SOS1, salt, and cryo-imaging of subcellular element distribution

For living organisms, proper control of element location is just as important as the control of enzyme location, but harder to study. A new study by Ramakrishna et al. uses an exciting new technology, cryo nanoscale secondary ion mass spectrometry ion microprobe, to investigate elemental distribution (measured by their major isotopes) in Arabidopsis and rice roots. The images are beautiful, and different elements can be clearly seen localized within different subcellular compartments, including the cytosol, vacuole, and apoplast. Here, the authors investigated the subcellular distribution of sodium (Na⁺) in root cells. Plant cells are very sensitive to cytosolic sodium levels and use pumps to maintain a low concentration in the cytosol. In wild-type plants in mild salt stress (2.5 mM), sodium is almost entirely found in the apoplast, whereas when the salt concentration is moderate (25mM) the sodium mainly accumulates in the vacuole. The authors also examined mutants of a sodium transporter SOS1 (SODIUM OVERLY SENSITIVE 1). SOS1 was previously shown to be a sodium/proton antiporter localized in the plasma membrane that contributes to sodium export from the cytosol. In these sos1 mutant plants the authors observed a very different pattern of sodium accumulation at moderate salt levels; the salt remained elevated in the apoplast with very little accumulating in the vacuole, indicating a role for SOS1 in moving sodium into the vacuole. This study demonstrates a new role for SOS1, and introduces a powerful new imaging technology. (Summary by Mary Williams @PlantTeaching.bksy.social @PlantTeaching) Nature https://doi.org/10.1038/s41586-024-08403-y