Deep Sequencing Of The Medicago truncatula Transcriptome Reveals a Massive Early Response To Nod Factor and Ethylene Signals

Legumes establish nitrogen-fixing endosymbiosis with rhizobium bacteria involving complex developmental changes in roots. Ethylene has been shown to play a key role in the early stages of the interaction, being a negative regulator of symbiotic development. In the present work we have applied RNAseq next-generation sequencing techniques to analyze the transcriptional changes in Medicago truncatula roots at very early symbiotic stages (from 30 min to 48 h after inoculation with Sinorhizobium meliloti). To discriminate between Nod-factor and ethylene signaling, we analyzed four plant genotypes: wild type A17, mutants in Nod-factor perception nfp and lyk3, and a mutant in ethylene perception, skl. In total we have identified almost 11,000 differentially-expressed genes, with more than 8,000 differentially regulated between wild type and skl samples. Among those numerous genes are involved in ethylene perception, signal transduction and ethylene biosynthesis, including a number of novel transcription factors of the AP2/ethylene-responsive factor (ERF) superfamily involved in very early responses (i.e., 6 h post-inoculation). Functional characterization of a number of candidate genes is currently underway to further investigate their role in nodulation. The availability of this detailed information describing the early transcriptional changes triggered by rhizobium will serve as a useful resource for the community, leading to the discovery of new genes involved in this complex symbiotic interaction process.