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Extracellular nucleotides (eNTP) occur outside the cytoplasm in the extracellular matrix, were discovered almost 100 years ago in animal cells. These studies showed that the nucleotides are not only building a strand of DNA/RNA or constitute reservoir of energy (eg ATP or GTP) for various biochemical processes, but also may play role in cell growth and development. Extracellular nucleotides are also involved in maintaining homeostasis in the whole animal organism by maintaining adequate blood pressure and immune response. Characterization of receptors that recognize extracellular nucleotides (purynocep- tors) and enzymes that act in the extracellular matrix caring out the hydrolysis of eNTP, and thus regulating their levels, made possible a better understanding of the role of extracellular nucleotides play in cells. Extracellular nucleotides are synthesized within the cytoplasm of the cell and then secreted into the extracellular matrix. Three different ways of eNTP transferring were identified, including their gradual secretion by exocytosis of secretory vesicles, rapid by the ion channels or in the association with multidrug resistance transporters. The prevalence of eNTP and the important role they play in animals allowed for assumption that a similar system of regulation of cell growth and development may occur in plants. First reports about the possibility of the presence of extracellular nucleotides in plants were given in the 70's last century. First studies were focused on the effect of exogenously administered ATP on closure of the trap flycatchers, on the modulation of stomatal guard cell aperture and on the cell division. However, evidences that they may serve as important function as in the animal cells appeared recently. Botanists have only just started to explore the mechanisms underlying their actions. This paper presents results of the researches that show the importance of the eNTP in plant response to biotic and abiotic stresses and on cell development and growth. The family of proteins called apyrases are identified in increasing number of plant species. Their mutations cause disruption in Arabidopsis seedling growth, and plants are sterile due to lack of germination of pollen grains. Studies on development of rice root hairs (that also show a tip growth as in a case of germination of pollen grains) revealed a major role of apyrases. Studies carried out on plant cells showed that extracellular nucleotides are signaling molecules inducing cell response production of reactive oxygen species or changes in calcium ion concentration in the cytoplasm. These mechanisms are the most universal and fundamental in the world of plants. Additionally, they proved to increase the concentration of eATP in wounding tissues or in a response to the attack of fungal pathogens and to osmotic stress. This means that extracellular nucleotides play an important role in plant defense and in the adaptation to new environmental conditions. The publications in recent years also suggest that extracellular nucleotides may be evolutionally very old and accompanied plants for millions of years. Some fungal elicitors induce an uptake of Ca2+ in plant cells, and this could be a mechanism for release of ATP from cells under the attack of fungal pathogens. Similar results were obtained for cells of algae in response to wounding. It is not known how ancient the reactive oxygen species strategy of plant pathogen defense is, but the fossil record indicates that plants had already developed fungal defense mechanisms in the Devonian era a billion years ago. Assembling information about the prevalence of extracellular nucleotides, their participation in the coordination of growth and development of plant cells, early evolutionary origin of eNTP and important functions in response to stress indicated that there are a key signal molecules in the plant world. The paper presents a review of articles supporting this claim.

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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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