Ligand-gated Ca2+ channels and Ca2+ signalling in higher plants

Ligand-gated Ca(2+) channels provide a possible mechanism for linking perception of stimuli to intracellular Ca(2+) mobilization. Evidence for ligand-gated Ca(2+) release in plant cells arises from radiolabeled ligand binding, microsomal ion flux, and electrophysiological approaches. Results from these diverse approaches demonstrate that two classes of ligand-gated channels are present at the plant cell vacuolar membrane. One class of channel is gated by inositol 1,4,5 trisphosphate (InsP(3)) and the second is gated by cyclic adenosine 5'-diphosphoribose (cADPR). Previous biochemical studies on plant InsP(3) binding sites have been hampered by low density of specific binding. The present work reports optimization of yield for solubilized InsP(3) binding sites with respect to detergent type and concentration, and the originating tissue. Further studies reveal a pharmacological similarity between cADPR-activated Ca(2+) release in plant and animal cells and demonstrate that the extent of cADPR-induced Ca(2+) release is dependent on the plant tissue type. In animal cells cADPR releases Ca(2+) through activation of at least one isoform of the so-called ryanodine receptor. It is shown here that ryanodine itself is able to activate single channel currents in vacuolar membranes. These observations are integrated into current models for ligand-gated Ca(2+) release in plant and animal cells and their role in Ca(2+)-based cell signalling.