Elemental cryo-imaging reveals SOS1-dependent vacuolar sodium accumulation

Increasing soil salinity causes significant crop losses globally; therefore, understanding plant responses to salt (sodium) stress is of high importance. Plants avoid sodium toxicity through subcellular compartmentation by intricate processes involving a high level of elemental interdependence. Current technologies to visualize sodium, in particular, together with other elements, are either indirect or lack in resolution. Here we used the newly developed cryo nanoscale secondary ion mass spectrometry ion microprobe1, which allows high-resolution elemental imaging of cryo-preserved samples and reveals the subcellular distributions of key macronutrients and micronutrients in root meristem cells of Arabidopsis and rice. We found an unexpected, concentration-dependent change in sodium distribution, switching from sodium accumulation in the cell walls at low external sodium concentrations to vacuolar accumulation at stressful concentrations. We conclude that, in root meristems, a key function of the NHX family sodium/proton antiporter SALT OVERLY SENSITIVE 1 (also known as Na+/H+ exchanger 7; SOS1/NHX7) is to sequester sodium into vacuoles, rather than extrusion of sodium into the extracellular space. This is corroborated by the use of new genomic, complementing fluorescently tagged SOS1 variants. We show that, in addition to the plasma membrane, SOS1 strongly accumulates at late endosome/prevacuoles as well as vacuoles, supporting a role of SOS1 in vacuolar sodium sequestration.

We thank F. Plane, A. Mucciolo and J. Daraspe for technical advice and assistance. We thank D. Salt for critical comments on the manuscript. We thank the Central Imaging Facility of the University of Lausanne and the Scientific Center for Optical and Electron Microscopy (ScopeM, ETH Zurich) for technical support. This work was supported by grant PID2022-140705OB-I00 (AEI-MCIN, Spain, co-financed by ERDF) to F.J.Q., and by in-house funding from ETH Zurich to C.S.-R., from the University of Lausanne to N.G. and from EPFL to A.M. The development of the CryoNanoSIMS was funded by EPFL, the University of Lausanne, grants from the Swiss National Science Foundation (206021_150762, 200021_179092 and

Peer reviewed