The Functional Analysis Of AtMGT7 In Arabidopsis Gene Family Encoding Putative Mg～(2+) Transporters

Magnesium( Mg²⁺)is the most abundant divalent cation in plant cells and plays a critical role in many physiological processes.Mg²⁺ is unique among the major biological cations in its chemical properties.Despite these critical cellular functions and unique chemical properties,Mg²⁺ uptake,transportand homeostasis in eukaryo -tes are poorly understood at both the physiological and the molecular level.Schock(2000) and Li(2001) independently identified a putative Mg²⁺ transport gene family (AtMGT) with 10-members in Arabidopsis.We report here some of the functional analysis results about one of the AtMGT members,AtMG77 gene.\AtMGT7 gene encodes two mRNAs transcripts resulted from alternative splicing variants, designated AtMGT7 and AtMGT7 -1 ,revealing there exist two splicing variant proteins in plant.2.The two mRNA variants were expressed with different patterns.AtMGT7 mRNA exists mainly in root, stem, and leaf tissues, and weakly. in the flower. However, total mRNA level was also very high in theflowers suggesting that AtMGT7-l mRNA accumulated preferentially inthe flower tissues. The AtMGT7 total mRNA was not detected in siliques.3.AtMGT7 functionally complemented a bacterial mutant lacking Mg²⁺ transport capability while AtMGT7-l did not.4.The ⁶³Ni²⁺ tracer analysis in the bacterial model showed that AtMGT7 mediated low-affinity transport of Mg²⁺ ,and also may be involved in the transport of Zn²⁺. Furthermore,AtMGT7 may be capable of transporting several other divalent cations,including Ni²⁺, Co²⁺, Fe⁺² ,Mn⁺²and Cu⁺² .However,the concentrations required for transport of these other cations are beyond normal physiological ranges. Consistent with the complementation assay result, ⁶³Ni²⁺ tracer analysis revealed that AtMGT7-1 did not transport Mg²⁺, This result also suggested that AtMGT7-1 might be able to transport Co²⁺ instead of Mg²⁺.5.Using green fluorescence protein as a reporter, we localized AtMGT7 and AtMGT7-1 proteins to the plasma membrane in Arabidopsis plants.This is the first molecular analysis of low-affinity Mg²⁺ transporter in plant, and the first report on alternative splicing variants of Mg²⁺ transporters with distinct functions.