Horizontal transmission of begomoviruses between Bemisia tabaci biotypes

We have previously shown that the monopartite Tomato yellow leaf curl virus (TYLCV), a begomovirus (family Geminiviridae, genus Begomovirus) infecting tomato plants can be transmitted in a gender-dependent manner among its insect vector the whitefly Bemisia tabaci type B (Gennaduis) (Aleyrodidae: Hemiptera) during mating. Viruliferous females were able to transmit the virus to non-viruliferous males and vice versa, in the absence of any other virus source. The recipient insects were able to infect tomato plants. In this communication, we present evidence that two bipartite begomoviruses infecting cucurbits, Squash leaf curl virus (SLCV) and Watermelon chlorotic stunt virus (WmCSV) can be transmitted in a gender-dependent manner among whiteflies. In addition we show that TYLCV can be transmitted during mating among individuals from the same biotype (from B-males to B-females and vice versa; and from Q-males to Q-females and vice versa). However, viruliferous males of the B biotype are unable to transmit the virus to females of the Q biotype (and vice versa); similarly, viruliferous males of the Q biotype are unable to transmit the virus to females of the B biotype (and vice versa). These findings support the hypothesis that a pre-zygotic mating barrier between the Q and B biotypes is the cause for the absence of gene flow between the two biotypes, and that virus transmission can be used as a marker for inter-biotype mating. To be transmitted during mating, the virus needs to be present in the haemolymph of the donor insect. Abutilon mosaic virus (AbMV), a bipartite begomovirus that can be ingested but not transmitted by B. tabaci, is absent in the whitefly haemolymph, and cannot be transmitted during mating. Mating was a precondition for horizontal virus transfer from male to female, or female to male. Virus was not transmitted when viruliferous B. tabaci were caged with the non-vector non-viruliferous whitefly Trialeurodes vaporariorum (Westwood) (Aleyrodidae: Hemiptera) and vice versa.     

Distribution of tomato-infecting begomoviruses and Bemisia tabaci biotypes in Morocco

Tomato yellow leaf curl virus (TYLCV) and tomato yellow leaf curl Sardinia virus (TYLCSV) (genus Begomovirus, family Geminiviridae) as well as their whitefly vector Bemisia tabaci were reported from the south‐west and central regions of Morocco. To establish a more comprehensive view of tomato begomoviruses and B. tabaci biotypes throughout Morocco, 32 tomato fields were surveyed for tomato yellow leaf curl disease (TYLCD) symptoms in southern and northern regions, and 54 samples of leaves from cultivated plants or weeds and 35 B. tabaci individuals were collected and analysed by PCR, randomly amplified polymorphic DNA and sequencing. Only TYLCV or TYLCSV were detected. TYLCV was detected in 15 plant samples whereas TYLCSV only in 4. Sequence analyses revealed the presence of the ‘Spanish’ strain of TYLCSV and distinguished two genetically distinct strains of TYLCV. The begomovirus infections were unevenly distributed throughout Morocco. In the north‐west and north‐central regions where tomato plants exhibiting TYLCD symptoms were rarely observed, only 1 sample out of 13 tested positive for the presence of a begomovirus. In the north‐east region, the ratio of begomovirus‐positive samples was higher, 6/13, and in the south‐west region, it was the highest, 13/14. Consistently the frequency of plants exhibiting TYLCD‐like symptoms in the northern regions was lower than that in the south‐west region. B. tabaci biotype Q is present throughout the country and in Algeria, whereas biotype B, identified for the first time in Morocco, was detected only in the north‐east region.     

