Physiological Plant Pathology最新文献

The chlorophyll-protein complexes and the polypeptides of the chloroplast thylakoid membranes from healthy and peanut green mosaic virus infected peanut leaves were examined. CPa, the chlorophyll-protein complex associated with the reaction centre of photosystem II, was markedly reduced in severely infected leaves and this reduction was accompanied by reductions in the light harvesting chlorophyll-protein complexes. The P₇₀₀ chlorophyll a-protein complex of photosystem I increased in infected leaves. Polypeptide analysis of the thylakoid membranes from healthy and infected leaves showed a significant reduction in the 47 kD polypeptide in severely infected leaves. The α- and β-subunits of the chloroplast coupling factor and the 23 kD polypeptide were marginally reduced while the 11, 16, 17 and 44 kD polypeptides increased in severely infected leaves. There was a correlation between the reduction in the amount of the 47 kD polypeptide and photosystem II activity in virus infected leaves. The results support the finding that decreased photosynthesis in infected tissue is in part due to decreased levels of chlorophyll a.

A homogeneous endopolygalacturonase preparation was obtained from culture filtrates of Geotrichum candidum by a single-step purification, using affinity chromatography on cross-linked polypectate. The enzyme has a molecular weight of 38 000, an isoelectric point at pH 7·8, and a K_m of 1·33 mg ml⁻¹ with sodium polypectate as the substrate. The purified enzyme, at pH 4·2, macerated the albedo of lemon peel and produced typical sour rot symptoms when injected into lemon peel. An endopolygalacturonase was purified from sour rot infected tissue which was identical with the enzyme obtained from culture filtrate as judged by isoelectric focusing, SDS polyacrylamide gel electrophoresis and antigenic properties.

Levels of trifolirhizin, a pterocarpanoid glycoside found in aseptically grown roots of red clover (Trifolium pratense) at concentrations as high as 1·6 mg g⁻¹ dry weight, decreased as symptom severity increased in roots inoculated with Fusarium roseum “Avenaceum” and harvested 6 days after inoculation. The aglycone maackiain, not detected in uninfected roots, was present in all but the least severely infected roots; however, trifolirhizin loss was evident even in these lightly damaged roots.

Radial mycelial growth of F. roseum was not inhibited by trifolirhizin and, although maackiain inhibited growth, the percentage inhibition decreased over the 4 day incubation period. Germ tube growth of F. roseum was more markedly inhibited by maackiain than was mycelial growth. In liquid culture, F. roseum hydrolysed trifolirhizin to maackiain and, under similar conditions, this pathogen caused a relatively rapid loss of maackiain from the medium.

Infection of eggplants (Solanum melongena) with root-knot nematodes (Meloidogyne javanica) decreased total plant and fruit dry weight, increased sucrose synthase activity in galls an in the whole root system and also increased invertase activity in galls but not in the whole root system.

No sucrose synthase activity was detected in homogenates of infective larvae of M. javanica but sucrose hydrolysing activity was detected in these nematodes. Thus sucrose synthase activity appears to be associated only with plant tissue whereas sucrose hydrolysing activity may be associated with both plant and nematode. The starch and sucrose content of galls of infected plants were much lower than that of adjacent roots or of the roots of uninfected plants.

Accumulation of the sesquiterpenoid phytoalexins rishitin and lubimin in potato tubers is elicited by the polyunsaturated fatty acids all-cis-5,8,11,14-eicosatetraenoic acid (arachidonic) and all-cis-5,8,11,14,17-eicosapentaenoic acid, which are present in the fungus Phytophthora infestans. The ability of these acids to elicit the accumulation of phytoalexins in members of the Convolvulaceae (sweet potato), Leguminosae (broadbean, French bean, pea, soybean), Solanaceae (pepper, potato, tobacco, tomato) and Umbelliferae (carrot, parsnip) was investigated. The acids elicited accumulation of rishitin and lubumin in potato, as previously reported, and the sesquiterpene capsidiol in pepper fruit. The acids did not elicit phytoalexin accumulation in the other hosts tested.

Phenylalanine ammonia-lyase (E.C. 4.3.1.5) (PAL) and hydroxycinnamate:CoA ligase (E.C. 6.2.1.12) activities were measured in extracts from maize mesocotyls resistant and susceptible to Helminthosporium maydis and resistant to H. carbonum. CoA ligase activity increased in response to infection with H. maydis in both the resistant and susceptible cultivars. Activity began to increase between 6 and 9 h after inoculation and in the resistant cultivar continued to increase throughout a 48-h period. In susceptible cultivars activity ceased to increase at approximately 12 h after inoculation. The results demonstrate that the increase in CoA ligase activity is detectable as early as the onset of penetration by the fungus.

No significant change in PAL activity was observed in either resistant or susceptible combinations with H. maydis, suggesting that PAL and CoA ligase are not coordinately regulated in interactions involving this fungus. Neither enzyme was found to change as a result of inoculation of any cultivar with H. carbonum.

A brief heat shock induced resistance to the scab pathogen, Cladosporium cucumerinum, in cucumber plants normally susceptible to the fungus. Immersion of seedlings in a 50 °C water bath for 40 or 50 sec was found to be the optimal treatment for the induction of resistance. Plants inoculated with C. cucumerinum as soon as 3 h after the heat shock exhibited increased resistance to the fungus; a 12 h interval from heat shock to inoculation allowed for development of maximum resistance. The resistance was still fully effective when plants were inoculated 48 h after heat shock. All scab susceptible cultivars that were tested became more resistant to C. cucumerinum after heat shock. There was a direct correlation between the activity of soluble peroxidase induced by heat shock and the resistance induced by the same treatment. Heat shocked cucumbers had an increase in activity of the same isoperoxidases seen to increase in cucumbers with systemic resistance induced by prior Colletotrichum lagenarium inoculation. The relationship of heat shock induced resistance to other stress responses and the role of peroxidases in induced resistance is discussed.

Systemic resistance was induced in cucumber plants by inoculating the first true leaves with Colletotrichum lagenarium. Changes in the antigen composition of the upper leaves were analysed quantitatively by immunoelectrophoresis. Eleven days after inoculation a “new” antigen appeared in the protected upper leaves, increased in concentration for a further two days and then decreased. This antigen appeared in the unprotected leaves of uninoculated plants only when plants started to flower. Several common antigens present in both protected and unhigher amounts in the leaves of protected plants than in the leaves of unprotected plants but decreased in both with the onset of flowering. The significance of these qualitative and quantitative changes is discussed.

Fifty-nine field isolates of Nectria haematococca mating population VI, all of which were able to demethylate pisatin, were assayed for the rate of demethylation over a short time interval after a prior treatment with pisatin. Isolates showed a continuous gradation in their demethylation rates, but all isolates highly virulent on pea demonstrated a measurable rate of pisatin demethylation in response to pisatin. This suggested that regulatory control of pisatin demethylation in N. haematococca might influence the virulence of this organism on pea.

An attempt to identify further differences in regulatory phenotypes, based on differential induction of pisatin demethylating activity by pisatin versus 6-methoxy-1-tetralone (a known inducer of pisatin demethylation), was unsuccessful. However, the time course of pisatin demethylation by freshly harvested mycelium in the absence of a previous pisatin treatment did differ among the isolates tested. The length of the lag period before initiation of pisatin demethylation might be used as an additional parameter to distinguish different regulatory phenotypes in N. haematococca.