Resting Spore Formation Research Articles

The role of diatom resting spores in pelagic–benthic coupling in the Southern Ocean

Abstract. Natural iron fertilization downstream of Southern Ocean island plateaus supports large phytoplankton blooms and promotes carbon export from the mixed layer. In addition to sequestering atmospheric CO2, the biological carbon pump also supplies organic matter (OM) to deep-ocean ecosystems. Although the total flux of OM arriving at the seafloor sets the energy input to the system, the chemical nature of OM is also of significance. However, a quantitative framework linking ecological flux vectors to OM composition is currently lacking. In the present study we report the lipid composition of export fluxes collected by five moored sediment traps deployed in contrasting productivity regimes of Southern Ocean island systems (Kerguelen, Crozet and South Georgia) and compile them with quantitative data on diatom and faecal pellet fluxes. At the three naturally iron-fertilized sites, the relative contribution of labile lipids (mono- and polyunsaturated fatty acids, unsaturated fatty alcohols) is 2–4 times higher than at low productivity sites. There is a strong attenuation of labile components as a function of depth, irrespective of productivity. The three island systems also display regional characteristics in lipid export. An enrichment of zooplankton dietary sterols, such as C27Δ5, at South Georgia is consistent with high zooplankton and krill biomass in the region and the importance of faecal pellets to particulate organic carbon (POC) flux. There is a strong association of diatom resting spore fluxes that dominate productive flux regimes with energy-rich unsaturated fatty acids. At the Kerguelen Plateau we provide a statistical framework to link seasonal variation in ecological flux vectors and lipid composition over a complete annual cycle. Our analyses demonstrate that ecological processes in the upper ocean, e.g. resting spore formation and grazing, not only impact the magnitude and stoichiometry of the Southern Ocean biological pump, but also regulate the composition of exported OM and the nature of pelagic–benthic coupling.

Impact of CO2 on the elemental composition of the particulate and dissolved organic matter of marine diatoms emerged after nitrate depletion

AbstractAlthough the dissolved inorganic carbon concentration, pH, and nutrient regimes of seawater dramatically change in coastal regions, the synergistic effects of changes in the CO2 and nutrient levels on the elemental dynamics of the particulate and dissolved organic matters (DOMs) produced by diatoms are rarely investigated. Here, we investigated the impacts of four different CO2 levels (180, 380, 600, and 1000 μatm partial pressure of CO2 : pCO2) on the allocation of carbon, nitrogen, phosphorus, and silicon between the particulate matter (PM) and DOM in two cosmopolitan coastal diatoms, Chaetoceros affinis and Ditylum brightwellii, under nutrient‐replete and nitrate‐depleted conditions. Under nutrient‐replete conditions, the specific growth rates of both species were positively correlated with pCO2 levels. The elemental compositions of the exponentially growing diatoms were stable under the different pCO2 conditions. After nitrate depletion, the particulate organic carbon to particulate nitrogen ratio and biogenic silica content per unit biomass in both species were positively correlated with the pCO2 value. Factors affecting the pCO2 dependent change in elemental composition were the variations in the partitioning of organic carbon between PM and DOM in C. affinis, and the physiological uncoupling of intracellular carbon and nitrogen and the intracellular silicon and nitrogen, as well as resting spore formation in D. brightwellii. Under high‐CO2 conditions, the faster growth rates of both diatom species could lead to their dominance in a phytoplankton community; their blooms could modify the first‐order processes in the biogeochemical cycling of bioelements after nitrate depletion.

