Polygalacturonase isoenzymes and oxalic acid produced by Sclerotinia sclerotiorum in soybean hypocotyls as elicitors of glyceollin

The polygalacturonases (PG) and oxalic acid produced by Sclerotinia sclerotiorum in infected soybean hypocotyls were investigated as elicitors of the phytoalexin glyceollin I. Purification to homogeneity through isoelectrofocusing and ion-exchange fast protein liquid chromatography revealed three endo-PG isoenzymes (PG-I, PG-II and PG-IV) and one exo-PG (PG-III) in 6-day-old etiolated soybean hypocotyls infected with the B-24 isolate of S. sclerotiorum. PG-I and PG-III, in the range of concentrations tested (0·15-1·2 reducing units ml-1), did not act as elicitors of glyceollin I synthesis. Some elicitor activity was shown by PG-II at 0·6-1·2 reducing units ml-1. PG-IV, at lower doses (0·038-0·30 reducing units ml-1), was even more effective in inducing phytoalexin synthesis. However higher concentrations of PG-IV induced tissue softening and decreased phytoalexin accumulation. PG-II and PG-IV released heat-stable elicitors from purified soybean cell walls supporting the evidence that uronides are intermediate inducers in elicitation by endo-PGs. Oxalic acid was an active elicitor of glyceollin I over the range of concentrations tested (0·31-20 mm) with the maximum at a concentration of 5 mm. The inability of oxalic acid to release uronides from purified cell walls makes it unlikely that uronide intermediate elicitors are involved in elicitation by oxalic acid.