An ATP signalling pathway in plant cells: extracellular ATP triggers programmed cell death in Populus euphratica

Abstract

We elucidated the extracellular ATP (eATP) signalling cascade active in programmed cell death (PCD) using cell cultures of Populus euphratica. Millimolar amounts of eATP induced a dose-and time-dependent reduction in viability, and the agonist-treated cells displayed hallmark features of PCD. eATP caused an elevation of cytosolic Ca 2+ levels, resulting in Ca 2+ uptake by the mitochondria and subsequent H2O2 accumulation. P. euphratica exhibited an increased mitochondrial transmembrane potential, and cytochrome c was released without opening of the permeability transition pore over the period of ATP stimulation. Moreover, the eATP-induced increase of intracellular ATP, essential for the activation of caspase-like proteases and subsequent PCD, was found to be related to increased mitochondrial transmembrane potential. NO is implicated as a downstream component of the cytosolic Ca 2+ concentration but plays a negligible role in eATP-stimulated cell death. We speculate that ATP binds purinoceptors in the plasma membrane, leading to the induction of downstream intermediate signals, as the proposed sequence of events in PCD signalling was terminated by the animal P2 receptor antagonist suramin.

Extracellular ATP signaling and homeostasis in plant cells

“…24 In Populus euphratica cells, ATP-induced cell death was seen when the applied dose of eATP greater than 500 mM, while low doses (10-200 mM) did not cause PCD over the observation period. 13 These contrasting responses suggest that plant ATP sensors were discriminatively activated in response to different strength of ATP stimulus. The assumption is consistent to the observations in animal systems that P2X receptors are usually activated at higher eATP concentrations (micromoles), as compared with P2Y that activated by nanomoles ATP.…”

“…15 Moreover, intracellular Ca 2+ mobilizations (including Ca 2+ influx, Ca 2+ elevation in the cytosol, Ca 2+ release from the vacuole and mitochondrial Ca 2+ uptake), and mitochondrial H 2 O 2 burst were implicated in the signaling pathway of eATP-induced PCD in P. euphratica cells. 13 Under salt shock, the release of intracellular ATP from P. euphratica cells and the activation of putative plasma membrane ATP receptors were found able to trigger the downstream signaling components, H 2 O 2 and Ca 2+ (Sun J., Deng S. and Chen S., unpublished data). Taking together, Ca 2+ , NO and H 2 O 2 are likely downstream signaling components of eATP in plants.…”

“…24 This is likely a defense response since overloading of Ca 2+ in the cytosol would trigger programmed cell death in plant cells. 13 ATP application into intact root vacuoles of beet plants was found to induce an obvious Ca 2+ release, which is possibly mediated by the activation of depolarization-activated slow vacuolar (SV) channels. 33 In P. euphratica cell cultures, eATP (1.0 mM) caused a release of vacuolar Ca 2+ into cytosol and this process was mediated by cADPR-dependent activation of vacuolar Ca 2+ channels.…”

“…16 Exogenous application of ATP to plant cells caused numerous physiological responses, including alteration of membrane potential, 17 14,15 and modulation of ion fluxes across the plasma membrane and tonoplast. 13,19,24 In addition to artificially increasing eATP concentrations in the ECM, a few studies focused on the effects of eATP depletion on plant and cellular responses. Slabas and colleagues reported that the removal of eATP caused expression of pathogenesis-related (PR) genes and cell death.…”

“…A recent report by Sun et al has shown that excess eATP (0.5-2.0 mM) can initiate cell death program in Populus euphratica cell cultures. 13 Another two reports disclosed the regulation of eATP on stomatal movement. 14,15 Several excellent reviews have comprehensively summarized recent progress in plant eATP signaling (see refs.…”