Diversity, temporal distribution and physiology of the centric diatom Leptocylindrus Cleve (Bacillariophyta) from a southern hemisphere upwelling system

The marine diatom Leptocylindrus is a major component of phytoplankton blooms in coastal ecosystems and upwelling regions worldwide, however, little is known about this genus in the southern hemisphere. Whilst Leptocylindrus danicus has been reported from south-eastern (SE) Australia since the 1930s, there has been neither unequivocal species identification nor focused examination of the temporal abundance of Leptocylindrus in this region. Such investigations are crucial in the context of climate change and the strengthening of the East Australian Current, which is expected to result in alterations to the seasonal abundance and distribution of Leptocylindrus along the east Australian coast. Thus we also describe the temporal distribution of Leptocylindrus based on 50 years of records, revealing that this diatom is a key component of the seasonal phytoplankton cycle, with greatest abundance in the austral spring and summer. Using light and transmission electron microscopy and molecular phylogenetics based on the nuclear-encoded ITS1–5.8S–ITS2 rDNA region, our study unambiguously revealed three species, L. danicus, Leptocylindrus convexus and Leptocylindrus aporus from 34 clonal isolates from SE Australia, with the majority (82%) of strains identified as L. danicus. Furthermore, we investigated the growth, auxospore and resting spore formation of the most commonly occurring species, L. danicus, under four temperature and irradiance scenarios. The diatom reached maximum growth rates (µMax, 1.71 divisions day−1) under relatively high temperatures (25°C) and light conditions (100 µmol photons m−2 s−1) between days 2 and 7 of the experiment. When temperature and light regimes were reduced (18°C, 50 µmol photons m−2 s−1) auxospores and resting spores were formed. The rapid growth rate and potential of L. danicus to form auxospores are important survival mechanisms in coastal upwelling systems and likely to result in the continued success of this species in Eastern Australia. The ecological, physiological and evolutionary response of this significant diatom group to further ocean warming should be the focus of future investigations.

温度对蚜科专化菌暗孢耳霉休眠孢子形成的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 温度对蚜科专化菌暗孢耳霉休眠孢子形成的影响 DOI: 10.5846/stxb201401070048 作者: 作者单位: 浙江农林大学 林业与生物技术学院,浙江农林大学 林业与生物技术学院,浙江农林大学 林业与生物技术学院,浙江农林大学 林业与生物技术学院,浙江农林大学 林业与生物技术学院 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金(31300550); 国际科学基金(IFS, C/5146-1) The effect of temperature on in vivo resting spore formation of the aphid-specific pathogen Conidiobolus obscurus Author: Affiliation: Zhejiang Agricultural and Forestry University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:为探究虫霉休眠孢子形成的关键影响因素,通过孢子浴接种和多浓度生物测定实验,观察蚜科专化菌暗孢耳霉(Conidiobolus obscurus)在5个温度处理(10,15,20,24,28 ℃)和3个接种浓度梯度下感染寄主桃蚜(Myzus persicae)的情况,并观测不同处理下蚜尸内产休眠孢子的比例。暗孢耳霉对桃蚜的毒力随温度和接种浓度升高而提高。同时,感病致死的蚜尸镜检结果表明:温度显著影响休眠孢子形成,温度越高,形成几率越大;接种浓度的影响次之,在15-24 ℃间,形成几率随接种浓度提高而增大。这一现象可能的解释:高温环境将使寄主种群增长停滞或消退,暗孢耳霉通过感知环境温度情况,倾向于在较高温时于寄主体内形成休眠孢子来规避接下来可能出现的寄主匮乏期。 Abstract:Fungi belonging to the order of Entomophthorales are found to induce host epizootics under natural conditions, restraining many agroforestry pest populations below economic damage level. For decades, researchers have focused on exploring these valuable resources with respect to their application in pest biocontrol programs, but the process has proven to be difficult. A poor understanding of the fungal epizootic mechanism and survival strategies in the nature, including the mechanism of in vivo resting spore formation, is one of the main obstacles hindering this effort. Entomophthorales can actively eject numerous conidia to infect nearby hosts, thus transmitting within the host populations. When the host population decreases dramatically, the fungi tend to form thick-walled spores (resting spores, RS) within the bodies of cadavers. Once the conditions improve, RS germinate to restart the infection cycle and regulate the host population. Thus, the RS formation is an important step for fungal survival and the subsequent initiation of the epizootic. Previous reports indicated that many biotic and abiotic factors influence in vivo RS formation, including temperature, humidity, light duration, host age, nutritive conditions, and inoculum levels. However, no consistent conclusion has been formed regarding the vital factors, as contradictory results have been reported for different pathogen-host systems. Temperature and inoculum levels were recently found to exert dominant effects on the formation of Pandora nouryi RS in Myzus persicae inhabiting fields in cold climates. To investigate the main factors influencing in vivo RS formation of the aphid-specific pathogen Conidiobolus obscurus, we performed a series of multi-concentration bioassays of C. obscurus against the host M. persicae under five different temperature treatments (10 ℃, 15 ℃, 20 ℃, 24 ℃, and 28 ℃). The results of the bioassays showed that aphid mortality increased with increasing conidial concentrations used to inoculate the aphids (inoculum level) under each temperature condition. The data were well fitted to the time-concentration-mortality model, and the lowest calculated median lethal concentration (LC50) with 95% confidence intervals was estimated to be 13.6 (11-16.8) conidia/mm2 on day 4 at 28 ℃. Within the crushed cadaver bodies, proportion of the RS formation significantly increased with increased temperature, reaching a peak of 0.66 at 28 ℃. The proportion also increased with inoculum levels at temperatures from 15-24 ℃. Thus, C. obscurus tends to form RS more readily at higher temperatures and inoculum levels. We explain this phenomenon as follows: the rise of temperature to high levels signals the upcoming variation in the host population to C. obscurus, because high temperatures always cause host diapause and population collapse in the fields. For survival in the successive period of host shortage, the fungi tend to form thick-walled RS in host cadavers. Further observation of other entomophthoralean fungi and their hosts is required to verify this prediction. The present results also showed that C. obscurus possesses high infectivity under higher temperature conditions and is thus suitable for aphid control in thefields. 参考文献 相似文献 引证文献

