Physiological Roles of Nonselective Cation Channels in the Plasma Membrane of Higher Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 242

adaptation to new environments, stress, gravitropic and hormonal re-
sponses, interaction with pathogens, and coordinated behaviour of cells
during growth and development. Ligand- and ROS-activated NSCC catal-
yse Ca 2+ influxtothecytosolinresponsetosignallingsubstances(lig-
ands) that could result in generation of a transient increase in Ca 2+ activ-
ity having signal-specific shape. Calcium transients probably trigger both
signal-specific gene expression and direct Ca 2+ -mediated changes in in-
tracellular reactions. Here, I briefly describe ligand- and ROS-activated
Ca 2+ -permeable NSCC that have been found in plants.
ROS (hydrogen peroxide, superoxide and hydroxyl radical) are impor-
tant signalling agents in plants that are involved in many physiological
phenomena (Apel and Hirt 2004). Plant ROS-activated NSCC were found
for the first time in intact cells of the freshwater alga Nitella flexilis (Demid-
chik et al. 1997, 2001). Extracellular Cu 2+ , which can generate OH • in intact
cell walls (Fry et al. 2002), caused significant influx currents that were non-
selective for monovalent cations, voltage-independent and instantaneously
activating (Demidchik et al. 1997). Calcium permeability of ROS-activated
NSCC was found for the first time in Arabidopsis guard cells (Pei et al.
2000). H 2 O 2 activated significant Ca 2+ influx currents that regulated stom-
atal closing. In contrast to the case for guard cells, H 2 O 2 did not activate
cation conductances in Arabidopsis root protoplasts, however OH • was ef-
fective in this preparation (Demidchik et al. 2003; Foreman et al. 2003).
Arabidopsis root OH • -activated channels were permeable for monovalent
and divalent cations, showed inward rectification and slow kinetics, and
were sensitive to lanthanides. They mediated an increase in [Ca 2+ ] cyt which
could encode information about stresses (Demidchik et al. 2003).
Glutamate and glycine are animal neurotransmitters and important sig-
nalling agents regulating a number of physiological functions (reviewed
by Dingledine et al. 1999). In plants, they can induce NSCC-mediated Ca 2+
influx conductances, plasma membrane depolarisation and cytosolic Ca 2+
transients (Dennison and Spalding 2000; Dubos et al. 2003; Demidchik et
al. 2004), most likely through the activation of plant ionotropic glutamate
receptors (Lam et al. 1998; Lacombe et al. 2001). Glutamate-activated Ca 2+
transients were greater in the mature epidermis than in inner layers of root
cells (Demidchik et al. 2004), suggesting that glutamate can be an external
stimulus from rhizosphere (from other plants, pathogens and parasites).
Apoplastic glutamate and glycine concentrations are at least 0.1−0.3 mM, as
measured in Arabidopsis roots (Demidchik et al. unpublished), therefore
they could play signalling roles at higher concentrations (about 1 mM).
Systems excreting and breaking down apoplastic amino acids have not yet
been identified in plants. Therefore, signalling roles of glutamate-/glycine-
activated NSCC are very likely restricted by perception of external stimuli
from other organisms (Fig. 16.1).