Ascocarp formation and survival and primary inoculum production in Erysiphe (sect. Microsphaera) pulchra in dogwood powdery mildew

Development of powdery mildew Erysiphe (sect. Microsphaera) pulchra in dogwood (Cornus florida) was assessed over a 5‐year period (1996–2000). Variations in the timing of initial infection, disease severity, ascocarp formation, and primary inoculum density were evaluated. Ascocarps formed late in the growing season (September‐November) when relatively low temperatures (< 27°C) persisted for at least 2 weeks, but ascocarp abundance was not influenced by disease severity. Studies conducted in a controlled environment showed that low temperatures triggered ascocarp formation and neither day length nor host plant age affected ascocarp formation. Ascocarps formed within 12–14 days at 18°C/ 10°C (day/night) and 23°C/15°C, but required 25 days at 26°C/18°C; no ascocarps formed at 28°C/ 20°C. Because ascocarps are an important source of primary inoculum for dogwood powdery mildew, ascocarp survival was evaluated in a 2‐year study (1998–2000). 60–80% of mature, dark‐coloured ascocarps survived at ‐10°C and ‐20°C and maintained viable spores for 4 months, but only 4–12% of partially developed, light brown ascocarps survived at ‐10°C and ‐20°C in the first experiment and only 30–40% survived in the second experiment. Immature ascocarp initials (cream‐yellow in colour) withered and disintegrated at all temperatures (24°C/20°C, 4°C, ‐10°C, and ‐20°C). Because ascocarps need time to mature, the timing of ascocarp initiation affects ascocarp maturity and thus winter survival and primary inoculum density. The evaluation of spring inoculum dispersal to spore traps and trap plants in 1999 and 2000 showed that rainfall patterns in early spring influenced primary inoculum and thus the timing of initial infection.     

Inversion of net ecosystem CO2 flux measurements for estimation of canopy PAR absorption

The fractional absorption of photosynthetically active radiation (f_PAR) is frequently a key variable in models describing terrestrial ecosystem–atmosphere interactions, carbon uptake, growth and biogeochemistry. We present a novel approach to the estimation of the fraction of incident photosynthetically active radiation absorbed by the photosynthetic components of a plant canopy (f_Chl). The method uses micrometeorological measurements of CO₂ flux and incident radiation to estimate light response parameters from which canopy structure is deduced. Data from two Ameriflux sites in Oklahoma, a tallgrass prairie site and a wheat site, are used to derive 7‐day moving average estimates of f_Chl during three years (1997–1999). The inverse estimates are compared to long‐term field measurements of PAR absorption. Good correlations are obtained when the field‐measured f_PAR is scaled by an estimate of the green fraction of total leaf area, although the inverse technique tends to be lower in value than the field measurements. The inverse estimates of f_Chl using CO₂ flux measurements are different from measurements of f_PAR that might be made by other, more direct, techniques. However, because the inverse estimates are based on observed canopy CO₂ uptake, they might be considered more biologically relevant than direct measurements that are affected by non‐physiologically active components of the canopy. With the increasing number of eddy covariance sites around the world the technique provides the opportunity to examine seasonal and inter‐annual variation in canopy structure and light harvesting capacity at individual sites. Furthermore, the inverse f_Chl provide a new source of data for development and testing of f_PAR retrieval using remote sensing. New remote sensing algorithms, or adjustments to existing algorithms, might thus become better conditioned to ‘biologically significant’ light absorption than currently possible.     

The First Record and Characterization of Powdery Mildew (Erysiphe pachypodiae sp. nov.) on Pachypodium lamerei (Apocynaceae)

During the summer and autumn of 1999, symptoms of powdery mildew disease were first observed on Pachypodium lamerei in the Czech Republic. White lesions of irregular shape appeared on leaf margins and spread towards the central vein of the leaf, often followed by necrosis of leaf tissue. In the spring of 2000, the sexual stage (cleistothecia) also appeared on infected leaves. Based on the observations of the morphology of its anamorph and teleomorph stage as well as on results from inoculation experiments, the identity and origin of this powdery mildew species are discussed. Based on the pseudoidium anamorph, this fungus may be clearly assigned to Erysiphe emendation, probably to section Uncinula, since the cleistothecial appendages are non‐mycelioid and occassionally circinate at the apex. The fungus is described as a new species Erysiphe pachypodiae sp. nov.     

