Sodium fluxes along the Arabidopsis root

Cuin TA, Bazihizina N and Shabala S

Tasmanian Institute of Agriculture, University of Tasmania, Hobart, Tasmania.

Despite decades of research into plant salinity tolerance, the patterns of root sodium fluxes remain unclear. The current consensus is that large quantities of sodium go passively into root cells and are subsequently expelled in a futile cycle that consumes large amounts of metabolic energy. However, no transport system for cellular sodium influx has been definitively identified, and only one transporter, SOS1, is recognised as mediating sodium efflux. However, this antiporter is localised only to the root tip. What happens along the rest of the root? Perhaps sodium does not enter the cell at all, but instead emanates from sodium cycling in the apoplast [1]. We need to assess how much and where sodium enters the root to discern whether cellular cycling is actually significant. There is no point developing crops with increased sodium export if its efflux is from the apoplast, not the cell. NaCl induces membrane depolarisation of at least 60 mV in Arabidopsis roots [2]. This implies large sodium influxes into cells. Also in [2], we demonstrated that sos mutations affect the ion transport patterns in the entire root, not just the tip. We now revisit this work with an improved sodium LIX [3], measuring net sodium fluxes along Arabidopsis roots. Using transport mutants and pharmacology, we are deciphering the extent of sodium cycling in the root and starting to answer questions as to the nature and location of root sodium transport. [1] Britto and Kronzucker, 2015. J. Plant Physiol 186-187: 1-12 [2] Shabala et al., 2005. Planta 222: 1041-1050 [3] Jayakannan et al., 2011. J. Plant Physiol 168: 1045-1051.