AtDTX25, a member of the multidrug and toxic compound extrusion family, is a vacuolar ascorbate transporter that controls intracellular iron cycling in Arabidopsis

Abstract

Iron (Fe) is an essential element, its transport is regulated by the cell redox balance. In seeds, Fe enters the embryo as Fe2+ and is stored in vacuoles as Fe3+. Through its ferric reduction activity, ascorbate plays a major role in Fe redox state and therefore Fe transport within the seed. We searched for ascorbate membrane transporters responsible for controlling Fe reduction by screening the yeast ferric reductase-deficient fre1 strain and isolated AtDTX25, a member of the Multidrug And Toxic compound Extrusion (MATE) family. AtDTX25 was shown to mediate ascorbate efflux when expressed in yeast and Xenopus oocytes, in a pH-dependent manner. In planta, AtDTX25 is highly expressed during germination and encodes a vacuolar membrane protein. Isolated vacuoles from AtDTX25-1 knockout mutant contained less ascorbate and more Fe than wild-type (WT), and mutant seedlings were highly sensitive to Fe deficiency. Iron imaging further showed that the remobilisation of Fe from vacuoles was highly impaired in mutant seedlings. Taken together, our results established AtDTX25 as a vacuolar ascorbate transporter, required during germination to promote the reduction of the pool of stored Fe3+ and its remobilisation to feed the developing seedling.