Plant cell wall-lysing enzymes in the apoplast of tomato leaves infected with Fulvia fulva

Abstract

Apoplastic fluid recovered from healthy (uninfected) tomato leaves and from leaves infected with races of Fulvia fulva (Cooke) Ciferri (syn. Cladosporium fulvum (Cooke)) contained heat-labile lytic activities that solubilised carbohydrates from plant cell walls. Total activity was slightly enhanced in apoplastic fluid from infected plants. Material solubilised from uniformly 14C-labelled plant cell walls by apoplastic fluid from compatible and incompatible interactions and from the uninfected plant was analysed. 14C-Labelled poly-, oligo-, and mono-saccharides were detected, suggesting that both exo- and endo-hydrolase activities were present; endoactivity predominated during the early stages of infection. Fluid collected from incompatible interactions as early as 5–7 days postinoculation contained enhanced levels of an activity that solubilised arabinose-rich polysaccharide; by 9–11 days postinoculation, this activity had decreased and was comparable to that of the healthy plant. A similar activity was also seen in fluid from compatible interactions, but this was enhanced slightly later in the infection and increased rapidly during days 9–11 postinoculation. We suggest that the early and short-lived enhanced expression of an endohydrolase that attacks an arabinose-rich polymer may be related to disease resistance. Key words: tomato, Fulvia fulva (Cooke) Ciferri (syn. Cladosporium fulvum (Cooke)), apoplastic fluid, cell wall degrading enzymes, compatibility, oligosaccharin.