Activity of polygalacturonase produced by Cryphonectria parasitica in chestnut bark and its inhibition by extracts from American and Chinese chestnut

A purified polygalacturonase (PG) from culture filtrates of a virulent strain of Cryphonectria parasitica, Ep 155, caused browning of the inner bark of American chestnut. The enzyme also hydrolysed isolated cell wall materials from both American and Chinese chestnut bark in vitro. PG activity was detected in infected chestnut bark by a cup-plate assay, and by isoelectric focusing (IEF) and overlay-gel activity staining. A hypovirulent strain of C. parasitica, Ep 713, induced small cankers on American chestnut, and produced small amounts of PG in vivo as compared to its isogenic virulent strain Ep 155. Lower PG activity was also detected in cankers on resistant Chinese chestnut as compared to those on susceptible American chestnut, IEF and immunoblotting of the PG from infected bark against antibodies raised against the purified PG produced in vitro indicated that these enzymes were the same. Tannin isolated from the bark of American chestnut inhibited this PG more than tannin isolated from Chinese chestnut bark. A proteinaceous extract from Chinese chestnut bark, however, was about 15 times more inhibitory to the C. parasitica PG than a similar protein extract from American chestnut. This PG inhibitor was active also against the PG of C. lindemuthianum, but not the PG from Rhizopus sp. The high level of this nominally specific PG inhibitor in the resistant host lends further support to the hypothesis that PG is a virulence factor for C. parasitica and that its inhibition may contribute to the resistance of Chinese chestnut.