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 Ca²⁺ mobilizing agents inositol 1,4,5-trisphosphate (InsP₃) and cyclic ADP-ribose (cADPR). In this study, by using a vesicular ⁴⁵Ca²⁺ flux assay, we demonstrate that microsomal vesicles from red beet and cauliflower also respond to nicotinic acid adenine dinucleotide phosphate (NAADP), a Ca²⁺-releasing molecule recently described in marine invertebrates. NAADP potently mobilizes Ca²⁺ 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 Ca²⁺ pool was derived from the endoplasmic reticulum. This exclusively nonvacuolar location of the NAADP-sensitive Ca²⁺ pathway distinguishes it from the InsP₃- 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 Ca²⁺ release is insensitive to heparin and 8-NH₂-cADPR, specific inhibitors of the InsP₃- and cADPR-controlled mechanisms, respectively. However, NAADP-induced Ca²⁺ release could be blocked by pretreatment with a subthreshold dose of NAADP, as previously observed in sea urchin eggs. Furthermore, the NAADP-gated Ca²⁺ release pathway is independent of cytosolic 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 not blocked by L-type channel antagonists. The existence of multiple Ca²⁺ mobilization pathways and Ca²⁺ release sites might contribute to the generation of stimulus-specific Ca²⁺ signals in plant cells.