References

16 Nonselective cation channels 243
Molecular characterisation of plant glutamate receptors is in progress.
Knocking out AtGLR1.1 (one of Arabidopsis glutamate receptor homologues)
showed a possible involvement of glutamate receptors in the seed
germination, N and C nutrition, and in abscisic acid regulation of the root
growth (Kang and Turano 2003; Kang et al. 2004). Changes in glutamateinduced
cation fluxes have not been tested in Atglr1.1. Kim et al. (2001) have
shown that AtGLR2 could be involved in Ca 2+ transport and utilisation in
Arabidopsis. According to Turano et al. (2002) AtGLR3.2 is expressed in
growing tissues and vessels where it could be implicated in Ca 2+ transport.
Phylogenetic and expression analyses of the Arabidopsis glutamatereceptor-like
gene family suggest a particular role of glutamate receptors
in the plant root physiology because all 20 AtGLRs were expressed in roots
(Chiu et al. 2002).
Cyclic nucleotides (cAMP and cGMP) are ubiquitous signalling molecules
in all living organisms. CNGC are thought to be a target for these
signalling agents (Talke et al. 2003). Genes encoding CNGC have been
identified in plants (reviewed by Demidchik et al. 2002). Knocking-out
AtCNGC2 (dnd1)andAtCNGC4 (hlm1) suppressed hypersensitive response
and altered resistance to pathogens (Clough et al. 2000; Balague et al. 2003;
Jurkowski et al. 2004). This suggests that CNGC participate in plant Ca 2+
transport. Indeed, heterologously expressed plant CNGC demonstrate activation
by cyclic nucleotides and permeability to monovalent and divalent
cations (Leng et al. 1999, 2002). In Arabidopsis guard cells and mesophyll,
Lemtiri-Chlieh and Berkowitz (2004) have recently characterised Ca 2+ -
permeable CNGC with inward rectification and sensitivity to lanthanides.
However, Maathuis and Sanders (2001) reported an inhibitory effect of
cyclic nucleotides on constitutive Na + -permeable NSCC in Arabidopsis
root-derived protoplasts. Therefore, the precise role of CNGC in the plant
signalling remains obscure.
Purines play a central role in energy metabolism: however, in animals,
they also function as signalling molecules activating ionotropic purinergic
receptors (which are NSCC) (Ralevic and Burnstock 1998). In plants, ABC
transporters could release ATP and ADP to the extracellular space, whilst
ectoapyrases could break these molecules down (Thomas et al. 1999). These
systemscanpotentiallyproviderapidrelease/removalofpurinesthatisnecessary
for purinergic signalling (Ralevic and Burnstock 1998). Does purine
signalling exist in plants? Three recent reports show that such signalling
may exist in plants. Lew and Dearnaley (2000) have found that externally
applied ATP and ADP depolarise the plasma membrane of Arabidopsis
root hair. Demidchik et al. (2003) have demonstrated that different purines
(including nonhydrolysable ATP analogues) induce transient elevations
in [Ca 2+ ]cyt. Jeter et al. (2004) have additionally shown that purines activate
Ca 2+ -mediated cytosolic signalling cascades. Purine-induced [Ca 2+ ]cyt