Physiological Plant Pathology最新文献

Etiolated soybean hypocotyls (cultivar Altona) treated with the systemic fungicide metalaxyl [N-(2,6-dimethylphenyl)-N-(methyl-acetyl) alanine methyl ester] developed restricted brown lesions with high levels of the phytoalexin, glyceollin, when inoculated with zoospores of Phytophthora megasperma f. sp. glycinea (race 6, compatible). These responses were very similar to those in interactions of untreated hypocotyls with incompatible races. When hypocotyls were supplied with a mixture of metalaxyl, at marginally inhibitory concentrations, and glyphosate (N-phosphonomethylglycine), an inhibitor of the shikimic acid pathway, the effectiveness of the metalaxyl treatment was reduced, the lesions spread and glyceollin concentrations were lower. The effectiveness of metalaxyl added directly to the inoculum drop was also less in glyphosphate-treated hypocotyls than in control hypocotyls. Glyphosate did not affect the fungitoxicity of metalaxyl in in vitro growth assays. It is concluded that at marginally inhibitory concentrations host defence mechanisms contribute to metalaxyl activity. This is most probably due to glyceollin accumulation, but as levels frequently remained above ED₉₀ values in spreading lesions and other aspects of host metabolism are known to be influenced by glyphosate, alternative explanations are possible. The possibility that the stimulation of host defence responses in metalaxyl-treated tissue may be due to inhibition of pathogen activities that suppress such responses in untreated tissues is discussed.

Systemic acquired resistance to tobacco mosaic virus (TMV) was induced in Nicotiana tabacum cv. Xanthi-nc plants by incubating them at 22°C for 9 days after inoculating them with TMV. After induction of resistance the plants were incubated for varying periods of time at either 22°C or 32°C before challenge inoculation with TMV on the five leaves above the primary inoculated ones. They were then incubated at 22°C. The level of acquired resistance, as measured by the reduction in lesion size relative to the controls, in plants that had been incubated at 32°C for up to 8 days, was similar to the level in plants that had been incubated continuously at 22°C. Thus, systemic acquired resistance is maintained for at least 8 days in plants incubated at 32°C.

Resistance to powdery mildew conditioned by the ml-o gene in barley is expressed mainly during primary penetration by the pathogen. Since barley epidermal cells usually respond to attempted penetration by the deposition of a papilla at the encounter site, the possible association of papilla formation with ml-o resistance was examined. Information on penetration efficiency, papilla frequency and papilla diameter was obtained using coleoptiles and leaves of near-isogenic resistant and susceptible barley breeding lines. Increased papilla frequency and, usually, increased papilla diameter were associated with a much lower penetration efficiency on resistant lines. Moreover, penetration failures in the presence of papillae were more frequent on resistant than on susceptible lines. These results establish a clear coincidence between resistance to penetration and an enhanced papilla response in ml-o barley and set the stage for the companion experimental study.

Hypersensitive response inducing (HR-inducing) strains of the heterologous (non-soybean) pathogens Pseudomonas syringae pv. phaseolicola and P. syringae pv. syringae attained populations in soybean leaves only three-fold to five-fold less than a compatible race of the soybean pathogen P. syringae pv. glycinea. Incompatible (HR-inducing) races of P. syringae pv. glycinea and a non-HR-inducing strain of the heterologous pathogen Corynebacterium flaccumfaciens pv. flaccumfaciens showed more restricted growth. A non-HR-inducing strain of the heterologous pathogen Erwinia carotovora subsp. atroseptica and a strain of the saprophyte Bacillus cereus did not grow in soybean leaves. Incompatible races of P. syringae pv. glycinea and the heterologous pseudomonads induced accumulation of isoflavonoids (daicizein, formononetin, genistein, glyceollin) and isoflavone glucosides (daidzin, genistin, ononin). Bacillus cereus induced accumulation of isoflavone glucosides alone, and C. flaccumfaciens pv. flaccumfaciens and E. carotovora subsp. astroseplica did not induce accumulation of either type of stress metabolite. Of the purified compounds tested, glyceollin alone had significant antibacterial activity, but only against Cram-positive bacteria. By use of in vitro bioassays, no evidence was obtained to indicate that induction of additional inhibitory compounds or inhibitory activity due to total isoflavonoid content of inoculated leaf tissue was responsible for resistance.

Immobilization of bacteria by highly electron-dense material in intercellular spaces of soybean leaves occurred to the greatest extent with E. carotovora subsp. atroseptica and B. cereus, and to a lesser extent with C. flaccumfaciens pv. flaccumfaciens. No bacterial immobilization was evident at 48 h after inoculation with incompatible races of P. syringae pv. glycinea or heterologous pseudomonads.

Purified lipopolysaccharides (LPS) from four races of Pseudomonas syringae pv. glycinea were assayed for phytoalexin elicitor activity in soybean cotyledons, and isolated O-chain fractions were chemically characterized. The LPS were weak elicitors of glyceollin accumulation, but no race specificity was noted. SDS-Polyacrylamide gel electrophoresis showed considerable size heterogeneity in the LPS but only slight differences were noted between preparations from the various races. The major components of the O-chain fractions from all races were rhamnose and N-acetylglucosamine. The ratio of these components in preparations from race 4 differed significantly from those of races 1, 5 and 6.

Wounding wheat leaves induced increases in the activities of peroxidase (PO) and phenylalanine ammonia-lyase (PAL) which were further enhanced by inoculating the wounds with Botrytis cinerea. Subcellular fractionation of PO revealed no preferential stimulation of activity in any particular fraction by wounding alone or by inoculating intact or wounded leaves. Although the increases in PO and PAL activities induced by wounding were confined to wound areas, inoculation induced increases in both activities away from damaged tissue and thus in tissues distant from fungal hyphae.

PO activity was localised at infected wound margins in the cell walls and in degenerating cytoplasm of the leaf tissue. Some weaker activity was observed in fungal cell walls growing close to the wound edge.

Addition of heat-killed spores or a cell-free germination fluid of B. cinerea to wounds elicited increases in PO activity which were of similar magnitude to those elicited by living spores but PAL activity was stimulated to levels only around 10% that of the wound-inoculated response. Macerase-resistant rings of tissue were induced around wound margins by these fungal preparations, although the response was weaker than that of wounds inoculated with living spores.

Incubation of wheat leaf protoplasts with live spores, heat-killed spores or germination fluid of B. cinerea revealed no stimulation of either PO or PAL activities; in some cases inhibitions were observed.

It is postulated that inoculation of wheat leaf wound areas with B. cinerea does not merely augment an underlying injury response. Some form of fungal recognition is involved in the mediation of an infection response which diverts wheat leaf metabolism towards lignification.