The energy cost of the tonoplast futile sodium leak.

Active removal of Na+ from the cytosol into the vacuole plays a critical role in salinity tissue tolerance, but another, often neglected component of this trait is Na+ retention in vacuoles. This retention is based on an efficient control of Na+ -permeable slow- and fast-vacuolar channels that mediate the back-leak of Na+ into cytosol and, if not regulated tightly, could result in a futile cycle. This Tansley insight summarizes our current knowledge of regulation of tonoplast Na+ -permeable channels and discusses the energy cost of vacuolar Na+ sequestration, under different scenarios. We also report on a phylogenetic and bioinformatic analysis of the plant two-pore channel family and the difference in its structure and regulation between halophytes and glycophytes, in the context of salinity tolerance.