Symbiotic competence in Lotus japonicus is affected by plant nitrogen status: transcriptomic identification of genes affected by a new signalling pathway

In leguminous plants, symbiotic nitrogen fixation performances and nitrogen environmental conditions are linked because nodule initiation, development and functioning are greatly influenced by the amount of available nitrogen sources. We demonstrate here that nitrogen supply also controls, beforehand, the competence of leguminous plants to perform the nodulation program. Lotus japonicus plants pre-incubated for ten days in high N condition, and then transferred to low N, prior to the Mesorhizobium loti inoculation, showed a reduced nodule formation capacity. This phenotype was maintained for at least six days and a complete reacquisition of the symbiotic competence was observed only after nine days. The time course analysis of the change of the symbiotic phenotype was coupled to a large-scale transcriptomic analysis of the gene expression profile. The identified differentially expressed genes are mostly involved in metabolic pathways. However, the transcriptional response, includes also genes belonging to other functional categories such as signalling, stress response and transcriptional regulation. Some of these genes show a molecular identity and a regulation profile, that suggest a role as possible molecular links between the N-dependent plant response and the nodule organogenesis program.