Degradation of pectic polysaccharides in various fruits by pectinase derived from Aspergillus niger

Aspergillus spp. are widely spread plant pathogens. In several kinds of fruit, especially in apple and peach, aspergillus rot is one of the postharvest desease in fruit, but the infection mechanism is not known. The degradation of cell walls by pathogens is significant in primary infection and afterinfection, and plays an important role in the advancement of mycelium into the host plant and the appearance of symptoms (11). Studies on the fruit softening in this field often have dealt with the enzymological and molecular biologocal investigations. Plant cell walls are mainly composed of polysaccharides, e. g. pectic and hemicellulosic polymers, cellulose and phenolic polymers such as lignin. Phytopathogens can infect the host plant by degrading the cell wall components and invading the tissues and cells. Concerning the relationship between the pathogenicity of fungus and cell degrading enzymes, reports have been made on pectinases (3), cellulases (11), and some glycosidases (3, 11), which can degrade cell wall polysaccharides. But the results showed that the pathogenicity was not always proportional to the enzyme activity. Although cell wall polysaccharides include different components and their linkages are also various, there have been few studies from the viewpoint of the variation of polysaccharides. For the extraction of pectic polymers in plant cell walls, water, EDTA or CDTA, HCl or Na2CO3 were used generally (6). In a previous paper (4), we analysed the pectic components in water and hot water extracts of cell wall pectic polysaccharides with the objection to obtaine the substrate, which does not include chemical reagents as above, for the enzyme reaction. It was found that the hot water extract included most of the pectic components (except pectic polymers soluble in chilating reagents) of cell wall polysaccharides in fruit flesh, which is consists maily of primary cell wall. Several reports suggested that the pectinase, cellulase and some β-glycosidase could be involved in the pathogenicity of this fungus. Prectically, pectinase produced by this fungus have been saparated and used for the clarification of fuit juice industrially. It is one of the important enzymes to develop the aspergillus rot on some fruits, resulting in the fruit softening and collapse of the flesh tissue by degrading cell wall pectic polymers. In this report, we compared the degrading activity of pectinase from Aspergillus niger on the hot water extract from various fruits. The objection of this study was to show that the difference in pathogenisity by pectinase could be caused by the components of host plant, from the viewpoint of the difference in the substrate polysaccharides.