Cassava mosaic geminiviruses (CMGs) are implicated in cassava mosaic disease (CMD), the main constraint to cassava production in Africa. Here, we report the complete nucleotide sequences of the DNA-A and DNA-B of a newly characterized CMG found infecting cassava in Madagascar, for which we propose the tentative name cassava mosaic Madagascar virus. With the exception of two recombinant regions that resembled a CMG, we determined that the non-recombinant part of the DNA-A component is distantly related to the other CMGs. Whereas the DNA-B component possesses one recombinant region originating from an unidentified virus, the rest of the genome was seen to be closely related to members of the species East African cassava mosaic Zanzibar virus (EACMZV). Phylogenetic analysis based on complete genome sequences demonstrated that DNA-A and DNA-B components are outliers related to the clade of EACMV-like viruses and that DNA-A is related to the monopartite tomato leaf curl begomoviruses described in islands in the south-west Indian Ocean.

Cassava is the staple food for hundreds of millions of people in Africa but its cultivation is seriously constrained by cassava mosaic disease (CMD) in Madagascar, and in Africa in general. This study identified the cassava mosaic geminiviruses (CMGs) involved in CMD in Madagascar and their associated epidemiological characteristics from countrywide surveys. Molecular characterization of CMGs in Madagascar revealed an unprecedented diversity and co-occurrence of six viruses: African cassava mosaic virus (ACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Kenya virus (EACMKV), East African cassava mosaic virus (EACMV), South African cassava mosaic virus (SACMV) and the recently described Cassava mosaic Madagascar virus (CMMGV). Distinct geographical distributions were observed for the six viruses. While ACMV was more prevalent in the central highlands, EACMV and EACMKV were prevalent in lowlands and coastal regions. Both EACMCV and SACMV occurred in almost all the localities visited. PCR diagnosis revealed that mixed infection (up to four co-infected viruses) occurred in 21% of the samples and were associated with higher symptom severity scores. Pairwise comparisons of virus associations showed that EACMCV was found in mixed infections more often than expected while ACMV and SACMV were mostly found in single infections. A greater abundance of whiteflies was observed in lowland and coastal areas. Nevertheless, infected cuttings remain the primary source of CMD propagation (95%) in Madagascar. (Résumé d'auteur

Two complete nucleotide sequences of an alphasatellite isolated from a cassava plant with mosaic disease symptoms in Madagascar are described and analyzed. While the helper begomovirus was identified as an isolate of East African cassava mosaic Kenya virus (EACMKV), its associated alphasatellite was most closely related (80 % nucleotide sequence identity) to cotton leaf curl Gezira alphasatellite. These satellite molecules have typical features of alphasatellites, with a single gene in the virion sense, an A-rich region and a stem-loop structure. According to the proposed species demarcation threshold of alphasatellites (83 % nucleotide identity), they are isolates of a new species for which we propose the name "Cassava mosaic alphasatellite".

Anther/pollen culture technology is being efficiently used for accelerating rice breeding progress and improving grain quality characters. Using pollen culture technique, we obtained elite rice (Oryza sativa L.) line DHP6, which is well adapted to the highlands of Madagascar. Here we show that certain grain, nutritional, and culinary quality characteristics have been significantly improved in line DHP6. Morphometric and physico-chemical analysis demonstrated that physical (colour, transluscency, and chalkiness), chemical (amylose content, alkali spreading value, and gel consistency) and cooking (volume expansion, elongation ratio) features were greatly improved compared to the parental line. Culinary and nutritional quality analyses showed a significant increase in nutritional values and mineral richness. Some desired concentrations of minerals like Mg and Cu were much higher (e.g. three times richer in Cu) than those of its parental line (variety IR58614). Taken together, such ameliorated grain quality traits will promote its wide-scale planting by the farmers and should thus help to improve the nutritional quality of the population of Madagascar.

In Madagascar, rice production is low (2 t ha·') and does not cover the needs of the population. To satisfy the food requirements, there is a need to develop agriculture on highland areas called "tanety". Soils of tanety have low nutrients availability, particularly phosphorus which is the main plant growth limiting factor (soil solution< 0.1 mg P 1·'). Arbuscular mycorrhizal (AM) symbiosis is widely recognized to have high significance for tropical agriculture, particularly for plant P nutrition. The mycorrhizal status of some wild and cultivated plants was determined. Roots were sampled in two locations: the highlands and in the cliff-east. AM development (internal vesicles and hyphae) was estimated by microscopic examination. For the first time several native plants from Madagascar were reported as forming arbuscular mycorrhizae. Crop plants from the highlands were less colonized by AM fungi than wild species, whereas cultivated and wild plants from fallows areas were highly colonized by AM fungi. In highlands, for a long time, dry plants were burnt in furrows to release mineral nutrients. This practice is suspected to have decreased AM fungi population. Four AM fungi were isolated from colonized roots. Two of them were identified as G. fasciculatum. They were successfully propagated on leek growing on red pouzzolane. The efficiency of these isolates was tested in comparison with a strain of Glomus intraradices and found to be more efficient.