Tomato yellow leaf curl viruses: ménage à trois between the virus complex,the plant and the whitefly vector

Tomato yellow leaf curl disease (TYLCD) is one of the most devastating viral diseases affecting tomato crops in tropical, subtropical and temperate regions of the world. Here, we focus on the interactions through recombination between the different begomovirus species causing TYLCD, provide an overview of the interactions with the cellular genes involved in viral replication, and highlight recent progress on the relationships between these viruses and their vector, the whitefly Bemisia tabaci. Taxonomy: The tomato yellow leaf curl virus‐like viruses (TYLCVs) are a complex of begomoviruses (family Geminiviridae, genus Begomovirus) including 10 accepted species: Tomato yellow leaf curl Axarquia virus (TYLCAxV), Tomato yellow leaf curl China virus (TYLCCNV), Tomato yellow leaf curl Guangdong virus (TYLCGuV), Tomato yellow leaf curl Indonesia virus (TYLCIDV), Tomato yellow leaf curl Kanchanaburi virus (TYLVKaV), Tomato yellow leaf curl Malaga virus (TYLCMalV), Tomato yellow leaf curl Mali virus (TYLCMLV), Tomato yellow leaf curl Sardinia virus (TYLCSV), Tomato yellow leaf curl Thailand virus (TYLCTHV), Tomato yellow leaf curl Vietnam virus (TYLCVNV) and Tomato yellow leaf curl virus(TYLCV). We follow the species demarcation criteria of the International Committee on Taxonomy of Viruses (ICTV), the most important of which is an 89% nucleotide identity threshold between full‐length DNA‐A component nucleotide sequences for begomovirus species. Strains of a species are defined by a 93% nucleotide identity threshold. Host range: The primary host of TYLCVs is tomato (Solanum lycopersicum), but they can also naturally infect other crops [common bean (Phaseolus vulgaris), sweet pepper (Capsicum annuum), chilli pepper (C. chinense) and tobacco (Nicotiana tabacum)], a number of ornamentals [petunia (Petunia×hybrida) and lisianthus (Eustoma grandiflora)], as well as common weeds (Solanum nigrum and Datura stramonium). TYLCVs also infect the experimental host Nicotiana benthamiana. Disease symptoms: Infected tomato plants are stunted or dwarfed, with leaflets rolled upwards and inwards; young leaves are slightly chlorotic; in recently infected plants, fruits might not be produced or, if produced, are small and unmarketable. In common bean, some TYLCVs produce the bean leaf crumple disease, with thickening, epinasty, crumpling, blade reduction and upward curling of leaves, as well as abnormal shoot proliferation and internode reduction; the very small leaves result in a bushy appearance.     

Characterisation and genetic diversity of pepper leafroll virus,a new bipartite begomovirus infecting pepper,bean and tomato in Peru

The complete genome of a novel bipartite begomovirus (genus Begomovirus, family Geminiviridae) was cloned from a severely diseased yellow Peruvian chili pepper (Capsicum baccatum cv. Pendulum) plant collected in the department of La Libertad, Northern Peru and full‐length sequenced. The two genomic components share a common region of 156 nucleotides with a 100% sequence identity. Analysis of the genome organisation and phylogenetic comparisons revealed that the virus is a typical New World begomovirus. The closest related begomovirus, an isolate of Tomato yellow vein streak virus (ToYVSV), shared only 76.8% nucleotide sequence identity for the DNA‐A component. Therefore, following species demarcation criteria of the International Committee on Taxonomy of Viruses, this virus isolate belongs to a new begomovirus species for which the name pepper leafroll virus (PepLRV) is proposed. Pepper plants infected with the cloned PepLRV isolate developed leaf roll symptoms similar to those observed in field‐infected plants suggesting this virus as the causal agent of the disease syndrome observed in the field. Widespread occurrence of PepLRV throughout Peru was demonstrated, infecting plants of diverse cultivated species such as tomato, pepper, common and pallar beans, and of the weed species Nicandra physaloides. Low genetic diversity was observed among PepLRV isolates present in this country with no evident geographical or temporal structure of the population, typical of a recent founder effect. This is the first report of a begomovirus infecting pepper and bean crops in Peru.     

