Longstanding theories of how plants rely on calcium waves to respond systemically to injuries and other stresses have taken on a new perspective.
Researchers at the John Innes Center have shown that calcium waves are not a primary response, but rather a secondary response to a wave of amino acids released from the wound.
These findings challenge established thinking about long-range signaling molecules and the mechanisms by which information travels from the point of stress through living and non-living plant tissues.
It has been observed for many years that injuries and other traumas initiate calcium waves that travel both short cell-to-cell distances and longer leaf-to-leaf distances.
These calcium waves are reminiscent of signals seen in mammalian nerves, but because plants have no nerve cells, the mechanism by which this happens has been questioned.
The new findings appearing in Scientific progress suggest that when a cell is injured, it releases a surge of glutamate, an amino acid. As this wave travels through plant tissues, it activates calcium channels in the membranes of the cells it passes through. This activation resembles a calcium wave, but is a passive response or “readout” of the moving glutamate signal.
Previous hypotheses to explain how calcium waves pass through plant cells involved active mechanisms to propagate the calcium signal. These hypotheses were based on the signal propagating along the cell membrane or through a pressure wave in the xylem, but there was no explanation for how the response went from one cell to another.
dr. Christine Faulkner, group leader at the John Innes Center said: “Every time active reproductive models were presented, I wondered how this wave traveled from cell to cell. It seemed to me that there was a hole in the theory, and this research reveals a new mechanism showing that the calcium wave is not what it seems.”
The group of Dr. Faulkner specializes in the study of plasmodesmata, the channels or bridges that connect cells, and the team speculated that a wound signal would travel from cell to cell through plasmodesmata. However, using quantitative imaging techniques, data modeling and genetics, they found that the mobile signal is a glutamate wave that propagates outside the cells, along the cell walls.
“The glutamate and calcium waves are interconnected — glutamate triggers the calcium response. You could imagine it with an analogy of a hallway. The glutamate rushes down the hallway, and when it passes a door, it kicks it open. The calcium response is the door Until now, the assumption was that what moved through the corridor was hydraulic pressure or a series of propagating chemical reactions, but our study shows that this is not the case,” said Dr Faulkner.
The study’s lead author, Dr. Analisa Bellandi, said: “We have shown that calcium waves are synchronous with glutamate waves and that their dynamics correspond to the transmission by diffusion and flow. This research makes us rethink what we know. I hope our research inspires the debate and empowers people take a fresh look at data that has been in the field for a long time.”
Diffusion and bulk flow of amino acids mediate calcium waves in plants Advances in science.