Potato Cell Wall Polysaccharides: Degradation with Enzymes from Phytophthora infestans

Phytophthora irtfestans culture filtrate contained 2 polygalacturonases, 4 galactanases, and 2 ipectinesterases. An endo-polygalacturonase (mol. wt. 350 000) and an endo- (l, 4') β-D-galactanase (mol. wt 55 000) were partially purified and used to degrade potato cell walls. The polygalacturon-ase released less than 6% of the cell wall carbohydrate, even after galactanase treatment or removal of the more soluble pectic fraction. Its limited degradative activity may be connected with the biotrophic habit of the blight fungus. The galactanase detached up to 23% of the cell wall, including uronic acid when calcium ions had been removed beforehand. Arabinose was also released, showing that there are arabinogalactan side-chains in the pectin. However galactanase degradation of a soluble pectin fraction left 63% of the arabinose attached to large rhamnogalacturonan fragments with only a little galactose. Many arabinan chains are thus perhaps attached directly to rhamnogalacturonan.