Tropisms are directional growth responses that allow plants to adjust to changes in their environment. Their underlying mechanisms involve intracellular redistribution of plant hormones, called auxins, to restrict cell elongation to one side of the shoot or root and thus cause it to bend toward or away from the signaling source in response to gravity, water and light. Now Testerink and colleagues show that plants use a similar mechanism, which they call halotropism, to avoid excess salt. They observed that Arabidopsis and tomato seedlings direct root growth away from NaCl gradients assembled in agar media or in soil. Auxin-responsive reporter constructs revealed that auxin selectively accumulates within cells of the lateral root tip on the side opposite the high salt; chemical disruption of this redistribution eliminates the halotropic response. The differential accumulation of auxins during plant development is partially mediated by PIN-FORMED auxin efflux carriers (PINs) in the plasma membrane. Two members of this family, PIN2 and PIN3, control auxin accumulation in the root in response to gravity signaling. Monitoring of the fate of PIN-GFP fusions in seedlings reveals that cell-surface localization of PIN2 but not of other carriers is reduced in root epidermal cells that face elevated salt levels. The authors find that directional root growth is modulated by PIN2 endocytosis, not degradation, as it is in response to gravity signals, results further suggesting that NaCl triggers a distinct signaling pathway. Moreover, PIN2's subcellular relocalization is not induced by mannitol, indicating that halotropism is distinct from the osmotic-stress response. PIN2 internalization and halotropism are both blocked by chemical inhibition of clathrin assembly. Indeed, differential clathrin recruitment to the plasma membrane is induced by salt treatment and is controlled by the phosphatidic acid–generating enzyme phospholipase D (PLD). PLD inhibitors or mutations that reduce its activity compromise clathrin recruitment and disrupt PIN2 internalization in response to salt stress. That PLD is also implicated in gravity, water and osmotic-stress responses suggests that its fluctuations may serve to integrate multiple environmental signals to optimize root growth in response to changing conditions. (Curr. Biol. doi:10.1016/j.cub.2013.08.042, 21 October 2013)