In response to my recent post about the dynamic life of plants, reader tmso asked about whether plants can sense and respond to smoke. I still haven’t found anything about an immediate response to smoke, but I’ve learned quite a bit about how smoke and fire affect germination. Many plants, especially in areas with a Mediterranean environment, are ‘fire followers’ that germinate after a fire has cleared the landscape. Some of them have seeds coated with flammable oils which release a lot of heat when they burn, causing the seed to germinate. Other plants, though, don’t respond to the fire itself, but germinate and grow better in response to smoke. Over the past decade, we’ve started to understand how these plants sense smoke, though there’s still a lot discover. (Thanks for the question, tmso!)

In the 1970s and 1980s, scientists discovered that burned wood helped the seeds of fire followers germinate better, but in time they discovered that the wood itself wasn’t necessary. Chemicals in smoke could do the job as well, and “smoke water” (made by bubbling smoke through water and then diluting it) also worked. In fact, smoke water improves germination and growth in a wide range of plants, including several that aren’t normally fire followers like banana, tomato, maize and rice.

In 2003, researchers finally isolated the chemicals responsible for the improved germination. They used a variety of chemistry techniques to separate the mish-mash of chemicals in smoke into different groups and then tested each group to see whether it still affected seeds. Eventually, they homed in on a chemical which was later named karrikin from ‘karrik’, the word for smoke in the language of the Noongar Aboriginals of southwest Australia. Karrikin turned out to be the first of a new class of signalling chemicals (the karrikins) that regulate plant growth; natural and synthetically produced karrikins both help seeds germinate better.

Even though karrikins were first identified in smoke, most of the karrikin is actually in the residue left after burning, like charred wood. Scientists think that karrikins don’t travel far with smoke itself, but only end up within the burnt area. After the fire has died down, the karrikins help dormant seeds germinate and fill up the cleared area. Given that many plants which aren’t fire followers also respond to smoke and karrikins, it’s possible that karrikins might be produced by other processes like microbial activities or plant metabolism. At the moment, we just don’t know.

Arabidopsis thaliana is one of the plants that responds to karrikins even though it isn’t a fire follower. That’s quite a fortunate fact, since A. thaliana is the workhorse of plant molecular biology and genetics. Working with Arabidopsis, scientists have started to uncover how plants sense and respond to karrikins. Earlier this year, researchers at the Salk Institute identified the gene for the karrikin receptor and showed how it changes shape when karrikin binds to it. They still have to work out what happens after that — how the signal from karrikin gets passed through the cell and what changes it causes. Researchers have figured out that karrikin signalling uses a major component of the signalling pathway of another plant growth regulator, the strigolactones, but they also know that plants can distinguish karrikins from strigolactones and respond differently to them. We’ve still got a lot to learn about karrikins and how they work, and I’m excited to see where these discoveries will lead!

Some plants don’t respond to karrikins even though they do respond to smoke, so there must be other chemicals in smoke that help seeds germinate. Research with these plants uncovered a second class of smoke-borne chemicals, the cyanohydrins, which also promote seed germination. Scientists think cyanohydrins may break down into cyanide, which then promotes germination, but we know even less about the cyanohydrins than the karrikins.

Smoke also has other chemicals which stop seeds from germinating. While a very small amount of karrikins is enough to help seeds germinate, the inhibitors are only effective at higher concentrations. This difference in sensitivity might help plants control when their seeds germinate after a fire. As the chemicals break down or get washed away, there eventually won’t be enough of the inhibitors left to block germination, but the remaining karrikins would still be enough to promote germination. Germination is also blocked by chemicals in the foliage and leaf litter until fire burns these away. The interlocked pathways orchestrating germination remind us that while fires may seem like a blight that leaves the land charred and barren, to many plants they represent a unique opportunity to step into the light.

Refs
Flematti GR, Ghisalberti EL, Dixon KW, & Trengove RD (2004). A compound from smoke that promotes seed germination. Science (New York, N.Y.), 305 (5686) PMID: 15247439

Chiwoca, Sheila DS, Dixon, Kingsly W, Flematti, Gavin R, Ghisablerti, Emilio L, Merritt, David J, Nelson, David C, Riseborough, Julie-Anne M, Smith, Steven M, & Stevens, Jason C (2009). Karrikins: A new family of plant growth regulators in smoke Plant Science, 177, 252-256 DOI: 10.1016/j.plantsci.2009.06.007

Nelson, David C, Flematti, Gavin R, Ghisalberti, Emilio L, Dixon, Kingsley W, & Smith, Steven M (2012). Regulation of Seed Germination and Seedling Growth by Chemical Signals from Burning Vegetation
Annual Review of Plant Biology, 64, 107-130 DOI: 10.1146/annurev-arplant-042811-105545

Guo Y, Zheng Z, La Clair JJ, Chory J, & Noel JP (2013). Smoke-derived karrikin perception by the α/β-hydrolase KAI2 from Arabidopsis. Proceedings of the National Academy of Sciences of the United States of America, 110 (20), 8284-9 PMID: 23613584