tonoplasten

Tonoplasten, more commonly referred to in English as tonoplasts, are the membranes surrounding the plant cell vacuole, forming the boundary between the vacuolar lumen and the cytoplasm. They are selective barriers composed of a lipid bilayer embedded with transport proteins and channels that regulate movement of ions, metabolites, and water between the cytosol and the vacuole. The tonoplast is rich in transporters that use proton gradients established by vacuolar H+-ATPases and H+-pyrophosphatases to drive secondary transport processes.

Key components include V-ATPases and V-PPases that acidify the vacuole, and transporters such as Na+/H+ and Ca2+/H+ antiporters (e.g., NHX and CAX families) that maintain ion homeostasis. Aquaporins, particularly tonoplast intrinsic proteins (TIPs), control water permeability and contribute to osmoregulation. The tonoplast also hosts transporters for organic acids, amino acids, sugars, and secondary metabolites, enabling sequestration of compounds such as pigments and toxins within the vacuole.

Functions: By maintaining an acidic vacuolar lumen and regulating ion balance, the tonoplast supports turgor pressure essential for cell expansion and rigidity. It participates in osmoregulation under drought or salinity, detoxification by sequestering harmful compounds, storage of nutrients, pigments, and metabolites, and various signaling and metabolic processes dependent on vacuolar ion flux.

Biogenesis and diversity: The tonoplast is formed from the plant endomembrane system and is present in plant cells containing a central vacuole. Its protein composition varies by tissue, developmental stage, and species, and is studied through tonoplast isolation and proteomics.