Diversité taxonomique et propriétés symbiotiques des rhizobia nodulant Acacia raddiana au nord et au sud du Sahara

Nous avons étudié une collection de 76 nouveaux isolats bactériens de nodules obtenus par piégeage sur plantes d'Acacia tortilis subsp. raddiana in vitro, inoculées avec des suspensions de sols prélevés dans divers sites au nord (Tunisie) et au sud (Sénégal, Mauritanie) du Sahara, en comparaison avec 54 souches de références. L'analyse des nouveaux isolats par RAPD et SDS-PAGE a montré une grande diversité taxonomique parmi les isolats d'A. tortilis subsp. raddiana. En effet, bien que toutes les souches puissent être qualifiées de « rhizobium à croissance rapide », elles se répartissent en au moins 9 groupes différents, dont plusieurs ne correspondent à aucun des groupes de rhizobia déjà décrits. Aucune relation entre la position taxonomique des souches, leur origine géographique, les conditions édaphiques ou la profondeur d'isolement n'a pu être établie. La caractérisation symbiotique des souches d'A. tortilis subsp. raddiana a, au contraire, montré une grande homogénéité des caractéristiques symbiotiques majeures. Toutes les souches nodulent également A. Senegal, Prosopis julifora et Leucaena leucocephala, qui constituent avec A. tortilis subsp. raddiana un groupe d'inoculation. L'analyse des profils chromatographiques des facteurs Nod (FN) d'une quinzaine de souches réparties dans les différents groupes taxonomiques montre qu'ils sont très similaires. L'homogénéité structurale des FN des souches d'A. tortilis subsp. raddiana a été confirmée par la détermination de la structure des FN de quelques souches représentatives. Cette étroite relation entre FN et plante d'isolement/spectre d'hôte indépendamment de la taxonomie des souches suggère l'existence d'une forte pression de sélection de la part de la plante hôte et confirme nos observations précédentes, à savoir que les facteurs Nod sont de bons indicateurs du potentiel de nodulation d'un rhizobium donné.We studied a collection of 76 new bacterial isolates from nodules developed on Acacia tortilis subsp. raddiana plants inoculated in vitro with soil suspensions harvested in different sites in North (Tunisia) and South (Senegal, Mauritania) parts of Sahara. RAPD and SDS-PAGE analyses in comparison with 54 reference strains from diverse origins showed great taxonomie diversity among the new isolates. All of them are fast-growing and form at least 9 different groups, several of which could correspond to undescribed groups. No relationship could be established between taxonomie groups, geographical origin (North/South of Sahara) of the strains, edaphic conditions, or depth of soil sampling (down to - 32 m). Opposite to their taxonomie diversity, the new isolates share identical main symbiotic properties. All strains nodulating A. tortilis subsp. raddiana can also nodulate Prosopis juliflora, Acacia Senegal and Leucaena leucocephala, these four plants then constituting a cross-inoculation group. By thin-layer chromatography, we showed that 20 A. tortilis subsp. raddiana strains, representative of the different taxonomie groups, share very similar Nod factor (NF) profiles, independently from their geographical origins. NF structure homogeneity was confirmed by NF structure determination of some representative strains. The consistency between NF and plant of isolation/nodulation spectrum, independently of the taxonomie position of the strains suggests a high selection pressure from the plant, and confirms our previous observations that NF are good markers for predicting the nodulation potential of a given rhizobial strain

Un arbre au désert

Cet ouvrage réunit les résultats de recherches menées sur Acada raddiana, l'un des principaux taxons spontanés du genre Acada en Afrique, et qui, malgré sa large distribution et son importance dans l'économie rurale, n'a jusqu'à présent fait l'objet d'aucune synthèse spécifique. Différentes disciplines (écologie, écophysiologie, génétique, microbiologie, entomologie, physiologie et foresterie) ont été associées pour préciser la taxonomie et la répartition géographique de ce taxon, analyser la diversité aussi bien de la plante-hôte que des micro-organismes qui lui sont associés, caractériser son fonctionnement et les interactions en milieu naturel en réponse aux facteurs eau, azote et phosphore, enfin pour réunir les éléments nécessaires à son utilisation dans les opérations de réhabilitation. Cet ouvrage, qui s'adresse aux chercheurs et aux enseignants, ainsi qu'aux organismes de développement, est divisé en cinq parties : diversités, écologie, usages; fonctionnement hydrique, phénologie ; micro-organismes associés : diversités, physiologie ; interactions Acacia-milieu ; multiplication de l'espèce : aspects fondamentaux et appliqués