Cortical Colonization by Plasmodiophora brassicae in Susceptible and Resistant Cabbage Cultivars

Three cultivars of cabbage (Brassica oleracea L. var. capitata) that differed in reaction to clubroot caused by Plasmodiophora brassicae were identifiedinfield trials. Cultivar Broncowas susceptible to pathotype 6 (all plants severely clubbed), 'Kilaherb' was resistant (no plants with symptoms), and 'B-2819' was intermediate (small clubs). The development of P. brassicae in the roots of these cultivars was then assessed under controlled conditions at 28 days after inoculation using root sectioning, staining with methy- lene blue, and bright field microscopy. Plasmodia of P. brassicae were present in the root tissue of each cultivar. There was extensive cortical colonization (26 %) and some resting spore formation in 'Bronco', and lower colonization (9 %) and only trace levels of resting spore formation in 'B-2819'. In the clubroot- resistant 'Kilaherb', root colonization was similar (7%)totheintermediate'B-2819',but norestingspores formed and no clubbing symptoms developed. This pattern of response differed substantially from that ob- served previously in commercial cultivars of spring canola (B.napus), and may indicate that the mechanism

A new oomycete species parasitic in nematodes, Chlamydomyzium dictyuchoides sp. nov.: Developmental biology and phylogenetic studies

