Mobilization of Ca2+ by cyclic ADP-ribose from the endoplasmic reticulum of cauliflower florets

The NAD(+) metabolite cADP-Rib (cADPR) elevates cytosolic free Ca2+ in plants and thereby plays a central role in signal transduction pathways evoked by the drought and stress hormone abscisic acid. cADPR is known to mobilize Ca2+ from the large vacuole of mature cells. To determine whether additional sites for cADPR-gated Ca2+ release reside in plant cells, microsomes from cauliflower (Brassica oleracea) inflorescences were subfractionated on sucrose density gradients, and the distribution of cADPR-elicited Ca2+ release was monitored. cADPR-gated Ca2+ release was detected in the heavy-density fractions associated with rough endoplasmic reticulum (ER). cADPR-dependent Ca2+ release co-migrated with two ER markers, calnexin and antimycin A-insensitive NADH-cytochrome c reductase activity. To investigate the possibility that contaminating plasma membrane in the ER-rich fractions was responsible for the observed release, plasma membrane vesicles were purified by aqueous two-phase partitioning, everted with Brij-58, and loaded with Ca2+. These vesicles failed to respond to cADPR. Ca2+ release evoked by cADPR at the ER was fully inhibited by ruthenium red and 8-NH2-cADPR, a specific antagonist of cADPR-gated Ca2+ release in animal cells. The presence of a Ca2+ release pathway activated by cADPR at higher plant ER reinforces the notion that, alongside the vacuole, the ER participates in Ca2+ signaling.