Potential osmosensing mechanisms in plant cells. (A) Histidine kinases could be activated by osmotic imbalance across the plasma membrane, initiating a signal transduction pathway similar to the high osmolarity glycerol response 1 pathway in Saccharomyces cerevisiae. (B) Ion channels that are opened in response to planar membrane tension or membrane bending could respond to both protoplast swelling or shrinking. The release of ions in response to hypoosmotic swelling would be similar to the MscS/MscL “osmotic release valve” paradigm in Escherichia coli. In addition, the entry of Ca²⁺ through an opened MS ion channel could have downstream signaling properties. (C) Cell wall integrity-monitoring mechanisms could involve receptor-like kinases with extracellular domains capable of sensing the movement or disruption of cell wall components. The cell wall is presented as a gray box. The plasma membrane is represented as a bilayer of gray lipid molecules.