The genus Chlamydomyzium is a little studied holocarpic oomycete parasite of nematodes of uncertain phylogenetic and taxonomic position. A new holocarpic species, Chlamydomyzium dictyuchoides, is described which has usually refractile cytoplasm and a dictyuchoid pattern of spore release. This new species infects bacteriotrophic rhabditid nematodes and was isolated from diverse geographical locations. Infection was initiated by zoospore encystment on the host surface and direct penetration of the cuticle. A sparsely branched, constricted, refractile thallus was formed which eventually occupied almost the entire host body cavity, often accompanied by complete dissolution of the host cuticle. Walled primary cysts formed throughout the thallus and each cyst released a single zoospore via an individual exit papillum, leaving a characteristic dictyuchoid wall net behind. At later stages of infection some thalli formed thick-walled stellate resting spores in uniseriate rows. Resting spore formation appeared to be parthenogenetic and was not accompanied by the formation of antheridial compartments. These spores had ooplast-like vacuoles and thick multi-layered walls, both of which suggest they were oospores. The maximum likelihood tree of sequences of the small ribosomal subunit (SSU) gene placed this new isolate in a clade before the main saprolegnialean and peronosporalean lines diverge. A second undescribed Chlamydomyzium sp., which has direct spore release forms a paraphyletic clade, close to C. dictyuchoides and Sapromyces. The fine structure of other documented Chlamydomyzium species was compared, including an undescribed (but sequenced) isolate, SL02, from Japan, Chlamydomyzium anomalum and Chlamydomyzium oviparasiticum. Chlamydomyzium as currently constituted is a paraphyletic genus that is part of a group of phylogenetically problematic early diverging clades that lie close to both the Leptomitales and Rhipidiales.

Effects of Cryopreservation at -80oC on the Formulation and Pathogenicity of the Obligate Aphid Pathogen Pandora nouryi

Cryopreservation at -80 degrees C is an alternative to liquid nitrogen storage for Entomophthorales. However, detailed studies about its effects on fungal pathogenicity and formulation are very limited. In the present study, the obligate aphid pathogen Pandora nouryi was formulated as mycelia grown on millet-gel granules after preservation as primary spores at -80 degrees C for 3-18 months, although its ability to produce infectious conidia gradually diminished. The sporulation capacity of this granular formulation was reduced to 18.5 x 10(4) conidia/mg after 18 months of storage, which was still higher than that of mycotized aphids. The half-decline time of sporulation capacity was computed as 13.6 months. The infectivity to the green peach aphid Myzus persicae had no significant decline in 12 months. The ability to yield resting spores within host carcasses remained unchanged, and the probability of resting spore formation increased with the conidial concentrations that infect aphids. Therefore, cryopreservation at -80 degrees C exerted a marginal impact on formulation and pathogenicity of P. nouryi and can substitute for costly liquid nitrogen storage in routine laboratory studies. The potential of the formulation in aphid biocontrol can be maintained although there is a risk of losing fungal sporulation ability in long-term preservation.

Chaetoceros rotosporus sp. nov. (Bacillariophyceae), a species with unusual resting spore formation

Li Y., Lundholm N. and Moestrup Ø. 2013. Chaetoceros rotosporus sp. nov. (Bacillariophyceae), a species with unusual resting spore formation. Phycologia 52: 600–608. DOI: 10.2216/13-168.1With more than 400 species, the diatom genus Chaetoceros Ehrenberg is one of the most species-rich genera of marine planktonic diatoms. Although many Chaetoceros taxa are considered to be cosmopolitan, research has mainly focused on polar and temperate areas. Studies on material from tropical waters suggested that a high and undescribed species diversity of Chaetoceros occur here. On the basis of plankton samples collected in Daya Bay, South China Sea, a new species, C. rotosporus sp. nov., is described. Vegetative cells and resting spores were examined by light microscopy as well as transmission and scanning electron microscopy. The most variable part of the nuclear large-subunit ribosomal DNA was sequenced for information on phylogenetic relationships. Characteristic traits of C. rotosporus were a 90° rotation of the resting spore during spore maturation and a nearly rectangular silica wing that extended around the girdle of the immature resting spore. Neither character has been observed in previously described Chaetoceros species. Other distinguishing characters were the very large apertures and the single chloroplast, which formed a shallow bowl with four arms within each cell and appeared H-shaped in broad girdle view. Chaetoceros rotosporus sp. nov. belongs to the subgenus Hyalochaete Gran, but it could not be placed in any of the traditional sections of Chaetoceros, indicating a need for revision of Chaetoceros using a combination of morphological and molecular data.

