Friend or Foe? Chloride Patterning in Halophytes

• Published in: Trends in plant science Vol. 24; no. 2; pp. 142 - 151
• Main Authors: Bazihizina, Nadia, Colmer, Timothy D., Cuin, Tracey Ann, Mancuso, Stefano, Shabala, Sergey
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.02.2019; Elsevier BV
• Subjects: Amino acids; Anion channels; Anions; breeding; Ca2+ signalling; Calcineurin; Cbl protein; Chlorides; Efflux; energy; Energy metabolism; Halophytes; Homology; influx; Ion channels; Ion transport; Kinases; membrane transport proteins; nitrates; Osmoregulation; phytomass; Plant biomass; Plant Roots; Post-translation; Protein kinase; Proteins; Salinity; Salinity effects; Salinity tolerance; Salt Tolerance; Salt-Tolerant Plants; Selectivity; Stomata; salinity tolerance; membrane transport proteins; halophytes; influx; Ca2+ signalling; efflux; Ca(2+) signalling
• ISSN: 1360-1385; 1878-4372; 1878-4372
• DOI: 10.1016/j.tplants.2018.11.003

In this opinion article, we challenge the traditional view that breeding for reduced Cl− uptake would benefit plant salinity tolerance. A negative correlation between shoot Cl− concentration and plant biomass does not hold for halophytes – naturally salt tolerant species. We argue that, under physiologically relevant conditions, Cl− uptake requires plants to invest metabolic energy, and that the poor selectivity of Cl−-transporting proteins may explain the reported negative correlation between Cl− accumulation and crop salinity tolerance. We propose a new paradigm: salinity tolerance could be achieved by improving the selectivity of some of the broadly selective anion-transporting proteins (e.g., for NO3−>Cl−), alongside tight control of Cl− uptake, rather than targeting traits mediating its efflux from the root. Interest in the Cl− aspect of salinity tolerance has traditionally focused on non-halophytes. Knowledge of Cl− regulation in ‘salt-loving’ halophytes is limited, even though these plants thrive and survive at much higher external Cl− than non-halophytes and use Cl− for osmoregulation. Proteins catalysing root Cl− transport have recently been characterised in non-halophytes, but this knowledge needs to be extended to halophytes. Of interest is that single amino acid polymorphisms in halophytic transporters alter their function compared to their non-halophytic homologues. Our understanding of post-translational regulation of anion channels in stomata is fairly advanced, but that of root Cl− transport remains elusive. The calcineurin B-like protein (CBL)–CBL-interacting protein kinase (CIPK) network regulates several ion transport systems in plants. Does it also modulate Cl− transport?