There are a couple of strategies that salt-tolerant plants can use to survive in high-salt conditions.
Plants can increase the concentration of compatible solutes in their cell to the point where the intracellular concentration of solute approaches the concentration of the water in the environment. By doing this, the plants are able to minimize water loss through osmosis, while ensuring that the cells do not reach harmful levels of salinity.
However, compatible solutes are often not enough to completely combat the effects of salt stress. Salt stress creates higher concentrations of oxygen free radicals, which can cause harm to DNA and other cell components. Mangrove trees have been found to have heightened expression of repair proteins and anti-oxidants when exposed to salt stress.
Additionally, mangroves have been found to be able to excrete salt from their root cells and their leaves. They have ion-transporters which actively pump sodium ions out of their cells into the extracellular space. They have salt-filtering mechanisms that allow them to uptake the water from salt-water without up-taking the salt in the water.
So in conclusion, salt-tolerant plants sequester some salt within their cells, but also use ion-pumps and compatible solutes to limit the concentrations of salt within their cells. They can selectively uptake water from salt water, and use repair mechanisms to limit the damage of high salt concentrations in their cells.
Kao, C.H. 2015. Mechanisms of salt tolerance in rice plants: compatible solutes and aquaporins. Crop, Environment & Bioinformatics 12:73-82.
Wang, L., Pan, D., Lv, X., Cheng, C.L., Li, J. Liang, W., Xing, J, and W. Chen. 2016. A multilevel investigation to discover why Kandelia candel thrives in high salinity. Plant, Cell & Environment 39:2486-2497.
Srikanth, S., Kaihekulani, Lum, Y., and Z. Cheng. 2016. Mangrove root: adaptations and ecological importance. Trees 30:451-465