References

16 Nonselective cation channels 239
root epidermis (Demidchik et al. 2002). These channels were clearly dominant
in Ca 2+ influx currents at resting membrane potentials. 44 Ca 2+ flux
measurements showed that the Ca 2+ uptake rate was not sensitive to verapamil
(HACC blocker) but was inhibited by Gd 3+ (nonspecific blocker
of cation channels). In addition, basal [Ca 2+ ]cyt revealed linear voltage
dependence (NSCC have linear current–voltage relationships) that again
implicates NSCC in Ca 2+ uptake. Constitutive Ca 2+ -permeable NSCC were
permeable to Mg 2+ , and could therefore participate in nutritional Mg 2+
influx into the root.
Glutamate-activated Ca 2+ influx channels have recently been identified in
Arabidopsis root epidermal protoplasts (Demidchik et al. 2004). Glutamateactivated
Ca 2+ conductance appeared as “spiky” currents in about 20%
of protoplasts. Protoplasts isolated from roots of aequorin-transformed
Arabidopsis plants demonstrated steady-state (measured over 2 h) elevation
of [Ca 2+ ]cyt in response to 0.2−2 mM glutamate, suggesting a significant role
of glutamate-activated channels in the regulation of the basal cytosolic Ca 2+
activity. Apoplastic glutamate concentration is between 0.3 and 1.3 mM, as
measured in a range of tissues and species (Lohaus et al. 1995; Lohaus and
Heldt 1997). Therefore, it is possible that apoplastic glutamate, and maybe
glycine too (Dubos et al. 2003), functions as a “permanent” activator of
Ca 2+ influx into the plant cells.
Purine-induced Ca 2+ influx has recently been discovered in Arabidopsis
roots (Demidchik et al. 2003). Application of ATP and its nonhydrolysable
derivativesresultedinamanifoldtransientincreasein[Ca 2+ ]cyt fully recovered
within 10−15 min after the application of purines. This suggests
that, in contrast to glutamate, purines do not cause steady-state changes in
Ca 2+ influx and do not affect nutritional Ca 2+ uptake.
Arabidopsis root and guard cell ROS-activated NSCC catalysed significant
Ca 2+ and Mg 2+ influx that could play a role in nutritional uptake of
these cations (Pei et al. 2000; Demidchik et al. 2003; Foreman et al. 2003).
16.4.3
Microelements and Trace Elements
It is generally accepted that Na + influx is catalysed by root epidermal
NSCC (reviewed by Demidchik et al. 2002; Tester and Davenport 2003).
Constitutive Na + -permeable NSCC have been demonstrated in a range of
tissues and species (Stoeckel and Takeda 1989; Elzenga and van Volkenburgh
1994; White and Lemtiri-Chlieh 1995; Tyerman et al. 1997; Roberts
and Tester 1997; Amtmann et al. 1997; Véry et al. 1998; Demidchik and
Tester 2002). In the majority of preparations these channels were weakly
selective for monovalent cations, voltage-insensitive (or slightly voltage