Hormonal dynamics during salt stress responses of salt-sensitive Arabidopsis thaliana and salt-tolerant Thellungiella salsuginea

Salt stress responses in salt-sensitive Arabidopsis thaliana (2-150 mM NaCl) and the closely related salt-tolerant Thellungiella salsuginea (Eutrema halophila, 150-350 mM NaCl) were compared to identify hormonal and transcriptomic changes associated with enhanced stress tolerance. Phytohormone levels, expression of selected genes, membrane stability, and Na+ and K+ concentrations were measured in shoot apices, leaves, and roots. Thellungiella exhibited higher salt stress tolerance associated with elevated basal levels of abscisic acid and jasmonic acid, and lower levels of active cytokinins (excluding cis-zeatin) in shoot apices. Analysis of the dynamics of the early salt stress response (15 min to 24 h) revealed that the halophyte response was faster and stronger. Very mild stress, in our hydropony arrangement 2-25 mM NaCl, affected the transcription of genes involved in cytokinin metabolism (AtIPTs, AtCKXs). Mild stress induced in Arabidopsis (50 mM) stress responses only in shoot apices, while in Thellungiella (150 mM) across the whole plant. Arabidopsis exhibited in hydropony evidence of severe stress above 75 mM NaCl and died in 150 mM, whereas the halophyte only became severely stressed above 225 mM. The responses of individual phytohormones (cytokinins, auxin, abscisic acid, jasmonic acid, salicylic acid and their metabolites) to salinity are discussed.