THE FUNCTION OF EXTRACELLULAR ENZYMES OF DUTCH ELM DISEASE PATHOGEN

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Abstract
Polygalacturonase and callulase are produced by Ceratocystis ulmi the cause of Dutch elm disease. The enzymes are extracellular, and their possible role in pathogenesis in the elm has been examined. The pectic enzyme is polygalacturonase and not pectin depolymerase, as shown by the appearance of reducing groups when pectins are incubated with the enzyme and by the appearance of galacturonic acid among the hydrolytic products. A cellulase is produced that is unable to attack native cellulose, but it hydrolyzes carboxy methyl cellulose to glucose. Both enzymes are most abundantly produced by the fungus when it grows on chopped elm twigs, and both show maximum activity in the range of pH about 5.5. Apparently neither enzyme is involved in pathogenesis in the elm because neither causes wilting of leaves of elm cuttings subjected to the enzymes, nor are the water-transporting cells of such cuttings plugged with hydrolytic products of enzyme action upon host cell walls. At most, the enzymes enable the fungus to obtain food from host cell walls. Mercuric ion inhibits the cellulase and N-t-tridecyl-azomethine inhibits the polygalacturonase. Neither enzyme is affected by 8-quinolinol benzoate, a chemotherapeutant for Dutch elm disease. Another chemotherapeutant, potassium-2-benzothiazolyl thioglycolate, inhibits the polygalacturonase but not the cellulase of C. ulmi.