Activation of additional fusicoccin-binding sites on the plasmalemma underosmotic stress

Novel fusicoccin (FC)-binding sites were found to appear on the plasmalemma of protoplasts isolated from suspension-cultured sugar beet (Beta vulgaris L.) cells after being placed in hypotonic medium. The number of novel FC-b inding sites was two or three times higher than that of preexisting sites, and this number was correlated with the decrease in the osmotic pressure of the extracellular medium. This effect was also observed after transferring the protoplasts from hypertonic medium back to an isotonic one. Protoplast s were found to behave as ideal osmometers when the osmolarity of extracell ular medium was changed from 250 to 650 mOsm. The novel FC-binding sites di sappeared 10-15 min after the osmotic stress, although protoplast sizes rem ained constant. It was shown that the novel FC-binding sites were stabilize d by FC and remained in plasmalemma isolated from the protoplasts. It is co ncluded that a new type of FC-binding sites, so-called "silent" receptors, are present in plant cells. They are activated under osmotic stress and mig ht be involved in plant cell osmoregulation.