Physiological Plant Pathology最新文献

The effects of hot water treatments of the roots, aerial parts or entire plant of tomato on the severity of Fusarium wilt symptoms was investigated.

Symptoms developing following root inoculation were markedly delayed by treatment of the roots or of the entire plant at time-temperature combinations which caused some cell death to parts of the root system. Treatments at 48–49°C for 30 sec were the least severe of the treatments which gave protection. Protection was not effective against stem inoculations but was associated with a reduction in stem colonization. Plants showed some degree of systemic protection against a root inoculation following heat treatment of the aerial parts at 48–49°C for 30 s, but to a lesser extent than that induced by root treatments. The highest levels of protection were obtained when roots or aerial parts were treated 12–48 h before root inoculation. Only the root treatments were effective when applied 48 h after inoculation. Drastic pruning of roots did not reduce the effectiveness of the heat treatment and in fact root pruning itself induced some protection.

The protection induced by the hot water treatments seems to depend on a transient state of resistance, which is similar to that induced by some biotic elicitors.

Cell suspension cultures of Dianthus caryophyllus when treated with an elicitor preparation from Phytophthora parasitica accumulated the phytoalexin dianthalexin whereas untreated cultures produced no dianthalexin. The sensitivity of the cells to the fungal extract was maximal during the linear phase of growth and cultures became less sensitive as they approached the stationary phase. Maximum accumulation at all stages occurred 24 h after application of the elicitor preparation.

A radioimmunoassay for the phytotoxin brefeldin A has been developed employing [7-³H] brefeldin A and an antiserum raised against 7-dehydrobrefeldin A conjugated to bovine serum albumine. The antisera allowed the determination of as little as 1 pmol of toxin and had a similar affinity for both 7-epi-brefeldin A and 7-dehydrobrefeldin A. Several other compounds, including some with structures similar to brefeldin A and certain toxins from other Alternaria species, were bound by the antiserum but at least 6000-fold less strongly than brefeldin A.

The radioimmunoassay was used to determine the amounts of brefeldin A in leaf tissues adjacent to and distant from the inoculation site in safflower leaves inoculated with the pathogen Alternaria carthami or with the non-pathogens Ascochyta imperfecta and Eupenicillium brefeldianum. High concentrations of brefeldin A, up to approximately 3 mM, accumulated in leaf tissues over a period of 17 days after inoculation with A. carthami. The other two fungi, although able to produce large quantities of brefeldin A in stationary culture, did not accumulate it in inoculated leaves and the small amount of toxin present with the inoculum disappeared with time. Alternaria carthami, on the other hand, failed to accumulate the toxin when inoculated onto leaves of Zinnia elegans, Helianthus annus, Calendula officinalis or Lactuca sativa.

These results support our earlier hypothesis that efficient production of brefeldin A is a factor in the mechanism of infection of safflower tissue by the pathogen A. carthami.

The effect of the host-specific toxin produced by Helminthosporium sacchari (HS-toxin) on cell membrane potential of susceptible sugarcane was investigated. Membrane depolarization was detected at concentrations as low as 50 nm HS-toxin. The onset of membrane depolarization was observed after a lag phase of 4–10 min depending on the toxin concentration.

The energy-dependent component of membrane potential (E_m–p) was inhibited by the toxin. However, membrane repolarized, i.e. E_m–p, was re-established after removing the toxin from the bathing solution. At low toxin concentrations E_m–p was lost in the dark but was partially recovered in the light and more fully in the light with the addition of fusicoccin.

The HS-toxin-induced membrane depolarization was prevented by temperatures above 30°C but subsequent lowering of temperature to 26°C resulted in membrane depolarization. Treatments with a nontoxic lower homologue of HS-toxin (lacking one galactose unit in the molecule) protected cells against HS-toxin-induced membrane depolarization. Pretreatments of tissues with galactose or raffinose did not prevent HS-toxin-induced depolarization.

It is postulated that HS-toxin is causing a loss of H⁺ gradient across the plasmalemma. However, it is unlikely that the H⁺ pump is the site of the toxin action since in the presence of the toxin the pump can be activated by light and fusicoccin.

Hyphal growth and polygalacturonase liberation by Geotrichum candidum in wounds in the peel of resistant and susceptible lemons were similar for 10 h after inoculation; a stimulation of hyphal growth and polygalacturonase activity in susceptible fruit was detected after 15 h. The infectivity of the inoculum was increased by the addition of sterile macerating enzymes of the pathogen, but not by addition of nutrients. No evidence was found that preformed factors in resistant lemons limited fungal growth or liberation of polygalacturonase. Sterile macerating enzyme preparations injected into the albedo produced more maceration in yellow or turgid fruits, which are relatively susceptible to infection, than in the more resistant, light green or subturgid fruits. Albedo tissue strength (by penetrometer) of light green fruits was higher and remained higher during the maceration process than in yellow fruits, even though the rate of tissue softening was slightly faster in the former. The effect of macerating enzymes on potassium leakage from albedo cells paralleled their effect on tissue strength.

