Physiological Plant Pathology最新文献

Histological changes in roots of potato plants with the gene H₁, which confers resistance to the potato cyst-nematode Globedera rostochiensis pathotype Ro 1, are described for periods of 24 h to 10 days after invasion by second-stage juveniles of G. rostochiensis. The changes are compared with those which occur in roots of a susceptible cultivar. Experiments were carried out on potato roots growing from sprouting tuber pieces on water agar. Root cells surrounding the invading nematode undergo a hypersensitive-like response and become necrotic. Cell walls of inner cortical and stelar cells adjacent to the stylet of the invading nematode break down to form a syncytial complex but cells surrounding the syncytium become necrotic, presumably limiting movement of nutrients from plant to nematode. Ultrastructural changes of cells in the cortex and the stele are illustrated and discussed.

Both the wild-type isolate of Erwinia amylovora (Burrill) Winslow and a noncapsulated avirulent form, mutant S, when in contact with pear fruit slices, induced electrolyte leakage and grew rapidly. A capsulated form of the mutant S, induced by growing it in d-galactose medium, was found to have reduced capacity to cause leakage and grew less rapidly. Loss of capsule by the pre-capsulated mutant occurred simultaneously with increased leakage of pear electrolytes and bacterial growth. Another capsulated avirulent form, mutant P, did not induce electrolyte leakage from pear tissue and failed to grow. In mixed inocula, the capsulated mutant P partially inhibited the action and growth of both the wild-type and the non-capsulated mutant S on pear tissue. Prevention of contact between bacterial inoculum of the wild-type for the non capsulated mutant S, and the pear tissue by means of a membrane filter, also prevented loss of electrolytes and bacterial growth. Cell-free fluids from cultures of bacteria in artificial media or from inoculated pear tissue suspensions, failed to cause loss of electrolytes from healthy pear slices. Wild-type bacteria isolated from infected apple plants contained a proportion of cells wholly or partly deficient in outer surface components, in contrast to bacteria grown in culture, which were more uniformly coated. It is suggested that host damage is caused by contact with partially or wholly noncapsulated virulent bacteria and that the amount of damage is regulated by the properties of the outer coat of the pathogen. It is also suggested that the regulation of host damage is a requisite for infection and the progress of the disease.

Fine structure observations of hyphal interactions between a recently described mycoparasite, Pythium nunn, and plant pathogenic fungal species suggested that enzymatic dissolution of cell walls of host fungi was involved. The fluorescence, localized about cell wall lysis at points of interaction when stained with Calcofluor White M2R New, also suggested enzymatic activity since this stain usually binds to oligomers in regions of incomplete cell wall polymers. Again, mycoparasitic activity of P. nunn releases ¹⁴CO₂ from labelled cell walls of a plant pathogenic species of Pythium and Rhizoctonia solani. Culture filtrates of P. nunn had high cellulose and β-1,3-glucanase activity when the fungus was grown on cell walls of Pythium spp. Chitinase and β-1,3-glucanase were produced by P. nunn in cultures containing cell walls of R. solani and Sclerotium rolfsii but low activity was associated with cell walls of Fusarium oxysporum f. sp. cucumerinum. These hydrolytic enzymes were produced in dual cultures of P. nunn with several Pythium spp., Phytophthora spp., Mucor sp., Rhizopus sp., R. solani and S. rolfsii but were not detected with 10 non-host deuteromycete fungi. This difference may be due to the outer cell wall layer of mucilaginous material associated with these non-host fungi since trypsin- and or KOH-treated hyphae of non-host F. oxysporum f. sp. cucumerinum released $\text{N}$-acetyl-d-glucosamine in the presence of crude enzymatic preparations from P. nunn. This suggests that the potential host range of P. nunn is limited by components on the outer layer of fungal cell walls. Pythium nunn also produced factors inhibiting growth and/or propagule germination of a plant pathogenic Pythium sp. in vitro and in soil.

Phascollinisoflavan, an isoflavonoid phytoalexin produced by Phaseolus vulgaris, was metabolized by Fusarium solani f. sp. phaseoli. A major product of this reaction, designated metabolite-1 (M-1), was isolated from fungal cultures; whereas phaseollinisoflavan could not be recovered from the fungal culture medium 21 h after its addition, the level of M-1 rose steadily between nine and 24 h. Bioassays of radial mycelial growth or of dry matter accumulation involving F. solani f. sp. phaseoli, Pythium myrioylum, Phytophthora cryptogea and/or Rhizoctonia solani indicated that M-1 was less fungitoxic than phaseollinisoflavan. In addition to its production in liquid cultures treated with phaseollinisolflavan, M-1 was also isolated from Fusarium-infected bean tissues; trace amounts of M-1 were detected two days after inoculation and levels rose to at least I mg g⁻¹ dry weight 8 days later. Purification of M-1 involved solvent partitioning, TLC, gel filtration and gas chromatography. Partial characterization indicated that the molecular weight of this metabolite was 342, 18 mass units greater than that of phaseollinisoflavan, suggesting the addition of the elements of water to the phytoalexin.