Abiotic and Biotic Factors Affecting Resting Spore Formation in the Mite PathogenNeozygites floridana

Neozygites floridana is an obligate mite pathogenic fungus in the Entomophthoromycota. It has been suggested that resting spores of this fungus are produced as a strategy to survive adverse conditions. In the present study, possible mechanisms involved in the regulation of resting spore formation were investigated in the hosts Tetranychus urticae and Tetranychus evansi. Abiotic and biotic factors mimicking conditions that we, based on earlier field studies, thought might induce resting spores in temperate and tropical regions were tested with isolates from Norway and Brazil. A total of 42 combinations of conditions were tested, but only one induced the formation of a high number of resting spores in only one isolate. The Brazilian isolate ESALQ1420 produced a large number of resting spores (51.5%) in T. urticae at a temperature of 11°C, photoperiod of 10L:14D, and light intensity of 42–46 (μmol m−2 s−1) on nonsenescent plants (nondiapausing females). Resting spores of the Brazilian N. floridana isolate ESALQ1421 were found at very low levels (up to 1.0%). Small percentages of T. urticae with resting spores (0–5.0%) were found for the Norwegian isolate NCRI271/04 under the conditions tested. The percentages of resting spores found for the Norwegian isolate in our laboratory studies are similar to the prevalence reported in earlier field studies.

The role of temperature on in vivo resting spore formation of the aphid-specific pathogen Pandora nouryi (Zygomycota: Entomophthorales) under winter field conditions

Resting spore formation of Pandora nouryi within the host Myzus persicae was investigated in an open cabbage field during two winters: 2007–2008 and 2010–2011. Multivariate correlation analysis of field data revealed that weekly mean temperature was the main abiotic factor governing the probability of resting spore formation in vivo, followed by host density and mean day length, while the effect of season was insignificant. Moreover, the temperature effect on the proportions of resting spore producing cadavers in those aphids infected with P. nouryi was well fit by a logistic equation, and high proportions (0.37–0.95) were found on most of weekly sampling occasions with mean temperatures of less than 10°C. Thus, these resultant analyses support the hypothesis that temperature is the most important abiotic factor controlling P. nouryi resting spore formation in M. persicae under field situations.

First report of Pandora neoaphidis resting spore formation in vivo in aphid hosts

The entomopathogenic fungus Pandora neoaphidis is a recognized pathogen of aphids, causes natural epizootics in aphid populations, and interacts and competes with aphid predators and parasitoids. Survival of entomophthoralean fungi in periods of unsuitable weather conditions or lack of appropriate host insects is accomplished mainly by thick-walled resting spores (zygospores or azygospores). However, resting spores are not known for some entomophthoralean species such as P. neoaphidis. Several hypotheses of P. neoaphidis winter survival can be found in the literature but so far these hypotheses do not include the presence of resting spores. Resting spores were found in an aphid population where P. neoaphidis was the only entomophthoralean fungus observed during surveys conducted in organic horticultural crops in greenhouses and open fields in Buenos Aires province, Argentina. This study sought to use molecular methods to confirm that these resting spores were, in fact, those of P. neoaphidis while further documenting and characterizing these resting spores that were produced in vivo in aphid hosts. The double-walled resting spores were characterized using light and transmission electron microscopy. The Argentinean resting spores clustered together with P. neoaphidis isolates with bootstrap values above 98 % in the small subunit ribosomal RNA (SSU rRNA) sequence analysis and with bootstrap values above 99 % the Internal Transcribed Spacer (ITS) II region sequence analysis. This study is the first gene-based confirmation from either infected hosts or cultures that P. neoaphidis is able to produce resting spores.

