Regulation of Plant Growth and Development by Extracellular Nucleotides - Communication in Plants: Neuronal Aspects of Plant Life - page 230

γ β Fig. 15.4. Effects of applied of ATP S on the elongation of hypocotyls of etiolated

Arabidopsis seedlings. The open circle at 1.0 mM indicates that at this concentration AMP
hadnoeffectongrowth. Error bars represent standard errors

SandADP

γ β sis . Figure 15.4 shows that even lower doses of ATP S, in

the 10−20 µM range, can stimulate hypocotyl growth, while doses above
400 µM inhibit growth. Comparing these results with the growth results
shown in Tang et al. (2003), we find that the threshold for inhibiting growth
is about 10 times lower when the poorly hydrolyzable nucleotides are ap-
plied. Taken together, the data show that nucleotide hydrolysis is not re-
quired for its growth effects, and suggest that the less sensitive responses
to ATP and ADP may be due to the rapidity with which these nucleotides
are hydrolyzed in the plant cell wall, where acid and alkaline phosphatases
abound.
To relate the previously mentioned results to physiology, Coco et al.
(2003) proposed that sites of active delivery of secretory vesicles are sites
of release of high concentrations of ATP, because secretory vesicles contain
concentrations of ATP near millimolar (Pugielli et al. 1999), and they would
unload this cargo of ATP into the ECM when they fuse with the plasma
membrane. If this is true also in plants, then one would predict that the
growth of plant cells would be accompanied by the delivery of ATP to plant
cell walls, because sites of active growth in plants are also sites of active
delivery of secretory vesicles (Clark et al. 2005).
SincehighlevelsofATPcaninhibitgrowth,onecouldalsopostulate
that control of eATP concentration at growth sites would be a mechanism
of growth control. Consistent with this hypothesis, treatment of wild-type
pollen with either micromolar levels of ATP

SandADP

γ S or with inhibitors of the ATP-

hydrolyzing enzyme apyrase inhibits pollen germination (Steinebrunner
et al. 2003). Knocking out AtApy1 and AtApy2, two closely related apyrases