When salt stress is applied to the tip of a lateral root, a wave of Ca2+ signaling moves through the plant (coded red in this movie). The Ca2+ increases are channeled in specific cell types, the cortex and endodermal cell layers and the wave spreads at about 0.4 mm/sec. Research has shown the Ca2+ wave carries stress information throughout the plants.

For more research in this area, see:
Choi et al. (2014) Proceedings of the National Academy of Sciences USA 111:6497-6502