NPF2 is involved in intracellular pH regulation and ion balance in the diatom Phaeodactylum tricornutum

: Cellular ion and pH homeostasis are crucial factors affecting and regulating metabolic reactions, protein stability, signalling and transport mechanisms. To balance intracellular ion concentrations, organisms need to constantly sense and respond to both the extracellular environment and cell compartments' requests. This fine-tuned system has been deeply studied in organisms, such as land plants and animals, but for diatoms, living in a highly variable environment, the key regulators of intracellular homeostasis have not yet been characterized. Previous results showed a pH response in the expression of the Phaeodactylum tricornutum NRT1/PTR Family transporter 2 (NPF2) gene, homologous to bacterial Proton-coupled Oligopeptide Transporters (POTs) involved in proton translocation across membranes. Here, the P. tricornutum NPF2 protein was shown to localize in the periplastidial compartment, between two of the four membranes of diatom chloroplasts, and to play a role in maintaining intracellular pH (pHi) homeostasis as well as in controlling photosynthetic and photo-acclimation processes. Moreover, transcriptomic analyses comparing the expression profiles of wild-type and a NPF2 knockout mutant strain highlighted an overall regulation of stress inducible genes, critical in maintaining redox homeostasis. These results allow us to propose PtNPF2 as a key player in maintaining pHi homeostasis, with important implications for cellular processes that are sensitive to pH changes.