Oligouronide signaling of proteinase inhibitor genes in plants: Structure-activity relationships of di- and trigalacturonic acids and their derivatives

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Polygalacturonic acid (DP ave ~ 20), α-1,4-di- and trigalacturonic acids, Δ4,5-α-1,4-di- and Δ4,5-a-trigalacturonic acids, and several chemically modified derivatives of these oligomers were prepared. Their proteinase inhibitor-inducing activities were determined by supplying solutions of the compounds to young, excised tomato plants through their cut stems. Digalacturonic acid, on a molar basis, was the most active oligomer (ED 50 ~ 1.5 μg/plant), being about three times more active than the parent oligogalacturonic acid (ED 50 ~ 5.5 μg/plant). The specific inducing activity of trigalacturonic acid was about half that of digalacturonic acid. Both Δ4,5-di- and Δ4,5-trigalacturonic acids were about half as active as di- and trigalacturonic acids, respectively. Reduction of the hemiacetal (carbonyl) group of the di- and trigalacturonic acids with sodium borohydride completely destroyed proteinase inhibitor inducing activities, indicating that the inducing activity of both acids depends upon an intact hemiacetal at the reducing termini. Reduction of the double bonds of Δ4,5-di- and Δ4,5-trigalacturonic acids by catalytic hydrogenation with H 2 (palladium catalyst) produced derivatives with specific inducing activities of approximately one-half that of the parent compounds. Thus, while the reducing termini of oligogalacturonides require an intact hemiacetal for proteinase inhibitor inducing activities, the nonreducing termini of the small oligouronides do not require a C4 hydroxyl nor a C5 proton to be active inducers.