Physiological Plant Pathology最新文献

The growth of Sclerotinia sclerotiorum in white beans (Phaseolus vulgaris) was much slower in the tolerant cv. Ex Rico-23 than in the susceptible cvs. Kentwood and Seafarer. The difference was paralleled by the rate of diffusion in leaf tissue of oxalic acid, a phytotoxin of the white mold fungus (Sclerotinia sclerotiorum). Uptake of oxalic acid into excised leaves through the petiole resulted in more severe brown-rot-like symptoms in the leaves of cv. Kentwood than in those of Ex Rico-23. When [¹⁴C] oxalic acid was fed through petioles, radioactivity in the interveinal tissue of Kentwood was about three times higher than that in Ex Rico-23. Autoradiographs of leaves showed that radioactivity in Ex Rico-23 was confined to major veins and that activity in the interveinal tissue was low, whereas in Seafarer and Kentwood radioactivity did not accumulate in the veins but was distributed uniformly throughout the leaf tissue, with a slight accumulation along leaf margins.

Callus cultures derived from seeds of lucerne (Medicago saliva L.) were challenged with a lucerne isolate of Verticillium albo-atrum using agar blocks or spore suspensions as inoculum. The rate of colonization of different callus lines by the fungus varied from complete overgrowth of tissues in susceptible lines, to a rapid browning reaction of host cells accompanied by a restriction of fungal growth in resistant lines. The phytoalexin medicarpin accumulated in both resistant and susceptible reaction types, but resistant callus lines accumulated higher levels of this compound upon challenge with spore suspensions of V. albo-atrum. Pretreatment of resistant calli with subherbicidal levels of the inhibitor glyphosate lowered the production of medicarpin, reduced the browning response of callus cells and enhanced colonization by the fungus. Possible links between these in vitro responses and those of intact lucerne plants to V. albo-atrum are discussed.

The constituents of the interface between a strain of Erysiphe pisiand two susceptible and five resistant lines of Pisum sativum were investigated by fluorescence microscopy after treatment with specific reagents. Fragments of epidermes were detached so that the haustoria and extrahaustorial membranes were exposed directly to the reagents. The specificities of the 23 reagents employed included α- and β-linkages in polysaccharides; constituent sugars; aldehyde, amino, sulphydryl and disulphide groups; proteins, lysine, arginine, lipid, calcium and anions; and anion transport sites.

The main conclusions are that in the susceptible cultivars the extrahaustorial membrane includes polysaccharides (β1–4 linked) with small amounts of α-glucose, α-mannose and galactose, β-linked N-acetylglucosamine (haustorial face only), protein, arginine, amino and sulphydryl groups and calcium. The extrahaustorial matrix includes varying quantities of β-linked polysaccharides. Haustorial walls contain β-linked polysaccharides including α-glucose, α-mannose, galactose and N-acetyl glucosamine (β-linked). The haustorial cytoplasm contains protein and releases fluorescein from fluorescein diacetate and 3-o-methyl fluorescein phosphate.

The main differences between the susceptible lines and the resistant lines is that the resistant line, show enhanced reactivity of terminal glucose, mannose and N-acetylglucosamine groups in the extrahaustorial membranes of some lines. The last is probably due to greater accessibility of lectin effected by rupture of extrahaustorial membranes. Constituents of extrahaustorial membranes fluorescing with 4-acetamido-4′-iso-thiocyanato-stilbene-2,2′-disulphonic acid developed ore slowly and included less cross-linked protein. Calcium was rare in the extrahaustorial membranes and host cytoplasm in resistant lines.

The results are discussed in relation to the molecular and functional properties of the interface.

The ml-o gene in barley conditions resistance to primary penetration by Erysiphe graminis f. sp. hordei as well as an enhanced papilla response. To determine if papillae are necessary for ml-o resistance, cytoplasmic aggregation and papilla formation Were inhibited in near-isogenic resistant-susceptible pairs of barley breeding lines by low-speed centrifugation of leaves and centrifugation and heat-shock treatments of coleoptiles. Papilla frequency was markedly reduced by both methods. Centrifugation separated papilla formation from resistance at most encounter sites, although a small increase in penetration efficiency occurred because haustoria developed at some sites where papilla formation was inhibited. Heat-shock caused a substantial increase in penetration efficiency. Here too, this increase occurred because haustoria developed at sites where papilla formation was inhibited. Taken together, these results demonstrate a strong link between papilla formation and ml-o resistance, but they do not uniformly support the hypothesis that ml-o resistance is based on papilla formation. The centrifugation results, in particular, suggest that papilla formation is not necessary for resistance.

Existing evidence implicates an extracellular protease of Nectria galligena Bres. in the initiation of the accumulation of antifungal amounts of benzoic acid in immature apple fruit. It is the purpose of the present work to examine further this relationship between protease and the host cells by immunocytochemical techniques.

Thin sections of infected apple tissue, fixed, and embedded in Lowicryl K4M at −40°C were reacted with monospecific antibodies raised in rabbits against the purified protease. Using a secondary tetramethylrhodamine isothiocyanate conjugated antibody, specific fluorescent localization of intercellular and intracellular fungal hyphae and host cell cytoplasm was demonstrated, thus establishing the presence of the protease in infected tissue. In a transmission electron microscope study, this was confirmed, and the localization and distribution of the enzyme further resolved by protein A-gold immunocytochemistry. Specific binding of colloidal gold was evident over, (1) intercellular and intracellular fungal hyphae, (2) electron-dense deposits located within and around hyphae and adjacent to partially degraded apple cell walls, and (3) peripheral vacuolar cytoplasm of intact host cells. These observations suggest that protease synthesized in hyphae of N. galligena is secreted into an extracellular matrix whence it may diffuse through host cell walls and detached, but entire, plasmalemmae observed in infected tissues. The protease is, therefore, able to reach sites in the host where it may elicit benzoic acid or where it may release an endogenous elicitor for that purpose. This is essential supporting evidence for the hypothesis that the accumulation of benzoic acid is initiated by fungal protease in vivo.

The carrot phytoalexin, 6-methoxymellein, accumulated in carrot suspension culture when it was incubated with a partial hydrolysate of carrot cells obtained by pectinase or trypsin treatment. 6-Methoxymellein also accumulated when these enzymes were added to the culture. In both cases, phytoalexin content decreased rapidly after maximum accumulation was attained, suggesting that the cultured carrot cells metabolized the phytoalexin.

Cultured carrot cells incubated with the phytoalexin formed three different metabolites: among them, two which accumulated in the medium, were identified as the β-glucoside of 6-methoxymellein and 6-hydroxymellein.

6-Methoxymellein was found to be toxic to the host plant and induced a lag period in the growth of cultured carrot cells at low concentration. When it was added at concentrations higher than 0·5 mm, it caused a rapid decrease in viable cell number.

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.