Regulation of Plant Growth and Development by Extracellular Nucleotides - Communication in Plants: Neuronal Aspects of Plant Life - page 231

that are both strongly expressed in wild-type pollen, also blocks pollen
germination (Steinebrunner et al. 2003). However, although it is likely
that Arabidopsis , like peas (Thomas et al. 1999) and like animals, has ec-
toapyrases that control the extracellular concentration of ATP, the subcel-
lular locale of AtApy1 and AtApy2 in Arabidopsis has not been confirmed,
so their regulation of pollen germination may or may not be through their
maintenance of a growth-promoting level of eATP concentration in pollen
walls. By sequence analysis there appear to be seven different apyrases in
Arabidopsis , and it will be important to determine which of these function
as ectoapyrases, and thus as potential regulators of extracellular nucleotide
agents that may serve as growth regulators in plants.
Legume apyrases have been strongly implicated in the process of Nod
signaling (Cohn et al. 2001), and they also appear to play a role in plant
defense against pathogens (Kawahara et al. 2003). Although these enzymes
have not been a focus of this review, to the extent that members of this
family control eATP concentration they could obviously play major roles in
growth control. Progress in defining the role(s) of apyrases in nodulation
will be synergistic with progress in defining the role(s) of apyrases in
growth control in Arabidopsis and other non-nodulating species. Quite
likely, the apyrase studies in legumes may also catalyze more rapid progress
in defining the role(s) of extracellular nucleotides in plant growth and
development.
15.4
Conclusions and Future Perspectives
Thedataathandstronglyfavortheconclusionthatextracellularnucleotides
can induce signaling changes in plant cells that are potentially growth
regulating. A key limiting factor for future progress in this exciting new field
is the lack of a clearly identified P2-like purinoceptor. The primary structure
of these receptors is not highly conserved even in mammals, and a close
homolog of the mammalian P2 purinoceptors has not been identified either
in plants or in other animals like Drosophila or Caenorhabditis elegans .
However, Demidchik et al. (2003) hinted that they were making progress on
this front, and if they are successful in identifying a receptor for extracellular
nucleotides, this would represent a major advance.
Another key area critical for future progress is defining the mechanisms
and physiological situations that promote the release of nucleotides to the
wall of plant cells and that control their concentration there. A prerequisite
forthisprogresswillbedevelopingtechniquesforrapidlyandquantita-
tively measuring changes in the concentration of extracellular nucleotides
in plant cell walls. Progress in this area has been relatively slow even in