Magnesium （Mg^2＋） is abundant in plant cells and plays a critical role in many physiological processes. A 10-member gene family AtMGT （also known as AtMRS2） was identified in Arabidopsis, which belongs to a eukaryote subset of the CorA superfamily, functioning as Mg^2＋ transporters. Some family members （AtMGT1 and AtMGT10） function as high-affinity Mg^2＋ transporter and could complement bacterial mutant or yeast mutant lacking Mg^2＋ transport capability. Here we report an AtMGT family member, AtMGT9, that functions as a low-affinity Mg^2＋ transporter, and is essential for pollen development. The functional complementation assay in Salmonella mutant strain MM281 showed that AtMGT9 is capable of mediating Mg^2＋ uptake in the sub-millimolar range of Mg^2＋. The AtMGT9 gene was expressed most strongly in mature anthers and was also detectable in vascular tissues of the leaves, and in young roots. Disruption of AtMGT9 gene expression resulted in abortion of half of the mature pollen grains in heterozygous mutant ＋/mgt9, and no homozygous mutant plant was obtained in the progeny of selfed ＋/mgt9 plants. Transgenic plants expressing AtMGT9 in these heterozygous plants can recover the pollen phenotype to the wild type. In addition, At- MGT9 RNAi transgenic plants also showed similar abortive pollen phenotype to mutant ＋/mgt9. Together, our results demonstrate that AtMGT9 functions as a low-affinity Mg^2＋ transporter that plays a crucial role in male gametophyte development and male fertility.
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.
