You might expect sensors of mechanical pressure to be located on the surface of cells. That is indeed the case in animal cells where ion channels called PIEZOs embedded in membranes help sense touch and pressure. Mechanosensitive ion channels such as PIEZOs help gauge pressure by opening and allowing the flow of ions across the membrane.
In humans, PIEZO channels are essential for life. They help sense light touch, shear force and compressive force. Dysfunctional PIEZO channels have been linked to multiple inherited human diseases, such as muscular atrophy, respiratory distress in newborns, abnormal lymph nodes, scoliosis, and anemia.
Biologists have now discovered a different location and mode of function of PIEZO channels in plants. In an article titled, “Plant PIEZO homologs modulate vacuole morphology during tip growth” published in the journal Science, the authors show that in plants PIEZO channels have unexpectedly retreated into the interior of the cell, at the growing tips in moss and pollen tubes of flowering plants.
PIEZO channels were first identified in plant genomes in 2010. This new research shines a spotlight on plant cells and explores how they differ from animal cells.
Deep in plant cells, PIEZO channels are found in the membranes of vacuoles—large organelles that help maintain cell turgor, isolate harmful substances, maintain pH and play other important roles.
The researchers developed mutant mosses where PIEZOs have lost their function, are hyper-functional or expressed excessively to show that in moss PIEZOs play a role in increasing the complexity of vacuolar membranes, called tonoplasts. The scientists identified two PIEZO channels (PpPIEZO1 and PpPIEZO2) that contribute to the normal growth, size, and calcium levels in cells at the growing tips.
“PIEZO channels in plants play a dramatic and critical role in regulating the shape of the vacuole and how much membrane there is,” says Elizabeth Haswell, PhD, professor of biology in Arts & Sciences and a Howard Hughes Medical Institute-Simons Faculty Scholar at Washington University in St. Louis, who is the senior author on the study. “This is the first example of PIEZO channels involved in regulating organelle morphology. The data we present could lead to new lines of investigation for both plant and animal PIEZO homologs.”
The researchers made their initial discoveries using cells at the growing tip of a plant, “spreading earth moss” (Physcomitrium patens). But they extend their findings to cells from other distantly related plants, including in pollen tubes in a classic model, the flowering plant Arabidopsis thaliana.
“Mosses are one of the groups that comprise the bryophytes, which are the second largest land plant lineage,” says Ivan Radin, PhD, a research scientist in the Haswell laboratory and first author on the paper. “When we can show that the same thing happens both in moss and a flowering plant, as we did here, the most likely conclusion is that the process is ancestral—it’s at least as old as the land plants are.”
The authors propose that in plant cells the membrane of the vacuole has more freedom of movement than the plasma membrane which is “plastered up against a stiff cell wall.” Therefore the location of mechanosensory proteins in the tonoplast makes them more effective at sensing mechanical forces.
As a next step in this research, scientists in the Haswell laboratory are conducting experiments to show how external and internal forces affect PIEZO channels in moss cells, find out the function of these vital channels in plant cell vacuoles, and explore the evolution of these channels in algae.
“Plant PIEZO channels are likely to be controlled by membrane tension in plants the same way they are in animals,” says Haswell. “We are looking at how PIEZO channel activation results in membrane elaboration and how it is regulated,” Haswell said. “We want to know how the localization evolved and what it does in other cell types. We plan to compare and contrast the structure and function with the animal channels and in organisms across the green lineage.”