Characteristics of Galacturonic Acid Oligomers as Elicitors of Casbene Synthetase Activity in Castor Bean Seedlings

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Abstract
Partial digestion of polygalacturonic acid with polygalacturonase isolated from Rhizopus stolonifer produces a mixture of α-1,4-d-galacturonide oligomers which act to elicit casbene synthetase activity in castor bean (Ricinus communis L). These oligomers were separated by anion exchange chromatography on DEAE Sephadex A-25 into discrete sizes and their degrees of polymerization were analyzed by fast atom bombardment mass spectrometry. A minimum degree of polymerization of nine units appears to be required for elicitor activity; trideca-α-1,4-d-galacturonide was the most active of the oligomers tested. Methyl-esterification of the carboxylate groups greatly diminishes the elicitor activity of the oligomers, a finding which suggests a requirement for the polyanionic character of the oligomers for full activity. The fact that a number of other polyanionic polymers tested as casbene synthetase elicitors did not show significant activity indicates that structural features other than the polyanionic character are also necessary for activity.