14 August 2017
Tejada-Jimenez, M., Gil-Diez, P., Leon-Mediavilla, J., Wen, J., Mysore, K. S., Imperial, J. and Gonzalez-Guerrero, M. - New Phytologist, 2017
Molybdenum is one of the scarcest oligonutrients in the biosphere (Esteifel, 2002). Plants use it as part of the molybdenum cofactor (Moco) in just five enzymes involved in nitrate assimilation, purine metabolism, phytohormone production, and sulfite detoxification (Tejada-Jimenez et al., 2013). This nutrient, unlike other transition metals, is recovered from soil as the oxyanion molybdate instead of in a cationic form. This determines that the transporters involved in this process are not related to the classical transporters required for iron, copper, or zinc uptake, but are members of other families. Molybdate shares some physicochemical characteristics with sulfate, leading to cross-inhibition of sulfate transport by molybdate, probably as a consequence of nonspecific molybdate transport through sulfate transporters in plants (Stout et al., 1951). Therefore, until recently it was thought that sulfate transporters might mediate molybdate transport in eukaryotic systems (Mendel & Hansch, 2002; Kaiser et al., 2005). The only known plant-type specific molybdate transporters belong to the Molybdate Transporter type 1 (MOT1) family and were identified in parallel in the green alga Chlamydomonas reinhardtii (Tejada-Jimenez et al., 2007) and in the higher plant Arabidopsis thaliana (Tomatsu et al., 2007). They share a high degree of homology with the sulfate transporter (SULTR) family, but lack the conserved sulfate transporter and anti-sigma factor antagonist domain (Tejada-Jimenez et al., 2007). In C. reinhardtii, CrMOT1 is responsible for high-affinity molybdate uptake, a process that is not severely affected by sulfate, indicating that MOT1 proteins are molybdate-specific transporters (Tejada-Jimenez et al., 2007). In A. thaliana, two members of the MOT1 family have been identified. One of them has been proposed to play a role in efficient molybdenum uptake from the soil (Tomatsu et al., 2007), although this function is not clear given the conflicting subcellular localizations reported for this transporter in the plasma membrane or in mitochondria (Tomatsu et al., 2007; Baxter et al., 2008). A second member of the MOT1 family is located in the vacuole of leaves and seems to be involved in intracellular and inter-organ molybdenum transport (Gasber et al., 2011). More recently, a MOT1 protein (LjMOT1) has been identified in Lotus japonicus, with a role in molybdate uptake from soil and translocation to the shoots; it is located in the plasma membrane when expressed in Nicotiana benthamiana Domin leaves (Gao et al., 2016; Duan et al., 2017). In addition, another molybdate transporter family, MOT2, belonging to the major facilitator superfamily, has been identified in C. reinhardtii (Tejada-Jimenez et al., 2011), indicating that molybdate transporters have appeared at least twice in evolution. However, their functionality as molybdate transporters has only been demonstrated in this alga.
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