brane from a variety of cell types derived from different tissues in different plant species.
Applied pressures of 1 to 10 kPa have been shown to trigger activation of channel cur-
rents in Arabidopsis,Allium,Vicia,Valonia,Lilium,and others. Apart from this common
mechanism of gating, however, the observed channel activities are quite diverse with re-
gard to selectivity, conductance, or voltage dependence. Some channels are unselective
(Spalding and Goldsmith 1993) and others have been demonstrated to show preference
for anions (Falke et al. 1988; Cosgrove and Hedrich 1991; Heidecker et al. 1999; Qi et
al. 2004) or cations (Garrill et al. 1994), whereas certain channels show high selectivity
for K+or Ca2+(Cosgrove and Hedrich 1991; Ding and Pickard 1993; Liu and Luan 1998;
Dutta and Robinson 2004). Measured conductances range from 3 to 15 pS for Ca2+-
selective channels (Cosgrove and Hedrich 1991; Ding and Pickard 1993; Dutta and
Robinson 2004) to 13 to 97 pS for anion channels (Falke et al. 1988; Cosgrove and
Hedrich 1991). Although most channels are seemingly independent of membrane poten-
tial (Falke et al. 1988; Garrill et al. 1994; Liu and Luan 1998; Heidecker et al. 1999;
Dutta and Robinson 2004; Qi et al. 2004), others show significantly reduced open prob-
abilities at either negative or positive voltages (Cosgrove and Hedrich 1991; Spalding and
Goldsmith 1993), whereas some are active only at higher voltages, irrespective of the po-
larity (Ding and Pickard 1993).
Unfortunately, most of these channels have not been analyzed beyond the initial char-
acterization. Thus, even though the molecular identity remains elusive, information on
how the regulation of these mechanosensors are modulated by other factors such as pH,
lipid environment, or the cytoskeleton could greatly enhance our understanding of how
plant cells tune their mechanosensitivity during differentiation or in response to chang-
ing environmental conditions.
Despite all the differences shown by the plant mechanosensitive conductances de-
scribed above, one interesting common feature of all these channels is that their acti-
vation is feasible in excised membrane patches devoid of cell wall or intact cytoskele-
ton. This observation suggests that plant mechanosensory channels are sensitive to
changes in lipid bilayer tension and their gating does not absolutely depend on tethering
to force transmitting elements, though both cell wall and cytoskeleton may have a role
in modulating membrane tension in vivo. The pressures used to open mechnosensitive
conductances in all these plant experiments are also comparable to those that gate the
bacterial MscL and MscS channels, consistent with a role for membrane tension in their
regulation.
Indeed, there are homologs of the MscS channels in plant genomes with, for exam-
ple, 10 MscS-like (MSL) proteins in Arabidopsisand 6 in rice (Haswell and Meyerowitz
2006). The MSLs represent very strong candidates for mechanosensitive channels in
plants, but to date there is no direct electrophysiological evidence that these proteins are
indeed channels and show mechanosensitivity. In Arabidopsis, MSL2 and 3 are local-
ized to plastids where they play a redundant role in plastid division (Haswell and
Meyerowitz 2006), perhaps reflecting conservation of these channels from the endosym-
biotic origin of the plastid. The other MSLs of Arabidopsislocalize to other regions of
the cell, but we must await analysis of their roles at the genetic and electrophysiological
levels to know whether they represent the elusive plant plasma membrane mechanosen-
sory channel.
frankie
(Frankie)
#1