A role for the AtMTP11 gene of Arabidopsis in manganese transport and tolerance

The Arabidopsis AtMTP family of genes encode proteins of the cation diffusion facilitator (CDF) family, with several members having roles in metal tolerances. Four of the 11 proteins in the family form a distinct cluster on a phylogenetic tree and are closely related to ShMTP8, a CDF identified in the tropical legume Stylosanthes hamata that is implicated in the transport of Mn2+ into the vacuole as a tolerance mechanism. Of these four genes, AtMTP11 was the most highly expressed member of the Arabidopsis subgroup. When AtMTP11 was expressed in Saccharomyces cerevisiae, it conferred Mn2+ tolerance and transported Mn2+ by a proton-antiport mechanism. A mutant of Arabidopsis with a disrupted AtMTP11 gene (mtp11) was found to have increased sensitivity to Mn2+ but not to Cu2+ or Zn2+. At a non-toxic but sufficient Mn2+ supply (basal), the mutant accumulated more Mn2+ than the wild type, but did not show any obvious deleterious effects on growth. When grown with Mn2+ supplies that ranged from basal to toxic, the mutant accumulated Mn2+ concentrations in shoots similar to those in wild-type plants, despite showing symptoms of Mn2+ toxicity. AtMTP11 fused to green fluorescent protein co-localized with a reporter specific for pre-vacuolar compartments. These findings provide evidence for Mn2+-specific transport activity by AtMTP11, and implicate the pre-vacuolar compartments in both Mn2+ tolerance and Mn2+ homeostasis mechanisms of Arabidopsis.