Structural Diversity and State of Knowledge of Flavonoids of the <Emphasis Type="Italic">Scutellaria</Emphasis> L. Genus

The review presents the results of scientometric analysis of data on the level of study and chemical diversity of flavonoids of the Scutellaria L. genus species of the world’s flora. Flavonoid composition in 63 species of skullcap is reported, together with the data on distribution in plants, structure, and sources of 301 flavonoids belonging to the groups of flavones, flavanones, flavanonols, flavonols, chalcones, isoflavones, flavolignans, and bioflavonoids. The greatest number of flavonoids was shown to be isolated from plants of S. indica, S. baicalensis, S. barbata, S. amoena, S. prostrata, S. galericulata, S. discolor, S. ramosissima, and S. supina. Scientometric studies indicate the constantly growing interest in the study of species of the Scutellaria L. genus by scientists of various branches of science, including phytochemists, biologists, and pharmacologists. Information provided in the review can be used to address issues of chemosystematics of plants of the Scutellaria L. genus.     

Temporal Effects of a Begomovirus Infection and Host Plant Resistance on the Preference and Development of an Insect Vector,Bemisia tabaci,and Implications for Epidemics

Persistent plant viruses, by altering phenotypic and physiological traits of their hosts, could modulate the host preference and fitness of hemipteran vectors. A majority of such modulations increase vector preference for virus-infected plants and improve vector fitness, ultimately favouring virus spread. Nevertheless, it remains unclear how these virus-induced modulations on vectors vary temporally, and whether host resistance to the pathogen influences such effects. This study addressed the two questions using a Begomovirus-whitefly-tomato model pathosystem. Tomato yellow leaf curl virus (TYLCV) -susceptible and TYLCV-resistant tomato genotypes were evaluated by whitefly-mediated transmission assays. Quantitative PCR revealed that virus accumulation decreased after an initial spike in all genotypes. TYLCV accumulation was less in resistant than in susceptible genotypes at 3, 6, and 12 weeks post inoculation (WPI). TYLCV acquisition by whiteflies over time from resistant and susceptible genotypes was also consistent with virus accumulation in the host plant. Furthermore, preference assays indicated that non-viruliferous whiteflies preferred virus-infected plants, whereas viruliferous whiteflies preferred non-infected plants. However, this effect was prominent only with the susceptible genotype at 6 WPI. The development of whiteflies on non-infected susceptible and resistant genotypes was not significantly different. However, developmental time was reduced when a susceptible genotype was infected with TYLCV. Together, these results suggest that vector preference and development could be affected by the timing of infection and by host resistance. These effects could play a crucial role in TYLCV epidemics.     

Hyperspectral and Thermal Imaging of Oilseed Rape (Brassica napus) Response to Fungal Species of the Genus Alternaria

In this paper, thermal (8-13 µm) and hyperspectral imaging in visible and near infrared (VNIR) and short wavelength infrared (SWIR) ranges were used to elaborate a method of early detection of biotic stresses caused by fungal species belonging to the genus Alternaria that were host (Alternaria alternata, Alternaria brassicae, and Alternaria brassicicola) and non-host (Alternaria dauci) pathogens to oilseed rape (Brassica napus L.). The measurements of disease severity for chosen dates after inoculation were compared to temperature distributions on infected leaves and to averaged reflectance characteristics. Statistical analysis revealed that leaf temperature distributions on particular days after inoculation and respective spectral characteristics, especially in the SWIR range (1000-2500 nm), significantly differed for the leaves inoculated with A. dauci from the other species of Alternaria as well as from leaves of non-treated plants. The significant differences in leaf temperature of the studied Alternaria species were observed in various stages of infection development. The classification experiments were performed on the hyperspectral data of the leaf surfaces to distinguish days after inoculation and Alternaria species. The second-derivative transformation of the spectral data together with back-propagation neural networks (BNNs) appeared to be the best combination for classification of days after inoculation (prediction accuracy 90.5%) and Alternaria species (prediction accuracy 80.5%).     

