Long-standing theories of how plants rely on calcium waves to respond systemically to injury and stress have received new insight.

Researchers at the John Innes Center have shown that calcium waves are not a primary response, but rather a secondary response to a surge of amino acids released by the wound.

These findings challenge established views about long-range plant signaling molecules and the mechanisms by which information travels from the point of stress through living plant tissues.

It has been observed for many years that wounds and other traumas initiate waves of calcium that travel both short distances from cell to cell and longer distances from leaf to leaf.

These calcium waves are reminiscent of the signaling seen in mammalian nerves, but since plants do not have nerve cells, the mechanism by which this occurs has been questioned.

Wounds and trauma to plants initiate calcium waves

The new findings, published in Scientific Advances, 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. This activation appears as a calcium wave, but it is a passive response, or a “readout” of the moving glutamate signal.

Previous hypotheses to explain how calcium waves pass through plant cells involved active calcium signal propagation mechanisms. These hypotheses were based on the fact that the signal propagated along the cell membrane or via a pressure wave in the xylem, but there was no explanation for how the response passed from one cell to another.

“Every time models of active propagation were presented, I wondered how this wave travels from one cell to another. It seemed to me that there was a missing piece in the theory, and this research uncovers a new mechanism that shows the calcium wave is not what it seems,” said Dr. Christine Faulkner, group leader at the John Innes Centre, according to EurekAlert.

Glutamate and calcium waves are connected within plants

The team speculated that a signal from a wound might travel from one cell to another via plasmodesmata. However, using quantitative imaging, data modeling and genetics techniques, they discovered that the mobile signal is a wave of glutamate that travels outside the cells, along the cell walls.

“Glutamate and calcium waves are connected – glutamate triggers the calcium response. You could imagine this with an analogy of a corridor. Glutamate rushes down the corridor and when he passes a door, he slams it. The calcium response is the opening of the door. Until now, it was assumed that what moved 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 was published in Science Advances.

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