In the halotolerant Lobularia maritima (Brassicaceae) salt adaptation correlates with activation of the vacuolar H(+)-ATPase and the vacuolar Na+/H+ antiporter

Abstract

Lobularia maritima (Brassicaceae) is a facultative halophyte related to Arabidopsis thaliana and may be a suitable model to identify molecular mechanisms that regulate tolerance to salt stress in plants. Under the same salt stress conditions, the accumulation of sodium was similar in shoots and roots of Lobularia maritima and Arabidopsis thaliana, whereas the sodium to potassium ratio was less in Lobularia maritima. Aquaporins, the NHX-type Na(+)/H(+) antiporter, and the vacuolar ATPase are well established targets of regulation under salt stress that have a central role in the control of water status and cytoplasmic sodium homeostasis. Therefore, salt-dependent expression of transcripts encoding a PIP2;1 aquaporin, the Na(+)/H(+) antiporter NHX, and V-ATPase subunit E (VHA-E) was characterized in Lobularia maritima. Transcription of LmPIP2;1 was repressed in leaves and roots by treatment with 500mM NaCl. In contrast, salt stress stimulated the expression of LmNHX1 and LmVHA-E. Cell-specificity of the transcription of LmNHX1 was analyzed by fluorescence in situ PCR in leaf cross sections of Lobularia maritima. Expression of the gene was localized to the phloem and to mesophyll cells. In plants treated with 500 mM NaCl, transcription of LmNHX1 was stimulated in the mesophyll. The findings indicate divergent transcriptional responses of key mechanisms of salt adaptation in Lobularia maritima and suggest distinct regulation of sodium homeostasis and water flux under salt stress.