Physiological Plant Pathology最新文献

A sensitive and semi-quantitative method for assaying the toxicity of metabolites from Endothia parasitica was developed using American and Chinese chestnut inner bark. Skyrin and rugulosin showed detectable toxic effects in high concentrations. However, diaporthin and other metabolites extractable in organic solvents showed no toxic affect on chestnut tissue. Skyrin, rugulosin and diaporthin were not detectable in samples taken from the advancing mycelium at the periphery of cankers. These compounds were found only in areas associated with well developed necrosis and active sporulation. Results presented here do not support the concept that skyrin, rugulosin, diaporthin or other extractable aromatic or heterocyclic compounds effect the advance of E. parasitica mycelium into chestnut inner bark.

Three classes of enzymatic activity were expressed in cultures of virulent and hypovirulent strains of Endothia parasitica. Polygalacturonase, cellulase, and protease activities accumulated in liquid cultures. Polygalacturonase activity was also detected in samples from active cankers resulting from infection by virulent strains of E. parasitica. The acidification of liquid media, as well as chestnut tissue in advance of growing mycelium, was determined and compared with the pH optima for the three enzyme activities. Additionally, polyphenolic extracts from Castanea mollissima were found to be more effective in eliminating polygalacturonase activity than were extracts from Castanea dentata. A specific inhibitor of polygalacturonase activity was also isolated. Polygalacturonase inhibitory activity extracted from C. mollissima inner bark was found to be more than twice as potent as the same activity extracted from C. dentata inner bark.

Stem inoculation of Burley tobacco with sporangia of the blue mold pathogen, Peronospora tabacina, elicited high systemic resistance in the foliage against disease caused by subsequent inoculation with the pathogen. Girdling the stem above the site of inoculation prior to 9 days after stem inoculation prevented the elicitation of resistance, whereas girdling below the site did not. Increased growth (height, number of leaves, reduced time to flowering) was observed in ungirdled, stem-inoculated plants, plants stem-inoculated and girdled above the site of inoculation at day 6 or later and in plants stem-inoculated and girdled below the site of stem inoculation. The factor responsible for resistance did not precent penetration by the fungus, and it may or may not be the factor responsible for enhanced growth. The resistance factor was graft transmissible between rootstock and scion. Induced scions (10 cm in length), when grafted onto control rootstocks, developed into fully grown plants that remained systemically protected against blue mold.

The hypersensitive reaction of potato tuber tissue, measured by necrosis and phytoalexin accumulation, was elicited by lipids and lipid-containing and lipid-free fractions extracted from mycelium of Phytophthora infestans. The most active fraction extracted after homogenization in phosphate buffer was composed of carbohydrate, protein and lipid. A heat-released preparation containing very low levels of eicosapentaenoic, arachidonic and dihomo-γ-linolenic acids was more active than a lipid extract of the mycelium. The majority of the elicitor of the mycelium remained associated with the wall residue after extraction.

No relation was found between the activity of the fractions and their content of specific fatty acid elicitors. The activity of most fractions could not be explained by synergism between the eliciting fatty acids and mycelial carbohydrate. Furthermore, the activity of the lipid fractions appeared to be attenuated by some fungal component. Many of the fractions were shown by analytical isoelectric focussing to contain carbohydrate and protein bands also present in an elicitor obtained from culture filtrates of P. infestans. It is suggested that both lipid and non-lipid materials in the mycelium of P. infestans are able to elicit the hypersensitive response in potato tuber tissue.

Combined treatment of cucumber protoplasts with actinomycin D and UV irradiation inhibited host RNA synthesis by more than 95%. Cucumber mosaic virus (CMV) replication was however, not affected in either susceptible, Ashley, or resistant, China (Kyoto), cucumber protoplasts, whether the treatment was administered before or immediately after inoculation, or whether the coculum was virus RNA or nucleoprotein. These inhibitors also failed to affect the expression of resistance in China (Kyoto) protoplasts suggesting the absence of an “active defence mechanism” operating at the single cell level.

The lower level of infectious RNA in China (Kyoto) protoplasts was reflected by decreased synthesis of total virus specific RNA rather than a reduction in any one of the CMV RNAs. The amount of the replicating satellite RNA 5 (CARNA 5) associated with this strain was similar, in proportion to genomic RNAs, in protoplasts from each cultivar and therefore could not account for the suppression of disease in China (Kyoto).

Abscisic acid (ABA) concentration was increased 1·1 to five-fold in Samsun tobacco infected with six strains of tobacco mosaic virus (TMV) which produced mosaic symptoms of widely different severity and which multiplied to different extents. The ABA accumulation was more closely related to the severity of mosaic symptoms (r = 0·93; 4 degrees of fredom, df) than to the extent of virus multiplication (r = 0·39, 4 df). The increase in ABA concentration was located outside the chloroplast; virus infection had little effect on, or reduced, chloroplastic ABA concentration.

