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root epidermis (Demidchik et al. 2002). These channels were clearly dom-
inant 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 ve-
rapamil (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). Glutamate-
activated 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 re-
covered 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 signifi-
cant 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 Volken-
burgh 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
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