cell components. A convincing role in pathogenesis, however, has been es­ tablished only for those enzymes that attack the pectic fraction of the plant cell wall. Several fundamental advances in our understanding of these enzymes have been made since the role of pectic enzymes in tissue degradation was reviewed in this series by Bateman & Millar (17) twenty years ago. In the past decade it was demonstrated that highly purified pectic enzymes could macer­ ate and kill plant tissues in a manner similar to that occurring in soft-rot diseases; that the same enzymes could be used to disassemble the dicot primary cell wall, thus revealing the structural importance of pectic polymers in cell-wall architecture; and that a relationship exists between the enzymatic vulnerability of pectic polymers in the cell wall and the regulation of pectic enzyme synthesis in pathogenic fungi. In the present decade it has been demonstrated that fragments released from the cell wall by pectic enzymes can elicit plant defense reactions and that the highly pectolytic bacteria, Erwinia chrysanthemi and E. carotovora, are amenable to a powerful array of molecular genetic manipulations. This paper considers these most recent advances and attempts to project future progress. The following steps occur during the interaction of a pectolytic pathogen and a potential host: (a) The entering pathogen possesses structural genes encoding pectic enzymes with particular physical and catalytic properties. (b)

The Role of Pectic Enzymes in Plant Pathogenesis

Some of the more important evidence is presented which supports the view that accumulation of phytoalexins at the site of attempted infection in plants. The accumulation of phytoalexins represents only one of a number of disease-resistance mechanisms in plants. Experiments are described demonstrating that the enzymes that catalyze the synthesis of phytoalexins are themselves synthesized de novo when plant cells are exposed to microbes of other effective stimuli. Evidence is also presented showing that phytoalexins, once accumulated, are catabolized or detoxified by many microorganisms as well as by the plants themselves. Elicitors are the major concern of this review. The literature is summarized in which elicitors are shown to be not only constituents of some microbes but also to be present in the cell walls of plants. A hypothesis is presented that suggests that all of the abiotic and some of the biotic elicitors stimulate phytoalexin accumulation by causing the release of an endogenous elicitor from the cell walls of plants. 108 references, 9 figures.

PHYTOALEXINS AND THEIR ELICITORS-A Defense Against Microbial Infection in Plants

Polysaccharide lyases (or eliminases) are a class of enzymes (EC 4.2.2.-) that act to cleave certain activated glycosidic linkages present in acidic polysaccharides. These enzymes act through an eliminase mechanism, rather than through hydrolysis, resulting in unsaturated oligosaccharide products. Acidic polysaccharides are ubiquitous and so are the lyases that degrade them. This review article examines lyases that act on acidic polysaccharides of plant, animal, and microbial origin. These lyases are predominantly of microbial origin and come from a wide variety of both pathogenic and nonpathogenic bacteria and fungi. The lyases discussed include alginate lyase (EC 4.2.2.3), pectin lyase (EC 4.2.2.10), pectate lyase (EC 4.2.2.2), oligogalacturonide lyase (EC 4.2.2.6), exopolygalacturonate lyase (EC 4.2.2.9), chondroitin lyases (EC 4.2.2.4 and EC 4.2.2.5), hyaluronate lyase (EC 4.2.2.1), heparin lyase (EC 4.2.2.7), heparan lyase (EC 4.2.2.8), and other unclassified lyases. This review examines the sources, regulation, purification, and properties of these polysaccharide lyases.

Polysaccharide lyases

https://mmbr.asm.org/content/mmbr/50/2/193.full.pdf

Signal exchange in plant-microbe interactions.

Stream fungi have the capacity to degrade leaf litter and, through their activities, to transform it into a more palatable food source for invertebrate detritivores. The objectives of the present study were to characterize various aspects of fungal modification of the leaf substrate and to examine the effects these changes have on leaf palatability to detritivores. Fungal species were grown on aspen leaves for two incubation times. Leaves were analyzed to determine the weight loss, the degree of softening of the leaf matrix, and the concentrations of ATP and nitrogen associated with leaves. The activities of a protease and 10 polysaccharide-degrading enzymes produced by each fungus were also determined. Most fungi caused similar changes in physicochemical characteristics of the leaves. All fungi exhibited the capability to depolymerize pectin, xylan, and cellulose. Differences among fungi were found in their capabilities to produce protease and certain glycosidases. Leaf palatability was assessed by offering leaves of all treatments to larvae of two caddisfly shredders (Trichoptera). Feeding preferences exhibited by the shredders were similar and indicated that they perceived distinct differences among fungi. Two fungal species were highly consumed, some moderately and others only slightly. No relationships were found between any of the fungal characteristics measured and detritivore feeding preferences. Apparently, interspecific differences among fungi other than parameters associated with biomass or degradation of structural polysaccharides influence fungal palatability to caddisfly detritivores.

Comparison of degradative ability, enzymatic activity, and palatability of aquatic hyphomycetes grown on leaf litter

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.

P.A. Wardman

Papers

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