GABA operates upstream of H⁺-ATPase and improves salinity tolerance in Arabidopsis by enabling cytosolic K⁺ retention and Na⁺ exclusion

The non-protein gamma-aminobutyric acid (GABA) rapidly accumulates in plant tissues in response to salinity. However, the physiological rationale beyond this elevation remains elusive. This study compared electrophysiological and whole-plant responses of salt-treated Arabidopsis thaliana mutants pop2-5 and gad1,2 that possess different abilities for GABA accumulation. The pop2-5 mutant that was capable to over-accumulate GABA in its roots showed a salt-tolerant phenotype. On the contrary, gad1,2 mutant lacking an ability for conversion of glutamate to GABA showed over-sensitivity to salinity. The differential salinity tolerance between two lines was explained by: (1) the role of GABA in the stress-induced activation of H+-ATPase thus leading to better membrane potential maintenance and reduced extent of stress-induced K+ leak from roots; (2) reduced rates of net Na+ uptake in pop2-5 roots; (3) higher expression of SOS1 and NHX1 genes in leaves of salt-tolerant pop2-5 plants, which contributed to reducing Na+ concentration in the cytoplasm by excluding Na+ to apoplast and sequestering Na+ in vacuole; (4) lower rate of H2O2 production and reduced ROS-inducible K+ efflux from root epidermis in the tolerant line; and (5) better K+ retention in the shoot associated with the lower expression level of GORK channels in plant leaves