The effects of detergents DDM and β-OG on the singlet excited state lifetime of the chlorophyll a in cytochrome b6f complex from spinach chloroplasts

The singlet excited state lifetime of the chlorophyll a (Chl a) in cytochrome b ₆ f (Cyt b ₆ f) complex was reported to be shorter than that of free Chl a in methanol, but the value was different for Cyt b ₆ f complexes from different sources (∼200 and ∼600 ps are the two measured results). The present study demonstrated that the singlet excited state lifetime is associated with the detergents n-dodecyl-β-D-maltoside (DDM) and n-octyl-β-D-glucopyranoside (β-OG), but has nothing to do with the different sources of Cyt b ₆ f complexes. Compared with the Cyt b ₆ f dissolved in β-OG, the Cyt b ₆ f in DDM had a lower fluorescence yield, a lower photodegradation rate of Chl a, and a shorter lifetime of Chl a excited state. In short, the singlet excited state lifetime, ∼200 ps, of the Chl a in Cyt b ₆ f complex in DDM is closer to the true in vivo.     

Aggregation and collapse of fungal wall vesicles in hyphal tips: a model for the origin of the Spitzenk?rper

The intracellular origins of polarity and branch initiation in fungi centre upon a localization in the supply of fungal wall constituents to specific regions on the hyphal wall. Polarity is achieved and maintained by accumulating secretory vesicles, prior to incorporation into the wall, in the form of an apical body or Spitzenkörper. However, neither the mechanisms leading to this accumulation nor the initiation of branching, are as yet understood. We propose a mechanism, based on experimental evidence, which considers the mechanical properties of the cytoskeleton in order to explain these phenomena. Cytoskeletal viscoelastic forces are hypothesized to be responsible for biasing vesicles in their motion, and a mathematical model is derived to take these considerations into account. We find that, as a natural consequence of the assumed interactions between vesicles and cytoskeleton, wall vesicles aggregate in a localized region close to the tip apex. These results are used to interpret the origin of the Spitzenkörper. The model also shows that an aggregation peak can collapse and give rise to two new centres of aggregation coexisting near the tip. We interpret this as a mechanism for apical branching, in agreement with published observations. We also investigate the consequences and presumptive role of vesicle–cytoskeleton interactions in the migration of satellite Spitzenkörper. The results of this work strongly suggest that the formation of the Spitzenkörper and the series of dynamical events leading to hyphal branching arise as a consequence of the bias in vesicle motion resulting from interactions with the cytoskeleton.     

Toxicity of methyl ketones from tomato trichomes to Tetranychus urticae Koch

The two methyl ketones, 2-tridecanone and 2-undecanone, are constituents of type VI glandular trichomes of the wild tomato, Lycopersicon hirsutum f. glabratum (PI 134417). They are known to cause mortality in several herbivorous insect species. In this study we investigated the effects of these chemicals on two strains of the two-spotted spider mite, Tetranychus urticae Koch, collected from tomato and cucumber crops in Dutch greenhouses. The two ketones were tested separately, in combination in the ratio found in L. hirsutum f. glabratum and in several other ratios to detect any synergistic interaction between them. We measured both the direct contact and residual toxicity, as well as the viability of the eggs produced by ketone-treated females. The methyl ketones can be considered as quite toxic to spider mites because their LC50 values were comparable to that found for the formulated acaricide amitraz. 2-Tridecanone was slightly more toxic than 2-undecanone but only in the tomato strain. Synergistic effects were not detected. In the bio assays for the residual effects of the two methyl ketones no significant mortality occurred but the mites avoided feeding on the treated surface and the eggs were laid almost exclusively on the untreated area. Furthermore, there was no significant egg viability for most of the treatments and when some egg viability occurred, it was not correlated to the dose of the chemical. Based on published data on the levels of methyl ketones in the trichome tips of wild and cultivated tomato (L. hirsutum f. glabratum and Lycopersicon esculentum Mill, respectively) it was calculated how many contacts are required to reach the LD50 (ng a.i. per mite). It appeared that on wild tomato just a few contacts suffice, but many are needed on cultivated tomato. Nevertheless, it seems justified to conclude that even in cultivated tomato, 2-tridecanone represents a naturally occurring acaricide. The cucumber and tomato strains of the two-spotted spider mite did not differ in their response to the methyl ketones. Because this is surprising, given the selection pressures on their respective host plants, we formulated two hypotheses, one explaining the absence of a difference due to cross-resistance and another one proposing that bioaccumulation obscures the underlying differences between the strains. This revised version was published online in November 2006 with corrections to the Cover Date.     

