title: Calcium release from the endoplasmic reticulum of higher plants elicited by the NADP metabolite nicotinic acid adenine dinucleotide phosphate.
abstract: Higher plants share with animals a responsiveness to the Ca(2+) mobilizing agents inositol 1,4,5-trisphosphate (InsP(3)) and cyclic ADP-ribose (cADPR). In this study, by using a vesicular (45)Ca(2+) flux assay, we demonstrate that microsomal vesicles from red beet and cauliflower also respond to nicotinic acid adenine dinucleotide phosphate (NAADP), a Ca(2+)-releasing molecule recently described in marine invertebrates. NAADP potently mobilizes Ca(2+) with a K(1/2) = 96 nM from microsomes of nonvacuolar origin in red beet. Analysis of sucrose gradient-separated cauliflower microsomes revealed that the NAADP-sensitive Ca(2+) pool was derived from the endoplasmic reticulum. This exclusively nonvacuolar location of the NAADP-sensitive Ca(2+) pathway distinguishes it from the InsP(3)- and cADPR-gated pathways. Desensitization experiments revealed that homogenates derived from cauliflower tissue contained low levels of NAADP (125 pmol/mg) and were competent in NAADP synthesis when provided with the substrates NADP and nicotinic acid. NAADP-induced Ca(2+) release is insensitive to heparin and 8-NH(2)-cADPR, specific inhibitors of the InsP(3)- and cADPR-controlled mechanisms, respectively. However, NAADP-induced Ca(2+) release could be blocked by pretreatment with a subthreshold dose of NAADP, as previously observed in sea urchin eggs. Furthermore, the NAADP-gated Ca(2+) release pathway is independent of cytosolic free Ca(2+) and therefore incapable of operating Ca(2+)-induced Ca(2+) release. In contrast to the sea urchin system, the NAADP-gated Ca(2+) release pathway in plants is not blocked by L-type channel antagonists. The existence of multiple Ca(2+) mobilization pathways and Ca(2+) release sites might contribute to the generation of stimulus-specific Ca(2+) signals in plant cells.
authors.affiliation: The Plant Laboratory, Biology Department, University of York, P.O. Box 373, York YO10 5YW, United Kingdom.
authors.forename: L
authors.initials: L
authors.lastname: Navazio
authors.forename: M A
authors.initials: MA
authors.lastname: Bewell
authors.forename: A
authors.initials: A
authors.lastname: Siddiqua
authors.forename: G D
authors.initials: GD
authors.lastname: Dickinson
authors.forename: A
authors.initials: A
authors.lastname: Galione
authors.forename: D
authors.initials: D
authors.lastname: Sanders
mesh_terms: D001937:Brassica / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D019663:Chenopodiaceae / Q000378:metabolism*; D018870:Endoplasmic Reticulum, Rough / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism; D009249:NADP / Q000031:analogs & derivatives* / Q000378:metabolism; D018517:Plant Roots / Q000378:metabolism
chemical_list: D009249:NADP; C024376:NAADP; D002118:Calcium