PLANT-CELL WALL-LYSING ENZYMES IN THE APOPLAST OF TOMATO LEAVES INFECTED WITH FULVIA-FULVA

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 C-14-labelled plant cell walls by apoplastic fluid from compatible and incompatible interactions and from the uninfected plant was analysed. C-14-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.