Green Alert! How plants propagate warnings
In antiquity, because hostile armies could attack at any time. Thus, sentinel soldiers would keep a close eye on potential dangers and send alerts as fast as possible. Later, to speed up delivery, people developed systems of light beacons. Yet, receiving warnings without knowing the exact nature of the danger was frustrating and dangerous. Humans kept inventing systems to sound the alert, including the telegraph, followed by radio and satellites. .
Nowadays, anticipating danger and instantly sending warning feel technologically advanced, doesn’t it?
But are human technologies that far at the forefront of alert systems?
To deal efficiently with a threat, early danger detection and awareness is the best strategy. This is also true in nature.
Although they don’t look like fighters, plants are good at taking defensive measures. Like humans, plants detect dangers, communicate warnings, and activate contingency plans. Let’s discover how plants do it.
Plant Warnings And Defenses, A Complete System
Let’s compare the plants in the figure above. The intact plants on the left were exposed to hungry caterpillars, with the results shown on the right. The cngc19–2 mutant plant was heavily eaten with only leaf stems remaining. The non-mutant plant (WT, wild type) kept some intact leaf tissue.
Why did the caterpillars eat more of the cngc19–2 mutant plant?
The answer lies in the nature of the defense-less cngc19–2 mutant. That plant was not able to send warnings about the presence of caterpillars to other leaves. It could only defend itself locally, where each caterpillar bite occurred. As global defenses were not activated, caterpillars could eat a large chunk of toxin-free leaves.
In the wild-type plant, a warning signal was sent and leaves armed up with toxic chemicals. Leaf taste changed from sweet and yummy to wasabi-like spicy — and therefore, caterpillars stopped eating.
Waving The Warning Flag
Now you might wonder: How did the plants send the alert? What is the plant equal of a warning flare, or radio alert? How did scientists know that the cngc19–2 plants were unable to send an alert?
They could see it and you will too (in the small movies below).
A few months ago, a series of short videos showed that plants communicate internally at high speed. Plants do that even though they don’t have a nervous system. This discovery was made possible by a recent technological advance. The observation of ion concentrations in living plant tissues in real-time. When ion concentrations increase, the tissues light up thanks to fluorescence. Using this technique, Masatsugu Toyota and co-authors [1] showed that plants send warning signals as waves of calcium ions.
Yet, how the calcium waves were initiated and propagated remained a mystery. That’s the question that Mukesh Kumar Meena and coauthors have been researching. They recently published their results in The Plant Cell [2].
Channels, The Masters Of Ion Logistics
The calcium waves are due to the quantity of calcium ion increasing and decreasing in the liquid within cells (called the cytosol). As cells are tiny compartments, the calcium ions are not free to move around. How do the calcium ions move within plant tissues, then?
Let’s start with an architecture comparison: To get light into a room, an architect will design a hole in a wall — that is, a window.
The same idea applies to cells. To import or export ions, plants have holes in their external cell wall. Moreover, like you can draw the curtains on a window, the holes can be opened and closed. These holes can even transport only some ions and not others, such as moving calcium without moving sodium. Some proteins called channels do that. They insert themselves into the cell wall and transport specific ions by creating a tiny bridge between the outside and the inside.
Meena and co-authors focused on one such channel, named CNGC19. They showed that it generates and propagates the calcium wave upon leaf wounding by cutting.
Like countries keeping an eye on their enemies, plant cells have ‘sentinels’, proteins that look out for troubles. When these sentinels perceive damage, they call for help, hereby activating CNGC19 channels. The activation of the channels allows calcium to pour into the cells. Like light beacons, CGNC19 proteins get activated in neighboring cells. They relay the distress signal that propagates first locally and then spreads. The signal reaches the leaf veins. Veins act as signal superhighways, sending the warning through the entire leaf and then the whole plant.
To confirm the effects of CNGC19, the researchers generated mutant plants without it — these are the cngc19–2 mutant plants we saw above. In this mutant, the calcium signal is local and of much smaller intensity. In short, the warning does not propagate well. As some of the signal is still transmitted without CNGC19, this channel is not working alone. Generating and propagating the calcium wave may be a group affair.
The role of this channel protein does not stop at transporting calcium. The researchers showed that CNGC19 is in close communication with plant hormones. CNGC19 is also implicated in the coordination of accumulation of glucosinolates. These toxic chemicals repel chewing insects by making the leaves less digestible.
Be warned: plants know how to master communication and defend themselves. This protein does not only provide high-speed warnings but also activate contingency plans.
Céline Caseys
Department of Plant Sciences
University of California, Davis
Orcid: 0000–0003–4187–9018
Twitter: C_Caseys
References:
[1] Toyota M, Spencer D, Sawai-Toyota S, Jiaqi W, Zhang T, Koo AJ, Howe GA, and Gilroy S. (2018). Glutamate triggers long-distance, calcium-based plant defense signaling. Science 361: 1112–1115.
[2] Meena MK, Prajapati R, Krishna D, Divakaran K, Pandey Y, Reichelt M, MK Mathew, Boland W, Mithöfer A and Vadassery J (2019). The Ca2+ channel CNGC19 regulated Arabidopsis defense against Spodoptera herbivory. Plant Cell DOI: https://doi.org/10.1105/tpc.19.00057
