Symbiotic nitrogen fixation occurs in specialized structures:nodulesNodules: special organs of the host plants that enclose thenitrogen-fixing bacteria.The oxygen level within the nodule is sufficient for respiration,but low enough to avoid inactivation of nitrogenaseLeghemoglobin: oxygen-binding heme protein present in thecytoplasm of nodule cells to transport enough oxygen forrespiration of symbiotic bacteria.Establishing symbiosis requires an exchange of signalsplant genes specific to nodules are called nodulin genesNod factors produced by bacteria act as signals for symbiosis