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Via the normal channels
Because of the very sensitive nature of the signalling mechanisms in which calcium is involved, its concentration must be very tightly regulated in space and in time. Indeed, there are several situations known in which calcium signalling involves fluxes in concentration over very short time periods. Much of this control is exerted by the operation of voltage-gated Ca2+ channels that are able to allow or to impede passage of calcium ions within the cell. However, the necessity to control, at concentrations lower than 1 µM, the calcium concentration in particular cell compartments is challenged by the abundance of calcium in the environment and hence the strong tendency for calcium to enter the cells. If Ca2+ signalling is not to be disrupted, there must be means of sequestering the calcium in locations where it is physiologically inert. Many plant species form calcium oxalate crystals and, as indicated by Volk et al. (Pullman, Washington State University, USA, pp. 741–753) it is thought that one function of these is to sequester calcium in order to prevent tissue damage. A leading question is how the calcium is transported to the site of storage. The authors have studied this problem in Pistia stratiotes, which stores calcium oxalate in specialized cells called idioblasts, the abundance of which is directly related to the concentration of supplied calcium. Study of calcium transport in protoplasts and in whole plants showed first that idioblasts have a greater capacity than other cells to accumulate calcium. Secondly, Ca2+ channel blockers almost completely inhibit calcium oxalate formation and thirdly, a fluorescently tagged Ca2+ channel-binding protein revealed intense labelling in the idioblasts but not in normal mesophyll. Finally, a protein was present in microsomal preparations from idioblasts, but not from mesophyll, which was recognized by antibodies raised against a mammalian Ca2+ channel subunit. All these data therefore point to the essential role of Ca2+ channels in Ca oxalate formation.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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