-sufficiency are being subverted by abiotic and biotic factors, including plant diseases. Several important rice diseases have emerged recently in SSA. The most notorious of these has been rice yellow mottle virus (RYMV), which is restricted to only Africa.
RYMV was first identified in East Africa in 1966, from where it is believed to have spatially spread westward into West Africa. The virus has now spread to all rice-growing countries in the region, and as such is described as an emerging disease
Clear symptoms of RYMV include severe leaf mottling, yellow-green streaking, decreased tillering, and stunting of plants during the vegetative stage. Reproductive stage symptoms include poor emergence of panicles and panicle sterility. RYMV yield losses range from 10% to 100%, depending on the time of infection, ecology, viral strain, and the rice genotype.
Plants that display severe symptoms during seedling and early vegetative stages often result in plant death. In lowland ecologies, susceptible genotypes can be severely damaged in a short period of time during periods of intense disease activity.
This has been observed in susceptible varieties grown in West African countries. For example, in Nigeria and Côte d’Ivoire, yield losses above of 90% have been reported in susceptible cultivars, while in Sierra Leone, losses of 82% were reported on varieties PN 623-3, TOX 516-12-SLR, and ROK 3. Most recently in Burkina Faso, losses of 84% were recorded on popular varieties, FKR56N, FKR62N, and TS2.
The response from the scientific community to the RYMV threat has been positive and has made progress. Yield losses have been addressed through adapting RYMV control measures to local situations. This includes crop improvement toward local production systems and utilization of specific agronomic practices including the use of varietal mixtures and pesticide sprays, early planting, destruction of previous plants, ratoons and volunteer crops, removal of infected plants, crop rotation, and optimum fertilizer application. These practices aim at disrupting the life cycle of the disease and improving crop health.
However, their use is limited and still ineffective, especially when the disease occurs in epidemic proportions. Genetic resistance is the best feasible option for economical and sustainable long-term RYMV management. Recent advances in genetics and molecular biology have contributed to the identification of RYMV resistance genes and quantitative trait loci that have been mapped and some cloned from O. sativa and O. glaberrima.
The virus evolves rapidly, and resistance-breaking variants have been observed across SSA. So much work remains to be done to identify the best strategies to limit the prevalence and yield losses caused by RYMV.
In this review, we present an update of RYMV genetics in relation to virus-rice interactions and discuss some interventions focusing on natural genetic resistance. In addition, we highlight the implications of using known genetic resistance in breeding for durable resistance to the virus in SSA.
RYMV continues to cause losses to rice production in SSA due to lack of resistant elite cultivars. Considering the high adaptive potential of this virus and frequent resistance breakdown in rice cultivars, breeding for resistance and its durability should be given top priority in SSA. Encouraging news comes from recent knowledge on rice-RYMV interactions, which has enabled identification and isolation of three major resistance genes (RYMV1, RYMV2, and RYMV3), and the subsequent definition of geographic specificities of these genes and RYMV strains.
There is need to build on these successes by further identifying the genomic localization of desired SNPs linked to the major resistance genes for use in genomic-assisted breeding. The speedy development of homozygous lines through double haploid breeding when coupled with MAS would be more effective for developing and deployment of durable RYMV resistant varieties.
Parallel efforts are needed to identify additional genes and extensively screen host-virus protein interactors to identify and validate additional host factors that assist or suppress the virus. With these interaction factors identified, a reverse genetics approach could be used to identify novel host S genes that could be modified by genome editing to impair susceptibility. These short and long-term strategies will provide immediate products and build a large germplasm base and knowledge necessary to respond to future RYMV attacks.
Read the study:
Odongo P, Onaga G, Oliva R, Natsuaki K, Alicai T, Geuten K. (2021) Insights into natural genetic resistance to Rice Yellow Mottle Virus and implications on breeding for durable resistance. Frontiers in Plant Science Vol. 12
