Understanding mechanisms of salinity tissue tolerance in plants: lessons from halophytes

S Shabala1, II Pottosin1,2, A Kiani-Pouya1 and R Hedrich1,3

  1. School of Land and Food, University of Tasmania, Australia
  2. Centro Universitario de Investigaciones Biomedicas, Universidad de Colima, Mexico
  3. Institute for Molecular Plant Physiology and Biophysics, University of Wurzburg, Germany

The challenge of feeding 9.3 billion people by 2050 cannot be achieved without a major breakthrough in crop breeding for salinity tolerance. We argue that the current trend of targeting Na+ exclusion mechanisms in breeding programmes for salinity tolerance in crops needs revising, and the progress in this area will be achieved only by learning from halophytes, naturally salt-loving plants. Contrary to crops, halophytes do not exclude Na+ from uptake but instead sequester it either in internal (vacuoles) or external (epidermal bladder cells; EBC) structures, thus achieving efficient osmotic adjustment at low metabolic cost. In this talk, we describe several new mechanisms that emerged as important traits that confer superior salinity tissue tolerance in halophytes over the last few years. This includes (1) better cytosolic K+ retention originating from intrinsically higher plasma membrane H+ pumping ability; (2) efficient control of SV and FV tonoplast channels to prevent futile Na+ cycling between the cytosol and vacuole; and (3) effective Na+ sequestration in external storage organs (such as salt bladders). We provide the evidence that post-translational regulation of H+-ATPase plays a critical role in salinity tissue tolerance, and show choline, a cationic precursor of glycine betaine, as a key component of vacuolar Na+ retention mechanism. We then provide direct evidence for the role of salt bladders in plant salinity tolerance by showing that removal of the bladders result in a salt-sensitive phenotype. It is suggested that the further progress in crop breeding for salinity tolerance may be achieved by delineating mechanisms underlying regulation of transport processes behind tissue tolerance in halophytes and incorporating these traits into them into crop breeding practices.