References

224 S.J. Roux, C. Song, C. Jeter
sensor aequorin, which can then sensitively report changes in [Ca 2+ ]cyt by
giving off light.
Demidchik et al. (2003) applied nucleotides to excised roots bathed in
a buffered solution containing 10 mM CaCl2 and found that as little as
300 nM ATP could induce a twofold increase in [Ca 2+ ]cyt in less than 10 s.
The nonhydrolyzable 2-meATP was almost as effective as ATP, indicating
that agonist hydrolysis was not required for the response (Fig. 15.1). Neither
AMP nor phosphate induced any significant change in [Ca 2+ ]cyt, andthe
pyrimidine UTP was ineffective below 100 µM, demonstrating the specificity
of the nucleotide action. The P2-receptor antagonist pyridoxalphosphate-6-azophenyl-2
′ ,4 ′ -disulfonic acid (PPADS) and the calcium channel
blocker gadolinium, both of which inhibit eATP action at some P2 receptors
in animal cells, completely suppressed the response of roots to eATP.
Takentogether,thedoseresponsiveness,substratespecificityandpharmacological
profile suggested that the response was mediated by a distinct cell
surface receptor, possibly an ion channel. The magnitude of the increase in
[Ca 2+ ]cyt, from approximately 100 nM to over 400 nM after treatment with
1 µM ATP, was certainly sufficient to turn on calcium-dependent signaling
pathways in plants.
Fig.15.1. Externally applied purines and UTP induce an increase in the concentration
of cytoplasmic calcium ions ([Ca 2+ ]cyt) inArabidopsis roots. Shown are the mean doseresponse
curves for effects of different agonists on [Ca 2+ ]cyt, as estimated by aequorin
luminescence. ATP analogs tested included BzATP, an analog with a modified Rib moiety;
αβmeATP, which has a methylene insert between the first and second phosphates; and
βγmeATP, which has a methylene insert between the second and third phosphate. (Taken
from Demidchik et al. 2003, with permission)