Molecular identification and genetic diversity among six Begomovirus isolates affecting cultivation of cucurbitaceous crops in Uttar Pradesh,India

Begomoviruses cause a number of serious diseases of cultivated crops and are considered as the major constraint for the cultivation of several crops all over the world. During a survey in the years 2007 and 2008, the typical symptoms of Begomovirus (yellow mosaic and yellow vein) were observed on six cucurbitaceous crops, viz. bitter gourd (Momordica charantia L.), pointed gourd (Trichosanthes dioica), pumpkin (Cucurbita pepo), pumpkin (Cucurbita maxima), sponge gourd (Luffa cylindrica) and ridged gourd (Luffa acutangula) being cultivated in northern India. Begomovirus infection was suspected due to significant infestation of whiteflies (Bemisia tabaci, the known vector of Begomoviruses) on these species. The presence of Begomovirus was detected from the total DNA extracted from six infected leaf samples of these species by polymerase chain reaction using the specific primers of a well-characterised Begomovirus. The ～800 bp amplicons of these isolates were cloned, sequenced and the data obtained were compared with each other and with sequence database available in GenBank for best sequence identities and phylogenetic relationships. Based on highest 97–99% sequence identities and closest phylogenetic relationships, four representative Begomovirus species were identified as Ageratum enation virus (from T. dioica), Squash leaf curl China virus (from C. maxima), Tomato leaf curl New Delhi virus (from M. charantia, L. cylindrica and L. acutangula) and Tomato leaf curl Palampur virus (from C. pepo). These results suggested the existence of a high genetic diversity among Begomoviruses infecting cucurbitaceous crops.     

降水差异对内蒙古温带草原植物根系和叶片功能性状的影响北大核心CSCD

植物能够调整叶片或根系功能性状的变化来适应气候变化,从而形成多样性的环境适应策略。该研究以内蒙古温带草原降水量存在差异的草原群落(较为湿润的多伦草原和较为干旱的正镶白旗草原)为研究对象,分别测定两个草原群落优势种(多伦20种和正镶白旗13种)和共有种(8种)的根系功能性状(根直径、比根长和根组织密度)和叶片功能性状(叶面积、比叶面积、叶干物质含量、叶碳含量、叶氮含量),分析根叶性状的变异规律及其关系,以探究草种对不同降水环境的适应策略,为不同降水区域草种的选择以及草原经营管理提供理论依据。结果显示:(1)随着降水量的变化,草种的根性状在两个群落间无显著差异,多伦草原植物的比叶面积显著高于正镶白旗草原,而叶干物质含量和叶碳含量则呈相反趋势;两个草原群落的单子叶禾草类植物根直径最小,比根长最大,叶干物质含量最高;单子叶百合科类植物的根组织密度最小,叶干物质含量和叶氮含量最低;双子叶非禾草类植物的根组织密度最大,比叶面积最高而叶面积最低。(2)随降水量的减少,两个群落共有种的叶面积、比叶面积降低,叶干物质含量和叶碳含量增加;其中,羊草、冰草、糙隐子草的直径增加,比根长和根组织密度降低,而猪毛蒿的直径和比根长则呈相反的趋势;糙隐子草、羊草、猪毛蒿的叶氮含量增加,冰草、克氏针茅叶氮含量降低。(3)主成分分析表明,单个草原或综合两个草原的植物根与叶性状几乎不相关。研究表明,单子叶禾草类植物的根系性状在降水变化中差异较为明显,而双子叶非禾草类植物基本没有变化;干旱对植物叶性状的影响大于根性状;在不同降水环境下,草原植物根系存在独特的资源获取方式,根与叶性状对环境变化的适应策略均具有独立性。     

Antagonism of Cucumber green mottle mosaic virus against Tomato leaf curl New Delhi virus in zucchini and cucumber

