Plant Vacuoles
Authors: Rea Philip A 
Published: June 2018

Vacuoles can represent 90% or more of the total volume of a mature plant cell. Derived from the endoplasmic reticulum and Golgi complex and delimited from the cytosol by the vacuolar membrane (tonoplast), they are versatile dynamic organelles capable of assuming different morphologies and serving a wide range of functions in different cells or in the same cells at different times. Many cells contain just one central vacuole, others contain several that differ in their relative contributions to lytic and storage functions and a few contain compound vacuoles – vacuoles within vacuoles – by means of which lytic and storage functions are partitioned within a single vacuole. The many processes in which vacuoles participate include the maintenance of tissue water balance and turgor regulation for growth, mechanical support, stomatal aperture control and thigmotropic responses; inorganic and organic nutrient storage and retrieval; macromolecule turnover and salvage; cytosolic inorganic ion and metabolite homeostasis; storage excretion and detoxification of endogenous waste products, heavy metals and other xenobiotics. Although most of the carrier‐ or channel‐mediated transport processes that occur across the vacuolar membrane are driven by the H
+
electrochemical gradient (proton motive force, PMF) established by ATP (adenosine triphosphate)‐energised V‐type H
+
‐ATPases and inorganic pyrophosphate‐energised H
+
‐PPases (V‐PPase), there are many that are directly energised by ATP through the action of P‐type ATPases or ATP‐binding cassette (ABC) transporters.

Key Concepts

Vacuoles often represent much of the volume of a plant cell.

As components of the endomembrane system, vacuoles are derived from the endoplasmic reticulum and/or Golgi complex.

Depending on their function and contents, plant vacuoles fall into two categories: lytic vacuoles (LVs) or protein storage vacuoles (PSVs). LVs, whose contents are acidic and dominated by degradative enzymes, primarily serve a lysosomal function. Protein storage vacuoles (PSVs), whose contents are less acidic and dominated by storage proteins, primarily serve a depot function.

Bounded by membranes rich in water channels (aquaporins) that enable rapid osmotic equilibration between the vacuole lumen and cytosol, vacuoles are crucial for establishing and maintaining cell turgor and for ensuring adequate hydration of the cytosol.

Plant vacuoles are vital for the temporary storage of both organic and inorganic nutrients and for maintaining their levels within the ranges compatible with metabolism in the cytosol and other intracellular organelles.

The plant vacuolar membrane (tonoplast) is unusual in containing two distinct categories of proton pumps – V‐type H
+
‐ATPases (V‐ATPases) and vacuolar H
+
‐pyrophosphatases (V‐PPases) – that are capable of contributing to vacuolar acidification and to the establishment of the transmembrane H
+
‐electrochemical gradient (proton motive force, PMF) required for secondary active solute transport into and out of the vacuole.

As the sole enzyme in plant cells capable of catalysing the direct hydrolysis of cytosolic inorganic pyrophosphate (PPi) to inorganic phosphate (Pi), the hydrolytic activity of the V‐PPase is critical if cytosolic biosynthetic reactions that generate PPi are to be sustained.

Vacuoles enable plant cells to accumulate both endogenous and exogenous toxins, for instance secondary metabolites and heavy metals, to levels that would be lethal if the same substances were to accumulate to similar levels in the cytosol or other intracellular compartments.

Multidrug and toxin efflux (MATE) transporters and ATP‐binding cassette (ABC) transporters mediate the vacuolar uptake of many secondary metabolites and xenobiotics.