AN ENDOPHYTIC FUNGUS INDUCES TOLERANCE AGAINST THE CLUBROOT PATHOGEN PLASMODIOPHORA BRASSICAE IN ARABIDOPSIS THALIANA AND BRASSICA RAPA ROOTS

ISHS V International Symposium on Brassicas and XVI International Crucifer Genetics Workshop, Brassica 2008 AN ENDOPHYTIC FUNGUS INDUCES TOLERANCE AGAINST THE CLUBROOT PATHOGEN PLASMODIOPHORA BRASSICAE IN ARABIDOPSIS THALIANA AND BRASSICA RAPA ROOTS

Improved sporulation of alginate pellets entrapping Pandora nouryi and millet powder and their potential to induce an aphid epizootic in field cages after release

Pandora nouryi is an obligate aphid fungus (Entomophthorales) forming resting spores in vivo but little is known about its epizootic potential. We manufactured small uniform alginate pellets (∼1 mg/dry pellet) entrapping ∼1% P. nouryi mycelia and ∼5% millet powder to facilitate sporulation and evaluated their epizootic potential in Myzus persicae population developing on cabbage plants inside meshed field cages (2 × 2 × 2 m). The pellets sporulated 8.5-fold more (28.4 × 10 4 conidia/pellet) than the millet-free pellets at 20 °C, and 3.6–13.6 fold more and much longer than mycosis-killed adult aphids at 4–28 °C under saturated humidity. These pellets, irrespective of drying or not, retained sporulation capability after 120-day storage at 6 °C. The releases of 5–20 pellets/plant in different field cages resulted in successful colonization of P. nouryi in aphid cohorts during 4-month field trial from late autumn through spring. Aphid mortalities attributed to P. nouryi mycosis were consistently detectable but very low in all cages, irrespective of the timing of a release and the density of released pellets, until ⩽13% mortalities occurred in late spring. Of those aphids infected with P. nouryi, high proportions (32–95%) were found producing resting spores on most of weekly sampling occasions. The low-level prevalence of P. nouryi in the released cages was thus attributed to the frequent resting spore formation in vivo, which may interrupt the infection cycle. The alginate pellet formulation with millet powder is recommended for improving preparations of fungal pathogens without resting spores, such as Pandora neoaphidis, for biological control of aphids.

Increase in Si:N drawdown ratio due to resting spore formation by spring bloom-forming diatoms under Fe- and N-limited conditions in the Oyashio region

Resting spore formation and Si:N drawdown ratios were investigated under iron (Fe)- and nitrogen (N)-limited conditions using a unialgal culture of Thalassiosira nordenskioeldii and natural phytoplankton assemblages during the spring bloom in the Oyashio region. In the unialgal culture of T. nordenskioeldii, 20% and 100% of the cells formed resting spores under Fe- and N-limited conditions, respectively. The Si:N drawdown ratios were 2- and 14-fold higher in Fe- and N-limited conditions, respectively, compared to Fe- and N-sufficient conditions. At the start of the natural phytoplankton incubation, 18 among 47 identified diatom species were known resting spore-forming species. Approximately 15 common diatom species formed resting spores under Fe- and N-limited conditions. During the natural phytoplankton incubation, the percentage of the resting spores increased with time under both Fe- and N-limited conditions, reaching 25% and 40% of total diatom abundance, respectively. The Si:N drawdown ratios significantly increased with an increase in the contribution of resting spores in both the unialgal culture and natural phytoplankton incubations. These results suggest that if the bloom dominated by neritic, resting spore-forming diatom species decline by either Fe- or N-depletion, Si may be utilized preferentially to N in the upper mixed layer due to the formation of heavily silicified resting spores.

