Molybdenum and zinc uptake by nitrogen-fixing cells in legume nodules

The massive use of nitrogen fertilizers in agriculture is responsible of feeding half of the world population. However, they are an important source of greenhouse gasses, one of the main expenses for farmers (unreacheable in many developing countries), and contribute to water eutrophyzation. Symbiotic nitrogen fixation (SNF) carried out by the rhizobia-legume interaction is one of the main alternatives to the overuse of these fertilizers, in a return to more sustainable practices of crop rotation with legumes. This process requires of relatively large amount of transition metals (iron, copper, zinc,…) that work as cofactors of many of the main enzymes participating in SNF. Given the prevalent low bioavailability of these metals in many of the main agricultural areas of the world, the real impact of crop rotation with legumes in soil nitrogen fertilization will depend in ensuring a continous and suficient delivery of essential transition metals. To improve this, we need to identify and manipulate the transporters mediating the delivery of these nutrients.

The CBGP group “Metal homeostasis in plant-microbe interactions”, in collaboration with researchers from CNRS (France), SLS (Switzerland), and the Noble Research Institute (USA), has recently published the characterization of two transporters involved in zinc (MtZIP6) and molydenum (MtMOT1.3) delivery to nitrogen-fixing cells in the nodules of model legume Medicago truncatula. These are plasma membrane transporters that introduce metals into the cell cytosol. Reduction of the expression levels of these transporters results in the reduction of nitrogen fixation capabilities in nodules. In the case of the mutant in the molybdate transporter, this is due to the loss of an essential cofactor for nitrogenase functioning. In the case of the silenced line in the zinc transporter, the loss the nitrogen-fixing capabilities would be due to the existence of yet-to-be-determined zinc-proteins, that would play an important role in nitrogenase function.

Original Paper:

Tejada-Jiménez, M; Gil-Díez, P; León-Mediavilla, J; Wen, J; Mysore, KS; Imperial, J; González-Guerrero, M. 2017. "Medicago truncatula Molybdate Transporter type 1 (MtMOT1.3) is a plasma membrane molybdenum transporter required for nitrogenase activity in root nodules under molybdenum deficiency". New Phytologist. DOI: 10.1111/nph.14739".

Abreu, I; Saéz, Á; Castro-Rodríguez, R; Escudero, V; Rodríguez-Haas, B; Senovilla, M; Larue, C; Grolimund, D; Tejada-Jiménez, M; Imperial, J; González-Guerrero, M. 2017. "Medicago truncatula Zinc-Iron Permease6 provides zinc to rhizobia-infected nodule cells". Plant, Cell & Environment. DOI: 10.1111/pce.13035".

Image: MtZIP6 localization in M. truncatula nodule infection zone. The transporter localization is shown in red, the rhizobia are in green, and DNA in blue.