Handbook of Plant and Crop Physiology

(Steven Felgate) #1

ing in the open conformation. The relation between the amount of current flowing through the membrane
at different membrane potentials and the values of these potentials is then analyzed to learn about the con-
ductivity, the gating, and the selectivity of the channels [23–25].
The direction of the net ion flux that is facilitated by opening of voltage-gated channels depends on
a number of factors. These include the charge of the transported ion and the prevailing Nernst potential
(EN) for the specific ion.
Let us consider the effect of depolarization and hyperpolarization on Kand Clfluxes via voltage-
gated channels in the plasma membrane of a plant cell. We assume that Khas been accumulated pas-
sively in the cytoplasm and is at equilibrium across the plasma membrane (EMENfor K, for exam-
ple,100 mV) and that Clhas been accumulated actively in the cytoplasm and would be at equilibrium
only at a very positive membrane potential (for example, 70 mV). Activation of Kchannels by depo-
larization (for example, to 0 mV) would then induce Kefflux from the cytoplasm to the free space. Net
Kefflux occurs because depolarization increases EMaboveENfor K. Depolarization-activated open-
ing of a Clchannel would also induce Clefflux. This is because even at EM0, the electrochemical
potential of Clinside is larger than outside.
Let us now consider hyperpolarization-activated Kand Clchannels. Hyperpolarization (changing
EMto more negative than the ENfor K) creates an inward-directed electrochemical potential gradient
for Kand results in net Kinflux. Activation of a Clchannel by hyperpolarization would, however,
again result in net Cl efflux through the open channel. This is because the outward-directed electrochem-
ical potential gradient of CIwould increase with the decrease in membrane potential.
Using patch clamp, a large variety of ion channels has been detected in the plasma membranes, tono-
plasts, and even membranes of chloroplasts isolated from plant tissues, ranging from mesophyll to root
hairs and pollen tubes. Among these channels, the best characterized are, of course, the most easily ac-
cessible channels, those in the plasma membranes.



  1. Plasma Membrane Channels


KCHANNELS There are two major kinds of voltage-activated Kchannels in the plasma membrane
of plant cells. One kind is activated by hyperpolarization and conducts Kinto the cells (termed also “in-
ward rectifying” channels). The second type is activated by depolarization and facilitates Kefflux (“out-
ward rectifying” channels). The gating of both types of channels is affected strongly by protons. Hyper-
polarization-activated K-influx channels in the stomatal guard cell membrane are activated by lowering
external pH (external acidification) [26,27] and those in phloem cells by increasing external pH [28]. De-
polarization-activated Kchannels of stomatal guard cells are inhibited by protons [26,29]. Ca^2 inhibits
the hyperpolarization-activated Kchannels in stomatal guard cells but does not affect the depolariza-
tion-activated Kchannels in these cells.


The hyperpolarization-activated Kchannels are not ideally selective for Kand admit other
cations, such as Rb, Naand Ca^2 [30,31]. The reported Napermeability relative to Kperme-
ability in K-influx channels ranged between 1/100 and 1/10 [30,32]. The depolarization-activated K
channels were usually less Kselective [33–36]. Since the electrochemical potential gradient of Ca^2 
is always inward (the usual concentration ratio of Ca^2 across the plasma membrane is at least 10^4 ),
even a tiny permeability of the Kchannels to Ca^2 may allow a nonnegligible influx of Ca^2 into the
cytosol.
A number of genes of channels have been cloned from plants (Arabidopsis thaliana, maize, potato).
They were expressed in yeast cells, in oocytes of the frog Xenopus, and in insect cell lines. Patch-clamp
and related electrophysiological methods were used to characterize the channels in these foreign (het-
erologous) systems (reviewed in Ref. 37). Molecular identification of a native (in situ) plant channel with
a channel clone has not been yet established unequivocally for any one of these plant channels. However,
theArabidopsisKAT1 channel clone [38] tends to be indentified with the guard-cell K-influx channels
[39]. Similarly, the AKT2/3 type of channels [40,41] are probably identical to the K-influx channels of
the phloem elements [28,42] and the ArabidopsisSKOR1 channel clone [43] with K-efflux channels in
xylem parenchyma (reviewed in Ref. 37). The ZMK1 channel clone from corn (of the AKT1 family) was
identified with the hyperpolarization-activated Kchannels in corn coleoptiles and implicated in gravit-
ropism [42].


MINERAL NUTRIENT TRANSPORT IN PLANTS 343

Free download pdf