Research on plant-parasitic nematode biology conducted by the United States Department of Agriculture-Agricultural Research Service

The recent de-registration of several chemical nematicides and the impending loss of methyl bromide from the pest-control market necessitate the development of new methods for controlling nematode-induced crop damage. One approach for developing novel target-specific controls is by exploiting fundamental differences between the biological processes of nematodes and their host plants. Researchers of the Agricultural Research Service (ARS) of the US Department of Agriculture are actively exploring these differences. Research accomplishments include the discovery of heat shock protein genes possibly involved in developmental arrest of the soybean cyst nematode, the identification of neuropeptides and female-specific proteins in the soybean cyst nematode, the disruption of nematode reproduction with inhibitors of nematode sterol metabolism, the development of novel morphological and molecular (heat shock protein genes and the D3 segment of large subunit ribosomal DNA) features useful for nematode identification and classification, and the elucidation of the population genetics of potato cyst nematode pathotypes. In addition, several ARS researchers are investigating biological determinants of nematode response to management strategies utilized in agricultural fields. These collective efforts should lead to new chemical and non-chemical alternatives to conventional nematode control strategies.     

Carbocyclic coformycin: a case study of the opportunities and pitfalls in the industrial search for new agrochemicals from nature

The work undertaken to isolate the novel, herbicidally active compound, carbocyclic coformycin, to obtain sufficient quantities of the compound for full biological evaluation, and to identify its biochemical mode of action is summarised. Although the compound was extremely active against some weed species, limitations in its spectrum of activity precluded further development. Carbocyclic coformycin exerts its biological action through a novel mode of action by the inhibition of the enzyme adenosine 5′-phosphate deaminase (EC 3.5.4.6) following phosphorylation in planta. From this work, the potential benefits of natural product research in the discovery of new agrochemicals are highlighted along with some of the possible pitfalls. © 1998 SCI.     

Plant-pathogenic bacteria as biological weapons - real threats?

At present, much attention is being given to the potential of plant pathogens, including plant-pathogenic bacteria, as biological weapons/bioterror weapons. These two terms are sometimes used interchangeably and there is need for care in their application. It has been claimed that clandestine introduction of certain plant-pathogenic bacteria could cause such crop losses as to impact so significantly on a national economy and thus constitute a threat to national security. As a separate outcome, it is suggested that they could cause serious public alarm, perhaps constituting a source of terror. Legislation is now in place to regulate selected plant-pathogenic bacteria as potential weapons. However, we consider it highly doubtful that any plant-pathogenic bacterium has the requisite capabilities to justify such a classification. Even if they were so capable, the differentiation of pathogens into a special category with regulations that are even more restrictive than those currently applied in quarantine legislation of most jurisdictions offers no obvious benefit. Moreover, we believe that such regulations are disadvantageous insofar as they limit research on precisely those pathogens most in need of study. Whereas some human and animal pathogens may have potential as biological or bioterror weapons, we conclude that it is unlikely that any plant-pathogenic bacterium realistically falls into this category.     

Recent developments in auxin biology and new opportunities for auxinic herbicide research

Auxinic herbicides mimic the effects of natural auxin. However, in spite of decades of research, the site(s) of action of auxinic herbicides has remained unknown and many physiological aspects of their function are unclear. Recent advances in auxin biology provide new opportunities for research into the mode of action of auxinic herbicides. Of considerable interest is the discovery of auxin receptors (TIR1 and possibly ABP1) that may lead to the discovery of auxinic herbicide site(s) of action. Knowledge of auxin-conjugating enzymes and auxin signal transduction components may shed new light on herbicide activity, selectivity in dicots, and mechanisms leading to phytotoxicity in sensitive plants. Analysis of genes induced in response to auxin may provide a novel approach for detection of off-target herbicide injury in crops. For example, the auxin-responsive gene GH3 is highly and specifically induced in response to auxinic herbicides in soybean, and may offer a novel method for diagnosing auxinic herbicide injury. Advances in our understanding of auxin biology will provide many new avenues and opportunities for auxinic herbicide research in the future.     

