External ATP Required for Viability

Abstract

In animal cells, adenosine triphosphate (ATP) serves not only as an energy source but also as an extracellular signaling molecule, a ligand for purinergic cell surface receptors. A role for extracellular ATP in plants is also beginning to emerge, and Chivasa et al. report that extracellular ATP is required for plant cell viability and that depletion of ATP may be one mechanism by which the process of pathogen-induced hypersensitive cell death is triggered (see commentary by Eckardt). Arabidopsis cells in culture released ATP into the medium. Cell-impermeant enzymes that consumed or metabolized ATP, such as hexokinase or apyrase, depleted extracellular ATP and caused cell death in cultures of Arabidopsis and maize. The nonhydrolyzable ATP analog βγ-methyleneadenosine 5′-triphosphate (AMP-PCP) also decreased cell viability. Application of apyrase, hexokinase, or AMP-PCP also caused localized cell death in leaves of Arabidopsis , tobacco, and bean plants. The entire Arabidopsis plant was susceptible to death if grown with hexokinase or AMP-PCP in the medium (hydroponically grown plants) or nutrient agar. Fumonisin B1 (FB1) is an elicitor produced by the maize pathogen Fusarium moniliforme , which causes programmed cell death and activation of the plant defense response. Arabidopsis cell cultures grown in the presence of FB1 had decreased extracellular ATP, and the decline preceded loss of membrane integrity. Addition of ATP rescued the cells from FB1-induced death, as did addition of other nucleoside triphosphates, which prevented ATP from decreasing by competing for ATP-degrading enzymes. Exogenous ATP also rescued Arabidopsis plants grown in the presence of FB1 from death, but did not rescue the plants from the growth retardation effects of FB1, suggesting that ATP selectively protects against FB1-mediated cell death. Mass spectrometry revealed that the abundance of several (34) intracellular proteins, including several implicated in the stress response, was altered by depletion of extracellular ATP. Thus, extracellular ATP appears to be one of the signals that inhibits what is known as the "default death pathway" in plants and connects cell viability to the pathogen elicitor-triggered cell death response. The mechanism by which ATP exerts this effect--for example, through phosphorylation of an extracellular protein, through serving as a cofactor in the formation of an extracellular complex, or through serving as a ligand for an extracellular receptor--remains to be determined. S. Chivasa, B. K. Ndimba, W. J. Simon, K. Lindsey, A. R. Slabas, Extracellular ATP functions as an endogenous external metabolite regulating plant cell viability. Plant Cell 17 , 3019-3034 (2005). [Abstract] [Full Text] N. A. Eckardt, Ins and outs of programmed cell death and toxin action. Plant Cell 17 , 2849-2851 (2005). [Full Text]