Abstract
The sodium/proton exchanger (NHX) mediates Na+ and H+ countertransport in plants and plays an important role in regulating intracellular pH and maintaining ion and osmotic balance. Previously, an NHX1 orthologue was isolated from the halophyte Nitraria sibirica Pall (referred to as NsNHX1), and its role in enhancing salt tolerance of transgenic Arabidopsis was confirmed. To further analyse its features and functions, the 1311-bp sequence of the NsNHX1 promoter was cloned, and histochemical staining showed that β-glucuronidase (GUS) expression driven by the NsNHX1 promoter was strongly induced by abiotic stress and phytohormones, such as salt, drought, gibberellins, and methyl jasmonate, in turn indicating that NsNHX1 might participate in the regulation of various signalling pathways. To determine how NsNHX1 regulates salt tolerance in forestry trees, NsNHX1 was introduced into 84K poplar, and salt tolerance analysis of transgenic poplars showed that overexpression of NsNHX1 increased the overall biomass, survival rate, and plant height, and the contents of chlorophyll, proline and water, all of which are consequences of antioxidant enzyme activity under salt stress conditions. This showed that overexpression of NsNHX1 enhanced the salt tolerance of transgenic poplars as a result of NsNHX1-mediated Na+ compartmentalisation, more efficient photosynthesis, greater activity of antioxidant enzymes, and improved osmotic adjustment. Moreover, overexpression of NsNHX1 enhanced the root development of transgenic poplars; this resulted in increased biomass and height under normal and salt stress conditions, likely due to coupling between NsNHX1 and membrane proton pumps. These results provided a theoretical and experimental basis for further understanding the function and regulatory mechanism of NsNHX1, as well as its application for genetic improvement of forestry trees.
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