Chloroplastic oxidative burst induced by tenuazonic acid, a natural photosynthesis inhibitor, triggers cell necrosis in Eupatorium adenophorum Spreng

Tenuazonic acid (TeA), a nonhost-specific phytotoxin produced by Alternaria alternata, was determined to be a novel natural photosynthesis inhibitor owning several action sites in chloroplasts. To further elucidate the mode of its action, studies were conducted to assess the production and involvement of reactive oxygen species (ROS) in the toxic activity of TeA. A series of experiments indicated that TeA treatment can induce chloroplast-derived ROS generation including not only ¹O₂ but also O₂⁻, H₂O₂ and OH in Eupatorium adenophorum mesophyll cells, resulting from electron leakage and charge recombination in PSII as well as thylakoid overenergization due to inhibition of the PSII electron transport beyond Q_A and the reduction of end acceptors on the PSI acceptor side and chloroplast ATPase activity. The initial production of TeA-induced ROS was restricted to chloroplasts and accompanied with a certain degree of chloroplast damage. Subsequently, abundant ROS were quickly dispersed throughout whole cell and cellular compartments, causing a series of irreversible cellular harm such as chlorophyll breakdown, lipid peroxidation, plasma membrane rupture, chromatin condensation, DNA cleavage, and organelle disintegration, and finally resulting in rapid cell destruction and leaf necrosis. These results show that TeA causing cell necrosis of host-plants is a result of direct oxidative damage from chloroplast-mediated ROS eruption.     

Phomapyrones from blackleg causing phytopathogenic fungi: isolation, structure determination, biosyntheses and biological activity

The isolation and structure determination of phomapyrones D-G, three 2-pyrones and a coumarin, from a group of isolates of the fungal pathogen Leptosphaeria maculans (Desm.) Ces. et de Not., asexual stage Phoma lingam (Tode ex Fr.) Desm, is reported. As well, phomenin B, infectopyrone, and polanrazines B and C were also obtained for the first time from these isolates. In addition, based on results of incorporations of 13C-labeled acetate and malonate, and deuterated methionine, a polyketide pathway is proposed for the biosyntheses of phomapyrones.     

Biosynthesis of photodynamically active rubellins and structure elucidation of new anthraquinone derivatives produced by Ramularia collo-cygni

Here we present the first isolation of the anthrachinone derivative rubellin A out of mycelium and culture filtrate of Ramularia collo-cygni. Furthermore, two compounds, rubellin E and 14-dehydro rubellin D were isolated and their structures elucidated. In comparison to the other rubellins, rubellin A shows increased photodynamic oxygen activation. By incorporating both [1-(13)C]-acetate and [2-(13)C]-acetate into the rubellins, we showed that such anthraquinone derivatives were biosynthesised via the polyketide pathway. The labelling pattern after being fed [U-(13)C(6)]-glucose proved the existence of fungal folding mode of the poly-beta-keto chain. The ability to produce rubellins is not limited to R. collo-cygni in the anamorph genus Ramularia.     

Spectral analysis of a series of partially protected and deprotected tetrapeptides, analogues of AS-I phytotoxin

Summary A series of six tetrapeptides, analogues of AS-I phytotoxin, pathogenic to sunflower, have been synthesized either in solution and/or by solid phase methods and have been tested for phytotoxic activity in various plants and cytotoxic activity in three cancer cell lines. These peptides were identified as model compounds by fast atom bombardment (FAB), plasma desorption (PD), electrospray ionization (ESI) mass spectrometry and by¹H,¹H-¹H,¹³C and¹H-¹³C NMR. The data presented show that in protected tetrapeptides the molecular ion was easily identified whereas some difficulties appeared with the fully deprotected peptides. NMR spectra are given.     

The phytopathogenic fungus Alternaria brassicicola: Phytotoxin production and phytoalexin elicitation

The metabolites and phytotoxins produced by the phytopathogenic fungus Alternaria brassicicola (Schwein.) Wiltshire, as well as the phytoalexins induced in host plants, were investigated. Brassicicolin A emerged as the most selective phytotoxic metabolite produced in liquid cultures of A. brassicicola and spirobrassinin as the major phytoalexin produced in infected leaves of Brassica juncea (whole plants). In detached infected leaves of B. juncea, the main component was N′-acetyl-3-indolylmethanamine, the product of detoxification of the phytoalexin brassinin by A. brassicicola. In addition, the structure elucidation of three hitherto unknown metabolites having a fusicoccane skeleton was carried out and the antifungal activity of several plant defenses against A. brassicicola was determined.     

