The researchers determined that roots are able to move against gravity and away from high salt due to a hormone called auxin that coordinates cell growth and development. When the plant's roots come in contact with a salty environment, the auxin goes to the non-salty side. They looked closer to see what was going on in the root cells exposed to salt solution. It turns out that the salt activates an enzyme (phospholipase D), which leads to a protein (PIN2) that normally sits at the surface cells to be collected in tiny bubbles within the cell. The movement of PIN2 away from the surface causes auxin to move from the salty side of the root to the non-saline side. PIN2 was previously shown to be important in response to gravity (gravitropism), which suggests that PIN2 may be important for plants' response to environmental cues.
1Galvan-Ampudia CS, Julkowska MM, Darwish E, Gandullo J, Korver RA, et. al. (2013) Halotropism is a response of plant roots to avoid a saline environment. Curr Biol. Oct 21;23(20), 2044-2050. doi: 10.1016