Alnus glutinosa

Casuarinaceae multilobed nodule. Each lobe has a lateral root structure with a central vascular bundle and cortical cells infected by Frankia (photo C. Franche)

The recent genome sequencing of three Frankia strains, done by the Genoscope, the Ecologie Microbienne Unit in Lyon and a consortium of American groups (JGI/DoE, Universities) permitted to highlight marked size differences correlated to host spectrum, an absence of symbiotic island and a lack of canonical nod cluster (Normand et al., 2007). Work is going on to identify events that take place in the bacterium during infection.

Alnus glutinosa root hairs 14 days after infection by Frankia alni. Numerous deformations at tips are now visibles, they will trap Frankia hyphae and thus permit penetration of the root hair and subsequent nodule induction (photo P. Pujic)

On the plant side, there is still little known about the symbiotic mechanisms involved except for the conservation of some determinants controlling infection of actinorhizal plants by Frankia and the infection of Legumes by Rhizobium (Svistoonoff et coll., 2004).

The objective of our project is to constitute databases of expressed genes (ESTs) of a significant size from Casuarina and Alder during the course of symbiotic interaction with Frankia, and to compare these genetic programmes to those of non-infected control roots. This genomic approach should in the coming years yield a global view of the workings of actinorhizal nodules. A comparative analysis of transcriptomes of actinorhizal and legumes should also help understand the nature of genes that predispose actinorhizal plants and legumes to develop nitrogen-fixing root nodules.

Alnus glutinosa root hairs 7 days post infection by Frankia alni. Deformation at extremities are only getting to be visible (photo P. Pujic)

Contacts: Patrick Wincker (Genoscope) - Didier Bogusz (IRD)

Bibliographie

Normand P, Lapierre P, Tisa LS, Gogarten JP, Alloisio N, Bagnarol E, Bassi CA, Berry AM, Bickhart DM, Choisne N, Couloux A, Cournoyer B, Cruveiller S, Daubin V, Demange N, Francino MP, Goltsman E, Huang Y, Kopp OR, Labarre L, Lapidus A, Lavire C, Marechal J, Martinez M, Mastronunzio JE, Mullin BC, Niemann J, Pujic P, Rawnsley T, Rouy Z, Schenowitz C, Sellstedt A, Tavares F,Tomkins J, Vallenet D, Valverde C, Wall LG, Wang Y, Medigue C, & Benson DR. 2007. Genome characteristics of facultatively symbiotic Frankia sp. strains reflect host range and host plant biogeography. Genome Research 17: 7-15

Svistoonoff S, Laplaze L, Liang J, Ribeiro A, Gouveia M.C, Auguy F, Fevereiro P, Franche C, & Bogusz D. 2004 . Infection-related activation of the cg12 promoter is conserved between actinorhizal and legume-rhizobia root nodule symbioses. Plant Physiol. 136: 3191-3197.