Polyamine and tyramine involvement in NaCl-induced improvement of Cd resistance in the halophyte Inula chrithmoides L

The aim of the present work was to analyze the impact of salinity on the plant response to Cd toxicity in the Mediterranean halophyte species Inula crithmoides. For this purpose, cuttings were cultivated hydroponically during 21d in the presence of 0, 25 or 50μM CdCl₂ combined or not with 0, 100, 200 and 400mM NaCl. The obtained data demonstrated that, in the absence of Cd, NaCl strongly increased plant growth (the maximal dry weight being observed at 100mM) and enhanced the Na⁺/K⁺ ratio in the shoot. Cd alone strongly affected plant growth in this halophyte. However, in Cd-treated plants, NaCl protected Inula crithmoides from Cd toxicity and contributed to reduce Cd absorption and translocation. Small aliphatic polyamine (putrescine, spermidine, spermine) increased in response to both NaCl and CdCl₂, the highest concentration in plants being observed when both agents are present in the medium. The recorded increase preferentially concerned the polyamine bound fraction, which might be related to their involvement in the protection of endogenous cellular structures. The aromatic monoamine tyramine also strongly increased in response to Cd toxicity and its putative role is discussed in relation to conjugation processes. Salinity and Cd increased ammonium/nitrate ratio in leaves and roots and the involvement of stress-induced modification of N nutrition on polyamine oversynthesis is also discussed.