Tomato leaf curl New Delhi begomovirus (ToLCNDV) (genus Begomovirus, family Begomoviridae) and Cucumber green mottle mosaic tobamovirus (CGMMV) (genus Tobamovirus, family Virgaviridae) cause diseases in cucurbit crops and are of increasing importance in many parts of the world. Both virus species belong to different families and have different modes of transmission, but share common hosts. We examined single and mixed infections of these viruses in cucumber and zucchini. Cucumber plants single-infected with CGMMV and co-infected with ToLCNDV, produced identical tobamovirus-specific symptoms, and had reduced growth and number of fruits when compared with single ToLCNDV infections. Zucchini infected with CGMMV remained symptomless but when infected with ToLCNDV only, most developed severe begomovirus-specific symptoms, and had reduced vegetative development and less fruits. Fewer zucchini plants with ToLCNDV co-infected with CGMMV produced symptoms than those infected with ToLCNDV only. When inoculated with CGMMV, this tobamovirus accumulated at similar rates in single and mixed infections with ToLCNDV in cucumber as well as zucchini, whereas the begomovirus accumulated significantly less when co-infected with CGMMV. The results suggest the existence of an antagonistic effect of CGMMV against ToLCNDV accumulation in cucumber. Such effect would also explain similar differences in viral loads, the vegetative and reproductive development, and the reduced symptom expression in zucchini.     

Silencing of a single gene in tomato plants resistant to Tomato yellow leaf curl virus renders them susceptible to the virus

A reverse-genetics approach was applied to identify genes involved in Tomato yellow leaf curl virus (TYLCV) resistance, taking advantage of two tomato inbred lines from the same breeding program—one susceptible (S), one resistant (R—that used Solanum habrochaites as the source of resistance. cDNA libraries from inoculated and non-inoculated R and S plants were compared, postulating that genes preferentially expressed in the R line may be part of the network sustaining resistance to TYLCV. Further, we assumed that silencing genes located at important nodes of the network would lead to collapse of resistance. Approximately 70 different cDNAs representing genes preferentially expressed in R plants were isolated and their genes identified by comparison with public databases. A Permease I-like protein gene encoding a transmembranal transporter was further studied: it was preferentially expressed in R plants and its expression was enhanced several-fold following TYLCV inoculation. Silencing of the Permease gene of R plants using Tobacco rattle virus-induced gene silencing led to loss of resistance, expressed as development of disease symptoms typical of infected susceptible plants and accumulation of large amounts of virus. Silencing of another membrane protein gene preferentially expressed in R plants, Pectin methylesterase, previously shown to be involved in Tobacco mosaic virus translocation, did not lead to collapse of resistance of R plants. Thus, silencing of a single gene can lead to collapse of resistance, but not every gene preferentially expressed in the R line has the same effect, upon silencing, on resistance.     

Begomoviruses Associated with Yellow Leaf Curl Disease of Tomato in Iran*

The occurrence of Tomato yellow leaf curl virus (TYLCV; genus Begomovirus, family Geminiviridae) in the major tomato‐growing areas of Iran was determined using TAS‐ELISA and PCR. The nucleotide sequences of the coat protein (CP) gene and intergenic region (IR) of eight Iranian isolates were determined. CP nucleotide identities among the Iranian isolates were 96–98%, and showed 94–96% identity with TYLCV‐IR [IR:Ira:98] and TYLCV‐IL [IL:Reo:86]. However, they showed low identity (68–69%) with ToLCIRV‐[IR:Ira]. Sequence analyses of IR indicated that seven Iranian isolates had sequence identity of 93–100% with each other, and 76% identity with the Jiroft isolate; identities of 75–79% with TYLCV‐IR[IR:Ira:98] were observed in every case, and 59–62% identity with ToLCIRV‐[IR:Ira]. The IR nucleotide sequences of Iranian isolates showed 92–93% identity with TYLCV‐IL[IL:Reo:86], except the Jiroft isolate (75%). The CP and IR sequence analyses suggested that eight Iranian TYLCV isolates probably differ from ToLCIRV‐[IR:Ira]. Based on IR sequence comparisons and phylogenetic analyses, the Iranian isolates were divided into two groups. The first major group (A), consists of seven virus isolates, was most closely related to TYLCV‐IL[IL:Reo:86], and relatively divergent from TYLCV‐IR [IR:Ira:98] and ToLCIRV‐[IR:Ira]. However, the Jiroft isolate from group B did not show high similarity with TYLCV‐IR[IR:Ira:98], ToLCIRV‐[IR:Ira], and TYLCV‐IL[IL:Reo:86], suggesting that the isolate may be a divergent variant. The differences are in a range that suggests different strains or species from TYLCV‐IR[IR:Ira:98] and ToLCIRV‐[IR:Ira] are probably associated with tomato yellow leaf curl disease in Iran.     