Leptosphaeria maculans causes stem canker on oilseed rape (Brassica napus). Studies using sodium polypectate and rape cell wall media show that the pathogen can produce a variety of cell wall-degrading enzymes in liquid culture. There is also an accumulation of cell wall-degrading enzymes in stem canker lesions. The levels of polygalacturonase, α-l-arabinanases, β-l-galactanases and carboxymethyl-cellulase were studied in three oilseed rape/L. maculans interactions up to 40 days after inoculation. The early accumulation of high levels of polygalacturonase in the most susceptible interaction indicates a probable role for this enzyme in severe canker formation, whereas high levels of the other enzymes monitored were only detected after the compatibility of the interaction had been established. The importance of regulation of cell wall-degrading enzyme activity in disease resistance is discussed.

Hemigossypol (HG), methoxyhemigossypol (MHG), desoxyhemigossypol (dHG) and desoxymethoxyhemigossypol (dMHG), the four major terpenoids formed in the stem stele of Verticillium dahliae-infected, wilt-resistant Seabrook Sea Island (SBSI) cotton, were tested at pH 6·3–7·5 in liquid nutrient media for toxicity to V. dahliae. The terpenoids dHG, HG, dMHG, and MHG at 25 °C killed all conidia after 18–40 h at 10, 45, 25 and 60 μg ml⁻¹, respectively; and all mycelia after 48 h at 15, 32, 25 and 45 μg ml⁻¹, respectively. Inhibition of conidia germination also occurred at concentrations well below the fungicidal concentrations. Dimethylsulfoxide at 2 or 5% was required to solubilize HG, MHG and dMHG at fungicidal concentrations. Only dHG had the water solubility apparently required to reach fingicidal concentration in the aqueous medium of infected xylem vessels and thus account for the death of V. dahliae conidia and mycelia in most infected vessels in the stem stele of SBSI cotton 10 days after inoculation. The dHG in the stem stele at 10 days after inoculation was in excess of fungicidal concentration.

A major component of cultures of Xanthomonas campestris pv. campestris grown on Watanabe broth was crystallized as colourless needles and identified as trans-3-methylthioacrylic acid. A second major component of the same culture filtrates was purified by high performance liquid chromatography and identified as 3-methylthiopropionic acid. Radiolabelling experiments with [¹⁴C] and [³⁵S] methionine gave approximately equally high levels of incorporation of radioactivity into the two above acids. When the amount of methionine suppleid to cultures was increased or decreased above or below that of Watanabe broth, a corresponding change in levels of production of these acids resulted. Neither 3-methylthiopropionic acid nor trans-3-methylthioacrylic acid was detected in cultures when methionine was omitted from the medium. When, for example, the amino acids leucine and phenylalanine were substituted for methionine, other carboxylic acids including isovaleric and phynylacetic acids were detected in cultures of the bacteria. The various carboxylic acid biotransformation products of X. campestris pv. campestris were bioassayed against protoplasts and seedlings of cabbage. All compounds expressed only weak phytotoxicity when prepared in unbuffered media and phytotoxicity was effectively eliminated by supplying the compounds under test in buffered solution at pH 6·5. Benzoic acid and 3-methylthiopropionic acid, which are structurally dissimilar but which possess similar pK_as, expressed similar degrees of activity. Contrarily, both within and between the various carboxylic acids tested there was good correlation between level of activity expressed and pH of the treatment solution.

The timing of formation of cytoplasmic aggregates induced by Erysiphe pisi appressoria in barley coleoptiles, and wall penetration by the appressoria, was investigated by light microscopy and scanning electron microscope (SEM) micromanipulation. Observations of E. pisi appressoria on living barley coleoptiles by light microscopy revealed the unique responses of host cells to this fungus: first, a small cytoplasmic aggregate formed below the appressorium, lasted for 20·9 ± 15·4 min, then vanished. After a further interval of 42·7 ± 21·0 min, a second cytoplasmic aggregate developed below the same appressorial lobe. At 21·4 ± 7·5 min after the initiation of the second cytoplasmic aggregate, a penetration-pore-like structure became visible in the lobe. Scanning electron microscope micromanipulation indicated that the penetration-pore-like structure observed by light microscopy represented the penetration pore or peg produced by the fungal appressorium. Observations of appressoria by differential interference contrast microscopy and micromanipulation at the SEM level lead us to conclude that the first and second cytoplasmic aggregates were initiated before the host wall was penetrated.

The production of the new 116 kD protein and its association with host chromatin in mosaic-diseased leaves of tobacco mosaic virus (TMV) infected tobacco was investigated. Whereas TMV was detected by ELISA in systemically infected leaves, 96 h after inoculation the 116 kD protein was not detectable until between 120 and 144h after. The accumulation of 116 kD protein appeared to coincide with the first visible appearance of symptoms (vein clearing). It was present both in the soluble protein and in the sedimentable membrane fractions but since its concentration in nuclei was about eight-fold higher than in the cytoplasm, it appears to be preferentially associated with chromatin. Moreover, its dissociation from the chromatin rewuired salt, in contrast to TMV coat protein which was released by urea only indicating that it is bound more tightly to chromatin than TMV coat protein is. Its close association with chromatin suggests that it may play a regulatory role in pathogenesis and symptom expression in developing leaves.