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Fluorine is the 13th-most abundant element on earth, found most often bound to other elements in its negatively charged form, fluoride. Fluoride compounds are used to improve dental health, to make steel, and to make useful materials like Teflon. Fluoride is also emitted into the environment as a byproduct of both natural and industrial processes. Fluoride even contaminates the fertilizer used to help plants grow. In high amounts, fluoride can be toxic. Single-celled organisms like bacteria protect themselves by making a transporter that specifically removes fluoride from the cell. Yeast have a similar transporter called FEX (fluoride exporter). Bacteria and yeast without these transporters die in the presence of the small amount of fluoride found in tap water. Plants are more complicated, but they also use FEX to keep fluoride from building up inside themselves. Plants without FEX can not make new seeds when grown in normal soil. 

Abstract Fluoride is everywhere in the environment, yet it is toxic to living things. How biological organisms detoxify fluoride has been unknown until recently. Fluoride-specific ion transporters in both prokaryotes (Fluoride channel; Fluc) and fungi (Fluoride Exporter; FEX) efficiently export fluoride to the extracellular environment. FEX homologs have been identified throughout the plant kingdom. Understanding the function of FEX in a multicellular organism will reveal valuable knowledge about reducing toxic effects caused by fluoride. Here, we demonstrate the conserved role of plant FEX (FLUORIDE EXPORTER) in conferring fluoride tolerance. Plant FEX facilitates the efflux of toxic fluoride ions from yeast cells and is required for fluoride tolerance in plants. A CRISPR/Cas9-generated mutation in Arabidopsis thaliana FEX renders the plant vulnerable to low concentrations (100-µM) of fluoride at every stage of development. Pollen is particularly affected, failing to develop even at extremely low levels of fluoride in the growth medium. The action of the FEX membrane transport protein is the major fluoride defense mechanism in plants.