Fascinating article from Science Daily. A little technical for moi,
but worth a look from those of us interested in theory and experiment.
Scientists Uncover How Plant Roots Respond To Physical Forces Such As
Gravity, Pressure, Or Touch
Researchers have identified two proteins responsible for
mechanosensitive ion channel activities in the roots of Arabidopsis
plants such as the one here. (Credit: iStockphoto)
ScienceDaily (Jun. 5, 2008) — Researchers at Washington University in
St. Louis are the first to identify two proteins responsible for
mechanosensitive ion channel activities in plant roots. Scientists
have long known that plant cells respond to physical forces. Until
now, however, the proteins controlling the ion channel response
remained a mystery.
As the name suggests, mechanosensitive channels are paths through the
cell membrane that respond to mechanical forces such as gravity,
pressure, or touch. Under certain forces, a channel opens, allowing
the flow of ions, such as calcium and potassium ions, into and out of
the cell. Different forces might close the channel, stopping the flow.
This cross-membrane ion flow has been measured electrophysically,
using a technique called the patch-clamp method. But the molecular
nature of the channels themselves was not known. Now, knowing the
proteins involved makes it possible to discover what the channels do
for the whole plant.
"People have been characterizing mechanosensitive channels in plants
for 20 years," said Elizabeth Haswell, Ph.D., assistant professor of
biology at Washington University in St. Louis and lead investigator of
this project, "This is the first time anybody has been able to show
which proteins underlie these activities."
Plants do it, bacteria do it
The two proteins governing ion channels in Arabidopsis root are MSL9
and MSL10, according to the study published in the May 20 issue of
Current Biology. MSL stands for MscS-Like proteins because of their
similarity to a family of bacterial channels known as MscS
(mechanosensitive channels of small conductance). Even though bacteria
and plants are not closely related in terms of evolution, this study
shows that bacterial and plant cells are probably using the same types
of proteins to respond to mechanical forces.
To establish that the channels were in fact mechanosensitive,
Haswell's French colleagues used the patch-clamp method to measure the
movement of ions across the membrane of Arabidopsis root cells as the
pressure inside the cell increased. These experiments demonstrated
that increasing cellular pressure also increased the ion flow across
the membrane. Likewise, as the pressure inside the cell went down, the
ion flow decreased.
To determine whether MSL9 and MSL10 were responsible for this ion
flow, Haswell created a mutant line of Arabidopsis without either type
of protein. When the root cells of the plants lacking MSL9 and MSL10
were tested, the researchers saw very little change in ion flow across
the membrane as the pressure inside the cell increased. In other
words, without these two proteins, very little channel activity was
seen. And the little channel activity they did measure was shown to be
caused by different proteins.
Two to tango
Having shown that MSL9 and MSL10 were responsible for the ion channel
activity, Haswell and colleagues set out to determine if both were
required for the response or if only one did most of the work.
Therefore, they tested plants that lacked only one of each protein and
were surprised to discover that cells with only MSL9 showed one type
of activity and cells with only MSL10 showed a different type of
activity. And, importantly, cells with both proteins showed a third
type, suggesting that both MSL9 and MSL10 are required to produce the
mechanosensitive channel activity seen in wild-type Arabidopsis root.
Haswell and colleagues propose that the channel is composed of
subunits of both proteins MSL9 and MSL10 and that this combined
structure results in the unique mechanosensitive ion channel behavior
observed in the wild-type plants and not in any of the mutant lines.
Despite identifying proteins that govern this ion channel response in
Arabidopsis root, mysteries remain. To determine how the mutant plants
might be defective and reveal the purpose of the channels, Haswell's
group grew the plants that lacked these channels under challenging
conditions, including high salt, root barriers, and dehydration. "We
tried hundreds of experiments, but we never saw a difference between
the mutant and wild-type," said Haswell, "but that's definitely one of
the next big steps -- to find out what the channels really do and why
This work was funded by the French Ministry of Research and the U.S.
Department of Energy.
Adapted from materials provided by Washington University in St. Louis.
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the following formats:
Washington University in St. Louis (2008, June 5). Scientists Uncover
How Plant Roots Respond To Physical Forces Such As Gravity, Pressure,
Or Touch. ScienceDaily. Retrieved June 6, 2008, from