Sodium is the critical factor leading to the positive halotropism of the halophyte Limonium bicolor

Previous studies indicate that the roots of nonhalophytes showed negative halotropism to salt stress to avoid salt damage. However, halotropism of euhalophytes and their possible reasons are little known. Limonium bicolor, a typical recretohalophyte with multicellular salt glands, was used to study halotropism compared with Arabidopsis thaliana under NaCl, KCl and Na2SO4 stress. The elongation of the roots in L. bicolor was significantly promoted by the appropriate concentrations of NaCl, KCl and Na2SO4, but those of A. thaliana was markedly inhibited. However, isosmotic mannitol with 200 mM NaCl did not affect the root growth of both L. bicolor and A. thaliana. The root activity of both L. bicolor and A. thaliana was enhanced by salts. Compared with K+, Cl-, and SO42-, Na+ played a critical role in halotropism of L. bicolor. Furthermore, the gravitropic setpoint angle of L. bicolor increased under NaCl, KCl and Na2SO4 treatments compared with controls, and the phenomenon was most apparent under NaCl treatments. The endogenous IAA content of the NaCl-treated L. bicolor seedlings was significantly higher than that of the controls. These results suggest that the recretohalophyte L. bicolor has positive halotropism and Na+ plays a pivotal role in L. bicolor's positive root halotropism by regulating IAA.