Biotic and abiotic regulation of resting spore formation in vivo of obligate aphid pathogen Pandora nouryi: Modeling analysis and biological implication

Entomophthoralean fungus Pandora nouryi is an obligate aphid pathogen that enables to produce resting spores (azygospores) for surviving host absence. To explore possible mechanisms involved in the regulation of resting spore formation in vivo, host cohorts consisting of 40-60 nymphs of green peach aphid Myzus persicae produced within 24h on cabbage leaf discs in petri dishes were exposed to spore showers of P. nouryi at the concentrations (C) from a very few to nearly 2000 conidia/mm(2) and then reared for 7-11 days at the regimes of 10-25 degrees C (T) and 8-16 h daylight (H(L)) or ambient (17.5+/-3.1 degrees C, 13:11 L:D). Aphid mortalities observed from 35-83 cohorts (showered separately) at each regime showed typical sigmoid trend and fit well a general logistic equation (0.79<or=r(2)<or=0.88), yielding similar LC(50) estimates of 1.7-6.1 conidia/mm(2). The proportions (P) of cadavers forming resting spores in the cohorts also fit the same equation (0.73<or=r(2)<or=0.85) at all tested regimes except at 10 degrees C, a low temperature for the host-pathogen interaction. This indicates the dependence of resting spore formation on the spore concentration. The effects of T and H(L) on P over C were well elucidated by the fitted modified logistic equations P=0.578/{1+exp[1.710-(0.136-0.0053T)C]} and P=0.534/{1+exp[1.639+(0.034-0.0053H(L))C]} (both r(2)=0.79). Our results highlight that the resting spore formation in vivo of P. nouryi is regulated primarily by the concentration of host-infecting conidia discharged from cadavers and facilitated by lower temperature and longer daylight.

Resting spore formation in the marine diatom Thalassiosira nordenskioeldii under iron- and nitrogen-limited conditions

Resting spore formation was investigated in the neritic and oceanic strains of Thalassiosira nordenskioeldii under iron- and nitrate-depleted conditions at 5°C and 10°C. Both strains immediately formed resting spores under nitrate-depleted conditions with almost 100% composition after 4-8 and 3-6 d cultivation periods at 5°C and at 10°C, respectively. However, resting spore formation in both strains under iron-depleted conditions increased with incubation time more gradually, and after 15 d of cultivation, spore composition ranged from 60% in the neritic strain at 5°C to 1% in the oceanic strain at 10°C. In addition, chlorotic cells with smaller cell volume compared with vegetative cells were observed under iron-depleted conditions. Sinking rates of vegetative cells, iron-limited cells and spores and nitrate-limited resting spores cultivated at 5°C were 1.24 + 0.14, 3.41 ± 0.43 and 9.22 ± 1.04 m d -1 , respectively, slightly faster than those at 10°C. The faster sinking rates in iron-limited resting cells and resting spores than in vegetative cells may prevent their habitat from expanding to high-nitrate low-chlorophyll oceanic regions with low iron concentrations.

Resting spore formation of aphid‐pathogenic fungus Pandora nouryi depends on the concentration of infective inoculum

Resting spore formation of some aphid-pathogenic Entomophthorales is important for the seasonal pattern of their prevalence and survival but this process is poorly understood. To explore the possible mechanism involved in the process, Pandora nouryi (obligate aphid pathogen) interacted with green peach aphid Myzus persicae on cabbage leaves under favourable conditions. Host nymphs showered with primary conidia of an isolate (LC(50): 0.9-6.7 conidia mm(-2) 4-7 days post shower) from air captures in the low-latitude plateau of China produced resting spores (azygospores), primary conidia or both spore types. Surprisingly, the proportion of mycosed cadavers forming resting spores (P(CFRS)) increased sharply within the concentrations (C) of 28-240 conidia mm(-2), retained high levels at 240-1760, but was zero or extremely low at 0.3-16. The P(CFRS)-C relationship fit well the logistic equation P(CFRS) = 0.6774/[1 + exp(3.1229-0.0270C)] (r(2) = 0.975). This clarified for the first time the dependence of in vivo resting spore formation of P. nouryi upon the concentration of infective inoculum. A hypothesis is thus proposed that some sort of biochemical signals may exist in the host-pathogen interaction so that the fungal pathogen perceives the signals for prompt response to forthcoming host-density changes by either producing conidia for infecting available hosts or forming resting spores for surviving host absence in situ.

