Long-held theories about how plants use calcium waves to respond systemically to wounding and other stresses have been given new life.
|After a leaf is severed, a fluorescent calcium wave travels through the plant, carrying information about the injury to other leaves. Credit: Annalisa Bellandi
Calcium waves, according to John Innes Center researchers, are a secondary response to an amino acid wave released from the wound, rather than a primary response.
These findings call into question long-held beliefs about long-distance plant signaling molecules and the mechanisms that allow information to travel from the point of stress to living and non-living plant tissues.
For many years, it has been observed that wounding and other forms of trauma initiate calcium waves that travel both short distances from cell to cell and longer distances from leaf to leaf.
These calcium waves are similar to signaling seen in mammalian nerves, but because plants lack nerve cells, the mechanism by which this occurs has been questioned.
The new findings, published in Science Advances, indicate that when a cell is injured, it emits a wave of glutamate, an amino acid. This wave activates calcium channels in the membranes of the cells it passes through as it travels through plant tissues. This activation appears to be a calcium wave, but it is actually a passive response to the moving glutamate signal, or "readout."
Previous hypotheses for explaining how calcium waves pass through plant cells involved active mechanisms for signal propagation. These hypotheses relied on the signal traveling along the cell membrane or through a pressure wave in the xylem, but there was no explanation for how the response moved from one cell to the next.
"Every time active propagation models were presented, I would question how this wave traveled from cell to cell," said Dr. Christine Faulkner, group leader at the John Innes Center. There appeared to be a gap in the theory, and this research reveals a new mechanism demonstrating that the calcium wave is not what it appears to be."
Dr. Faulkner's group studies plasmodesmata, which are channels or bridges that connect cells, and the team hypothesized that a wound signal would travel from cell to cell via plasmodesmata. However, they discovered that the mobile signal is a glutamate wave that travels outside of cells, along the cell walls, using quantitative imaging techniques, data modeling, and genetics.
"The glutamate and calcium waves are linked—the glutamate response is triggered by calcium." You could think of it in terms of a corridor. The glutamate rushes down the corridor, kicking open a door as it passes. The calcium response is the opening of the door. "Previously, it was assumed that what moved down the corridor was hydraulic pressure or a series of propagating chemical reactions, but our research shows that this is not the case," Dr. Faulkner said.
Dr. Analisa Bellandi, the study's first author, stated, "We've shown that calcium waves are synchronous with glutamate waves, and their dynamics match transmission by diffusion and flow." This study forces us to reconsider our assumptions. I'm hoping that our research will spark debate and allow people to look at data that has been in the field for a long time in a new light."