Elevated expression of <i>SaMTP8.1</i> is involved in internal Mn detoxification in the hyperaccumulating ecotype of <i>Sedum alfredii</i>

Sedum alfredii (Sa) is known as a Cd/Zn hyperaccumulator, which usually grows in soil with high Mn in its natural habitat. However, it is unclear how S. alfredii copes with high Mn at both physiological and molecular levels. In this study, we characterized the Mn accumulation and tolerance in the hyperaccumulating ecotype (HE) of S. alfredii by comparing it with a non-hyperaccumulating ecotype (NHE). HE and NHE accumulated similar Mn in the leaves after exposure to high Mn, but the young leaves of NHE showed toxicity symptoms (brown spot), whereas no such symptom was observed in HE. Functional characterization of SaMTP8.1 showed that SaMTP8.1 from both HE and NHE was localized to the tonoplast and showed similar transport activity for Mn in yeast. However, SaMTP8.1 from HE showed a higher expression level and increased genomic copy number compared with NHE. Ectopic expression of SaMTP8.1 in rice osmtp8.1 mutant complemented the mutant phenotype of Mn sensitivity, while overexpression of SaMTP8.1 in Arabidopsis enhanced tolerance to high Mn. Taken together, our results suggest that higher expression of SaMTP8.1 is involved in enhanced Mn tolerance through increased vacuolar sequestration of Mn in the leaves of HE S. alfredii.