The MAR1 transporter is an opportunistic entry point for antibiotics

The vast quantities of antibiotics used in modern agriculture contaminate the environment and threaten human health. Recent studies have shown that crop plants grown in soil fertilized with manure from antibiotic-treated animals can accumulate antibiotic within the plant body, thus making them an additional antibiotic exposure route for consumers. Until recently, mechanisms of antibiotic entry and subcellular partitioning within plant cells were virtually unknown. We have uncovered and characterized a transporter gene in Arabidopsis thaliana, MAR1, which appears to control antibiotic entry into the chloroplast. Antibiotic resistance via MAR1 is specific to the aminoglycoside class, and is conferred by loss-of-function mutations, which is rather unusual, since most transporter-based antibiotic resistance is conferred by overexpression or gain-of-function mutations in efflux pumps with poor substrate specificity. Since MAR1 overexpression lines exhibit various iron starvation phenotypes, we propose that MAR1 transports an iron chelation molecule that is mimicked specifically by aminoglycoside antibiotics, and this facilitates their entry into the chloroplast. Knowledge about MAR1 enhances our understanding of how antibiotics might enter the plant cell, which may aid in the production of crop plants that are incapable of antibiotic accumulation, as well as further the development of new plant-based antibiotic resistance markers.Key words: antibiotic, contamination, transport, import, chloroplast, membrane, iron, chelation, nicotianamine