Na+ compartmentation strategy of Chinese cabbage in response to salt stress

Na+/H+ antiporter (NHX), responsible for counter-transport of Na+ and H+ across membranes (Na+ compartmentalization), plays a central role in plant salt-tolerance. In order to explore the Na+ compartmentalization modes and salt tolerance strategy in Chinese cabbage (Brassica rape L. ssp. pekinensis), the seedlings of a salt-susceptible cabbage cultivar (Kuaicai 38) and a salt-tolerant cabbage cultivar (Qingmaye) were exposed to 100-400 mM NaCl for 30 days. Both of these cultivars showed a gradual decrease in fresh weight and water content and an increase in root-shoot ratio with the increasing NaCl-treatment concentration. The distribution of Na+ in these two cultivars was similar, with the green leaves showing the highest Na+ content, followed by inflated midribs, stems, and roots. The Na+ concentration in the apoplast was higher than that in the protoplast of the leaves. The expression levels of BrNHX1-1 and BrNHX1-2 in the leaves of Qingmaye were the highest among all BrNHX members, and increased after salt treatment. However, only BrNHX1-1 was expressed in Kuaicai 38. These results indicate that Na+ compartmentation into vacuoles is the major salt-adaptation strategy in Chinese cabbage. Coordinated overexpression of BrNHX1-1 and BrNHX1-2 may confer greater salt-tolerance for Chinese cabbage.