The cytochrome b6f complex. Novel aspects

The Cyt b ₆ f complex from plant chloroplasts, the green alga Chlamydomonas reinhardtii , and the thermophilic cyanobacterium, Mastigocladus laminosus , can be isolated in a highly active state, in which it is dimeric and contains one bound chlorophyll a molecule per monomeric unit. The latter feature is a distinguishing trait compared to the b ₆ f complex of bacterial photosynthesis and the respiratory chain. In contrast to the trans-membrane domains of the b ₆ f complex, and of most other integral membrane proteins, which are characterized by an a -helical structure, the p -side peripheral domains, consisting of Cyt f and the Rieske protein, have a predominantly β-strand secondary structure motif. One consequence of this motif is an extension of these polypeptides from the membrane surface. For example, the length of Cyt f is 75 Å. The heme Fe is 45 Å from the α-carbon of Arg250 at the membrane bilayer interface and, even though Cyt f may be tilted relative to the membrane plane, the heme electron transfer reactions are carried out far from the membrane surface. The presence of an internal 5 water chain, which has the properties of a proton wire, with one water H-bonded to the histidine-25 heme ligand, also suggests that the pathway of long distance H⁺ translocation traverses the extended p -side protein domain of the b ₆ f complex. A mechanism of H⁺ transfer in the chain that is coupled to the redox state of the heme, in which a proton is transferred into the chain to compensate the extra electron in the ferro-heme, is proposed.     

抑制Pm8抗性表达的遗传学研究及Pm8抗性抑制基因的染色体定位

对不同１ＢＬ·１ＲＳ易位系品种中Ｐｍ８抗性表达的差异进行了遗传学及生化研究。结果表明,小麦抗白粉病基因Ｐｍ８位于１ＲＳ染色体上,但在一些１ＢＬ·１ＲＳ易位系品种中,Ｐｍ８的抗性表达受一对位于小麦染色体组中的显性抑制基因所控制。生化研究结果显示,在种子蛋白ＳＤＳ－ＰＡＧＥ电泳图谱上的低分子量区域,有一条醇溶蛋白带（称为ＳＢ带）与Ｐｍ８的抗性表达紧密相关。所有ＩＢＬ·ＩＲＳ感白粉病基因型都含有ＳＢ带。利用ＳＢ蛋白带做生化标记,可以准确预测１ＢＬ·１ＲＳ易位系品种对白粉病的抗感性。通过时１ＢＬ·１ＲＳ易位系品种格蓝森８１单体１Ａ种质的综合分析,进一步将Ｐｍ８抗性抑制基因定位于其相应的部分同源染色体１Ａｓ上。这一结果可能预示着小麦部分同源染色体之间的某些内在联系。     

Polyamines in discrete regions of barley leaves infected with the powdery mildew fungus, Erysiphe graminis

The concentrations of putrescine, spermidine and spermine and the activities of arginine decarboxylase (ADC; EC 4.1.1.19) and ornithine decarboxylase (ODC: EC 4.1.1.17) were determined in discrete regions of barley leaves ( Hordeum vulgare L. cv. Golden Promise) infected with the powdery mildew fungus ( Erysiphe graminis f.sp. hordei Marchal). Polyamine concentrations and the activities of both enzymes were always greatest within the region surrounding the fungal pustule, with the lowest values always being found in the region furthest away from the pustule. Although the concentrations of the three amines and ADC and ODC activities within the fungal pustule were always less than values from the zone surrounding the pustule, these differences were never significant. Polyamine concentrations and ODC activity were not significantly reduced, and ADC activity remained unchanged in mildewed leaves with all surface fungal growth removed. It would appear therefore that not only does most of the increase in amines and ODC activity reside in the leaf itself, but that very little of this increase is due to fungal growth and sporulation. Furthermore, it seems possible that the increase in polyamines in mildewed barley could be involved in 'green-island' formation, where regions around mildew pustules remain green and physiologically active while the rest of the leaf senesces.     

Inhibition of nitrogenase by nitrite and nitric oxide in Rhodopseudomonas sphaeroides f. sp. denitrificans

In cells of Rhodopseudomonas sphaeroides f. sp. denitrificans nitrite and nitric oxide, the products of denitrification, inhibit activity of nitrogenase enzyme.Ferredoxin-linked CO₂ fixation, with H₂ as a reductant, was also inhibited by nitrite and NO in denitrifying cells.EPR spectroscopy of cell preparations treated with NO showed that it reacts with non-haem iron-sulphur proteins to form iron-nitrosyl complexes. Nitrite also reacts with these iron-sulphur proteins, but the formation of ironnitrosyl complexes was dependent on the presence of dithionite. Since nitrite is reduced to NO by dithionite it is likely that nitrogenase and CO₂ fixation reactions are inhibited not only by nitrite itself, but also by nitric oxide.Abbreviation DPPH 1,1-diphenyl-2-picrylhydrazyl