Elucidation of arsenic detoxification mechanism in Marchantia polymorpha: the role of ACR3

The arsenic-specific ACR3 transporter plays pivotal roles in As detoxification in yeast and a group of ancient tracheophytes, the ferns. Despite putative ACR3 genes being present in the genomes of bryophytes, whether they have the same relevance also in this lineage is currently unknown. In this study, we characterized the MpACR3 gene from the bryophyte Marchantia polymorpha L. through a multiplicity of functional approaches ranging from phylogenetic reconstruction, expression analysis, loss- and gain-of-function as well as genetic complementation with an MpACR3 gene tagged with a fluorescent protein. Genetic complementation demonstrates that MpACR3 plays a pivotal role in As tolerance in M. polymorpha, with loss-of-function Mpacr3 mutants being hypersensitive and MpACR3 overexpressors more tolerant to As. Additionally, MpACR3 activity regulates intracellular As concentration, affects its speciation and controls the levels of intracellular oxidative stress. The MpACR3::3xCitrine appears to localize at the plasma membrane and possibly in other endomembrane systems. Taken together, these results demonstrate the pivotal function of ACR3 detoxification in both sister lineages of land plants, indicating that it was present in the common ancestor to all embryophytes. We propose that Mpacr3 mutants could be used in developing countries as low-cost and low-technology visual bioindicators to detect As pollution in water. ENVIRONMENTAL IMPLICATION Inorganic arsenic (As) is a recognized carcinogen, considered the most hazardous element at the global level due to its ubiquitous presence as environmental pollutant of either natural or anthropic origin. The current work extends previous studies carried out in our lab for the creation of a series of biosensors based on mutants of the ACR3 arsenite transporter in the bryophyte Marchantia polymorpha. Mpacr3 mutants are extremely susceptible to As at levels that in some countries can be found in drinking water, resulting in a cheap and low-technology biosensor potentially useful to people worldwide for detecting As-contaminated water.