References

238 V. Demidchik
Ligand-activated and ROS-activated NSCC can potentially play an important
role in the nutritional K + influx. Demidchik et al. (2004) have demonstrated
that glutamate-activated NSCC in Arabidopsis root epidermis catalyse
significant K + influx currents. High K + permeability of glutamateactivatedNSCCisprobablyresponsibleforaglutamate-induceddepolarisation
of the plasma membrane in Arabidopsis root cells (Dennison and
Spalding 2000). OH•-activated NSCC were shown to mediate large K + and
NH + 4 influx currents (Demidchik et al. 2003). Mechanosensitive NSCC in
Allium cepa bulb epidermis are well permeable to K + (Ding and Pickard
1993) but their role in nutritional K + uptake is difficult to predict owing to
verylimiteddataonthisgroupofchannels.
16.4.2
Calcium and Magnesium
Extracellular Ca 2+ and Mg 2+ concentrations are similar (between 0.1–
2mM)but[Ca 2+ ]cyt (about 100 nM) is dramatically lower than cytosolic
Mg 2+ activity (0.4−2 mM)(Bergmann 1992; Marschner 1995). This results
in very positive ECa (from 87 to 125 mV) and EMg about 0 mV. At negative
membrane potentials, both ions can be passively transported to the cell but
only Ca 2+ can pass a membrane through the channels at positive potentials.
According to Romano et al. (1998) depolarisation-activated K + -permeable
channels in Arabidopsis mesophyll protoplasts can catalyse a significant
Ca 2+ influx. Similar results were found in root protoplasts (Roberts
and Tester 1997; Gilliham et al. 2004). Selectivity series for single Ca 2+ -
permeable outwardly rectifying conductances in the maize root showed that
they were catalysed by NSCC (Roberts and Tester 1997). So, depolarisationactivated
NSCC could catalyse some Ca 2+ influx.
Passive transport of Ca 2+ fornutritionalneedsisakeyquestionofplant
mineral nutrition (Marschner 1995). HACC activate at membrane potentials
more negative than −150 mV (Véry and Davies 2000). When [Ca 2+ ]cyt
increasesHACCactivateatmorepositivevoltagesandthereforeprobably
play a significant role in Ca 2+ uptake in growing tissues (where [Ca 2+ ]cyt is
elevated) (Véry and Davies 2000; Demidchik et al. 2002). DACC have low
amplitude and are only functional in about 35% of protoplasts (Thion et al.
1996). DACC reveal maximal activation at −70 to −80 mV and could be involved
in signalling, having no significant impact on nutritional Ca 2+ influx.
Typically plasma membrane potentials in Arabidopsis root epidermal cells
vary between −101 and −144 mV (Maathuis and Sanders 1993; Demidchik
et al. 2002), thus HACC or DACC cannot be a major Ca 2+ influx pathway
in these conditions. A key role of constitutive Ca 2+ -permeable NSCC
in nutritional Ca 2+ influx has recently been demonstrated in Arabidopsis