The chloroplast-located HKT transporter plays an important role in fertilization and development in Physcomitrium...

Cell survival depends on the maintenance of cell homeostasis that involves all the biochemical, genomic and transport processes that take place in all the organelles within a eukaryote cell. In particular, ion homeostasis is required to regulate the membrane potential and solute transport across all membranes, any alteration in these parameters will reflect in the malfunctioning of any organelle, and consequently, in the development of the organism. In plant cells, sodium transporters play a central role in keeping the concentrations of this cation across all membranes under physiological conditions to prevent its toxic effects. HKT transporters are a family of membrane proteins exclusively present in plants, with some homologs present in prokaryotes. HKT transporters have been associated to salt tolerance in plants, retrieving any leak of the cation into the xylem, or removing it from aerial parts, including the flowers, to be transported to the roots along the phloem. This function has been assigned as most of the HKT transporters are located at the plasma membrane. Here, we report the localization of the HKT from Physcomitrium patens to the thylakoid membrane, reminiscent of the prokaryote origin of these family of transporters. Mutation of PpHKT leads to several alterations in the phenotype of the organism, including the lack of sporophyte formation, and changes in expression of many genes. These alterations suggest that the breakdown in chloroplast ion homeostasis triggers a signalling cascade to the nucleus to communicate its status, being important for the moss to complete its life cycle.