Pectin lyase, produced by Erwinia chrysanthemi strain EC183 following mitomycin C treatment, was purified to near homogeneity from the culture lysate. The purification procedure involved ultrafiltration, ammonium sulphate fractionation and ion exchange chromatography on DEAE-BioGel A and CM-sepharose. The molecular weight and pI of the pectin lyase were estimated to be 34 500 and 9·45, respectively. The enzyme was most active on Link (98% esterified) pectin and least active on nonesterified pectic acid. Pectin lyase cleaved Link pectin in an endo manner, with optimum pH of 8·3 and optimum temperature of 33 †C. The K_m and V_max of the enzyme with Link pectin as the substrate were estimated to be 7·3±2·0 mg ml⁻¹ and 47·6±10·7 A₂₃₅ min⁻¹, respectively. Antipectin lyase antibody reacted with pectin lyases from 11 of 12 E. chrysanthemi strains and all four Erwinia carotovora subsp. atroseptica strains tested but not with enzymatic preparations from E. carotovora subsp. carotovora, Erwinia milletiae, Erwinia rhapontici or Aspergillus sojae. These data indicate that pectin lyase of E. carotovora subsp. carotovora is immunologically distinct from that produced by E. chrysanthemi.

The biosynthesis of ethylene by grapefruit (Citrus paradisi Macf) cv. “Marsh Seedless” 6 days after inoculation with either an ethylene producing or a non-ethylene-producing isolate of Penicillium digitatum, was studied. Relatively low levels of ethylene were produced by the apparently healthy region, at the edge of rot in fruits infected with either the ethylene producing or the nonethylene-producing isolate of the fungus. Higher levels of production were found in regions of the peel showing infection symptoms, but only in fruit infected with the ethylene-producing isolate. The production of symptoms and the ACC content of the peel of fruits inoculated with the ethylene-producing isolate was similar to that in fruits inoculated with the non-producing isolate.

The use of radiolabelled precursors showed that the ethylene produced by the healthy parts of the fruit originated from methionine, while the ethylene produced by the infected parts of the fruit originated mostly from glutamic acid. Furthermore, ethylene production by the healthy part of the fruit was markedly enhanced by 1-aminocyclopropane-1-carboxylic acid (ACC), and to a lesser extent by CuSO₄, but it was inhibited by aminoethoxyvinylglycine (AVG). In contrast the production of ethylene by infected peel and by P. digitatum in culture was not affected by ACC, but was markedly inhibited by CuSO₄, and, to a lesser extent, by AVG.

These results suggest that ethylene production in the healthy part of the fruit is of plant origin, whereas the markedly enhanced production of ethylene by the P. digitatum-infected regions, is mostly or entirely offungal origin.

An agglutinin against extracellular polysaccharides (EPS) of Xanthomonas campestris pv. citri, the casual agent of citrus canker, was precipitated with 50% saturated ammonium sulphate from an acid (pH 4·0) homogenate of citrus leaf tips. The agglutinin was selectively withdrawn from the crude preparation at pH 4·0 by agglutination with the bacterial EPS, and then separated from the EPS-agglutinin complex on a DEAE cellulose column with a buffer (pH 7·0) containing d-glucosamine, a hapten sugar of the agglutinin. Bio-Gel P-100 gel chromatography of the agglutinin-containing fraction at pH 7·0 and 4·0, yielded a single peak consisting of 96% protein and 4% carbohydrate with estimated molecular masses of 63 000 and 16 000, respectively. The agglutinin ran on SDS-gel electrophoresis as a single band with a molecular mass of 32 000, and on isoelectric focusing gel electrophoresis as a single band with an isoelectric point of pH 4·8.

The purified agglutinin was active with EPS at pH values lower than 6·0, forming a fibrous material. Agglutination was specifically prevented by d-glucosamine. At pH 7·0, the agglutinin associated with EPS via a d-glucosamine-sensitive site without visible precipitation.

A role for the agglutinin in the parasitism of citrus tissue by X. campestris pv. citri is presented.

Twenty-one-day-old cultures of Verticillium dahliae yielded an extract which was toxic to the susceptible cv. of cotton (KH-2) but not to three resistant cvs (H-4, Varahlaxmi and C-Indore). Cuttings from 8-week-old cotton plants of the susceptible cv KH-2 developed typical wilt symptoms 8 h after being treated with the toxin preparation. Loss of electrolytes from leaf and root tissues was significantly greater than that from water treated controls and began immediately after exposure to the toxin preparation. Root tissues of the susceptible cv. also showed a significant loss of β-cyanin within 15 min of exposure to the toxin preparation. Significant losses of K⁺ and Na⁺ occurred from leaf tissues, beginning immediately after treatment, but permeability to Ca²⁺ was not affected.

Ouabain, at 0.2 mm concentration, induced wilting in cuttings of the susceptible cultivar similar to that caused by the toxic preparation, but the resistant cvs were not affected. Ouabain also induced loss of electrolytes from leaf and root tissues of the susceptible cultivar but it had no effect on permeability to β-cyanin.

The results suggest that Vd-toxin sensitive K⁺ and Na⁺ ion transport systems are present in the plasma membranes of the susceptible cv. but not in the resistant cv. These sites are also sensitive to ouabain treatments. Other sites in the plasma membranes of the susceptible cv. determine permeability to β-cyanin, but these are not affected by ouabain treatments. Thus, the Vd-toxin preparation does not induce non-specific changes in the cell membranes leading to uncontrolled losses of all constituents, because permeability to Ca²⁺ ions is not affected.

Purine metabolism in the barley powdery mildew fungus (Erysiphe graminis sp. hordei) differs from that of its host. In the pathogen [³H]adenine and [³H]adenosine were extensively metabolised to inosine and to a lesser extent hypoxanthine but this was not the case in the host. This difference was correllated with the presence of adenosine deaminase in the mildew fungus but not leaves of the barley host. Infection brings together two pathways of purine metabolism with the result that purines are diverted from the host to pathogen. These changes are largely independent of any recognition events involved in determining compatibility/incompatibility. The importance of adenosine deaminase in directing these changes is discussed.