Endo-polygalacturonase from Cladosporium cucumerinum elicits lignification in cucumber hypocotyls

Abstract

The cucumber pathogen Cladosporium cucumerinum produces one endo-polygalacturonase and at least two exo-polygalacturonases during growth in a liquid medium containing citrus pectin as the carbon source. The endo-polygalacturonase was purified nine-fold by ion-exchange chrumatography and gel filtration. The enzyme elicits lignification in cucumber hypocotyls down to a concentration of about 0·08 units ml −1 which corresponds to about 70 ng protein ml −1. It also releases elicitors of lignification from polygalacturonic acid and cucumber cell walls. The enzyme has a pH-optimum between 5·0 and 5·5, a molecular weight of about 38 000 and contains neutral hexose and protein in a ratio of 15:85 ( w w ). The endo-polygalacturonase elicits lignification equally effectively in susceptible as in resistant cucumber hypocotyl segments, and releases about the same amount of elicitor from the cell walls of resistant and susceptible cucumber hypocotyls. This does not exclude the involvement of endo-polygalacturonase in the lignification reaction of resistant cucumber plants towards C. cucumerinum, but the specificity of the reaction must apparently be determined by other molecules.