Insect Enemies of Aquatic Weeds

Abstract

The Agricultural Research Service of the USDA has been actively engaged in research on the biological control of aquatic weeds since 1959. Research emphasis until 1967 was on alligatorweed, Alternanthera philoxeroides (Mart.) Griseb. As a result of this research, three species of insects from Argentina were released in the southeastern United States. Alligatorweed control with these natural enemies has been variable; but indications are that control will be substantial over most of the range of the weed.

One species of weevil, Neochetina eichhorniae Warner, has been released against water hyacinth, Eichhornia crassipes (Mart.) Solms, in the USA as a result of research in Argentina. A second closely related species, N. bruchi Hustache, is presently undergoing final clearance procedures. Several other arthropods are being studied in Argentina as possible biological control agents of water hyacinth.

A USA sponsored programme in Yugoslavia under the direction of Dr M Leki? resulted in the discovery of two insect species that show potential as control agents for eurasian watermilfoil, Myriophyllum spicatum L. Further testing for host specificity of these insects will be carried out in the USA in quarantine in Florida.

A USA sponsored Pakistan programme under Dr M. A. Ghani is investigating insects and other phytophagous organisms attacking Hydrilla verticillata Royle. The results are encouraging. PANS 20: 444–450, 1974.     

The need for effective marking and tracking techniques for monitoring the movements of insect predators and parasitoids

Predators and parasitoids move through the landscape at a wide range of rates and over a broad range of spatial scales. Understanding the dispersal characteristics of such 'beneficials' is of crucial importance for classical, augmentative, inundative and conservation biological control. It is a key practice, following the release of 'classical' (exotic) and augmentative (indigenous) biological control agents, to assess whether they have become successfully established, and also to what extent the agent has spread from the release points. It is important in conservation biological control to understand the role that natural enemy movements play in the improvement of pest control achieved through the provision of pollen, nectar, shelter and/or alternative prey/hosts. Other aspects of predator and parasitoid movement pertinent to biological control are the role refugia play in influencing, via natural enemy movements, control of pest population numbers. In these areas and others, some form of marking and/or tracking of insect predators and parasitoids is usually required so that movements can be studied. This special issue considers the usefulness and limitations of marking and tracking techniques, through up-to-date reviews written by an international team of researchers who are actively involved in the study of predator and parasitoid ecology.     

Fusarium wilt of banana: biology,epidemiology and management

Fusarium oxysporum is a soil borne hyphomycete that causes vascular wilts in several crop plants. A variety of remedial measures such as the use of fungicides, soil amendments and biological antagonists have proved insufficient in controlling F. oxysporum. Ever since it was first reported in banana crop, the only effective control strategy known is planting of resistant cultivars. However, presumably due to the high mutation rates and rapid co-evolution with its host, Fusarium wilt has surmounted host defense barriers and has already begun infecting even the resistant Cavendish varieties that dominate export markets worldwide. Transgenic banana plants showing enhanced resistance to Fusarium wilt have been developed in recent past, but they remain largely confined to the laboratory. The importance of banana as source of food and income in developing countries world over and the need to develop Fusarium wilt tolerant cultivars by novel biotechnological approaches is detailed herein. In this communication, we review the biology and management of Fusarium wilt in banana with the aim of providing the baseline of information to encourage much needed research on integrated management of this destructive banana crop disease problem.     

