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International audience; Nitrogen and phosphorus availability are frequent limiting factors in plant growth and development. Certain bacteria and fungi form root endosymbiotic relationships with plants enabling them to exploit atmospheric nitrogen and soil phosphorus. The relationships between bacteria and plants include nitrogen-fixing Gram-negative proteobacteria called rhizobia that are able to interact with most leguminous plants (Fabaceae) but also with the non-legume Parasponia (Cannabaceae), and actinobacteria Frankia, which are able to interact with about 260 species collectively called actinorhizal plants. Fungi involved in the relationship with plants include Glomeromycota that form an arbuscular mycorrhizal (AM) association intracellularly within the roots of more than 80% of land plants. Increasing numbers of reports suggest that the rhizobial association with legumes has recycled part of the ancestral program used by most plants to interact with AM fungi. This review focuses on the most recent progress made in plant genetic control of root nodulation that occurs in non-legume actinorhizal plant species.

Phyllanthus odontadenius Mull. Arg is one of the most important medicinal plants in the genus Phyllanthus (Phyllanthaceae). It is used in different regions in the world for the treatment of various diseases for example malaria caused by Plasmodium falciparum causing millions deaths in tropical and subtropical regions. In this work, we experimented P. odontadenius seeds germination and seedlings infected by Agrobacterium rhizogenes A4RS/pHKN9. We obtained 36% of seeds that germinated for 84.6% of seedlings transformed which showed positive gfp activities. The number of hairy roots formed in infectious sites for seedlings infected with A. rhizogenes A4RS/pHKN29 is valued at 6.3±2.71 against 0 for control seedlings. Diagrams showed that mean length for the new-formed roots were 1.92±0.55 cm and 1.59±0.49 cm for hairy root. Key words: Phyllanthus odontadenius, hairy roots, Agrobacterium rhizogenes, malaria, secondary metabolites.

International audience

Actinorhizal symbioses are mutualistic interactions between plants and the soil bacteria Frankia spp. that lead to the formation of nitrogen-fixing root nodules. The plant hormone auxin has been suggested to play a role in the mechanisms that control the establishment of this symbiosis in the actinorhizal tree Casuarina glauca. Here, we analyzed the role of auxin signaling in Frankia spp.-infected cells. Using a dominant-negative version of an endogenous auxin-signaling regulator, INDOLE-3-ACETIC ACID7, we established that inhibition of auxin signaling in these cells led to increased nodulation and, as a consequence, to higher nitrogen fixation per plant even if nitrogen fixation per nodule mass was similar to that in the wild type. Our results suggest that auxin signaling in Frankia spp.-infected cells is involved in the long-distance regulation of nodulation in actinorhizal symbioses.

The Casuarinaceae family is a group of 96 species of trees and shrubs that are tolerant to adverse soil and climatic conditions. In the field, Casuarinaceae bears nitrogen-fixing root nodules (so called actinorhizal nodules) resulting from infection by the soil actinomycete Frankia. The association between Casuarina and Frankia is of tremendous ecological importance in tropical and subtropical areas where these trees contribute to land stabilization and soil reclamation. During differentiation of the actinorhizal nodule, a set of genes called actinorhizal nodulins is activated in the developing nodule. Understanding the molecular basis of actinorhizal nodule ontogenesis requires molecular tools such as genomics together with gene transfer technologies for functional analysis of symbiotic genes. Using the biological vectors Agrobacterium rhizogenes and A. tumefaciens, gene transfer into the two species Allocasuarina verticillata and Casuarina glauca has been successful. Transgenic Casuarinaceae plants proved to be valuable tools for exploring the molecular mechanisms resulting from the infection process of actinorhizal plants by Frankia.

Only species belonging to the Fabid clade, limited to four classes and ten families of Angiosperms, are able to form nitrogen-fixing root nodule symbioses (RNS) with soil bacteria. This concerns plants of the legume family (Fabaceae) and Parasponia (Cannabaceae) associated with the Gram-negative proteobacteria collectively called rhizobia and actinorhizal plants associated with the Gram-positive actinomycetes of the genus Frankia. Calcium and calmodulin-dependent protein kinase (CCaMK) is a key component of the common signaling pathway leading to both rhizobial and arbuscular mycorrhizal symbioses (AM) and plays a central role in cross-signaling between root nodule organogenesis and infection processes. Here, we show that CCaMK is also needed for successful actinorhiza formation and interaction with AM fungi in the actinorhizal tree Casuarina glauca and is also able to restore both nodulation and AM symbioses in a Medicago truncately ccamk mutant. Besides, we expressed auto-active CgCCaMK lacking the auto-inhibitory/CaM domain in two actinorhizal species: C. glauca (Casuarinaceae), which develops an intracellular infection pathway, and Discaria trinervis (Rhamnaceae) which is characterized by an ancestral intercellular infection mechanism. In both species, we found induction of nodulation independent of Frankia similar to response to the activation of CCaMK in the rhizobia-legume symbiosis and conclude that the regulation of actinorhiza organogenesis is conserved regardless of the infection mode. It has been suggested that rhizobial and actinorhizal symbioses originated from a common ancestor with several independent evolutionary origins. Our findings are consistent with the recruitment of a similar genetic pathway governing rhizobial and Frankia nodule organogenesis. Fil: Svistoonoff, Sergio. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia Fil: Benabdoun, Faiza Meriem. Mentouri University. Departement of Biology and Ecology; Argelia. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia Fil: Nambiar Veetil, Mathish. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia Fil: Imanishi, Leandro Ezequiel. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Vaissayre, Virginie. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia Fil: Cesari, Stella. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia. Institut National de la Recherche Agronomique; Francia Fil: Diagne, Nathalie. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia. Institut National de la Recherche Agronomique; Francia Fil: Hocher, Valérie. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia Fil: de Billy, Françoise. Centre National de la Recherche Scientifique; Francia. Institut National de la Recherche Agronomique; Francia Fil: Wall, Luis Gabriel. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Ykhlef, Nadia. Mentouri University. Departement of Biology and Ecology; Argelia Fil: Rossemberg, Charles. Centre National de la Recherche Scientifique; Francia. Institut National de la Recherche Agronomique; Francia Fil: Bogusz, Didier. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia Fil: Franche, Claudine. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia Fil: Gherbi, Hassen. Centre National de la Recherche Scientifique. Institut de Recherche pour le Développement; Francia