植物病毒病媒介昆虫的传毒特性和机制研究进展

植物病毒病是农作物的“癌症”, 至今缺少有效的防治方法。目前已知80%的植物病毒病依赖于媒介昆虫传播, 而媒介昆虫对植物病毒的传播是一个昆虫、 病毒、 寄主植物互作的过程, 历经获毒、 持毒和传毒等多个阶段, 昆虫体内一系列病毒受体或蛋白参与了这个过程。昆虫传播病毒的方式有口针携带式、 前肠保留式和体内循环式3类, 它们各自对应的持久性为非持久性、 半持久性和持久性, 不同昆虫获取这3类病毒的获毒时间、 在体内存留位置和传毒时间也各不相同。 这个过程受到媒介昆虫的性别及龄期、 寄主植物、 环境条件、 昆虫体内共生菌等多种因素的影响。与之相关的蛋白主要有病毒衣壳蛋白(CP)、 次要衣壳蛋白（CPm）、 GroEL蛋白、 辅助因子(HC)和下颚口针蛋白等。近年来对植物病毒基因组的研究也取得了很大的进展, 对昆虫传毒机制的研究正受到越来越广泛的关注。本文综述了近年来该领域内的相关研究进展, 包括昆虫传播植物病毒的传毒方式、 影响传毒效率的因素、 传毒机制特别是昆虫体内与病毒传播可能相关的受体等。     

Biological control of fungal plant diseases

Research has demonstrated the agricultural potential of biological control. For airborne pathogens as well as for soilborne pathogens similar strategies based on different targets in the life cycle of the pathogen can be distinguished, viz. (1) microbial protection of the host against infection, (2) microbial reduction of pathogen sporulation and (3) microbial interference with pathogen survival. Some successes and failures with respect to these targets will be discussed and include (1) biocontrol of seedling diseases, root pathogens, and post-harvest diseases (2) biocontrol of powdery mildew and Botrytis cinerea (3) biocontrol of sclerotial pathogens. Despite of a lot of research on biological control of plant diseases, the number of products available is limited and their market size is marginal. The market for biological control products is not only determined by agricultural aspects such as the number of diseases controlled by one biocontrol product in different crops but also by economic aspects as cost-effective mass production, easy registration and the availability of competing means of control including fungicides. The future development of low-chemical input sustainable agriculture and organic farming will determine the eventual role of biological control in agriculture.     

Nematode vectors of plant viruses in New Zealand

Abstract

A survey of virus vector nematodes based on a total of 320 published and recent records revealed that 3 Longidorus, 1 Paralongidorus, 6 Xiphinema, 2 Trichodoms, and 3 Paratrichodorus species are present in New Zealand. Xiphinema diversicaudatum and Paratrichodorus minor are the most common and widespread species. Longidorus orongorongensis, an undescribed Paralongidorus species and Trichodoms cottieri appear to be endemic species, and Longidorus taniwha, Paratrichodorus lobatus and probably one (still unidentified) species of the X. americanum group are also indigenous to New Zealand. Most of the other species (e.g. Longidorus elongatus, X. diversicaudatum, and Trichodoms primitivus) were probably introduced from Europe. Maps showing the distribution of the 15 species in New Zealand, and illustrations and tables of the morphological characters for species identification, are presented. Several species are of economic importance through direct damage they cause to cultivated plants, and all can be considered potential vectors of plant viruses.     

Heterologous encapsidation in transmission of plant viral particles by aphid vectors

Recently, significant progress has been made in recognizing virus-aphid specificities and identifying the proteins involved in virus transmission by aphids. An essential role of the viral capsid protein in this process has been proved. Heterologous encapsidation between viruses in mixed infections may allow transmission by aphids of normally non-aphid-transmissible viruses or change virus-vector interactions. This review describes the most characteristic examples of the phenomenon. Recent findings regarding transmission by aphids of viroid encapsidated in the viral capsid protein, and of virus encapsidated in transgenic coat protein, are presented.     

