A phylogenetically conserved group of NF-Y transcription factors interact to control nodulation in legumes

Abstract

The endosymbiotic association between legumes and rhizobia leads to the formation of root nodules in which differentiated bacteria convert atmospheric nitrogen into a form that can be assimilated by the host plant. Successful root infection by rhizobia and nodule organogenesis require the activation of symbiotic genes that are controlled by a set of early transcription factors (TFs). MtNF-YA1 and MtNF-YA2 are two TFs playing partially redundant functions during several steps of the symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti. NF-Y proteins are part of a transcriptional complex composed of three proteins (NF-YA, NF-YB and NF-YC) which bind DNA at CCAAT-boxes, a motif present in most eukaryotic promoters. In plants, each subunit is encoded by small gene families, potentially leading to a multitude of heterotrimeric NF-Y complexes. Here, using yeast two hybrid screenings, we identified the MtNF-YB and MtNF-YC subunits that interact with MtNF-YA1 and A2. Further, we confirmed, both in yeast and in planta, the formation of trimeric NF-Y complexes and showed that these complexes are functional during nodulation using reverse genetic approaches and ChIP-PCR. Finally, as orthologs of the characterized NF-Y subunits also control nodulation in other legumes, we showed in common bean that similar NF-Y trimers could form in planta. Our results suggest that we have identified a group of evolutionary conserved NF-Y proteins that interact to control nodulation in leguminous plants.