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The molecular mechanism behind how the karrikins form is something of a mystery. Flematti tells C&EN that the compounds are generated from burning cellulose, a major component of plants. Johannes van Staden, a botany professor and director of the Research Centre for Plant Growth & Development at South Africa’s University of KwaZulu-Natal, Pietermaritzburg, thinks that the compounds are formed as part of the Maillard reaction—the reaction of amino acids and sugars that gives browned food its delectable flavor. Along with coworker Marnie E. Light, van Staden led a team that identified KAR1 in the reaction products from heating certain amino acids with sugars (J. Agric. Food Chem. 2005, 53, 5936). They also found the compound in toast and instant-coffee extract, which are well-known examples of food that’s undergone Maillard chemistry. “I believe it comes from very basic molecules. Anytime you heat plant material that contains sugars and amino acids, you get this molecule,” van Staden says. On the other hand, Flematti and coworkers have reported that KAR1 is produced from cellulose in the absence of amino acids. “How the molecule forms, I believe, will be—whenever it is solved—a brilliant piece of chemistry,” van Staden contends.
HEALING HAZE
The smoke left behind after a fire in Western Australia holds molecules that will jump-start the bushland’s regrowth.
SMOKE SIGNALS Seeds sprout in the ashes of forest fires, thanks to SMALL MOLECULES in smoke
BETHANY HALFORD, C&EN NORTHEAST NEWS BUREAU
THE ORIGIN of the karrikins isn’t the only
THE GRAY HAZE that lingers after a for-
est fire isn’t just a hallmark of devastation. Within that smoke’s wispy tendrils also drifts a family of signaling molecules ready to summon the forest back to life. Smoke, it seems, has the power to make seeds sprout. For decades, scientists have been hunting for the molecules responsible for this growth-triggering power as well as those compounds’ mechanism of action. By delving into the science of smoke signals, these researchers are hoping to harness the compounds to revitalize barren soil and boost agriculture. Smoke’s ability to spur seed germination after a forest fire first caught the attention of ecologists 20 years ago. Sifting through the thousands of chemicals in smoke, the scientists sought the molecular source of this botanical awakening. In 2004, a team led by the University of Western Australia’s Gavin
puzzle associated with these plant promoters. Scientists are also trying to figure out just how the karrikins prompt seeds to sprout. A group led by Steven M. Smith, a professor of plant genomics at the University of Western Australia, recently published work suggesting that the molecules enhance seeds’ and seedlings’ response to light (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/ pnas.0911635107). The research group discovered that the butenolides stimulate the expression of a small set of genes. “That set of genes was quite characteristic of genes that are comO O O O O monly regulated O O O by light,” Smith explains. “That KAR2 KAR3 KAR4 gave us a clue that the karrikins The karrikins are somehow ac-
R. Flematti reported the first smoke-derived compound responsible for promoting seed germination (Science 2004, 305, 977). That molecule, 3-methyl-2H-furo[2,3-c] pyran-2-one, or KAR1, turned out to be just one in a family of butenolides that triggers seed germination and controls seedling growth at parts-per-billion concentrations (J. Agric. Food Chem. 2009, 57, 9475). The researchers named the compounds “the karrikins,” for “ karrik,” an aboriginal word for smoke.
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tivating the light response system.” The researchers then tested how the seeds of Arabidopsis thaliana—a small flowering plant commonly used as a model system in plant biology—respond to light. Seeds treated with karrikins KAR1 and KAR2 become much more sensitive to light, the team discovered, as do their seedlings, which tend to grow in a more vigorous and robust fashion. This light responsiveness makes sense, Smith explains, because the quality of light changes after a fire. “Before the fire, you have a canopy of leaves that tends to filter out some of the shorter wavelengths of light,” he says. “Once the fire has gone through, the canopy is lost and the spectrum of the light changes.” It shifts from far red to the ultraviolet region. “Plants have an exquisite sensing system that allows them to detect different light qualities,” Smith continues. “Essentially, the karrikins activate that sensing system so the seeds and seedlings become much more sensitive to the light.” The molecules provide seeds with information about the environment above the soil, he says.
“These molecules are a new group of growth stimulants over and above those we already know about.” “It’s a fantastic trick that nature has come the inhibitor will override the karrikin. up with,” Smith adds. “The karrikins are Plants that respond to the karrikins aren’t signaling that there’s been this change in the limited to those native to fire-prone areas, environment, and they’re basically saying to which has prompted some to speculate the seeds, ‘Now would be a good time to gerthat there’s a source of karrikins other than minate’ because there’s plenty of light, and smoke. Although no other source has been there are presumably plenty of nutrients. So discovered to date, researchers have suggestthe environment is good for a small seedling ed that microbes in the soil might produce to germinate.” the compounds. “Why would all these difThe karrikins aren’t the only signaling ferent species respond if there wasn’t some molecules in smoke. Van Staden’s group reason to?” Smith asks. recently published the structure of a differEven crops such as corn, tomatoes, letent butenolide found in postfire tuce, and onions have been shown to haze (J. Nat. Prod., DOI: 10.1021/ flourish when exposed to the comO O np900630w). “This very small pounds. “We have found that the molmolecule actually acts as an inecules have tremendous invigorating hibitor of the stimulant” for gerproperties,” van Staden says. “Seeds Inhibitor mination, van Staden says. “In that have been stored for a long time our opinion, this is really a cruand treated with these molecules becial little molecule that determines whether come totally invigorated. They grow as if germination of a seed is switched on or off.” they never aged.” He adds that the karrikins So what would be the purpose of such also boost vegetable yields and help seedan inhibitor molecule? Van Staden speculings tolerate a wider range of temperatures. lates that the molecules that stimulate and “I believe that these molecules are a new inhibit germination work synergistically, group of growth stimulants over and above triggering germination when the environthose we already know about,” van Staden mental conditions are best for growing. says. He and Smith both say it’s possible that the compounds could one day be used FOREST FIRES are actually beneficial for in horticulture and farming. ecosystems, provided they don’t happen too The inhibitor compound could be used in often. “They are invigorating for the natural conjunction with the karrikins, van Staden vegetation,” van Staden says. “After a fire, says, to ensure that the seeds emerge when plants germinate and grow incredibly fast.” conditions are ideal. “These molecules will, Researchers have observed this phenomin my opinion, revolutionize agriculture for enon in the western cape of South Africa the simple reason that they provide plants and elsewhere, but the seeds will germinate with greater tolerance to a stressed environproperly only if there is sufficient rainfall. ment,” he adds. “Why is it that you need a certain In Australia, Smith says, researchers amount of rain before you get these masare already using the karrikins to restore sive blooms in these semidesert areas?” vegetation at bauxite mines, where the land van Staden asks. “There is something else has been stripped for aluminum ore. The that serves as, shall we say, a guard at the compounds are used to pretreat the seeds door to protect these seeds.” being planted in the hostile soil. A karrikin, once it’s been taken up by a Unfortunately, at the moment the comseed, can’t be washed out, van Staden expounds are limited to specialist applicaplains. “It is stuck somewhere in that seed, tions because they are expensive to make, and it starts stimulating physiological proSmith notes. Their synthesis proceeds cesses,” he says. The inhibitor butenolide, with low yields, van Staden explains, and on the other hand, is very water soluble. “So, the starting materials are costly. “There when a certain amount of rain gets into the are very clever chemists in the world,” he soil, the inhibitor will wash away,” he adds. says, “and I’m sure once they start thinking This leaves behind the promoter to stimuabout it, someone’s going to come along late growth. If there’s not enough rain, then with a solution to that problem.” ■ WWW.CEN-ONLINE.ORG
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