Saprotrophy of Conidiobolus and Basidiobolus in leaf litter

This study of the putative saprotrophs of Conidiobolus and Basidiobolus aids the understanding of their ecological roles in litter, and their relationship with the entomogenous fungi of the Entomophthorales. A total of 47 isolates (ten spp.) were screened for their ability to utilise pure compounds, arthropod cadavers, and plant leaf fragments as substrates. Isolates co-occurred in a larch plantation ( Larix sp.) or were from adjacent habitats. Of the 21 isolates (nine spp.) tested on potential prime carbon sources, none could utilise common plant structural polymers. Conidiobolus adiaeretus, C. iuxtagenitus, and B. ranarum from litter and some soil isolates of C. heterosporus, C. pumilus, and C. firmipilleus could use starches and glycogen. In marked contrast, all could utilise animal chitin, gelatine, casein, N-acetyl glucosamine, and trehalose. The lipids tributyrin and sunflower oil also supported growth. Conidia on cadavers usually led to high levels of colonisation as was the case for 30 isolates (ten species). Collembola were more frequently and rapidly colonised than mites. Cadavers of many other arthropods were also internally colonised. The ability to utilise cadavers of diverse arthropods indicates that trophic competition between co-occurring test species may be minimal. Niche differentiation may depend more on non-trophic features of their life history. Negative correlation of performance with the presence of naturally occurring, non-test fungi suggests competition with (or antibiosis from) at least some of the other fungi. In washed or unwashed plant fragments of larch litter (F-layer) only occasional local growth and resting spore formation occurred. Extra nutrients did not facilitate colonisation. Alternative forms of repetitional conidia showed a strong association with plant fragments but not with cadavers.

Susceptibility of hairy root lines of Brassica species to Plasmodiophora brassicae and in an in vitro subculture system

To investigate the susceptibility of hairy root lines of Brassica species to Plasmodiophora brassicae, hairy roots were induced in a number of Brassica species with Agrobacterium rhizogenes. Turnip hairy root was highly susceptible to P. brassicae; infection rates were high and large galls formed. In contrast, the rates of root hair infection and gall formation on intact Brassica plants did not differ significantly from the control. To induce resting spore formation, turnip hairy roots were incubated at 15°, 20°, or 25°C after 3 weeks of incubation at 25°C. The number and fresh mass of the galls per hairy root were higher and formation of resting spores was greatest after a 7-week incubation at 20°C. To subculture P. brassicae using turnip hairy root, turnip hairy roots were reinoculated with resting spores and gall with resting spores then formed on the hairy roots. In this way, P. brassicae using hairy roots could be subcultured in vitro two or three times on three single-spore isolates of P. brassicae. This is the first report of in vitro subculture of P. brassicae using hairy root.

Molecular Cloning of PbSTKL1 Gene from Plasmodiophora brassicae Expressed during Clubroot Development

AbstractInfection of crucifers by the obligate plant pathogen Plasmodiophora brassicae Woron. results in the formation of clubroot disease in these plants. Plasmodiophora brassicae gene expression during disease development was studied by differential display analysis of total RNA extracted from the roots of Chinese cabbage inoculated with the pathogen. In a series of experiments, 30 differentially expressed bands of cDNA were detected, and the expression of clone no. 17 was confirmed in clubbed roots. Southern blot analysis showed that this clone was a single‐copy gene in the P. brassicae genome. Putative amino acid sequence analysis of the full‐length cDNA of clone no. 17 (4.6 kb, designated PbSTKL1) revealed a serine/threonine kinase‐like domain at the C‐terminal region and a coiled‐coil structure in the middle region of the putative protein. PbSTKL1 expression increased strongly beginning 30 days after inoculation and was coincident with resting spore formation.