Identification of an Arabidopsis solute carrier critical for intracellular transport and inter‐organ allocation of molybdate

Abstract

Plants represent an important source of molybdenum in the human diet. Recently, MOT1 has been identified as a transport protein responsible for molybdate import in Arabidopsis thaliana L.; however, the function of the homologous protein MOT2 has not been resolved. Interestingly, MOT2‐GFP analysis indicated a vacuolar location of this carrier protein. By site directed mutagenesis at the N‐terminal end of MOT2, we identified a di‐leucine motif that is essential for driving the protein into the vacuolar membrane. Molybdate quantification in isolated vacuoles showed that this organelle serves as an important molybdate store in Arabidopsis cells. When grown on soil, leaves from mot2 T‐DNA mutants contained more molybdate, whereas mot2 seeds contained significantly less molybdate than corresponding wild‐type (Wt) tissues. Remarkably, MOT2 mRNA accumulates in senescing leaves and mot2 leaves from plants that had finished their life cycle had 15‐fold higher molybdate levels than Wt leaves. Reintroduction of the endogenous MOT2 gene led to a Wt molybdate phenotype. Thus, mot2 mutants exhibit impaired inter‐organ molybdate allocation. As total concentrations of the molybdenum cofactor (Moco) and its precursor MPT correlates with leaf molybdate levels, we present novel evidence for an adjustment of Moco biosynthesis in response to cellular MoO42− levels. We conclude that MOT2 is important for vacuolar molybdate export, an N‐terminal di‐leucine motif is critical for correct subcellular localisation of MOT2 and activity of this carrier is required for accumulation of molybdate in Arabidopsis seeds. MOT2 is a novel element in inter‐organ translocation of an essential metal ion.

Routes to the Tonoplast: The Sorting of Tonoplast Transporters in Arabidopsis Mesophyll Protoplasts

“…Additionally, di-Leu motifs in the N-terminal region of the monosaccharide transporter ERD SIX-LIKE1 (ESL1) (Yamada et al, 2010) and of the molybdate transporter MOT2 (Gasber et al, 2011) were shown to be responsible for the correct sorting of these transporters to the tonoplast.…”

The Molybdenum Cofactor

“…The latter is questionable, however, as the insertion of molybdenum into the Moco backbone takes place in the cytosol. For Mot2, GFP fusion proteins have been used to show that the protein localizes to the vacuolar membrane (17). Molybdate quantification in isolated vacuoles demonstrated that this organelle serves as an important molybdate storage compartment in Arabidopsis thaliana cells, where Mot2 was shown to be required for vacuolar molybdate export into the cytosol.…”

Quantitative trait loci (QTL) and candidate genes associated with trace element concentrations in perennial grasses grown on phytotoxic soil contaminated with heavy metals

“…Molybdenum is one of the least abundant essential micronutrients in plants and soils, but it has critical roles in plant growth, nitrogen assimilation, and nitrogen-fixing symbiotic bacteroids (Kaiser et al 2005). Molybdenum is an essential element for nearly all organisms, necessary for Mo cofactor dependent enzymes including nitrate reductases, sulphite oxidase, xanthine oxidase, and dimethyl sulphoxide reductase (Schwarz et al 2009 (Gasper et al 2011) and another sulphate transporter from the nitrogen-fixing forage legume Stylosanthes hamata has also been shown to have some molybdate transporter activity (Fitzpatrick et al 2008). However, we were unable to identify MOT2 orthogenes in barley and the only known Mo-specific transporter likely to affect Mo assimilation is the MOT1 orthogene.…”