Effect of salt stress on K+/Na+ homeostasis, osmotic adjustment, and expression profiles of high-affinity potassium transporter (HKT) genes

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Abstract
Salt stress is one of the major threats affecting crop yield. We assessed the behaviour of three barley genotypes, Ardhaoui, Manel, and Testour under 200 mM NaCl with the aim of evaluating the physiological and molecular mechanisms involved in barley salinity tolerance. Results revealed that salinity stress significantly decreases plant growth and water-holding capacity, particularly in the salt-sensitive genotype Testour. Tissue ionic content assessment demonstrated significantly distinct salinity-induced responses. The salt-tolerant genotype Ardhaoui accumulated more K+ and less Na+ content in both leaves and roots compared with the two other genotypes, leading to an increased K+/Na+ ratio. Furthermore, the genotype Ardhaoui exhibited a stronger selectivity transport capacity of K+ over Na+ from root to leaf compared to both Manel and Testour. This effect was due to enhanced K⁺ retention and Na⁺ exclusion, regulated by HvHKT expression. Indeed, higher HvHKT2;1 gene transcript abundance was detected in both leaves and roots of the Ardhaoui genotype, as well as an upregulation of HvHKT1;1 and HvHKT1, mainly in Ardhaoui roots. In view of the severe impact of salinity on plant development, these findings could be applied to the genetic improvement of plant salinity tolerance.

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