A U.S. study published on Thursday in the journal Science revealed a new model for how plant cells communicate like and unlike animal neurons.
The study, led by University of Maryland (UMD) researchers, has shown proteins of many plant cells, resemble glutamate receptors which help to relay nerve signals from one neuron to another in animals.
While plants lack a true nervous system, previous studies have shown that plants need these glutamate receptor-like proteins (GLRs) to do important things such as mate, grow, and defend themselves against diseases and pests.
Researchers experimented with Arabidopsis thaliana pollen cells and found that GLRs relied on another group of proteins, called "cornichon" proteins, to shuttle GLRs to different locations and regulate GLR activity within each cell.
With the help of cornichon proteins, GLRs act as valves that carefully manage the concentration of calcium ions, a vital aspect of many cell communication pathways, within various structures inside the cell, the study found.
"Calcium concentration is one of the most important parameters inside all cells. It is so well regulated that it allows cells to encode information. Put another way, calcium is the lingua franca of cell communication," said Jose Feijo, a professor of cell biology and molecular genetics at UMD and the senior author of the study.
"Our results suggest that GLRs play a role in this basic communication system in plants, and we also propose a mechanism for how the system works in plant cells," said Feijo.
The similarities between GLRs and animal glutamate receptors suggest that the proteins date back to a common ancestor, or a single-celled organism that gave rise to both animals and plants.
However, Feijo's team identified some important differences between GLRs and their counterparts in animal neurons.
Glutamate, the most common neurotransmitter in the human brain, does not play a major role in the plant system.
Also, while glutamate receptors are known to sit on the outer surface of animal neurons, some of Feijo's earlier experiments suggested that GLRs might instead be located on various structures inside plant cells.
"Our results suggest that GLRs are indeed redistributed to other compartments inside plant cells, forming a complex network that cooperates to regulate calcium concentrations and enable calcium signaling. This is a novel insight that opens completely new avenues to understand calcium signaling in plants," said Feijo.
Feijo and Michael Wudick, a postdoctoral researcher in cell biology and molecular genetics at UMD and lead author of the paper, investigated cornichon proteins, which are linked to the activity of glutamate receptors in animals.
Initially identified in fruit flies, cornichon proteins are named for the pickle-like appearance that a specific cornichon gene mutation imparts to fruit fly embryos.
In their experiments with Arabidopsis pollen cells, Feijo's team found that cornichon proteins actively shuttled GLRs from one location to another within the cell, enabling various compartments inside the cell to maintain different calcium ion concentrations.
Cornichons also act as gatekeepers for GLRs, switching the receptor molecules off and on like a valve in response to changing conditions inside the cell, which is unlike anything found in animals.
While animal neurons use glutamate receptors to conduct signals from cell to cell, Feijo suggested that plants depend on communication strategies that operate at the single-cell level.
"Individual plant cells have a level of autonomy that animal cells do not," Feijo said.
"Each plant cell has its own immune system, for example. And they have more communication channels to deal with the fact that they are stuck in place. Every flowering plant has more GLRs than animals have glutamate receptors."
"Our proposed model for plant cell communication suggests one reason for this abundance of GLRs," said Feijo.
Further progress toward decoding plant communication could result in reliable tests to diagnose diseases, nutrient deficiencies and other maladies in plants, Feijo said.
Such measures could help to ensure food security, as climate change and other stressors begin to take a toll on major agricultural crops.