The influence of cell walls from host and non-host plants on the production and activity of polygalacturonide-degrading enzymes from fungal pathogens

Abstract

The role of plant cell walls in non-host resistance was examined by determining their influence on production and activity of polygalacturonases (PG) and pectin lyases (PL) from 6 fungal pathogens able to cause significant wall degradation in vivo. Botrytis fabae, B. cinerea, Sclerotinia fructigena, S. trifoliorum, Fusarium oxysporum f.sp. lycopersici and F. oxysporum f.sp. pisi were grown in shake cultures containing as sole carbon source cell walls extracted from host and non-host plants (broad bean, tomato, apple, clover, pea and dwarf bean). Walls from various species had a differential effect on the rate and extent of synthesis of some enzymes over 15 days, but this showed no consistent relationship with compatibility or incompatibility of the host-parasite interaction. Thus greatest PG production by B. fabae, B. cinerea and F. oxysporum f.sp. lycopersici was on host cell walls, whereas walls from host plants supported the lowest accumulation of PL by F. oxysporum f.sp. lycopersici , PL by F. oxysporum f.sp. pisi and PG by S. fructigena . Occasionally enzyme levels were especially low during growth on non-host walls, e.g. B. fabae PG with apple and F. oxysporum f.sp. pisi PG on clover. Growth of F. oxysporum ff.sp. varied on cell walls from different species but was not related to enzyme production or host-parasite compatibility. Binding or inactivation of PG and PL by cell walls was investigated with 3 wall preparations extracted to contain: inactivated wall proteins; covalently-linked but not ionically-bound proteins; total wall proteins. Incubation with cell walls usually reduced soluble enzyme activity, e.g. ≤30% for PG of A. niger, Botrytis spp. and S. fructigena , ≤60% for F. oxysporum ff.sp. PG, ≤86% for PL of F. oxysporum ff.sp., ≤93% for PG from S. trifoliorum . Often much of the lost activity could be recovered by desorptiou with 0·2 M NaCl; however, complete activity was not always regained which could suggest inactivation or firmer binding. Cell walls containing ionically-bound proteins were not consistently more inhibitory than other wall preparations, except against PG from F. oxysporum ff.sp. Walls without active proteins were often equally effective in reducing activity. Effects of walls from host and non-host plants were essentially similar. These findings are discussed in relation to non-host resistance, PG inhibitors in cell wa ll and the restriction of wall-degrading enzymes during pathogenesis.