Physiological Plant Pathology最新文献

Resistance against Erysiphe graminis was induced in wheat and barley by treating the plants with metabolites from a selected isolate of Bacillus subtilis. The induced resistance did not affect conidial germination or the formation of appressoria, but it did reduce the formation of primary haustoria and the number of colonies which subsequently developed. The colonies formed were smaller with less mycelium per unit area of leaf than the colonies on untreated plants. Fewer haustoria and conidiophores were produced per unit area of colony, and numbers of conidia per conidiophore and per haustorium were also reduced. In terms of the quantity of fungal structures formed per haustorium, the efficiency of the haustoria was reduced by 46% by the induced resistance. A higher percentage of haustoria in the induced resistant plants showed an enlarged extrahaustorial matrix, although the structure of the haustoria themselves was not altered. The results suggest that the resistance induced by treatment with the microbial metabolites impairs the development of E. graminis in much the same way as genetically determined partial resistance.

Pear cell-suspension cultures (PCSC) inoculated with pathogenic strains of Erwinia amylovora developed necrosis in 4–10 days. Necrotoxin preparations, produced by fractionation of necrotic PCSC, induced necrosis of pear seedling cuttings and inhibited the growth of PCSC. Fractionation of PCSC medium yielded a fraction that induced similar responses. Solutions of the inorganic salts used in the medium, similar in electrical conductivity to the necrotoxin preparations, affected pear seedling cuttings and PCSC in a manner that was indistinguishable from that of necrotoxin preparations. Toxic activity was retained following destruction of the organic compounds of a necrotoxin preparation by ashing. Upon fractionation of necrotoxin preparations by gel-permeation chromatography, all toxic activity migrated with the low molecular weight fractions that contained inorganic salts. Therefore, the toxic activity of the necrotoxin preparation was due to the inorganic salts from the tissue culture medium, concentrated to toxic levels by the purification procedures. No phytotoxin of bacterial origin was found in our preparations.

Glyceollin isomers I, II and III were extracted from soybean cotyledons (cv. Harosoy 63) inoculated with Phytophthora megasperma f.sp. glycinea race 1, and separated and purified by column, high performance and thin layer chromatography and quantitated by spectrophotometry. The extinction coefficient of glyceollin I was 10 800, that of glyceollin II and III conformed to published values. Glyceollin I (ED₅₀ approx. 33 μg ml⁻¹) was almost twice as inhibitory as glyceollin II and III to growth of P. megasperma f.sp. glycinea race 1 on V8 juice agar. All three isomers were less active in soybean hypocotyl extract agar. Glyceollin II (ED₅₀ 7 μg ml⁻¹) was the most active against zoospore germination, and caused zoospores to burst; ED₅₀ values for glyceollin I and III were 12·2 and 13·9 μg ml⁻¹ respectively. Three isolates (1·1, 1·2, 1·3) were obtained from sectors in colonies on agar medium amended with glyceollin I, II or III respectively. Isolates 1·1 and 1·3 grew more rapidly on medium amended with glyceollin I, III or a natural glyceollin mixture than did wild type P. megasperma fsp. glycinea race I. All isolates and wild type had similar growth rates on medium amended with glyceollin II. The isolates did not lose their tolerance when grown on control medium and are presumably genetically controlled variants. Tolerance of glyceollin did not increase the aggressiveness of the isolates on soybeans.

A membrane-rich fraction from wounded potato tubers showed increasing activity of NADPH-dependent cytochrome c, but not of acetylated cytochrome c reducing activity, during ageing after slicing (wound-induced activity, WIA). In the fraction from aged tissues inoculated with an incompatible, but not a compatible, race of Phyiophthora infestans, as increase in native and acetylated cytochrome c reducing activities was closely associated with the occurrence of hypersensitive cell death and phytoalexin production (infection-induced activity, IIA). Treatment of aged tissues with hyphal wall components (HWC), a hypersensitivity-eliciting factor of the fungus, also activated both native and acetylated cytochrome c reducing activities similarly to infection. IIA and WIA were inhibited by NADP⁺, but only the former was appreciably inhibited by superoxide dismutase (SOD).

