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Posted: Apr 30, 2013
Nanodomains in plant cell membranes made visible
(Nanowerk News) Plants are generally firmly rooted in the ground so that they cannot just move to a different place when the conditions become too dry or uncomfortable in any other way. Therefore, they must be alert to environmental changes and react appropriately.
When the conditions are favorable, the root absorbs nutrients and life-sustaining water. Above the surface of the ground, the shoot adjusts to the current light conditions, performs photosynthesis and produces components for growth, development and reproduction.
Under stress, the plants switch over from this standard program to survival mode. For this purpose, they need the ability to sense stress factors – such as heat, drought or the presence of pathogens – and to take appropriate action. They are able to do this with the help of sensors, which are each connected to a specific network.
Interconnected platforms in the membranes
"According to current knowledge, the cell membranes contain numerous tiny platforms, where certain signaling proteins interact. To some extent, these platforms possess preset interconnections. Depending on the respective signal, they are then reconfigured," explains Dr. Ines Kreuzer, a plant biologist at the University of Würzburg. Because these membrane platforms are so tiny, they are also known as nanodomains.
Reconfiguration of the nanodomains observed
As reported in the journal PNAS ("Arabidopsis nanodomain-delimited ABA signaling pathway regulates the anion channel SLAH3"), Kreuzer's study group showed for the first time that the components of the drought stress signaling pathway occupy such nanodomains. In cooperation with Professor Gregory Harms at Wilkes University in Pennsylvania (USA), they were also able to trace the change in the domain composition induced by the hormone abscisic acid (ABA) under the laser microscope.
The ABA hormone is used to communicate changes in the water status between different parts of the plant. In dry conditions, high ABA levels ensure that the plant reduces its loss of water to the minimum.
Several signaling proteins involved
The team of the young Würzburg researcher identified several signaling proteins in the nanodomains as main components of the ABA signaling pathway. Kreuzer: "We are talking about the ion channel SLAH3, which is activated by the protein kinase CPK21. This kinase is controlled by the protein phosphatase ABI1. As soon as the receptor recognizes the presence of the ABA hormone, it deactivates the phosphatase and sends out the kinase to activate the ion channel. The opening of the ion channel converts the 'water shortage' signal into a flow of ions – an electrical response, in other words."
Phosphatase as "doorkeeper"
In this process, the nanodomains are a kind of "meeting place", where the reactants are given the opportunity to meet. In the absence of the drought stress hormone ABA, the phosphatase ensures that the ion channel and the kinase are no longer allowed into the membrane domains – there is no cellular response. "The processing of the hormone signal is obviously regulated on the basis of the fact that certain proteins either have or don't have access to special membrane areas, in which mechanism the phosphatase seems to perform the function of a 'doorkeeper', as Kreuzer summarizes.
The next steps of the research
Further studies are intended to show how the process in the nanodomains impacts on the nucleus. It is conceivable that drought tolerance genes are activated there, ensuring the survival of the plant even when there is a shortage of water.
Ines Kreuzer and her study group conduct research at the Department of Botany I – Molecular Plant Physiology and Biophysics of the University of Würzburg, headed by Professor Rainer Hedrich. Their studies are funded by the German Research Foundation within the Research Training Group 1342 (Molecular and Functional Analysis of Lipid-Based Signal Transduction Systems).