Salt-stress signalling and the role of calcium in the regulation of the Arabidopsis ATHB7 gene

In plants changes in cytosolic calcium ion concentrations ([Ca2+](cyt)) have been detected after various stress treatments, including salt treatment. The involvement of a Ca2+ signal as an essential component of signalling pathways leading to downstream responses, such as gene expression, is supported only by a few studies. In this study the possible involvement of the salt stress-induced increase in [Ca2+](cyt) in the signalling pathway leading to the induction of ATHB7, a homeobox gene encoding a homeodomain leucine zipper (HDZip) transcription factor was analysed. The salt-induced expression of ATHB7 was found to be independent of the Ca2+ signal evoked by salt. Instead, it was found that ATHB7 expression in shoots was not dependent on a direct contact with salt or osmoticum, whereas in roots, ATHB7 seemed to be induced by the direct contact, indicating that signals from roots cause systemic induction of ATHB7. Abscisic acid (ABA) or ABA-dependent components were found to, at least partly, to function as the systemic signal.