The Effect of Plant Polygalaturonase-Inhibiting Protein on the Rate of Oligouronide Formation

The effects of the polygalacturonase-inhibiting protein (PGIP) on the rate of oligouronide formation were studied in a model system containing polygalacturonic acid and polygalacturonase (PG) from the culture medium of phytopathogenic fungi. PGIP preparations were prepared from stored potato tubers and sprouts and also from apple fruits. The PGIP effects on oligouronide synthesis depended markedly on the physiological state of the source plant. Apple cultivars differing in their earliness differed in PGIP effects as well. The PGIP from potato tubers, which were in deep dormancy, suppressed oligouronide formation. The inhibitory PGIP action was decreased after dormancy release and tuber sprouting, which resulted in the oligouronide accumulation. The effects of PGIP from apple fruits on the oligouronide synthesis in the system containing PG from various phytopathogenic fungi were not correlated with tissue damage induced by these fungi. The PGIP effects on oligouronide formation are evident; however, their role in plant-cell processes related to the pectin compound conversions and plant resistance to diseases remains to be elucidated.