Injured thale cress roots warn the plant's other roots with a pressure wave at 75 millimetres a second

At about 75 millimetres per second, the hydraulic wave covers 4.5 metres a minute ^[3][1]. According to the Würzburg account, that is roughly 10,000 times faster than previously reported calcium waves ^[3]. Taken at face value, the ratio puts those calcium waves near 7.5 micrometres per second ^[2]. The team's case for why speed matters is simple: a fast wave lets roots that are still intact start their defences early ^[17].

The cause is the pressure plants keep inside their cells. "As soon as a root cell is injured, there is an immediate drop in intracellular pressure, which is very high in plants," Baudon said ^[4]. That loss then spreads through the vascular system ^[3].

In this study, neighbouring roots are the same plant's other roots. The authors set out to explain how damage is signalled over long distances within one root system ^[5]. "While the defense mechanisms of aboveground plant parts have already been extensively studied, communication within the root system remained a mystery for a long time," Baudon said ^[16].

The root also uses a different sensor from the leaf. In the shoot, the anion channel MSL10 plays the key role; in the root, the calcium-conducting channel MCA1 dominates ^[10]. The signal travels differently as well. "In the leaves, warning signals often propagate over long distances without significant attenuation. In the root, however, the signal strength decreases exponentially with distance from the site of injury," Baudon said ^[11]. He argues the limit makes biological sense, because soil pests move through compact soil much more slowly than insects move across leaf surfaces ^[12].

I think the cleanest experiment in the study is the one with no wound in it. The team used optogenetics to fire calcium signals in intact root cells with light ^[8]. Nothing was cut. So when those roots later gave weaker electrical responses to glutamate or to a wound, the change can be pinned on the calcium signal and not on earlier tissue damage ^[8]. The plant still sensed the injury; its response was smaller, and the effect was temporary ^[9].

The thing this doesn't tell you is whether a real pest makes the same wave. The experiments applied mechanical injury to Arabidopsis ^[2]. Insect larvae and nematodes are the motivating threat, because hidden root damage often leads to major crop losses ^[15]. Whether a feeding larva drops root pressure the way a lab wound does is a separate experiment. The press account does not report sample sizes, how much weaker the dampened responses were, how large the hormone response was, or any test with live pests.

Baudon points toward crops. "A deeper understanding of how crops perceive mechanical damage allows for a more targeted optimization of natural resistance to root-feeding insects," he said ^[14]. For now, the MCA1 result comes from thale cress ^[2].