Proton ATPase - Physiological Roles in Plants

Physiological Roles in Plants

Plasma membrane H+
-ATPases are found throughout the plant in all cell types investigated, but some cell types have much higher concentrations of H+
-ATPase than others. In general, these cell types are specialised for intensive active transport and accumulate solutes from their surroundings. Most studies of these roles come from genetic studies on Arabidopsis thaliana. H+
-ATPases in plants are expressed from a multigene subfamily, and Arabidopsis thaliana for instance, have 12 different H+
-ATPase genes.

Some important physiological processes the plant H+
-ATPase is involved in are:

• Phloem loading. The phloem is a tissue specialised for long-distance transport of organic compounds, and is well known for its involvement in the transport of sugar from leaves or other source areas. Here the H+
  -ATPase powers the sucrose/H+
  cotransporters and is found to be essential for the loading of sucrose into the phloem.
• Solute uptake in roots. H+
  -ATPases energize the uptake of nutrients from the soil into the roots, and is also involved in the further loading of these solutes into the xylem, a tissue specialised for long-distance transport of water and micronutrients.
• Tip-growing systems. Pollen tubes and root hairs are examples of plant tip-growing systems, where a single cell expands in one direction only. The direction of growth is controlled by an asymmetrical proton gradient, where protons enter at the extreme tip and are pumped out just below the tip.
• Size of stomatal aperture. The somatal pore controls the diffusion of CO₂ into the leaves to be utilized for photosynthesis. The pore is formed by two guard cells, which control the size of the pore by swelling in response to the activity of the H+
  -ATPase. Opening and closure of the pore is partly controlled by regulation of the H+
  -ATPase.
• Plant movements. Like the somatal pore, other movements of plant organs are controlled by motor cells' changing cell turgor. These cells control phenomena such as solar tracking by the plant to optimize orientation of photosynthetic leaves, and the swift and spectacular reactions to touch found in some plant species (e.g., carnivorous plants). All of these swelling and shrinking processes take place by massive water and ion fluxes through channels. Here, activation of the H+
  -ATPase leads to plasma membrane hyperpolarization and the opening of voltage sensitive potassium channels. The K+
  influx leads to water uptake and turgor increase in the cell.
• Salt and osmotolerance. Salinity imposes two stresses on the cell: one is the loss of turgor due to the hypertonicity of the extracellular medium, and the other is a direct effect of toxic ions on metabolism. Therefore plants have developed several defence mechanisms. The Na/H+
  antiporter is heavily involved and is powered by the action of the H+
  -ATPase, which is highly expressed in leaves and roots during salt stress.
• Intracellular pH regulation. Intracellular pH remains constant during cell growth, it is presumed, to ensure optimal activity of the cytoplasmic enzymes. This is controlled by the proton pump.
• Acid growth. Acidification of the external medium caused by activation of the plasma membrane H+
  -ATPase initiates cellular expansion. It is believed that the plant hormone auxin activates the proton pump. The apoplastic acidification leads to loosening of the cell wall and hyperpolarization of the plasma membrane inducing K+
  uptake and swelling.