Loss of NifQ Leads to Accumulation of Porphyrins and Altered Metal-Homeostasis in Nitrogen-Fixing Symbioses

Symbiotic nitrogen fixation between legumes and rhizobia involves a coordinated expression of many plant and bacterial genes as well as finely tuned metabolic activities of micro- and macrosymbionts. In spite of such complex interactions, symbiotic proficiency remains a resilient process, with host plants apparently capable of compensating for some deficiencies in rhizobia. What controls nodule homeostasis is still poorly understood and probably varies between plant species. In this respect, the promiscuous Sinorhizobium (Ensifer) fredii strain NGR234 has become a model to assess the relative contribution of single gene products to many symbioses. Here, we describe how a deletion in nifQ of NGR234 (strain NGR Delta nifQ) makes nodules of Vigna unguiculata, V. radiata, and Macroptilium atropurpureum but not of the mimisoid tree Leucaena leucocephala, purple-red. This peculiar dark-nodule phenotype did not necessarily correlate with a decreased proficiency of NGR Delta nifQ but coincided with a 20-fold or more accumulation of coproporphyrin III and uroporphyrin III in V. unguiculata nodules. Porphyrin accumulation was not restricted to plant cells infected with bacteroids but also extended to the nodule cortex. Nodule metal-homeostasis was altered but not sufficiently to prevent assembly and functioning of nitrogenase. Although the role of NifQ in donating molybdenum during assembly of nitrogenase cofactor FeMo-co makes it essential in free-living diazotrophs, our results highlight the dispensability of NifQ in many legume species.