U. WARWICK (UK) — Efforts to develop drought-resistant seeds are now focused on a protein long considered irrelevant.
A team at the University of Warwick examined two proteins that are members of the large family of “major intrinsic proteins”, or MIPs, which are widespread among living organisms and are known to act as water channels governing water uptake.
The first protein type they examined known as PIPs are intrinsic proteins typically found in a cell’s outer casing, or plasma membrane. These plasma membrane intrinsic proteins (PIPs) have been considered the probable prime gatekeepers of the water transport into and out of cells.
The Warwick team also studied a second group known as tonoplast intrinsic proteins, or TIPs, which are typically found in an inner cell layer called the tonoplast that surrounds a vacuole in a cell. Vacuoles are enclosed compartments in a cell filled with water and containing inorganic and organic molecules.
Despite the fact TIPs appear capable of governing water uptake, their concentration in the tonoplast reduced the likelihood that they were key players in water uptake. The tonoplast generally does not present a major problem for intracellular water flow, as its water permeability is considered much higher than the outer plasma membrane.
As a result, very little was known about how TIPs acted in processes such as seed maturation and germination.
The work by Warwick researchers resulted not only in the most complete plant TIP expression map produced to date, but also led to the surprising discovery that TIP plays a crucial role in managing water during seed maturation and germination. The findings are reported in the journal Molecular Plant.
As PIP, but not TIP, are generally found at the plasma membrane of plant cells, one would expect the involvement of PIP in seed de/rehydration. Intriguingly, however, the researchers studied microarray datasets and found that—out of 13 PIPs encoded by the Arabidopsis genome—only 3 (PIP1;2, PIP1;4, PIP1;5) were detected in their seeds.
They also found that those 3 PIPs did not show up until 60 hours after germination—only after the end of the most important phases of water uptake in a germinating seed.
In contrast the researchers found very high levels of TIP3 protein present in the plasma membrane during seed development and germination.
Lead researcher Lorenzo Frigerio hypothesizes that TIP3, besides residing in the tonoplast, is recruited to the plasma membrane to compensate for the absence (or very low concentration) of PIP.
The work was funded by the Leverhulme Trust and the European Union.
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