United States Department of Agriculture-Agricultural Research Service research on pest biology: weeds

Over 125 permanent full-time scientists conduct research within the USDA Agricultural Research Service (ARS) on issues related to weeds. The research emphasis of most of these scientists involves ecology and management or biological control of weeds. Many scientists perform research on weed biology as components of their primary projects on weed control and integrated crop and soil management. Describing all ARS projects involved with weed biology is impossible, and consequently only research that falls within the following arbitrarily chosen topics is highlighted in this article: dormancy mechanisms; cell division; diversity of rangeland weeds; soil resources and rangeland weeds; poisonous rangeland plants; horticultural weeds; weed traits limiting chemical control; aquatic and semi-aquatic weeds; weed/transgenic wheat hybrids; seedbanks, seedling emergence and seedling populations; and weed seed production. Within these topics, and others not highlighted, the desire of ARS is that good information on weed biology currently translates or eventually will translate into practical advice for those who must manage weeds.     

分子模拟在农药半抗原设计及其免疫识别机制中的应用

免疫半抗原的分子设计与合成是建立小分子免疫化学分析方法的关键步骤。在介绍农药半抗原设计一般策略的基础上,指出分子模拟技术在农药半抗原合理设计中的适用性在于其能通过对分子结构和动力学行为的模拟,获得能表示和解释农药分子免疫原性和生物活性的参数。重点阐述了分子模拟在免疫原和竞争原设计中的应用,揭示了采用分子模拟技术进行半抗原设计的优势在于其能够克服目前半抗原设计方法的经验性和主观随意性。运用分子模拟技术能够更好地研究抗原抗体反应中的分子间作用力,因此该技术有利于促进抗原抗体反应识别机制研究的深入进行。分子模拟技术还能够应用于解释抗体的交叉反应率,从而能应用于提高抗体宽谱性研究。     

Importance of the life cycle in sympatric host race formation and speciation of pathogens

ABSTRACT Numerous morphological species of pathogenic fungi have been shown to actually encompass several genetically isolated lineages, often specialized on different hosts and, thus, constituting host races or sibling species. In this article, we explore theoretically the importance of some aspects of the life cycle on the conditions of sympatric divergence of host races, particularly in fungal plant pathogens. Because the life cycles classically modeled by theoreticians of sympatric speciation correspond to those of free-living animals, sympatric divergence of host races requires the evolution of active assortative mating or of active host preference if mating takes place on the hosts. With some particular life cycles with restricted dispersal between selection on the host and mating, we show that divergence can occur in sympatry and lead to host race formation, or even speciation, by a mere process of specialization, with strong divergent adaptive selection. Neither active assortative mating nor active habitat choice is required in these cases, and this may explain why the phylo-genetic species concept seems more appropriate than the biological species concept in these organisms.     

Technical and commercial aspects of biocontrol products

The global agrochemical market in 1991 was $26800 million, yet biological products were reported to account for only $120 million of sales per annum—less than 0.5 % of the total. The majority of these sales are attributed to bio-insecticides of which Bacillus thuringiensis accounts for over 90%, but B. thuringiensis could be described as a biologically produced insecticide, rather than a true biocontrol agent. Biological products have technical limitations, including extreme specificity, sensitivity to environmental factors and problems with robustness of the formulations, but ironically, it is these limitations which also give biological control an image of environmental acceptability. Nonetheless, some of the limitations will be overcome and sales will increase, but primarily in niche situations such as the control of soil-borne diseases and the control of insect pests showing resistance to agrochemicals. In order for significant inroads to be made into such niche markets it is imperative that progress with biological products is not impaired by over-regulation, and a rational approach by all regulatory bodies is required. Overall, though, agrochemicals are likely to continue to be the major method of crop protection for the foreseeable future, and the biological control field now needs clear, well-defined goals if current successful niche products can be the basis for future success rather than a limited experiment in alternative technology.     

