Nitrate metabolism and regulation of legume nitrogen fixation

The interaction between nitrate and symbiotic nitrogen fixation is complex, diverse, and tightly regulated at multiple levels. The effects of nitrate are strongly dependent on the specific legume-rhizobia symbiosis and on the concentration and exposure time to nitrate. This has hampered studies on nitrate inhibition of nodule development and function. Here, we critically review most of the proposed mechanisms, which include carbohydrate deprivation, inactivation of nitrogenase and/or leghemoglobin by nitrite and/or nitric oxide, feedback regulation of nodule activity by an increase of nitrogenous metabolites in the phloem, and oxygen limitation through an oxygen diffusion barrier (ODB) mainly located in the inner cortex. Several hypotheses to explain the limitation of oxygen access to the nitrogen-fixing zone include glycoprotein occlusion of intercellular spaces, shifts in K+ and H2O2, and enhanced 'protective' respiration and activity of the ascorbate-associated enzymes in the cortical cells. Important progress has been made with the identification of nitrate transporters (NPF and NRT2 families) and transcription factors involved in nitrate regulation of nodulation (NIN, NLPs, and FUN) and in nitrate-induced nodule senescence (NAC094 and SNAP1/2/3/4). However, with the exception of a genetic study showing that suberin deposition in the endodermis is involved in nodule oxygen permeability, the molecular mechanisms underlying the ODB remain a mystery.