References

222 S.J. Roux, C. Song, C. Jeter
7-pass TM heterotrimeric G-protein linked receptors. The binding of ATP
or ADP (among other NTPs and NDPs) activates these receptors, initiating
secondary messenger systems and downstream signaling cascades, thereby
affecting changes in gene expression and culminating in the induction of
cell-type specific responses.
The first recognized physiological activity of these signaling agents was
that of a co-neurotransmitter, and so this type of signaling was originally
known as purinergic transmission (Burnstock 1972). Derivatives of ATP,
including ADP and adenosine, were also shown to have biological effects.
For example, adenosine functions as a negative regulator of neurotransmitter
release at specialized P1 purinceptors, functioning together with ATP
to modulate smooth muscle contraction; and ADP signals blood platelet
aggregation during thrombosis (reviewed in Burnstock 1996; Ralevic and
Burnstock 1998).
Extracellular ATP (eATP) has been reported to have numerous effects
on the physiology of plants, too, altering both developmental programs
andresponsestoenvironmentalstimuli.Earlystudiesshowedthatexogenous
application of ATP could induce the closure of the Venus’s-flytrap
(Jaffe 1973), affect cytoplasmic streaming in Chara cells (Williamson 1975),
modulate stomatal aperture in Commelina communis (Nejidat et al. 1983),
and stimulate pollen tube generative nuclear divisions in Lilium lingiflorum
(Kamizyo and Tanaka 1982). However, most of these reports assumed that
the applied ATP was somehow, directly or indirectly, altering the energy
charge of the cell, and thus was still playing its standard role of driving
energy-dependent reactions.
More recent results have revealed that the hydrolysis of eATP is not
required to induce responses in plant cells (Demidchik et al. 2003; Jeter
et al. 2004). This has led plant scientists to recognize the potential role
of eATP as an agonist, exerting its effects through interaction with cell
surface receptors, similar to what happens in animal cells. The purpose of
this chapter is to review this more recent literature and discuss potential
mechanisms by which extracellular nucleotides could alter plant growth
and development as cell–cell signaling molecules.
15.2
Rapid Responses of Plants to Applied Nucleotides
15.2.1
Induced Changes in the Concentration of Cytoplasmic Calcium Ions
In animal cells, activation of purinoceptors by extracellular nucleotides
rapidly leads to changes in membrane potential and increases in the con-