Pectolytic enzymes of Sclerotinia sclerotiorum and their localization in infected bean

Abstract

Sclerotinia sclerotiorum produced at least three pectolytic enzymes, each of which was similar in both diseased tissue and in culture. An endopolygalacturonase (endo-PG) (PG, EC 3.2.1.15) was most abundant 24 h after inoculation of bean plants and after 2–3 days incubation in culture but decreased thereafter. Production in culture was suppressed by glucose. The activity of this enzyme was localized in the advancing margins of young lesions in infected plants but was usually not present later than 48 h after inoculation. An exo-PG was detected at lesion margins after 48 h and also in cultures of S. sclerotiorum in Na polypectate (NaP) medium and on autoclaved bean hypocotyls. Increases in exo-PG activity with time of incubation was correlated with rapid growth of the fungus. Pectin methylesterase (PME, EC 3.1.1.11) was detected early in pathogenesis and at the margins of advancing lesions. PME activity increased with age of diseased tissue and age of cultures on NaP medium and bean hypocotyl medium. Fungal PME was distinguished from healthy bean PME on the basis of differing pH optima for activity and lack of NaCl or CaCl2 activation of the fungal PME. The action of PG and PME on apple tissue infected with S. sclerotiorum was detected histochemically. Alteration of pectic substances in the middle lamella of infected apple fruit tissue was detected one to two cells in advance of the invading hyphae. The importance of these pectolytic enzymes in relation to pathogenesis and colonization of host tissues is discussed.