CCAAT-Box Binding Factors Control Symbiotic Signalling and Nodule Development In Medicago truncatula

MtNF-YA1& 2 encode transcription factors (TFs) of the CCAAT-box binding factor family which are strongly expressed in Medicago truncatula during early phases of the symbiotic interaction with Sinorhizobium meliloti. In animal systems, NF-YA proteins are known to associate with NF-YB and NF-YC subunits to form heterotrimeric hubs which interact with other TFs, and promote specific processes, in particular cell cycle progression. Whereas in animals each NF-Y subunit is encoded by a single gene, structural diversification in plants has led to the emergence of gene families comprising between 8 and 39 members depending on the subunit and the plant species. This diversification is also apparent at the functional level where NF-Y -encoding genes are involved in processes as diverse as embryo development, flowering time control, and drought or ER-stress. We have analysed in detail the symbiotic expression patterns of the two closely related MtNF-YA1& MtNF-YA2 genes by q-RTPCR, mRNA in situ hybridization and promoter-GUS analysis. These analyses together with functional RNAi and mutant characterization have shown that these two TFs play partially overlapping roles during Nod factor signalling, as well as during subsequent rhizobial infection and nodule development. Furthermore, in order to better understand their functions and the symbiotic pathways regulated by these TFs we have recently started to characterise their symbiotic protein partners as well as their potential targets during different steps of the symbiotic legume-Rhizobium interaction.