Water-soluble glucans (WSG), a hypersensitivity-inhibiting factor from compatible, but not incompatible, races of P. infestans, significantly inhibited the IIA but not the WIA in vitro.

These results suggest that a novel O₂⁻ generating NADPH oxidase system in the membrane of potato tissues may be activated following an incompatible cell reaction, which results in hypersensitive cell death and phytoalexin production. The lack of IIA activation in the compatible interaction may result from an inhibition of the reaction system by the water-soluble glucans from the fungus, thus resulting in the establishment of a compatible interaction.

Extractable levels of phenylalanine ammonia-lyase (E.C. 4.3.1.5) (PAL) increased in a susceptible combination of the maize mesocotyl with Helminthosporium maydis, but not in resistant combinations with either H. maydis or H. carbonum. The increase in extractable PAL was detected only in tissue incubated in the dark and was prevented or significantly reduced by cycloheximide and cinnamic acid. The in vivo effect of H. maydis on the mesocotyl was compared with that of the PAL inhibitor a-aminooxyacetate (AOA). AOA caused an increase in the level of extractable PAL and prevented the expression of resistance of the mesocotyl to H. carbonum. H. maydis also altered the expression of resistance to H. carbonum, in that it allowed a significant increase in post-penetration growth of H. carbonum. We suggest that susceptibility of maize to H. maydis is due to the fungus interfering with the plant's resistance response.

Application of digitonin to the leaf surface of potato plants was demonstrated to activate O₂⁻ generation of the leaf tissues as determined by extracellular cytochrome c reduction which was inhibited by superoxide dismutase (SOD). Protoplasts prepared from stem shoots of potato enhanced NADPH-dependent reduction of extracellular cytochrome c immediately after being treated with digitonin. The reduction was also inhibited by SOD. The similar activation of SOD-sensitive cytochrome c reducing activity was observed in leaves or protoplasts of some other plants treated with digitonin.

Potato leaf tissues pre-treated with digitonin by application to the upper or lower surfaces, or through the petiole, were protected from disease caused by infection with compatible races of Phytophthora infestans. Treatment of the wound surface ofpotato tuber tissue with digitonin induced the generation of the superoxide anion and, at the same time, sporangial germination, appressorial formation and invasion by P. infestans was greatly reduced. This effect was partially negated by the presence of SOD in the inoculum and seemed not to depend on antifungal compounds.

These results suggest that plant tissues possess an O₂⁻ generating NADPH oxidase which is activated by digitonin and that its activation in potato may contribute to an enhanced resistance against attack by compatible races of P. infestans at pre- or post-infection stages.

The distribution of peroxidase(s) in non-inoculated cotton bolls and flowers was qualitatively determined. The soluble and bound forms of cotton peroxidase(s) were investigated for changes elicited by fungal infection. Inoculation of cotton bolls, 30 days post anthesis, with Aspergillus flavus, Fusarium equiseti, Fusarium moniliforme, Fusarium semitectum or Rhizoctonia solani followed by a 6-day incubation period stimulated soluble peroxidase activity by 2- to 6-fold. In addition, fungal stress elicited two to five distinct soluble peroxidase isozymes not observed or at very low levels in non-inoculated controls. The pl values by isoelectric focusing for these isoperoxidases were 4·25, 4·35, 4·50, 4·65, 4·75 and 4·83. The pI 4·50 form was produced in controls and was also the major isozyme in the stress profiles. Each cotton-pathogen combination produced a unique peroxidase profile. No differences between control and fungal-stressed bolls were observed with either the specific activity or the peroxidase profile in the case of ionically-bound or covalently-bound classes of the enzyme.

