The cost of being salt tolerant

R Munns1,2,3, T Colmer2, R James3, M Gilliham1,4 and S Tyerman1,4

  1. ARC Centre of Excellence in Plant Energy Biology
  2. School of Plant Biology, University of Western Australia
  3. CSIRO Agriculture
  4. University of Adelaide

Plants adopt various strategies to grow in a saline soil. The essential achievement is osmotic adjustment. All cells must accumulate sufficient solutes to balance the extra osmotic pressure of the soil solution, to maintain turgor and volume, while avoiding the risk of Na or Cl toxicity in key cytoplasmic compartments – the cytosol, mitochondria and chloroplasts. To achieve this, there are two alternative strategies: (1) excluding most of the Na and Cl but relying on energy-expensive organic solutes for osmotic adjustment or (2) allowing the uptake of sufficient Na and Cl to balance that in the soil solution while having strict ionic regulation in the cytoplasmic compartments. In this talk we consider the most cost-effective strategy. Calculations show that plants relying on the first strategy cannot grow in salinities above 150 mM NaCl as the energy demanded for osmotic adjustment using organic solutes would be more than the measured respiration rate. Salt-tolerant plants must use Na and Cl for osmotic adjustment of the vacuole. Yet genetic variation indicates that the costs of cellular compartmentation are considerable. ATP costs for the various processes involved in salt tolerance are discussed.