On the Use of Agricultural System Models for Exploring Technological Innovations Across Scales in Africa: A Critical Review

The major challenge of the 21st century is to achieve food security under, roughly, a doubling in food demand by 2050 compared to present, and producing the additional food under marked shifts in climatic risks and with environmentally sound farming practices. Sustainable intensification of agricultural production is required that meets the dual goal of improved environmental sustainability and economic efficiency. Ex ante evaluation of technological innovations to support agricultural production and food security taking into account the various future risks can substantially contribute to achieve this. Here we perceive technological innovations as new or improved agro-technologies and –management practices, such as new breeds, integrated soil fertility practices or labour-saving technologies meeting the goals of sustainable intensification. In this report we present results from three systematic reviews: one on the use of biophysical modelling, the second and third on the use of bio-economic modelling at farm scale and agro-economic modelling at higher aggregation levels, for ex ante evaluation of the effects of (agro ) Technological Innovations (ag-TIs) on sustainable agriculture and food security indicators. To this end, we searched the SCOPUS database for journal articles published between 1996 and 2015. We considered modelling studies at different spatial scales with particular attention to local to national scale studies for the twelve PARI focal countries in Africa . But we also included studies for all other African countries as well as a few studies at supra-national/continental scale. Both, “quick wins” as well as long term benefits from ag-TIs were of interest. The various ag-TIs were furthermore grouped into four classes: (1) water/soil moisture (2) soil nutrients/conservation (3) crop/cropping system, (4) other ag-TIs or (5) combinations of 1 to 4. For each paper, we tried to identify the primary ag-TI analysed, and if there was equal emphasis to more than one, we classified them as combinations. It should be borne in mind that there is some subjectivity in classifying the papers in this way. Results. After various steps of refining “search strings”, screening on relevance and supplementing databases from additional sources, we found 140 relevant biophysical modelling studies, whereby coverage of sub-regions and ag-TIs varied markedly. Most studies were found for East and West Africa, followed by Southern Africa; hardly anything was found for Northern and Middle Africa . A similar pattern appeared for the integrated agro-economic modelling studies at farm scale, for which we found 40 relevant ones. Agro-economic modelling studies at higher aggregation levels showed a somewhat different pattern – and more generally contained little detail on technological innovations. Regarding the share of different primary agro-technologies explored in the biophysical studies we found 45 on crop management, 35 on combined agro-technologies, 31 on soil nutrient management and conservation, 23 on water/soil moisture management, and 6 on other technologies. We found similar shares among the various agro-technology groups for the integrated agro-economic modelling studies at farm scale. Looking at the outcomes from ex ante evaluations we found that many studies are (mostly) positive on effects of single and “conventional” ag-Tis. The majority of biophysical studies is performed at “field scale” and focuses on the effects on productivity (sometimes yield stability); many of these studies were performed in climate variability and change /adaptation research context. Most agro-economic modelling studies that look specifically at ex ante evaluations of ag-TIs are performed at farm or regional (sub-national) scales. While the number of biophysically oriented studies has grown exponentially over the considered period 1996-2015, this is not the case for the agro-economic modelling studies. Looking in more detail at the twelve focal countries of PARI (=Programme of Accompanying Research on Agricultural Innovations) we also find an unbalanced distribution, with most studies found in Kenya, Ethiopia, Mali and Ghana (biophysical modelling studies), and respectively in Kenya and Uganda (agro-economic modelling studies), whereas nothing or little was found for both types of studies in Togo, Zambia and Nigeria. Very few of the biophysically-oriented studies include other information than effects on crop yields, and there are few studies for both biophysical and agro-economic modelling that comprise multi-scale or higher scale analyses; if multi-scale, there are more studies that scale up from field/farm to regional/sub-national level than from field/farm to nation scale or beyond. There is definitely a need to overcome the lack of meaningful integrated multi-scale modelling along the lines proposed in chapters 5-6 of this report. Moreover, less than half of all integrated /agro-economic modelling studies at farm scale explicitly address risk – another clear shortcoming, which requires attention by the research community. A more general conclusion is that there is no application yet of true transdisciplinary research approaches in practice. Hence, there is need for participatory, collaborative (cross-sectoral) and combined modelling approaches with adequate stakeholder involvement throughout the research process. In this respect, some lessons might be learned from pioneering work conducted in Asia and Europe