When you step on a plant, douse it with salt water, or pluck its leaves off, it doesn’t look like the plant responds, as it stands immobilized by its roots in the ground. But plants are in-tune to their environment in many ways, including relatively slow biological reactions—like leaf regrowth—that are turned on in response to physical damage. Now, researchers have discovered a new chemical network that carries information through plants in response to external stimuli much more quickly. Waves of calcium ions drive this never-before-detected network, researchers report in a new PNAS paper.
“The surprising thing isn’t just that this wave is washing through the plant,” says Simon Gilroy, senior author of the new paper. “It’s that it’s channeled through cells which are specialized for this conduit of information.” The function of these cells as an information channel was never before known.
Researchers had long suspected that plants had a fast, electrical signaling network, akin to that in animals, that helped them respond to changes in their environment more quickly than known chemical signals. But sensors didn’t exist to detect any potential ions moving throughout a plant—until recently, that is.
Gilroy’s team was using a new calcium ion sensor—one that could measure levels of calcium within living plant cells—as a control in a different experiment on Arabidopsis thaliana, a common model plant organism. The researchers thought the sensor was so sensitive that it would be saturated with maximum levels of calcium at all times, hence making it a good control. But instead of staying steady, the sensor showed calcium ion fluctuations within far-off cells when the plant was exposed to different stresses, such as salt water. The group began probing where these fluctuations were arising from.
When Arabidopsis seedling roots tips were exposed to salt, Gilroy and his colleagues found, a wave of calcium moved throughout the whole plant, at around 400 microns per second. As the calcium signals traversed the seedlings, genes were rapidly turned on and off inside cell nuclei. When the researchers mutated a calcium channel suspected to be involved, the wave could no longer move through the plants, and stress-related genes failed to turn on or off in response to salt at the root tips.
“This is the sea change which is going on about how plants operate,” says Gilroy. The discovery of the new calcium signaling system brings about numerous questions, he says, about where the information encoded by the calcium ions travels, whether its route is predictive of its final response, and what other proteins are involved.
“Right now, we know one molecular component that makes this wave work,” says Gilroy. “That’s like in the animal world knowing a single channel that makes neurons work.”