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Extracellular ATP has been found to elevate cytosolic free Ca2+ in Arabidopsis thaliana and trigger gene transcription, suggesting that it acts as a plant cell regulator. Recent findings place extracellular ATP upstream of Arabidopsis thaliana NADPH oxidase activity and plasma membrane Ca2+-permeable channels in the root epidermis. Here we show that increasing extracellular ATP concentration evokes a larger but more irregular Ca2+ influx conductance in root epidermal protoplasts. This may help modulate changes in cytosolic free Ca2+ as a second messenger and help explain the dose-dependent effects of extracellular ATP on cell function. The receptors for ATP and the downstream plasma membrane Ca2+ channels remain unknown at the protein or gene level. No equivalents of animal ATP receptors have been identified in higher plant genomes. We propose here that annexins could perceive extracellular ATP and participate in Ca2+ influx.
Extracellular purine nucleotides are well established as animal cell regulators, acting in a variety of process from cell death to neurotransmission.1 Signaling is effected by activation of ionotropic or metabotropic plasma membrane receptors (purinoceptors) and can result in elevation of cytosolic free calcium ([Ca2+]cyt) as a second messenger.1 It is now clear that, although apparently lacking homologues of purinoceptors, plant cells also respond to extracellular ATP. The latter is now implicated in regulation of cell viability,2 growth,3–5 gravitropism6 and stress responses.7–10 In both roots and leaves, extracellular ATP increases transcription of stress-responsive genes such as MPK3 (mitogen-activated protein kinase 3).9,10 There may be cell- and dose-dependent effects on growth and development.4
Very early work on plant responses to extracellular ATP interpreted effects (such as enhanced closure rate of Venus fly trap) as the uptake of ATP to act as an additional energy source.11,12 Indeed tracer experiments confirmed ATP influx. Establishing a bona fide effect of extracellular ATP is made difficult by the ability of ATP to chelate cations, which could in turn trigger a signaling response. Chelation of extracellular Ca2+ promotes production of reactive oxygen species (ROS) by Arabidopsis thaliana roots13 and low cation availability is known to trigger adaptive responses by roots, often involving ROS.14
Such processes could readily confound the delineation of purine nucleotide effects. By ensuring that extracellular Ca2+ is held constant or is in excess, it has been shown that extracellular ATP can induce a transient increase in [Ca2+]cyt of Arabidopsis roots and leaves.8,15 Recently it was found to elevate [Ca2+]cyt in root epidermal protoplasts from Arabidopsis which may indicate that, in this tissue, the cell wall is not involved in ATP perception.10 The current model is that an initial event downstream of ATP perception is release of Ca2+ from intracellular stores. This can then activate the root epidermal plasma membrane (PM) NADPH oxidase AtRBOHC, resulting in production of reactive oxygen species (ROS). ROS then activate PM Ca2+-permeable channels which can further contribute to [Ca2+]cyt elevation.10
Low levels (20 µM) of extracellular ATP evoke a hyperpolarization-activated, time-dependent and regular inwardly-directed PM Ca2+ conductance in root epidermal protoplasts.10 Here we show that increasing the ATP concentration to 2 mM (reviewed in ref. 10) changes the conductance profile, making it increasingly irregular over the course of the voltage clamping step as membrane voltage becomes more hyperpolarized (Fig. 1). The control mean ± SE current at −200 mV was −177 ± 40 pA and after ATP addition it increased to −610 ± 87 pA (n = 4; Fig. 2). This represents an approximate threefold increase, compared to the two fold increase observed at this voltage in response to 20 µM ATP.10 This result illustrates the point that the [Ca2+]cyt response to changing extracellular [ATP] could be fine-tuned at the level of the PM Ca2+ influx pathway.
It is held that the combined but differentially regulated activities of passive and active transporters plus Ca2+-binding proteins effect stimulus-specific spatiotemporal patterns of [Ca2+]cyt as a second messenger, known as the “Ca2+ signature”.16 The PM Ca2+ influx response to extracellular ATP does now appear to be dose-dependent with the possibility of pulses of Ca2+ entry at higher [ATP] were the PM voltage to remain constant. The molecular identity of the underlying channels remains unknown. However, it is feasible that annexins could contribute to some or all of the ATP-evoked Ca2+ conductance or even be the “receptor”. Annexins are soluble proteins capable of Ca2+-dependent membrane association with or insertion into membranes.17 Animal and now plant annexins have been found to facilitate passive Ca2+ transport in vitro.18 Two annexins of the Arabidopsis root epidermis, AtANN1 and AtANN2, are possible candidates as they are predicted to be extracellular18 and have been found to be in the cell wall in proteomic studies on other cell types.19,20 AtANN1 can reside in the plasma membrane21 and, as it can bind ATP,22 could insert into the membrane to form a transport route. AtANN1 has cation transport capacity in vitro23 although its ability to translocate Ca2+ is unknown. AtANN2 is predicted to bind nucleotide triphosphates24 and could also be a transport route or could regulate levels of extracellular ATP. With loss of function mutants now available, these possibilities could be addressed.
This work was supported by the Royal Society and the University of Cambridge Brooks Fund.
Previously published online: www.landesbioscience.com/journals/psb/article/9680