MtMOT1.2 is responsible for molybdate supply to Medicago truncatula nodules

Abstract

Symbiotic nitrogen fixation in legume root nodules requires a steady supply of molybdenum for synthesis of the iron-molybdenum cofactor of nitrogenase. This nutrient has to be provided by the host plant from the soil, crossing several symplastically disconnected compartments through molybdate transporters, including members of the MOT1 family. Medicago truncatula Molybdate Transporter (MtMOT) 1.2 is a Medicago truncatula MOT1 family member located in the endodermal cells in roots and nodules. Immunolocalization of a tagged MtMOT1.2 indicates that it is associated to the plasma membrane and to intracellular membrane systems, where it would be transporting molybdate towards the cytosol, as indicated in yeast transport assays. Loss-of-function mot1.2-1 mutant showed reduced growth compared with wild-type plants when nitrogen fixation was required but not when nitrogen was provided as nitrate. While no effect on molybdenum-dependent nitrate reductase activity was observed, nitrogenase activity was severely affected, explaining the observed difference of growth depending on nitrogen source. This phenotype was the result of molybdate not reaching the nitrogen-fixing nodules, since genetic complementation with a wild-type MtMOT1.2 gene or molybdate-fortification of the nutrient solution, both restored wild-type levels of growth and nitrogenase activity. These results support a model in which MtMOT1.2 would mediate molybdate delivery by the vasculature into the nodules.

Nicotianamine Synthase 2 Is Required for Symbiotic Nitrogen Fixation in Medicago truncatula Nodules

“…This includes not only nitrogenase (Rubio and Ludden, 2005), but also nicotinamide adenine dinucleotide phosphate (NADPH)-oxidases that participate in nodule signaling (Montiel et al, 2016), leghemoglobin that maintains nodule O 2 homeostasis (Appleby, 1984), high-affinity cytochrome oxidases providing energy to the bacteroids (Preisig et al, 1996), as well as many enzymes involved in free radical control (Dalton et al, 1998;Santos et al, 2000;Rubio et al, 2007). Consequently, deficiencies in the uptake of these nutrients or alterations in the metal delivery pathways lead to defects in nodulation and/or nitrogen fixation (Tang et al, 1991;O'Hara, 2001;Senovilla et al, 2018;Gil-Díez et al, 2019).…”

Sulfate is transported at significant rates through the symbiosome membrane and is crucial for nitrogenase biosynthesis

“…Subsequently, SST1 was localized to the SM (Wienkoop & Saalbach, 2003) and suggested to be responsible for the transport of sulfate from the plant to the bacteroids . These authors proposed that SST1 is also able to transport molybdate, but this seems not to be the case because, very recently, molybdate-specific transporters have been detected in Medicago truncatula nodules (Gil-Díez et al, 2018;Tejada-Jiménez et al, 2017).…”

From the Eukaryotic Molybdenum Cofactor Biosynthesis to the Moonlighting Enzyme mARC

“…In Medicago truncatula , MtMOT1.3 and MtMOT1.2 are responsible for Mo supply to the nitrogen-fixing tissues. While MtMOT1.2 mediates Mo that is released by the vasculature to the organs where the nitrogen fixation takes places, the nodules [21]; MtMOT1.3 is responsible for Mo transport into nodule cells from the nodule apoplast [19].…”