Tobacco mosaic virus infection inhibited plant growth. Reduction in growth rate was related both to increase in extrachloroplastic ABA concentration (r = 0·69, 4 df) and to virus accumulation (r = 0·73, 4 df). It is concluded that both these factors are important in the inhibition of growth produced by TMV infection.

The translational activity of total RNA and polyadenylated (polyA⁺) RNA isolated from susceptible leaves of Hordeum vulgare infected with Erysiphe graminis f.sp. hordei has been investigated. When equal quantities of total RNA from control and inoculated leaves were included in assays of protein synthesis using rabbit reticulocyte lysates, the amount of protein synthesised in assays programmed with RNA from infected leaves was reduced to 61% of the controls at 1 day after inoculation. This effect increased with infection until at 5 days after inoculation, RNA from infected leaves had only 11% of the activity of that of controls. Analysis of translation products by SDS-polyacrylamide gel electrophoresis and fluorography indicated reduced synthesis of most labelled polypeptides in assays of RNA from infected leaves. Analysis of total RNA on agaroseformaldehyde gels indicated no significant degradation of differences in RNA from control and inoculated leaves. The proportion of total RNA present as polyA⁺ RNA also declined during infection but a decrease in the translational activity of equal amounts of polyA⁺ RNA from control and infected leaves was not observed until 5 days after inoculation. This decreased activity was due mainly to reduced synthesis of the M_r 20000 protein which was the major product in assays of polyA⁺ RNA from controls. The reduced activity of the polyA⁺ RNA at this later stage of infection was not evident in the presence of greater than 0·5 mm 7-methyl guanosine 5′ phosphate and the translation products were identical in the presence of this inhibitor. These data suggest that infection of barley by the powdery mildew fungus caused a rapid and extensive decline in the amount of available host mRNA.

Water solutions of the capsular polysaccharide (EPS) of Erwinia amylovora exhibit long flow times (t) in a kinematic viscometer. Addition of salts to the solutions greatly decreases the flow times. The salt-induced decrease in flow time is directly related to the ionic strength of the solution and independent of the ionic species present. The salt effect does not result from a change in the molecular weight of the EPS. Addition of NaCl, at concentrations sufficient to reduce the flow time, decreases or eliminates the capacity of EPS solution to cause wilt in the cut shoot assay. Treatment of EPS with either of two depolymerase phages decreases both t and the molecular weight of the EPS (from 100 × 10⁶ D to less than 4 × 10⁴ D). Such phage-produced fragments (ψdp) retain their capacity to cause wilt in the cut shoot assay but, like EPS, lose this ability in the presence of salts. Radiolabeled EPS and ψdp is retained at the end of the cut shoot when wilt occurs but is distributed throughout the shoot when wilt is inhibited by salt.

Aggressiveness of Bipolaris sorokiniana conidia on wheat seedlings was decreased when conidia were incubated on either a sandy loam soil at −50 mb matric potential for 15 days or a sand bed through which a salts solution was percolated for 5, 10 or 15 days. Seedlings inoculated with treated conidia showed significantly (P = 0·05) less disease and had longer roots and coleoptiles in most treatments, than seedlings inoculated with nontreated conidia. Conidia that had been incubated on soil or leached sand for 15 days showed less germination than nonincubated conidia in a salts solution or wheat seedling exudate. The germination rate of treated conidia on potato dextrose agar was lower than that of nontreated conidia, but viability was not affected. Loss of ¹⁴C from labelled conidia incubated for 15 days on soil or leached sand was 11·2 and 12·0%, respectively, of total label. The results indicate that incubation of conidia under conditions of nutrient stress can attenuate the aggressiveness and germinability of B. sorokiniana conidia, and that these changes were associated with loss of endogenous carbon from the conidia.

Biochemical and physiological changes and their relation to ethylene biosynthesis were studied in grapefruit (Citrus paradisi Macf. cv. Marsh Seedless) peel, 5–6 days after inoculation with Penicillium digitatum Sacc. In both the albedo and flavedo tissues of the peel, fungal invasion was associated with increases in free galacturonic acid but with reductions in pH and soluble proteins. The extent of the changes was smaller the greater the distance from the maceration front. Two parallel and distinct maceration fronts could be defined in the peel, the one in the albedo preceding that in the flavedo. Fungal glucosamine was present in the apparently healthy region of the albedo up to 15 mm ahead of the flavedo maceration front.

Fungal invasion was associated with increases in both 1-aminocycloproprane-1-carboxylic acid (ACC) and ethylene production but the ability of the tissue to convert ACC to ethylene decreased with the development of the infection. The early relatively low rate of ethylene production in infected fruit seems to originate mostly from the fruit tissue while a later and higher rate of ethylene production originates mostly from the fungus.