Influence of chlorine substituents on biological activity of chemicals: a review

A number of well known polychlorinated chemicals are toxicologically and environmentally unsafe. Because of their persistence they are in the focus of public discussions against chlorine chemistry. However, chlorinated organic chemicals in the molecular weight range between 200 and 600 constitute an important and indispensable segment in the arsenal of existing biologically active chemicals used as pharmaceuticals or crop‐protection agents. Over the course of time it has been found empirically that the introduction of a chlorine atom into one or more specific positions of a biologically active molecule may substantially improve the intrinsic biological activity. In some cases the presence of a chlorine atom is crucial for significant activity in compounds derived both from nature and chemical synthesis. But in other cases chlorination diminishes or abolishes biological activity, as shown for chlordane homologues. Thus a chlorine atom, like any other substituent, is a modulator of activity. Almost all non‐reactive chlorinated chemicals and chlorine‐free chemicals are devoid of any biological activity at the highest concentration typically used in primary screening tests for discovery of useful biological properties. The influence of a substituent such as chlorine on the biological activity of a potential drug or crop protection agent still has to be established empirically in biological experiments designed to detect desired activity or toxicological properties. Sometimes chlorine does prove to be the optimum for improvement of activity. Long‐term rigorous investigations of several hundred chlorinated compounds, registered by the authorities as pharmaceutical drugs or crop‐protection agents, show that the generalisation ‘all chlorinated chemicals are dangerous’, deduced from the negative toxicological properties of a hundred chlorinated and reactive compounds of low molecular weight that are relevant in terms of safe working conditions in the chemical industry and for ecological safety, is not justified. Chlorinated compounds are not necessarily toxic or dangerous. Highly reactive chemicals or polychlorinated compounds cannot be compared with regard to toxicological properties with unreactive compounds having a low degree of chlorination. The chlorine atom, as one of many possible substituents used in synthetic organic chemistry, will remain in the future one of the important tools for probing structure–activity relationships in life science research and as a molecular component in commercialised compounds, in order to provide safer, more selective and more environmentally compatible products with higher activity for medicine and agriculture. © 1999 WILEY‐VCH Verlag GmbH     

Evolving understanding of the evolution of herbicide resistance

A greater number of, and more varied, modes of resistance have evolved in weeds than in other pests because the usage of herbicides is far more extensive than the usage of other pesticides, and because weed seed output is so great. The discovery and development of selective herbicides are more problematic than those of insecticides and fungicides, as these must only differentiate between plant and insect or pathogen. Herbicides are typically selective between plants, meaning that before deployment there are already some crops possessing natural herbicide resistance that weeds could evolve. The concepts of the evolution of resistance and the mechanisms of delaying resistance have evolved as nature has continually evolved new types of resistance. Major gene target‐site mutations were the first types to evolve, with initial consideration devoted mainly to them, but slowly ‘creeping’ resistance, gradually accruing increasing levels of resistance, has become a major force owing to an incremental accumulation of genetic changes in weed populations. Weeds have evolved mechanisms unknown even in antibiotic as well as other drug and pesticide resistances. It is even possible that cases of epigenetic ‘remembered’ resistances may have appeared. Copyright © 2009 Society of Chemical Industry     

分子生物学技术在松材线虫研究中的应用

根据松材线虫(Bursaphlenchus xylophilus)分子生物学的研究基础,以及基于不同的研究目的,对松材线虫不同DNA区域,包括基因组DNA、线粒体DNA、核糖体DNA和卫星DNA研究做了综述,并分析、比较一些常用的分子生物学技术在松材线虫研究中的适用性,以及在生物防治研究中的进展。     

蓟马的分子鉴定和种群遗传学研究进展

蓟马是一类个体微小的昆虫,形态学特征观察困难,种内遗传多样性高,且部分害虫类蓟马寄主广泛,繁殖能力强,为害严重,已成为重要的农林类害虫。采用分子生物学手段对蓟马进行物种鉴定,可以解决传统形态学鉴定困难、局限性较大等问题,能提高物种鉴定的效率与准确性。目前,关于蓟马分子生物学鉴定的研究较多,其中,基于线粒体细胞色素氧化酶亚基（ I cytochrome oxidase I,COI）基因片段的DNA条形码技术被广泛用于蓟马的快速准确鉴定、属内近缘种的区分及不同品系或生物型的鉴别。此外,开展蓟马类害虫种群遗传学的研究不仅有利于全面了解其遗传分化、适应性、入侵来源和扩散路径,还可为制订合理有效的监测预报和综合防治策略提供理论依据。目前,多种蓟马的种群遗传学研究表明地理隔离、寄主植物和微生物等因素影响其种内和种间的遗传多样性和遗传分化。本文综述了国内外有关蓟马的DNA条形码鉴定与种群遗传学研究进展,对目前蓟马物种鉴定中存在的问题进行讨论,并对未来的发展趋势进行展望。     