Early molecular events in the interaction of enveloped viruses with cells

The fluorescence depolarization of 1,6-diphenyl-hexatriene was used to study the dynamic properties of the hydrophobic regions of the lipid envelopes of ortho- and paramyxoviruses as well as of the Rous sarcoma virus and of the membrane lipids of susceptible and nonsusceptible cells.The systems investigated where active and inactive influenza viruses, and NDV virus acting on chick embryo fibroblasts and Rous sarcoma virus acting on susceptible (C/E) and nonsusceptible (C/B) chicken-cell.Polarization degrees and mean rotational correlation times of DPH embedded in viral lipids were significantly higher than those of DPH in the cell membranes, due to a higher rigidity of the virus envelopes. When suspensions of labelled viruses and unlabelled cells or unlabelled viruses and labelled cells were mixed, a characteristic change of the fluorescence polarization degrees with time was observed. This behaviour was ascribed to a label transfer from virus to cell membranes or vice versa.While the rate constants of label transfer from virus to cells and cells to virus were about the same for the penetrating viruses the rate constants of label release from inactive virus to cells were much larger than for the migration in the opposite direction.     

Mechanisms of arthropod transmission of plant and animal viruses.

A majority of the plant-infecting viruses and many of the animal-infecting viruses are dependent upon arthropod vectors for transmission between hosts and/or as alternative hosts. The viruses have evolved specific associations with their vectors, and we are beginning to understand the underlying mechanisms that regulate the virus transmission process. A majority of plant viruses are carried on the cuticle lining of a vector's mouthparts or foregut. This initially appeared to be simple mechanical contamination, but it is now known to be a biologically complex interaction between specific virus proteins and as yet unidentified vector cuticle-associated compounds. Numerous other plant viruses and the majority of animal viruses are carried within the body of the vector. These viruses have evolved specific mechanisms to enable them to be transported through multiple tissues and to evade vector defenses. In response, vector species have evolved so that not all individuals within a species are susceptible to virus infection or can serve as a competent vector. Not only are the virus components of the transmission process being identified, but also the genetic and physiological components of the vectors which determine their ability to be used successfully by the virus are being elucidated. The mechanisms of arthropod-virus associations are many and complex, but common themes are beginning to emerge which may allow the development of novel strategies to ultimately control epidemics caused by arthropod-borne viruses.     

Ecdysteroids and juvenile hormones of whiteflies, important insect vectors for plant viruses

Ecdysteroids and juvenile hormones (JHs) regulate many physiological events throughout the insect life cycle, including molting, metamorphosis, ecdysis, diapause, reproduction, and behavior. Fluctuation of whitefly ecdysteroid levels and the identity of the whitefly molting hormone (20-hydroxyecdysone) have only been reported within the last few years. An ecdysteroid commitment peak that is associated with the reprogramming of tissues for a metamorphic molt in many holometabolous and some hemimetabolous insect species was not observed in last nymphal instars of either the sweet potato whitefly, Bemisia tabaci (Biotype B), or the greenhouse whitefly, Trialeurodes vaporariorum. Ecdysteroids reach peak levels 1-2 days prior to the initiation of the nymphal-adult metamorphic molt. Adult eye and wing differentiation which signal the onset of this molt begin earlier in 4th instar T. vaporariorum (Stages 4 and 5, respectively) than in B. tabaci (Stage 6), and the premolt peak is 3-4 times greater in B. tabaci ( approximately 400 fg/microg protein) than in T. vaporariorum ( approximately 120 fg/microg protein). The JH of B. tabaci nymphs and eggs was found to be JH III, supporting the view that JHs I and II are, with rare exception, only present in lepidopteran insects. In B. tabaci eggs, JH levels were approximately 10 times greater on day 2/3 (0.44 fg/egg or 0.54 ng/g) than on day 5 (0.04 fg/egg or 0.054 ng/g) post-oviposition. Approximately, 1.4 fg/2nd-3rd instar nymph (0.36 ng/g) was detected. It is probable that the relatively high level of JH in day 2/3 eggs is associated with the differentiation of various whitefly tissues during embryonic development.     

Subviral molecules of RNA associated with plant ss(+)RNA viruses

Plant ss(+)RNA viruses besides their genome RNAs often are associated with additional subviral RNA molecules which occur naturally or are generated de novo during infection. There are such molecules like: satellite, defective, defective interfering and chimeric RNAs. Subviral RNAs can not replicate and encapsidate by oneself. Helper viruses supply the protein complexes that are necessary to these processes. The subviral molecules are characterized by small size. Recombination, deletion and accumulation of mutation are the main ways of arising subviral elements, although the origin of satRNAs is unknown. The unique feature of subviral RNAs is their ability to modify of infection progress caused by helper virus. They can attenuate or enhance the intensity of disease symptoms. The overall influence on disease development depends on three-component complex consisting of: plant host-virus' strain--subviral RNA. This article is a synthetic review of information concerning subviral RNA molecules of plant viruses, their structure, functions and origin.     