Allelopathy as a competitive strategy in persistent thickets of Lantana camara L. in three Australian forest communities

Field experiments were established to assess possible allelopathic suppression by Lantana camara L. of two indigenous tree species. The design allowed comparison of allelopathic effects with density-dependent resource competition effects. Fire and its role in competitive interactions was included as an experimental treatment. Allelopathic responses were measured in L. camara thickets by germinating and growing Alectryon subcinereus (A. Gray) Radlk. in dry rainforest ecotones (Macleay River) or Cryptocarya rigida (Meissner) in warm temperate rainforest and wet sclerophyll forest (Lake Macquarie) at 10, 20 and 30 seedlings m^-2, where L. camara was either physically removed (LR), burnt (LB), or cut and left in place (LT). Germination for both species increased significantly by completely removing L. camara (LR) whereas burning (LB) was significant only for C. rigida. Seedling growth for both species was negatively related to increasing density when all L. camara was removed (LR) but was positively related in the other two treatments (LB and LT). C. rigida seedling biomass increased 47.4% (1.75%2.58 g) and 68.6% (1.98%2.95 g) with increasing seedling density for LT and LB respectively and decreased 23.2% (2.93–2.25 g) for LR. A. subcinereus seedling biomass increased 29.7% (1.95–2.53 g) and 34.7% (2.25–3.03 g) with increasing seedling density for LT and LB respectively and decreased 27.9% (3.30–2.38 g) for LR. Phytotoxin dilution effects were inferred in LT and LB rather than density-dependent intraspecific competition, whereas the reverse was true for LR. Seedling biomass for C. rigida resulting from potential phytotoxin dilution at high seedling density was not significantly different from the response of LR at low seedling density but, for A. subcinereus, the phytotoxin dilution response was significantly less than LR at low seedling density. Moderately intense fire (LB) was not significantly different from the LT treatment at both locations, emphasising that moderate to low intensity fires should not be used to control existing invasions of L. camara. Competitive strategies for invasive populations are identified that may modify succession following disturbance, thereby allowing thicket formation and long-term persistence to affect community dynamics. Such strategies need to be recognised in managing natural communities, particularly for biodiversity conservation.Nomenclature: Harden (1990).     

Specific binding of a Verticillium dahliae phytotoxin to protoplasts of cotton, Gossypium hirsutum

Summary The occurrence of specific, high-affinity binding sites for a protein-lipopolysaccharide (PLP) phytotoxin purified from culture filtrates of a virulent Vertidllium dahliae isolate has been demonstrated in cotton protoplasts. Binding of the ¹²⁵I-radiolabelled PLP-complex to protoplasts from cotyledon tissue was saturable and with an affinity (K_d = 17.3 nM) comparable with the concentration required for biological activity. A single class of binding site, accessible at the surface of the intact protoplasts, was found and the maximal number of binding sites were estimated as 2.41 × 10^–16 moles per protoplast. The binding affinity to protoplasts proved near identical to that found with purified plasma membrane fractions from roots. When cultivars exhibiting resistance or susceptibility towards the pathogen were compared, no significant differences were found in the affinity of binding, but five times as many binding sites per protoplast and sixteen times as many binding sites per mg membrane protein were found in the resistant cultivar.Abbreviations PLP protein-lipopolysaccharide - k_d dissociation constant - B_max maximal number of binding sites - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol     

Suppression of mitogen-induced lymphocyte proliferation by syringomycin-E

Abstract Syringomycin E (SR-E) is low molecular weight bacterial lipodepsipeptide with antifungal properties. Owing to immunosuppressive activities of such compounds as cyclosporine, FK506 and rapamycin, we studied the effect of SR-E on proliferation of human blood lymphocytes in vitro. SR-E, by itself, had no effect but the mitogen-induced lymphocyte proliferation was significantly suppressed. The suppressive effect was more pronounced with pokeweed mitogen (PWM) as compared to phytohemagglutinin (PHA) or monoclonal antibody to CD3 (anti-CD3). Since these mitogens induce cellular immunity (T cell-dependent), SR-E may potentially be a novel immunosuppressive compound.     

The selective allelopathic interaction of a pasture-forest intercropping in Taiwan

Summary An allelopathic interaction of a pasture-forest intercropping system was evaluated by experiments conducted in field and by laboratory assays. A study site was situated in the farm of Hoshe Forestry Experiment Station at Nantou County, Taiwan. After deforestation of Chinese fir (Cunninghamia lanceolata), a split plot design of 4 treatments, namely litter removed, litter removed and kikuyu grass (Pennisetum clandestinum) planted, litter left, and litter left and grass planted, was composed. Field meaurements showed that the fir litter left on the ground did not significantly inhibit the growth of weeds, kikuyu grass, and fir seedlings in the first four months following deforestation, while kikuyu grass significantly suppressed the growth of weeds longer than four months but did not reduce growth of fir seedlings. The aqueous extracts of fresh fir leaves, fir litter, and kikuyu leaves were bioassayed by using lettuce and rice seeds and stolon cuttings ofBrachiaria mutica. Bioassays showed that fresh fir leaves produced significant phytotoxicity while fir litter and kikuyu grass gave limited toxicity. Nine phytotoxic phenolics and many unidentified flavonoids were found in the leaf and litter of Chinese fir and kikuyu leaves. A good correlation between the degree of phytotoxicity and phytotoxins was obtained, indicating an allelopathy was involved. This finding suggests that allelopathy may contribute benefits in the intercropping system to reduce the need for herbicides and to lessen the labor cost for weed control. Paper No. 304 of the Scientific Journal Series of the Institute of Botany, Academia Sinica, Taipei, Taiwan, Republic of China. This study was supported in part by grants of Academia Sinica, Taipei, and Council of Agriculture, Executive Yuan of the Republic of China.