Insight into phytohormonal modulation of defense mechanisms to salt excess in a halophyte and a glycophyte from Asteraceae family

The aim of this study was to compare the efficiency of three defense mechanisms (ionic balance, osmotic adjustment and counteracting oxidative stress) under low, moderate and high soil salinity in two related species of contrasting tolerance to salinity: the halophyte Aster tripolium and the glycophyte Aster alpinus, and to elucidate their phytohormone-mediated regulation. The phytohormonal profiling was performed to asses correlations between the pool of plant growth regulators and parameters depicting ionic homeostasis, osmotic adjustment and antioxidant system. Defense mechanisms in both species were based on accumulation/activity of distinct compounds (Na+ and K+ ions, antioxidants), but differences among species concerned particularly soluble carbohydrates and betaines. The halophyte accumulated mannitol, uronic acids and sucrose, while the glycophyte mostly glucose and proline-betaine. The halophyte responses also correlated with changes in the content of plant growth promoting PGRs, as well as jasmonates and benzoic acid. The glycophyte responses corresponded with changes in content of abscisic acid and ethylene precursor, as well as salicylic acid. We provided evidence that benzoic acid rather than salicylic acid is involved in salt tolerance in the halophyte and elevated SA content may enhance vulnerability to salt excess. An important element of tolerance trait is also JA-GA network that influences the intensity of defense responses. This study uncovers new aspects of internal phytohormonal regulation of plant reaction to soil salinity and enables further insight into extremophyte biology.