合成生物学助力链霉菌天然产物农药创制与产业化

病虫草害防控是国家总体安全建设的重要组成部分。微生物天然产物农药是病虫草害绿色防控体系的重要成员, 在保障国家粮食安全、生态安全和农产品质量安全, 筑牢国家生物安全屏障中具有重要作用。链霉菌以能够产生丰富的次级代谢产物而著称, 是天然产物农药的资源宝库, 也是重要的天然产物药物的工业生产菌。然而, 随着病虫草害抗药性增强, 新发、突发病虫草害增多等问题不断涌现, 以及天然产物研究面临新骨架、高活性化合物发现难度增大, 工业菌株产量提升困难等瓶颈, 链霉菌天然产物农药创制与应用正面临巨大挑战。合成生物学作为一门交叉学科, 突破了生物学研究的传统模式, 为天然产物药物研发提供了新的思路与策略。本文综述了近年来合成生物学在链霉菌研究领域的技术革新, 以及合成生物学在推动链霉菌天然产物资源发现、天然产物高效生物制造等方面的研究进展, 并对合成生物学助力链霉菌天然产物农药的创制与产业化进行了展望。     

Some aspects of the taxonomy and biology of dracunculoid nematodes parasitic in fishes: a review

The nematode superfamily Dracunculoidea includes 166 recognized species, of which 150 (90%) are parasitic in about 300 species of freshwater, brackish-water and marine fishes. Fish dracunculoids are placed in 31 genera (86% of all dracunculoid genera) belonging to eight of the nine dracunculoid families: Anguillicolidae, Daniconematidae, Guyanemidae, Lucionematidae, Micropleuridae, Philometridae, Skrjabillanidae, and Tetanonematidae; the genus Lockenloia is considered incertae sedis. Because of difficulties in studying fish dracunculoids, associated with their morphological and biological peculiarities, most species of these largely histozoic parasites are poorly known and males of the majority of species and of eight genera have not yet been discovered. It is apparent that the present classification system of dracunculoids as a whole does not reflect phylogenetic relationships and a taxonomic revision of this nematode group, based on detailed morphological (including SEM and TEM), life history and molecular studies of individual species, is quite necessary. Data on the biology of fish dracunculoids is scarce. In known cases, their life cycles involve copepods, ostracods or branchiurids as intermediate hosts and, sometimes, fish paratenic hosts are known to occur in dracunculoid species parasitizing as adults piscivorous definitive hosts. However, nothing is known about the life cycles of representatives of 20 genera. Some species of dracunculoids, particularly of philometrids, are highly pathogenic and are known as agents of serious fish diseases. During recent years, especially the importance of Philometra spp. parasitizing the gonads of many species of marine fishes has increased due in particular to the rapid development of marine aquaculture, because they may significantly decrease fish reproduction or even cause full parasitic castration. Therefore, further detailed studies on fish dracunculoids are significant not only from the theoretical viewpoint, but they may also have practical implications.     

