The role of calcium in oligogalacturonide-activated signalling in soybean cells
α-1,4-Linked oligogalacturonides (OGs) are pectic fragments of the plant cell wall that are perceived by the plant cell as signalling molecules. Using cytosolic aequorin-expressing soybean (Glycine max L.) cells, we have analysed cytosolic Ca2+ changes and the oxidative burst induced by OGs with different degrees of polymerization. Our results provide evidence that different OGs are sensed through transient elevations of cytosolic Ca2+ that show different kinetics. Specificity of the Ca2+ signature relies also on the precise structural characteristics of the OG molecules, such as the methylesterification of galacturonic acid residues and the steric conformation. Inhibition of the OG-induced Ca2+ transient also blocks the oxidative burst, indicating that the cytosolic Ca2+ increase is one of the earliest steps in OG-activated signalling. However, a phosphorylation event seems to precede the Ca2+ rise, because the Ca2+ transient could be abolished by the protein kinase inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB). A pharmacological approach with different antagonists that interfere with the induction of the cytosolic Ca2+ rise indicates that both extracellular Ca2+ influx and intracellular Ca2+ release participate in transducing the OG signal. Treatment of cells with OGs establishes a refractory state, which impairs the ability of the cell to respond to a second stimulus with the same elicitor for up to 16 h. This desensitization period could be prolonged with the phosphatase inhibitor okadaic acid, and eliminated with the protein kinase inhibitor Ro 31-8220, suggesting that phosphorylation events may be involved in the establishment of the cell refractory state.
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