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 rapa 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.