Helper component-transcomplementation in the vector transmission of plant viruse

ABSTRACT Plant viruses are most frequently transmitted from one host plant to another by vectors. In noncirculative vector transmission, the virus does not process through a cycle within the vector body. Instead, upon acquisition by the vector, viruses are retained in the mouth parts or the anterior gut; from there, they will be subsequently released in a new host plant. Two molecular strategies have been described for the virus-vector interaction. In the capsid strategy, the virus coat interacts directly with binding sites in the vector mouth parts, whereas an additional nonstructural protein, designated helper component (HC), is required in the helper strategy. The HC and virus particles can be acquired sequentially, and this property introduces the possibility that an HC acquired first by the vector assists the transmission of virus particles located in the same cell, in other cells, or even in other host plants probed by the vector. Such a phenomenon is here called HC-transcomplementation. Surprisingly, the existing definition of HC does not explicitly include the concept of HC-transcomplementation, and it is generally omitted in the literature in any consideration of the virus biology other than the molecular interaction with the vector. Here we propose an extended definition of HC and emphasize the concept of HC-transcomplementation that distinguishes the helper strategy from any other type of vector transmission and may have consequences at the level of the virus population genetics and evolution.     

Some aspects of the taxonomy and biology of adult spirurine nematodes parasitic in fishes: a review

About 300 species belonging to four superfamilies (Gnathostomatoidea, Habronematoidea, Physalopteroidea and Thelazioidea) of the nematode suborder Spirurina are known as the adult parasites of freshwater, brackish-water and marine fishes. They are placed in four families, of which the Gnathostomatidae, including Echinocephalus with a few species and the monotypic Metaleptus, are parasites of elasmobranchs, whereas Ancyracanthus contains one species in teleosts; the Physalopteridae is represented in fish by four genera, Bulbocephalus, Heliconema, Paraleptus and Proleptus, each with several species in both elasmobranchs and teleosts. The majority of fish spirurines belongs to the Rhabdochonidae, which includes 10 genera (Beaninema, Fellicola, Hepatinema, Heptochona, Johnstonmawsonia, Megachona, Pancreatonema, Prosungulonema, Rhabdochona and Vasorhabdochona) of species parasitizing mainly teleosts, rarely elasmobranchs, and the Cystidicolidae with about 23 genera (Ascarophis, Caballeronema, Capillospirura, Comephoronema, Crenatobronema, Cristitectus, Ctenascarophis, Cyclozone, Cystidicola, Cystidicoloides, Johnstonmawsonoides, Metabronema, Moravecnema, Neoascarophis, Parascarophis, Prospinitectus, Pseudascarophis, Pseudoproleptus, Salvelinema, Similascarophis, Spinitectoides, Spinitectus, Sterliadochona), with many species parasitic in teleosts only. Because of difficulties in studying fish spirurines, associated with their morphological and biological peculiarities, most species of these parasites are poorly known. It is apparent that their present classification system does not reflect phylogenetic relationships and a taxonomic revision of this nematode group, based on detailed morphological (including SEM and TEM), life history and molecular studies of individual species, is quite necessary. In Cystidicolidae, several genera have been based on details in the cephalic structures visible only with the aid of SEM, but it will be evident whether or not these tiny features are of generic importance only when more cystidicolids are described using SEM and comparative molecular data become available. Data on the biology of fish spirurines are scarce. In known cases, their life cycles involve aquatic arthropods (crustaceans or insects) as intermediate hosts, in which, sometimes, the larvae undergo a precocious development and may even attain adulthood and become gravid in these invertebrates; sometimes, fish paratenic hosts are known to occur in cystidicolids parasitizing as adults piscivorous definitive hosts. Some spirurine species are pathogenic and are known as causative agents of serious fish diseases. Consequently, further detailed studies on fish spirurines are significant not only from the theoretical viewpoint, but they may also have practical implications.     

异常珍珠线虫的鉴定和生物学

异常珍珠线虫(Nacobbus aberrans)主要发生在南美洲和北美洲的温带和热带地区,在我国没有分布。其寄主范围广,主要危害马铃薯和甜菜。许多国家和地区都将其列为检疫性对象。本文主要介绍了异常珍珠线虫的生物学特性,形态学特征,分子生物学和防治方法。