Science https://doi.org/10.1126/science.abe6310 (2021)
Animal PIEZO proteins form mechanosensitive plasma membrane cation channels with important sensory roles. Whether their plant relatives also form mechanically gated channels is unknown, but the rigid cell wall that encloses plant cells is expected to prevent the deformation of the membrane required to activate PIEZOs. Writing in Science, Elizabeth Haswell and colleagues report that disruption of two PIEZO genes (PpPIEZO1 and PpPIEZO2) of the moss Physcomitrium patens causes delayed growth, particularly of tip-growing caulonemal cells. Tipward Ca2+ oscillations characteristic for this cell type were altered after loss of PpPIEZOs, suggesting that they mediate Ca2+ influx. However, GFP-tagged PpPIEZOs could not be detected at the cell surface, and instead localized to the vacuolar membrane. Vacuoles—plant-specific storage organelles with high intracellular Ca2+—change their morphology in response to different stimuli and have been implicated in plant mechanosensation. In caulonemal tip cells, vacuoles are typically tubulated and fragmented. By contrast, vacuoles in the PpPIEZO knockouts exhibited large, expanded vacuoles—a phenotype that could be rescued by overexpression of PpPIEZOs, or of a homolog from Arabidopsis thaliana (AtPIEZO1), which also localized to the vacuolar membrane. AtPIEZO1 is widely expressed in Arabidopsis tissues, including its pollen tubes, a well-studied model system for tip growth. Its genetic disruption also led to vacuole expansion in this cell type, which indicates a conserved function in diverse plants. These findings are compatible with other recent studies that report a role for plant PIEZOs in root cap mechanotransduction (Mousavi et al., https://doi.org/10.1073/pnas.2102188118, and Fang et al., https://doi.org/10.3390/ijms22010467) and elegantly explain how these channels can be activated despite the restrictions imposed by the cell wall.
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