Linoleate oxidation by avocado lipoxygenase was inhibited in vitro in the presence of the specific inhibitor 5,8,11,14-eicosatetraynoic acid (ETYA). Infiltration with ETYA of avocado discs inoculated with Colletotrichum gloeosporioides delayed symptom development at concentrations where the fungus itself was not affected. Subsequently, a natural inhibitor of avocado lipoxygenase was isolated from peels of unripe avocado fruits and identified as epicatechin. It inhibited avocado lipoxygenase with a K_i of 0·64 μm. The concentration of epicatechin in unripe fruits was 514 μg g⁻ fresh weight of peel; this decreased during ripening to 8 μg g⁻¹ fresh weight, before symptoms of C. gloeosporioides infections were expressed. A comparison of two cultivars with differing susceptibility to C. gloeosporioides showed that the concentration of epicatechin decreased faster in the cultivar in which symptoms appeared first. An atmosphere containing 50 μg l⁻ ethylene enhanced the decrease of the lipoyxygenase inhibitor in avocado fruits and shortened the period before symptoms of disease were expressed. In over-mature, firm and naturally infected fruits hanging in the orchard the concentration of epicatechin was 260 μg g⁻ in the area of the peel without symptoms and only 27 μg g⁻ in that showing symptoms of infection.

The results are discussed in relation to the hypothesis that the latency of the infection of avocado fruit by C. gloeosporioides may be accounted for by the degradation of the preformed antifungal compound, cis,cis-1-acetoxy-2-hydroxy-4-oxo-heneicosa-12, 15-diene, which is catalysed by avocado lipoxygenase, and that the in vivo lipoxygenase activity may increase during ripening owing to the decline in the levels of its endogenous inhibitor epicatechin.

Primary leaves of seedlings with different genetic backgrounds were inoculated with avirulent strains and then incubated in growth cabinets at constant temperatures of 15, 18, 22, 26 and 30 °C. Infection types in Sr15-bearing seedlings were low at 18 °C and below, mesothetic at 22 °C and high at 26 °C and above. Infection types in Sr14-bearing seedlings were high in most replicates at 15 °C and low at 22 °C and above; necrosis occurred in some replicates at 15 °C and all replicates at 18 °C and above. Infection types in Sr9b-bearing seedlings differed between the two strains of rust fungus used. With one strain, infection types decreased with increasing temperature from 18 °C; with the other strain, infection types decreased slightly at 30 °C. These observations are discussed in relation to others demonstrating two patterns of temperature sensitivity and then for their implications for hypotheses about the molecular bases of parasite--host interactions.

To investigate the role of silica-rich wall deposits in resistance to the cowpea rust fungus, French bean plants were grown hydroponically in nutrient solutions supplemented with or depleted in silicon. Primary leaves supplied with adequate silicon responded to fungal infection by the autofluorescence of guard cell walls, the limited autofluorescence of mesophyll cell walls (both visualized in cleared tissue), and the deposition in and on the latter of silica. If infected, silicon-depleted plants, light microscopy, electron microscopy, and energy dispersive X-ray analysis indicated that silica deposits were absent. The incidence of autofluorescence of guard cells was comparable to that in the silicon-supplemented plants, but the incidence and extent of mesophyll wall autofluorescence was greatly enhanced. The autofluorescence of mesophyll cells, but not guard cells, corresponded to areas of the wall that gave a colour reaction with toluidine blue indicative of phenolic compounds. Callose (aniline blue positive material), in the form of papillae, was also extremely common at infection sites in silicon-depleted leaves. Infection hyphae rarely formed haustoria in either silicon-depleted plants or those given adequate silicon, although these hyphae grew as well, and appeared equally healthy, in both types of plants. In silicon-depleted plants, pre-inoculation heat treatments or injection of intercellular fluids from bean rust-infected bean leaves, increased the incidence of haustorium formation and decreased the incidence of all observed plant responses. The results suggest that either silica deposition is not the primary barrier to haustorium formation in normal plants, or that a second barrier, such as the impregnation of the plant wall with phenolic materials, comes into play if silica deposition is prevented.