AtMGT7:An Arabidopsis Gene Encoding a Low-Affinity Magnesium Transporter

Magnesium （Mg^2＋） is one of the essential cations in all cells. Although the Mg^2＋ transport mechanism has been well- documented in bacteria, less is known about Mg^2＋ transporters in eukaryotes. The AtMGT gene family encoding putative magnesium transport proteins had been described previously. We report here that one of the Arabidopsis MGT family members, the AtMGT7 gene, encodes two mRNAs that have resulted from alternative splicing variants, designated AtMGT7a and AtMGT7b. Interestingly, the two mRNA variants were expressed with different patterns with AtMG77a expressing in all organs, but AtMGT7b appearing only in root and flowers. The AtMGT7a variant functionally complemented a bacterial mutant lacking Mg^2＋ transport capacity, whereas AtMGT7b did not. The 63Ni^2＋ tracer uptake analysis in the bacterial model showed that AtMGT7a mediated low-affinity transport of Mg^2＋. Consistent with the complementation assay result, 63Ni^2＋ tracer uptake analysis revealed that AtMGT7b did not transport Mg^2＋. This study therefore has identified from a higher plant the first low-affinity Mg^2＋ transporter encoded by a gene with alternatively spliced transcripts that produce proteins with distinct functions.

The aquaporin MePIP2;7 improves MeMGT9-mediated Mg2+ acquisition in cassava

Aquaporins are important transmembrane water transport proteins which transport water and several neutral molecules. However, how aquaporins are involved in the synergistic transport of Mg2+and water remains poorly understood. Here, we found that the cassava aquaporin Me PIP2;7 was involved in Mg2+transport through interaction with Me MGT9, a lower affinity magnesium transporter protein. Knockdown of Me PIP2;7 in cassava led to magnesium deficiency in basal mature leaves with chlorosis and necrotic spots on their edges and starch over-accumulation. Mg2+content was significantly decreased in leaves and roots of Me PIP2;7-RNA interference(PIP-Ri) plants grown in both field and Mg2+-free hydroponic solution. Xenopus oocyte injection analysis verified that Me PIP2;7 possessed the ability to transport water only and Me MGT9 was responsible for Mg^(2+)effux.More importantly, Me PIP2;7 improved the transportability of Mg^(2+)via Me MGT9 as verified using the CM66 mutant complementation assay and Xenopus oocytes expressing system. Yeast twohybrid, bimolecular fuorescence complementation,co-localization, and co-immunoprecipitation assays demonstrated the direct protein–protein interaction between Me PIP2;7 and Me MGT9 in vivo. Mg2+fux was significantly elevated in Me PIP2;7-overexpressing lines in hydroponic solution through non-invasive micro-test technique analysis. Under Mg^(2+)-free condition, the retarded growth of PIP-Ri transgenic plants could be recovered with Mg^(2+)supplementation. Taken together, our results demonstrated the synergistic effect of the Me PIP2;7 and Me MGT9 interaction in regulating water and Mg2+absorption and transport in cassava.