When plants are under attack - say, for instance, by an insect making a tasty leaf meal - their defence systems are raised in other parts. How do they know to do that?
According to new research, plants use the same signalling molecules that animals use in their nervous system. Our green friends don't have nerves, exactly - but they certainly have something surprisingly similar.
The research involved using fluorescent proteins to mark and watch the signals as they travel in waves through plants in response to a stressor. (Yes, there's a glorious video so you can see it in action for yourself.)
"We know there's this systemic signalling system, and if you wound in one place the rest of the plant triggers its defence responses. But we didn't know what was behind this system," explained botanist Simon Gilroy from the University of Wisconsin-Madison.
What they did know is that if a plant gets wounded, an electrical charge fires, propagating across the plant. The unknown part was what triggered that charge and helped propagate it - but that's not even what the researchers were trying to study.
What they were originally looking at was how plants respond to gravity by studying increases in calcium. So botanist Masatsugu Toyota genetically engineered a mustard plant that would let the researchers observe changes in calcium concentration in real-time.
He introduced a protein that only fluoresces in the presence of calcium. And then the researchers cut a leaf to see if they could detect calcium changes.
In animals, an excited nerve cell releases an amino acid called glutamate, which triggers a wave of electrically charged calcium ions that propagate to cells farther and farther away from the site.
As you can see from the videos, what happened to the plants is nothing short of incredible. Waves of light flow out from the source of the wound, spreading through the plant at the speed of about a millimetre per second.
It's a lot slower than animal nerve signals, which can travel up to 120 metres per second (268 mph), but for plants this is super speedy communication.
The researchers discovered that once the wave hits, defensive hormones rise in that region of the plant.
This tells the plant to mount its defences, such as an increase in noxious chemicals that will make the plant unpalatable to munching insects, or - as is known in the case of grass - the release of smelly volatiles that signal parasitic wasps to come and lay their eggs in insects that might be eating it.
But what was triggering the calcium waves? Just like in animals, the researchers believed it might be glutamate, which is also found in plants. And previous research published in 2013 revealed that plants missing glutamate receptors also did not have an electrical threat response.
So the research team wounded plants without glutamate receptors to see if there was an effect on calcium flow.
"Lo and behold, the mutants that knock out the electrical signalling completely knock out the calcium signalling as well," Gilroy said.
This research puts together various pieces to solve the puzzle of how plants cells communicate. The wound spills glutamate, which is taken up by receptors that trigger calcium ion propagation, which in turn switches on the plant's defence system to try and protect against further damage.
And they do it all without actual nerves. Pretty cool, huh? Looks like our green brethren are a lot more complex and dynamic than they seem.
The team's research has been published in the journal Science.