Aquaporins: Plants

In plants water is taken up from the soil through the roots, where it passes from the cortex into the vascular tissues. There are two routes for water to flow in these tissues, known as the; apoplastic and symplastic pathways. The presence of aquaporins in the cell membranes seems to serve to facilitate the transcellular symplastic pathway for water transport. When plant roots are exposed to mercuric chloride, which is known to inhibit aquaporins, the flow of water is greatly reduced while the flow of ions is not, supporting the view that there exists a mechanism for water transport independent of the transport of ions; aquaporins.

Aquaporins in plants are separated into four main homologous subfamilies, or groups,

  • Plasma membrane Intrinsic Protein (PIP)
  • Tonoplast Intrinsic Protein (TIP)
  • Nodulin-26 like Intrinsic Protein (NIP)
  • Small basic Intrinsic Protein (SIP)

These four subfamilies have continued to be broken up into smaller evolutionary subgroups that are directly related to their DNA sequence specificity. PIPs cluster into two subgroups, PIP1 and PIP2, whilst TIPs cluster into 5 subgroups, TIP1, TIP2, TIP3, TIP4 and TIP5. Each subgroup is again split up into isoforms e.g. PIP1;1, PIP1;2.

The silencing of plant aquaporins has been linked to pore plant growth and even death of the plant.


Gating of Plant Aquaporins

The gating of aquaporins is carried out to stop the flow of water through the pore of the protein. This may be carried out for a number of reasons, i.e. plant contains low amounts of cellular water due to drought. The gating of an aquaporin is carried out by an interaction between a gating mechanism and the aquaporin which causes a 3D change in the protein so that it blocks the pore and thus disallows the flow of water through the pore. In plants it has been seen that there are at least two forms of aquaporin gating. These are gating by the dephosphorylation of certain serine residues, which has been linked as a response to drought, and the protonation of specific histidine residues in response to flooding. The phosphorylation of an aquaporin has also been linked to the opening and closing of a plant in response to temperature.

PIPs

Plasma membrane intrinsic proteins are found, as their name suggests in the plasma membrane of plant cells. There are two PIP subgroups, PIP1 and PIP2, due to the distinct differences in their peptide sequence. PIP1s commonly have lower water channel activity than PIP2s although it is not understood why. Also not understood, but the water channel activity of PIP1s has been seen to increase when in the tetramer form with PIP2s.












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