The role of rhizobial (NifV) and plant (FEN1) homocitrate synthases in Aeschynomene/photosynthetic Bradyrhizobium symbiosis

In the most studied rhizobium-legume interactions, the host plant supplies the symbiont with homocitrate, an essential co-factor of the nitrogenase enzyme complex, via the expression of a nodule-specific homocitrate synthase FEN1. Photosynthetic bradyrhizobia interacting with Nod factor (NF) dependent and NF-independent Aeschynomene legumes are able to synthesize homocitrate themselves as they contain a nifV gene encoding a homocitrate synthase. Here, we show that in the model strain ORS285, nifV is required for free-living and symbiotic dinitrogen fixation with NF-independent Aeschynomene species. In contrast, in symbiosis with NF-dependent Aeschynomene species, the nifV requirement for efficient nitrogen fixation was found to be host plant dependent. Interestingly, orthologs of FEN1 were found in both NF-dependent and NF-independent Aeschynomene species. However, a high nodule specific induction of FEN1 expression was only observed in A. afraspera, a host plant in which nifV is not required for symbiotic dinitrogen fixation. These data indicate that efficient symbiotic nitrogen fixation in many of the tested Aeschynomene species requires rhizobial homocitrate synthesis. Considering that more than 10% of the fully sequenced rhizobium strains do contain a nifV gene, the Aeschynomene/photosynthetic Bradyrhizobium interaction is likely not the only rhizobium/legume symbiosis where rhizobial nifV expression is required.