Rice yellow mottle virus resistant Oryza glaberrima germplasm
Rice yellow mottle virus (RYMV) is an endemic pathogen in Africa and affect rice production in the continent. The extent of yield losses due to RYMV varied between 10 and 100% depending on rice varieties, time of infection, cropping system, vector abundance, and isolates pathogenicity. A partial resistance, characterized by a delay in symptom expression and virus accumulation, has been reported in Oryza sativa spp. japonica genotypes, which involves several quantitative trait loci (QTLs) High resistance, characterized by a lack of symptoms, undetectable virus content and no yield losses upon field infection, has only reported in two O. sativa spp. indica genotypes and a few O. glaberrima rice accessions. The high level of RYMV resistance is conferred by three genes known as RYMV1 (Ndjiondjop et al. 2007), RYMV2 (Thiémélé et al. 2010), and RYMV3 (Pidon et al. 2017). RYMV1 is the first known resistant gene, which has four alleles conferring resistance in O. sativa (rymv1-2) and O. glaberrima (rymv1-3, rymv1-4 and rymv1-5) genotypes.
Thiémélé and colleagues (Thiémélé et al. 2010) evaluated 337 O. glaberrima accessions using a less virulent isolate from Benin (B27) under screenhouse conditions at the AfricaRice station, Cotonou, Benin, and a highly aggressive isolate from Burkina Faso (BF1) under glasshouse conditions at the IRD laboratory, Montpellier, France. Using disease symptoms (the area under the symptom progression curve) from) and virus content estimated via enzyme-linked immunosorbent assay (ELISA), the authors identified 26 accessions that possess Rymv1-1-Og (15 accessions), rymv1-3 (6 accessions), rymv1-4 (3 accessions) and rymv1-5 (1 accession) recessive resistance alleles. Since rymv1-1 allele from RYMV1 gene causes susceptibility, RYMV2 and/or RYMV3 genes may be the source of resistance, which remains to be done. The latter was confirmed through an allelism test involving one of those accessions that revealed RYMV2 gene as a second major resistance gene. Markers specific to the different alleles of the RYMV1 gene are available for marker-assisted introgression of the resistant alleles (Thiémélé et al. 2010).
Agnoun and colleagues screened 160 O. glaberrima landraces and 55 intraspecific genotypes along with a resistant and susceptible checks against five RYMV isolates collected in Benin. The screening was done under screenhouse conditions at the AfricaRice research station, Cotonu, Benin. Using mean disease severity as selection criterion, two O. glaberrima landraces and 16 intraspecific genotypes were found to be highly resistant to all five RYMV isolates (Agnoun et al. 2019). Here, we compiled list of the most promising RYMV resistant O. glaberrima landraces (accessions) which could be used as donors in developing RYMV resistant rice germplasm using marker-assisted selection.
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