This paper presents a consolidated balance-sheet on the botanical and historical evidence for the role of the cultivated African rice species (Oryza glaberrima Steud) in West and Central Africa as well as its geographical distribution. Because Oryza glaberrima has survived without the help and interference from human, it has developed adaptive and protective mechanisms for resisting major biotic and abiotic stresses. Oryza glaberrima is also very plastic with two major ecotypes: the floating and the non floating, and presents natural hybridization and admixture with its wild parents and the Asian cultivated rice Oryza sativa. Several ecotypes showed good aptitude in response to climatic change effects such as drought, flood, pests and diseases. This review presents several phenotypic diversity aspects on O. glaberrima and highlights its ecological and genetic structuring as well as the management of its diversity. Even if the African rice has undergone less diversification than the Asian rice in their evolutionary process, it presents some interesting assets which are being suggested to exploit through varietal improvement programs.
Two Outstanding Oryza glaberrima (2n = 24, AA) varieties TOG5681 and TOG5674 were used as male donor parents with IR64, the high-yielding improved Asian rice variety used as recurrent female parent by the International Rice Research Institute (IRRI) to develop 18 BC 3 F 1 interspecific lowland rice progenies. The proportion of parental genomic contribution and the extent of genetic differences among these lines were assessing using 36 microsatellites markers. The average genomic contribution of the donor TOG5681 and that of the recurrent IR64 within their 12 interspecific lines derived from IR64xTOG5681 cross were estimated to 13.2% and 79.8% respectively. Using 33 out of the 36 SSR markers, the average genome introgression rate of TOG5674 and that of IR64 within their 6 progenies were estimated to 8.7% and 85.5% respectively. In addition, heterozygosity and non-parental alleles were also identified. Clustering analysis technique using NTSYS classified the progenies into six groups and group five is closely related to IR 64.
Rice cultivation in Africa suffers from various biotic and abiotic constraints. The African rice, Oryza glaberrima, is a potential source of useful genes for resistance to a range of these stresses. The development of interspecific varieties based on crosses between the high yielding cultivated Asian rice species (Oryza sativa) and O. glaberrima showed some gaps in the resistance compared to O. glaberrima. In order to valorize its genetic potential, intraspecific lines were developed using three O. glaberrima lines Tog5681, Tog5672 and Tog7291 as parents. Phenotypic evaluation of the progeny (total of 55 F 6 lines) and checks (O. glaberrima and O. sativa) using 18 quantitative characters, showed a higher degree of variation in plant height, leaf dimensions, panicle fertility and grain weight. Based on the discriminating characters proposed here for further exploitation, intraspecific lines were grouped into five clusters, two of which (G4 and G5) contained the best lines for vegetative (G4) and productive (G5) characters.
Rice yellow mottle virus (RYMV) is the most harmful virus that affects irrigated and lowland rice in Africa. The RBe24 isolate of the virus is the most pathogenic strain in Benin. A total of 79 genotypes including susceptible IR64 (Oryza sativa) and the resistant TOG5681 (O. glaberrima) as checks were screened for their reactions to RBe24 isolate of RYMV and the effects of silicon on the response of host plants to the virus investigated. The experiment was a three-factor factorial consisting of genotypes, inoculation level (inoculated vs. non-inoculated), and silicon dose (0, 5, and 10 g/plant) applied as CaSiO3 with two replications and carried out twice in the screen house. Significant differences were observed among the rice genotypes. Fifteen highly resistant and eight resistant genotypes were identified, and these were mainly O. glaberrima. Silicon application did not affect disease incidence and severity at 21 and 42 days after inoculation (DAI); it, however, significantly increased plant height of inoculated (3.6% for 5 g CaSiO3/plant and 6.3% for 10 g CaSiO3/plant) and non-inoculated (1.9% for 5 g CaSiO3/plant and 4.9% for 10 g CaSiO3/plant) plants at 42 DAI, with a reduction in the number of tillers (12.3% for both 5 and 10 g CaSiO3/plant) and leaves (26.8% for 5 g CaSiO3/plant and 28% for 10 g CaSiO3/plant) under both inoculation treatments. Our results confirm O. glaberrima germplasm as an important source of resistance to RYMV, and critical in developing a comprehensive strategy for the control of RYMV in West Africa.
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