Mepiquat chloride-priming induced salt tolerance during seed germination of cotton (Gossypium hirsutum L.) through regulating water transport and K+/Na+ homeostasis

The present study aimed to demonstrate the role of mepiquat chloride (MC) on the alleviation of the damaging effects of salinity on the germination of four cotton cultivars. Based on the ecophysiological data, CCRI44, CCRI49, and Z571 were less sensitive to salt stress and relied on a salt tolerance strategy to combat salinity at germination. Contrary to those salt-tolerance cultivars, seed germination of CZ91 was significantly inhibited by salt stress but most un-germinated seeds recovered completely when transferred to distilled water, indicating CZ91 follow a salt-avoiding strategy. Priming seeds with MC resulted in nullifying the negative effects of salt stress in salt-tolerance cultivars (especially in the cultivar Z571), but no significant response was observed in the salt-avoiding cultivar CZ91. At the mechanistic level, three complementary physiological traits are modulated by MC-priming upon exposure to salt stress: (i) better OA via increasing the water permeability and solute (i.e., Na+ or K+) mobility probably by regulating PIPs-regulated water channels; (ii) higher tissue tolerance ability via positively maintaining Na+ and K+ homeostasis by co-ordinated regulation of multiple ion transporters (e.g. SOS, HKT1, NHX1, AKT1 and HAK5); and (iii) further enhance the function of ion transport systems via inducing the gene expression of H+-ATPase. The strength of the three physiological traits varied among the MC pretreated cultivars, conferring differential ameliorating roles. Taken together, our observations confirmed that the effect of MC-priming on the alleviation of salinity stress is quite complex and dependent on the cultivar. It also shed light on transcriptional regulation of water and key ion transport systems involved in the maintenance of water absorption and K+/Na+ homeostasis as a key attribute for inducing salt tolerance by MC-priming in cotton cultivars.