Rice yellow mottle virus (RYMV) is responsible of a major disease of rice in Africa. A few Oryza glaberrima accessions and only one O. sativa variety named Gigante were found to be highly resistant to RYMV (Thottappilly and Rossel 1993, Ndjiondjop et al. 1999). In Gigante and in the O. glaberrima variety Tog5681, resistance is controlled by the recessive gene Rymv1 (Ndjiondjop et al. 1999) which maps on chromosome 4 (Albar et al. 2003) and encodes the translation initiation factor eIF(iso)4G (Albar et al. 2006). Three different alleles were identified in the O. sativa resistant variety Gigante (allele Rymv1-2), and the O. glaberrima accessions, Tog5681 (Rymv1-3) and Tog5672 (Rymv1-4). Compared to susceptible varieties (allele Rymv1-1), they are characterized by amino-acid substitutions or a small deletion in the conserved domain of the gene. We report on the genetic basis of resistance of Bekarosaka, a local variety from Madagascar, recently identified as highly resistant to RYMV.

Phenotypic and molecular analysis indicated that Bekarosaka is an indica variety. Microsatellite markers RM1, RM5, RM11, RM18, RM19 and RM167 (Wu and Tanksley 1993, Panaud et al. 1996), relevant to discriminate between indica and japonica varieties, were used to compare Bekarosaka to representative japonica, indica and O. glaberrima varieties. Marker amplification and revelation was performed as described in Albar et al. (2003). Bekarosaka showed alleles usually found in indica varieties but appeared clearly distinct from Gigante (Fig. 1).

The genetic basis of resistance was studied in two F2 populations developed from the IR64 x Bekarosaka and Gigante x Bekarosaka crosses. Symptoms were observed after mechanical inoculation of two-week old plants with a severe RYMV isolate, as described in Albar et al. (2003). The segregation of resistance in the IR64 x Bekarosaka population revealed nine resistant and 38 susceptible plants, showing monogenic segregation (1:3 ratio ; Chi 2 = 0.86), in agreement with the hypothesis that resistance is recessive and monogenic. Molecular analysis of the population with RM252 and RM273 markers, flanking Rymv1 (Albar et al. 2003), supported the co-localisation of the resistance gene of Bekarosaka with Rymv1 (Fig. 2). In addition, the three F1 hybrids and 30 F2 plants derived from the Gigante x Bekarosaka cross were all resistant, confirming that the resistance genes of Bekarosaka and Gigante were allelic. Thus, results of the genetic and mapping analyses indicated that the same gene, Rymv1, controls the resistance of Bekarosaka, Gigante and Tog5681.

Rymv1 allele in the Bekarosaka variety was characterized by sequencing the conserved part of the gene, which is mutated in the previously identified resistant varieties. Bekarosaka has the same mutation than Gigante (Fig. 3), which strongly suggested that Bekarosaka and Gigante possess the same Rymv1-2 allele. Thus, the resistance of Bekarosaka is controlled by Rymv1, which is yet the only gene showing high level of resistance to RYMV. Moreover, Bekarosaka and Gigante probably share the same resistance allele, Rymv1-2. This result underlines the key role of Rymv1, encoding the translation initiation factor eIF(iso)4G, in rice/RYMV interactions. Further investigation on the diversity of Rymv1 in genetic resources of rice is suggested.

References

Albar L., M.-N. Ndjiondjop, Z. Esshak, A. Berger, A. Pinel, M. Jones and A. Ghesquière, 2003. Fine genetic mapping of a gene required for Rice yellow mottle virus cell-to-cell movement. Theor. Appl. Genet. 107: 371-378.

Albar L., M. Bangratz-Reyser, E. Hèbrard, M.-N. Ndjiondjop, M. Jones and A. Ghesquière, 2006. Mutations in the eIF(iso)4G translation initiation factor confer high resistance of rice to Rice yellow mottle virus. Plant J. 47: 417-426.

Ndjiondjop M.-N., L. Albar, D. Fargette, C. Fauquet and A. Ghesquière, 1999. The genetic basis of high resistance to Rice yellow mottle virus (RYMV) in cultivars of two cultivated rice species. Plant Dis. 83: 931-935.

Panaud O., X. Chen and S. R. McCouch, 1996. Development of microsatellite markers and characterization of simple sequence length polymorphism (SSLP) in rice (Oryza sativa L.). Mol. Gen. Genet. 252: 597-607.

Thottappilly G. and H. W. Rossel, 1993. Evaluation of resistance to Rice yellow mottle virus in Oryza species. Indian J. Virol. 9: 65-73.

Wu K. S. and S. D. Tanksley, 1993. Abundance, polymorphism and genetic mapping of microsatellites in rice. Mol. Gen. Genet. 241: 225-235.