Abstract

In most legume nodules, the N2-fixing rhizobia are present as organelle-like structures inside their root host cells. Many processes operate and interact in the symbiotic relationship between the plant and the nodules, including nitrogen (N)/carbon (C) metabolisms, oxygen flow through the nodule, oxidative stress and phosphorous (P) levels. These processes, which influence the regulation of the N2 fixation are very finely tuned on a whole plant basis, are extensively reviewed in this paper. The carbonic anhydrase (CA)-phosphoenolpyruvate carboxylase (PEPC)-malate dehydrogenase (MDH) is a key pathway inside the nodule involved in this regulation, and malate seems to play a crucial role in many aspects of symbiotic N2 fixation control. How plants specifically sense N-status and how they translate into activity of all the regulatory factors are key issues for understanding the N2 fixation regulation on a whole plant basis. This requires to be thoroughly studied in the future since there is no unifying theory that explains all the aspects of the regulation of N2 fixation rates to date. Finally, high-throughput functional genomics and molecular tools (i.e., miRNAs) are currently very valuable for the identification of a large amount of regulatory elements that are good candidates for dissecting with high accuracy the particular N2 fixation control mechanisms associated to the physiological responses to abiotic stresses. In combination with the existing information, the use of these large amounts of genetic molecular tools will enable to identify the specific mechanisms underlying the regulation of N2 fixation.