An injured plant root can transmit a mechanical warning through its vascular system far faster than the calcium signals normally associated with plant defence.
Experiments with Arabidopsis show that damage produces a hydraulic pressure change travelling at about 75 millimetres per second. Mechanosensitive MCA1 channels in undamaged tissue detect that disturbance and convert it into electrical and calcium signals, while glutamate-receptor-like channels amplify the response and help activate stress hormones. Repeated signals can also reduce subsequent sensitivity, adding a form of local adaptation to the alarm network.
https://www.earth.com/news/plants-plant-roots-send-hidden-warning-signals-when-they-are-attacked/
