The Fire of Life
Forest fires are powerful, highly destructive events that while being life altering also have come to be appreciated for their clearing effects, from which explosive new forest growth evolves. But why does the forest respond in this way to such a dramatic event, and what triggers the natural growth that follows? Is there some sort of communication among plants to signal that the coast is clear for growth?
Those are the questions a team of researchers from the Salk Institute for Biological Studies and the University of California, San Diego, set out to answer with their work on molecular signaling in the aftermath of a fire. The research helps explain how fires lead to regeneration of forests and grasslands, and it may aid in the development of plant varieties that can help maintain and restore ecosystems.
How fires lead to regeneration “is a very important and fundamental process of ecosystem renewal that we didn’t really understand,” said co-senior investigator Joseph Noel, professor and director of the Jack H. Skirball Center for Chemical Biology and Proteomics at Salk. “Now we know the molecular triggers for how it occurs.”
“What we discovered,” added Joanne Chory, co-senior investigator and professor and director of Salk’s Plant Molecular and Cellular Biology Lab, “is how a dying plant generates a chemical message for the next generation, telling dormant seeds its time to sprout.”
Noel, Chory and colleagues Yongxia Guo and Zuyu Zheng of Salk, and James La Clair of UCSD, said the work advances the tools of protein chemistry to characterize how fire leaves its chemical message. As it turns out a chemical emitted in the fire, karrikin, and a protein (KAI2) found in most plants, play key roles.
“Our work provides clear biochemical and structural evidence that the KAI2 protein, functions as the receptor for smoke-derived germination-promoting chemicals called karrikins,” Noel said. “In fact, the work allowed us to visualize in three dimensions and at atomic resolution—we can see each atom of the KAI2 protein and the karrikin chemical—how karrikins alter the shape of KAI2 to generate a signal within plants.”
Using single crystal diffraction techniques, the researchers determined the structure of KAI2, which binds to karrikin in dormant seeds. They then compared the karrikin-bound KAI2 protein to the structure of an unbound KAI2 protein providing clues as to how KAI2 allows a seed to perceive karrikins in its environment. The chemical structures the team solved revealed all of the molecular contacts between karrikin and KAI2.
“But more important than that,” said Gou, “we also know that when karrikin binds to the KAI2 protein it causes a change in its shape.”
This shape change sends a new signal to other proteins in the seeds.
“These other protein players together with karrikin and KAI2 generate the signal causing seed germination at the right place and time after a wildfire,” explained Zheng.
“Understanding how a single molecular signaling event generates, translates and then transduces information in plants provides vital information to further probe how plants sense and adapt to their local environments,” Noel said. “This particular system is one example of how organisms generally collect and process natural chemical information.”
“In a sense, karrikin can be thought of as a sentence or even paragraph that plants can read and then respond to when the growing conditions after a fire goes out are ripe for seed germination and growth due to the nutrients left behind by fires,” he added. “By gaining insight into the biochemical and structural mechanisms by which dormant seeds recognize the chemical information from burning plants to germinate, we gain a much clearer understanding of ecosystems, all of which depend on plants and possibly can exploit this information to manage the renewal/transition/reservation of plants via chemical and genetic approaches.”
The work provides a solid foundation for the discovery of key plant related chemical signals, said Noel, who studies the biosynthetic pathways plants and microbes use to produce a vast array of compounds that allow them to survive and prosper in various and changing ecosystems.
“Because plants are sessile, (they cannot move in response to changing environmental signals) they have evolved the most sophisticated forms of metabolism and chemical sensing of any group of living organisms on Earth,” Noel said. “These chemicals produced by plants, other organisms in their environment or even by so-called abiotic events (fires), hold a tremendous amount of information that plants have evolved to ‘interpret’ to regulate their growth and development at the appropriate times conducive to viability and fitness, the latter being their ability to propagate, reproduce and continue their gene pool.”
In addition, he said, the development of molecular screening systems such as those already used to detect toxins in marine systems or diseases in humans, could also be developed to provide a greater understanding of the health and well being of forests.
“One could imagine a day when ELISA-based assays are used to guide seed planting as well as to probe the health of a forest,” Noel added. “The work may not only aid in the development of plant varieties that help maintain and restore ecosystems, but also can guide us in the application of karrikin-like natural chemicals to forest and grassland renewal, providing the fundamental food necessary to support all living things on Earth.”