Regulation of synthesis of cell wall degrading enzymes by Veticillium albo-atrum and Fusarium oxysporum f. sp. lycopersici

Abstract

Induction and catabolite repression of synthesis of cell wall degrading enzymes by the vascular wilt fungi Verticillium albo-atrum and Fusarium oxysporum f. sp. lycopersici have been studied. In cultures containing inorganic salts and tomato stem cell walls each fungus produced a range of extracellular, polysaccharide degrading enzymes. In V. albo-atrum cultures, the enzymes appeared sequentially over 2 to 9 days in the order endopolygalacturonase (endo-PG), exo-arabinanase, endo-pectin- trans-eliminase (endo-PTE), endoxylanase and cellulase; β- d-galactosidase, galactanase and β- d-glucosidase were also produced. Under similar conditions F. oxysporum produced endo-PG, endo-PTE, arabinanase, β- d-galactosidase, xylanase and cellulase. Synthesis of each enzyme was almost always induced specifically by the sugar or uronic acid unit predominant in the specific polymeric substrate for the enzyme. This became evident only when inducers were supplied at rates which prevented their accumulation in cultures. Exceptionally, cellulase was induced by cellobiose but not glucose, and β-galactosidase a and galactanase were induced both by d-galactose and the structurally related l-arabinase. Synthesis was repressed when inducers were present in slight excess of requirements for growth. Synthesis by V. albo-atrum of endo-PG and endo-PTE increased with inducer supply until inducer began to accumulate, then decreased sharply and was almost completely repressed at supply rates three times the optimal value. Synthesis of exo-arabinanase and β-galactosidase decreased as supply of arabinose increased above the optimum but remained at much higher levels relative to those for endo-PG and endo-PTE. Constitutive synthesis of enzymes by V. albo-atrum was also catabolite repressed but to a lesser degree. The significance of induction and catabolite repression in pathogenesis in vascular wilts is discussed.