Regulation of Plant Growth and Development by Extracellular Nucleotides - Communication in Plants: Neuronal Aspects of Plant Life - page 229

et al. 2004), this leads to the question of whether eATP, which can induce
superoxide production when applied to unwounded leaves, has any growth
effects. Tang and colleagues (Tang et al. 2003; Tang 2004) provided an
initial answer to this question. They demonstrated that relatively high
concentrations of applied ATP and ADP (3 mM) and lower concentrations
of the relatively nonhydrolyzable nucleotides ATP γ β S(0.3mM)

could inhibit the straight growth of roots, and that concentrations of these
nucleotides about 3 times lower could inhibit gravitropic growth of roots
without significantly inhibiting their straight growth.
Because of the strong relationship of auxin transport to growth, Tang et al.
(2003) tested whether the growth inhibition induced by applied ATP could
be mediated by the eATP effects on auxin transport. Their results indicated
that both in maize and in Arabidopsis roots the same concentrations of ATP
that inhibited gravitropic growth also inhibited auxin transport.
To investigate by what mechanism ATP was having its effects on growth,
Tang et al. (2003) carried out a variety of controls to render unlikely some
trivial explanations, such as that the effects were due to ATP-induced pH
changes or chelation of divalent cations or to phosphate released from the
hydrolysis of the applied nucleotides. They proposed two possible mecha-
nisms of ATP action. One, based on the results of Thomas et al. (2000), was
that eATP could reduce the steepness of the ATP gradient across the plasma
membraneandthusinhibitthetransporteffectivenessofamultidrugre-
sistance transporter that has been implicated in auxin transport (Noh et
al. 2001). The other was that the applied ATP could be acting through the
more traditional mechanism of activating a P2 purinoceptor.
Regarding the receptor hypothesis, none have been identified so far in
any plant. If one were identified as mediating the growth effects reported
by Tang et al. (2003), either it would have to be far less sensitive than the
mammalian ones, or only a small fraction of the applied ATP and ADP
would be reaching the receptor site, with the rest being rapidly hydrolyzed
or otherwise altered. In the animal literature P2X purinoceptors were orig-
inally thought to be quite insensitive, responding only to millimolar levels
of ATP, but now it appears that P2X receptors can respond to nanomo-
lar levels of ATP, but these same levels desensitize the receptors so that
they subsequently will respond only to much higher (millimolar) doses
(Rettinger and Schmalzing 2004). The same desensitization phenomenon
could significantly raise the threshold of plant responsiveness to external
nucleotides.
Schopfer (2001) has presented data favoring a promotive role for super-
oxide in plant growth. Can the low levels of eATP that promote superoxide
production in Arabidopsis leaves promote growth of Arabidopsis ?Tang
(2004) showed that concentrations of ATP in the range 100−200 µM could
significantly promote hypocotyl growth in etiolated seedlings of Arabidop-

SandADP