Physiological Roles of Nonselective Cation Channels in the Plasma Membrane of Higher Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 237

16.4
The Role of NSCC in Plant Mineral Nutrition

Uptakeofcationsisacrucialphysiologicalprocessthatisinvolvedinplant
cell metabolism, photosynthesis, productivity and resistance to stresses
(Bergmann 1992; Marschner 1995). Four macronutrients can be taken up
by roots in cationic form - N, K, Ca, and Mg (as NH 4 ,K + ,Ca 2+ and Mg 2+ ).
Most micronutrients are cations that can potentially permeate NSCC (Na + ,
Fe 2+ ,Cu 2+ ,Ni 2+ ,Co 2+ ,Zn 2+ ,andMn 2+ ). With the exception of K + and NH 4 ,
passiveinfluxofcationsinplantrootsisprobablydominatedbytheactivity
of NSCC. Cations can be transported into the cytosol by hyperpolarisation-
activated and voltage-insensitive NSCC that are functional at voltages al-
lowing passive cation influx. Depolarisation-activated NSCC are unlikely
to play significant roles in the influx of monovalent cations, but they could
transport divalent cations (because the activity of divalent cations in the
cytosol is low and they cross the plasma membrane passively at positive
membrane potentials).

16.4.1
Potassium and Ammonium

Many constitutive NSCC are permeable to K + (reviewed by Demidchik et
al. 2002). With few exceptions, for example, NSCC in pea leaf epidermis
(Elzenga and van Volkenburgh 1994) and maize root cortex (Roberts and
Tester 1997), K + permeates through NSCC better than other cations. Per-
meability to NH 4 has been measured in several preparations (Cerana and
Colombo 1992; Demidchik and Tester 2002; Roberts and Tyerman 2002;
Zhang et al. 2002), often with a slightly lower permeability than for K +
P NH 4 /

0. 7. However, a slightly greater selectivity for NH 4 over K + was
found in the symbiosome membrane of Lotus japonicus (Roberts and Ty-
erman 2002). In this membrane NSCC probably transport fixed NH 4

P K ≈

from

Rhizobia bacteria and the legume host.
According to Hirsch et al. (1998) the 86 Rb + uptake rate was about 10,
5 and 2 times higher at 10, 100 and 1,000 µM external Rb + respectively in
wild-type than in knockout lacking inwardly rectifying K + channels ( akt1 ).
So, the role of KIR in K + uptake is very high at moderately low external
K + concentrations but, under average soil K + concentrations (0.1−1 mM;
Bergmann 1992) NSCC could mediate about 20-35% of total accumulated
K + .ManyKIRarehighlypermeableforNH 4 but not all of them (Bertl et
al. 1997); therefore, in some cases, passive NH 4 influx is likely mediated by

NSCC (Kronzucker et al. 2001).