Smoke signals: How burning plants tell seeds to rise from the ashes
In the spring following a forest fire, trees that survived the blaze explode in new growth, and plants sprout in abundance from the scorched earth. For centuries, it's been a mystery how seeds, some long dormant in the soil, know to push through the ashes to regenerate the burned forest.
But a team led by Joseph P. Noel and Joanne Chory has now cracked the mystery behind this fundamental "circle of life." In addition to explaining how fires lead to regeneration of forests and grasslands, their findings, reported in the Proceedings of the National Academy of Sciences, may aid in the development of plant varieties that help maintain and restore ecosystems that support all human societies.
"This is a very important and fundamental process of ecosystem renewal around the planet that we really didn't understand," explains Noel. Now we know the molecular triggers for how it occurs."
"What we discovered," Chory adds, "is how a dying plant generates a chemical message for the next generation, telling dormant seeds it's time to sprout."
In previous studies, scientists had discovered that chemicals known as karrikins are created as trees and shrubs burn during a forest fire, and they remain in the soil after the fire, ensuring that the forest will regenerate. This new study sought to uncover exactly how karrikins stimulate new plant growth. First, the Salk researchers determined the structure of a plant protein known as KAI2, which binds to a karrikin in dormant seeds. Then, comparing the karrikin-bound KAI2 protein to the structure of an unbound KAI2 protein allowed the team to speculate how KAI2 allows a seed to perceive karrikins in its environment.
The chemical structures they solved revealed all the molecular contacts between a karrikin and KAI2. The study also showed that when a karrikin binds to the KAI2 protein, it causes a change in its shape, which may send 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.
More research is needed to understand exactly how the change in shape of the KAI2 protein activates a genetic pathway that regulates germination, says Chory. "But," she notes, "this finding is an absolutely critical step in understanding this genetic program and how plant ecosystems, forests and grasslands renew themselves."