photograph by iStockphoto

Time to Sprout

Researchers discover how burning plants tell seeds to start growing.

A forest fire can devastate swaths of land, felling centuries-old trees and anything else in its path. Researchers have come to learn that this kind of destruction is not all bad, and can signal rebirth. Now a team at the Salk Institute for Biological Studies has revealed how a protein in dormant seeds gets a wake-up call from smoke and ash.

“Fire is an important way for an ecosystem to replenish itself every now and again,” says HHMI Investigator Joanne Chory, “because when you get unchecked undergrowth, these opportunistic plants reduce nutrients available to the large stands of older trees that create the forest ecosystem.” 

As it burns, this undergrowth releases smoke and ash, which contain chemicals that remain in the soil after the fire dies down. One group of those chemicals, known as karrikins, tells dormant seeds that it’s time to germinate. “The ash left over includes not only these natural chemicals,” says HHMI Investigator Joseph Noel, “but also fantastic natural fertilizers to spur the growth of the older trees and ensure the next generation of trees.”

To dig deeper into karrikins, Chory and her postdoc Zuyu Zheng teamed up with Noel and his postdoc Yongxia Guo as well as James La Clair, a chemist at University of California, San Diego. It helped that Guo and Zheng are a married couple and that the groups had collaborated in the past. Chory, a geneticist, studies how plants adapt their growth rate and body plan to their environment; and Noel, a biophysicist and biochemist, is interested in how plant metabolic pathways evolve and adapt to local ecosystems.

If we could modify the protein to keep seeds sleepy until the right time for each situation—say when it was warm enough or there was enough rain—it would do a lot to establish the seedling in the best possible environment.

Joanne Chory

For this study, they wanted to know exactly how karrikins, named after the aboriginal word for smoke, awaken dormant seeds. Using x-ray crystallography to elucidate the structure of the protein that binds to karrikins, they were able to see how dormant seeds sense karrikins.

Genetic studies from other groups identified plants with kai2 mutations that are insensitive to karrikins, implying a role for the KAI2 protein in sensing karrikins. The Noel-Chory team mapped out how karrikins, once they enter seeds, stick to the KAI2 protein, causing KAI2 to change its shape. They hypothesized that seeds know that karrikins are in their midst because KAI2’s shape changes, likely serving as the signal to other resting proteins in the seed to power up for seed germination. The team published its findings May 14, 2013, in the Proceedings of the National Academy of Sciences.

Curiously, KAI2 looks like a type of protein known as a hydrolase enzyme that would normally orchestrate chemical reactions in plant cells. However, the team’s structural and biochemical studies show that KAI2 doesn’t function as an enzyme in response to karrikins; instead, evolution seems to have shaped it for another function: it acts as a receptor that binds karrikins and relays critical information to a network of proteins in plant seeds.

To verify their results, the scientists changed the shape of KAI2 slightly to see how karrikins would slip into the restyled KAI2 lock. They also looked at another essential group of plant compounds known as strigolactones (SL) that are similar to karrikins, but are involved in a chemical change instead of a shape change. By modeling SLs into their karrikin-bound KAI2 structures, they also gained a clearer understanding of how the two related systems operate to transmit distinct environmental signals in seeds and plants.

Plant evolution adapted the shape and function of ancestral receptors whose signatures remain as vestiges in the contemporary KAI2 family of proteins. Noel calls them “imprints of molecular fossils,” because they allow researchers to peer into plants’ storied pasts. At some point in the history of plants, it appears likely that only one protein receptor bound to chemicals that share the atomic structure of both strigolactones and karrikins. Through hundreds of millions of years of evolution, however, one became two.

Understanding plants’ deep evolutionary past may help scientists shape a more sustainable and environmentally friendly future, Noel says. “We can learn from nature and harness the tools of synthetic biology to develop plants that can respond to changing times, ultimately creating hardier and more nutritious plants.”

Chory also hopes their research findings may lead to better ways to sprout seeds when needed to meet the needs of Earth’s growing population. “If we could modify the protein to keep seeds sleepy until the right time for each situation—say when it was warm enough or there was enough rain—it would do a lot to establish the seedling in the best possible environment,” she says. 

Scientist Profile

Salk Institute for Biological Studies
Genetics, Plant Biology
Salk Institute for Biological Studies
Biochemistry, Plant Biology