The use of abrasives in the transmission of plant viruses

The effect of different abrasives on the transmissibility of several plant viruses was tested. Celite and animal charcoal were as effective as carborundum in increasing the number of lesions produced by a given inoculum: 400-mesh carborundum was the most effective among the different sizes which were tested; this gave a result equivalent to increasing the virus content a hundred times. Some preparations of carborundum and charcoal reduced infectivity.
Uninjured plants resisted infectivity when virus solutions were sprayed over them. Leaves previously nibbed without abrasives developed only few lesions whereas leaves rubbed with abrasives developed large numbers. Three hours after rubbing with abrasives leaves had regained their resistance to sprayed virus solutions.
The effects of rubbing leaves with abrasives are described and their significance discussed.     

Outcomes of fungal interactions are determined by soil invertebrate grazers

Saprotrophic fungal community composition, determined by the outcome of competitive mycelial interactions, is one of the many key factors affecting soil nutrient mineralisation and decomposition rates. Fungal communities are not generally predicted to be regulated by top-down factors, such as predation, but rather by bottom-up factors, including resource availability. We show that invertebrate grazers can exert selective pressures on fungal decomposer communities in soil, reversing the outcomes of competitive interactions. By feeding selectively on the cord-forming fungus Resinicium bicolor, isopods prevented the competitive exclusion of Hypholoma fasciculare and Phanerochaete velutina in soil and wood. Nematode populations also reversed the outcomes of competitive interactions by stimulating growth of less competitive fungi. These represent two opposing mechanisms by which soil fauna may influence fungal community composition and diversity. Factors affecting soil invertebrate communities will have direct consequences for fungal-mediated nutrient cycling in woodland soils.     

Association of cocksfoot mild mosaic and cocksfoot streak viruses in Dactylis glomerata and their simultaneous transmission by aphids

Macrosiphum (Sitobion) fragariae transmitted cocksfoot mild mosaic and cocksfoot streak viruses two to three times more frequently from plants in which they occurred together than from plants in which they occurred alone. In cells of doubly infected plants, particles of the two viruses commonly occurred in close association.     

Biological events and molecular signaling following MLKL activation during necroptosis

Necroptosis is a form of programmed necrotic cell death mediated by the kinase RIPK3 and its substrate MLKL. MLKL, which displays plasma membrane (PM) pore-forming activity upon phosphorylation, functions as the executioner during necroptosis. Thus, it was previously assumed that MLKL phosphorylation is the endpoint of the necroptotic signaling pathway. Here, we summarize several events that characterize the dying necroptotic cells after MLKL phosphorylation, including Ca²⁺ influx, phosphatidylserine (PS) externalization, PM repair by ESCRT-III activation, and the final compromise of PM integrity. These processes add several unexpected regulatory events downstream of MLKL signaling. We have also observed that CoCl₂, which may mimic hypoxia, can induce necroptosis, which suggests that in vivo triggers of necroptosis might include a transient lack of O₂.     

Insect transmission of plant viruses: a constraint on virus variability

Genetic diversity in viruses is shaped by high rates of recombination and is constrained by host defenses and the requirements of transmission. Recent studies of insect-transmitted plant viruses demonstrate highly conserved molecular motifs in viral genomes that regulate the specificity of insect transmission. In contrast, advances in our understanding of host plant response to virus infection reveal some generalized patterns of host defense to a diversity of viruses.     

Gut microbiota and parasite transmission by insect vectors

In the gut of some insect vectors, parasites ingested with the bloodmeal decrease in number before coming into contact with host tissues. Many factors could be responsible for this reduction in parasite number but the potentially important role of the large communities of naturally occurring microorganisms that exist alongside the newly ingested parasites in the vector midgut has been largely overlooked. Some previous reports exist of the inhibition of parasite development by vector gut microbiota and of the killing of Trypanosoma cruzi and Plasmodium spp. by prodigiosin produced by bacteria. Based on this evidence, we believe that the microbiota present in the midgut of vector insects could have important roles as determinants of parasite survival and development in insect vector hosts and, therefore, contribute to the modulation of vector competence for many important diseases.