Calcium release from the endoplasmic reticulum of higher plants elicited by the NADP metabolite nicotinic acid adenine dinucleotide phosphate

Higher plants share with animals a responsiveness to the Ca2+ mobilizing ag ents inositol 1,4,5-trisphosphate (InsP(3)) and cyclic ADP-ribose (cADPR). In this study. by using a vesicular Ca-45(2+) flux assay, we demonstrate th at microsomal vesicles from red beet and cauliflower also respond to nicoti nic acid adenine dinucleotide phosphate (NAADP), a Ca2+-releasing molecule recently described in marine invertebrates. NAADP potently mobilizes Ca2+ w ith a K-1/2 = 96 nM from microsomes of nonvacuolar origin in red beet. Anal ysis of sucrose gradient-separated cauliflower microsomes revealed that the NAADP-sensitive Ca2+ pool was derived from the endoplasmic reticulum. This exclusively nonvacuolar location of the NAADP-sensitive Ca2+ pathway disti nguishes it from the InsP(3)-and cADPR-gated pathways. Desensitization expe riments revealed that homogenates derived from cauliflower tissue contained low levels of NAADP (125 pmol/mg) and were competent in NAADP synthesis wh en provided with the substrates NADP and nicotinic acid. NAADP-induced Ca2 release is insensitive to heparin and 8-NH2-cADPR, specific inhibitors of the InsP(3)- and cADPR-controlled mechanisms, respectively. However. NAADP- induced Ca2+ release could be blocked by pretreatment with a subthreshold d ose of NAADP, as previously observed in sea urchin eggs. Furthermore, the N AADP-gated Ca2+ release pathway is independent of cyto solic free Ca2+ and therefore incapable of operating Ca2+-induced Ca2+ release. In contrast to the sea urchin system, the NAADP-gated Ca2+ release pathway in plants is no t blocked by L-type channel antagonists. The existence of multiple Ca2+ mob ilization pathways and Ca2+ release sites might contribute to the generatio n of stimulus-specific Ca2+ signals in plant cells.