Cell wall-degrading enzymes of vascular wilt fungi. II. Properties and modes of action of polysaccharidases of Verticillium albo-atrum and Fusarium oxysporum f. sp. lycopersici

Abstract

Single polysaccharidases of Verticillium albo-atrum degraded cell walls from vascular and other tissues of tomato stems without previous or simultaneous action of other enzymes. Endoxylanase, exo-galactanase and exo-arabinase of the pathogen had pH optima c. 5, 3 and 5 respectively. Cellulase degraded soluble and insoluble cellulose at optimum pH c. 7. Polygalacturonase (PG) and pectin lyase (PTE) behaved as typical endo-enzymes causing a rapid decrease in viscosity of solutions of substrates at rates proportional to the degree of substrate polymerisation. Although PTE degraded pectin at random, only unsaturated dimer was released from polypectate and cell walls. PG released galacturonic acid and oligogalacturonides which were rapidly degraded to the monomer but an exo-enzyme was not detected; this activity compares with the dual endo-exo action of homogeneous polygalacturonases from other pathogens and “multiple attack” of certain α-amylases. Wide and narrow range isoelectric focusing resolved PG into four endo-components of pI 5·0 5·9 6·2 and 6·7 and PTE into two fractions with activity profiles almost identical with the acidic PG components. PTE of V. albo-atrum and Fusarium oxysporum f. sp. lycopersici had a partial Ca 2+ requirement and pH optimum c. 9 which altered with substrate methylation. Activity of endo-PG of V. albo-atrum but not exo-PG of F. oxysporum was restricted by Ca 2+; calcium pectate gels were resistant to degradation. Pectic enzymes of V. albo-atrum macerated tomato stem tissue after long lag periods; other cell wall-degrading enzymes were ineffective. PTE of F. oxysporum macerated tissue more rapidly and was used alone or with other enzymes to compare degradation of tissue and cell walls isolated from susceptible or isogenically wilt-resistant tomato cultivars. There was no relation between varietal resistance and the susceptibilities of cell walls to enzymic degradation.