Preliminary Investigations about the Mode of Transmission and Spread of Xanthomonas campestris pv. graminis on Forage Grasses

Extensive studies showed that no disease was caused when seeds of different forage grasses were inoculated with Xanthomonas campestris pv. graminis. The disease could easily be induced by infecting the plants in the root system, leaves or flower. The inoculation site in the leaf proved to be of vital importance for the development of the disease. Wilting symptoms were quickly induced when the pathogen was inoculated near the leaf base. Plants in root-contact with diseased plants showed disease symptoms. It is not known whether these symptoms were caused by the bacteria or by toxins released by nearby diseased plants. Cross inoculation trials on different grass varieties revealed that different pathovars exist in the group of xanthomonads, pathogenic to forage grasses. Some have a broad host range whereas others are more limited to a single plant genus. Field trials suggest that in Belgian climatic conditions, the losses caused by bacterial wilt are rather limited.     

Different response of Alyssum montanum and Helianthus annuus to cadmium bioaccumulation mediated by the endophyte fungus Serendipita indica

Environmental pollution by heavy metals is a severe issue worldwide. Microbe-assisted phytoremediation is a safe, inexpensive, and promising strategy in refinement of metal-polluted regions. Current in vitro work was installed to study effects of the endophytic fungus Serendipita indica on some physiological traits and cadmium (Cd) bioaccumulation of Alyssum montanum and Helianthus annuus seedlings grown in MS medium, under varying levels of Cd (0, 20, 40 and 60 mg Cd/l medium). Even though Cd stress induced phyto-toxicity in both tested species, but a significant improvement was found in biomass accumulation, photosynthetic pigments content, and chlorophyll fluorescence indicators in inoculated seedlings by S. indica under different doses of Cd in media. The non-infected A. montanum plantlets accumulated more Cd in shoot than root, and illustrated the properties of an accumulator species as evidenced by translocation factor (TF) and bioaccumulation factor of shoot (BFS) higher than 1. Contrary to this, un-colonized H. annuus seedlings had higher amount of Cd in root than shoot and showed a phyto-stabilizer feature, as evidenced by TF˂1 and bioaccumulation factor of root (BFR) higher than 1. Presence of S. indica significantly enhanced Cd accumulation in root, while it noticeably diminished Cd amounts of shoot in both A. montanum and H. annuus seedlings, so that inoculated plants had higher values for BFR against lower values for BFS and TF, in compare to non-inoculated ones. These findings indicated that S. indica can be considered as a bio-fertilizer to improve the physiological characteristics of tested species under Cd stress, as well as a bio-stabilizer of Cd in the roots of A. montanum and H. annuus in the regions exposed to toxic levels of Cd.     

Acquisition of an Agrobacterium Ri Plasmid and Pathogenicity by Other α-Proteobacteria in Cucumber and Tomato Crops Affected by Root Mat

Root mat of cucumbers and tomatoes has previously been shown to be caused by Agrobacterium radiobacter strains harboring a root-inducing Ri plasmid (pRi). Nine other pRi-harboring α-Proteobacteria have subsequently been isolated from root mat-infected crops. Fatty acid profiling and partial 16S rRNA sequence analysis identified three of these strains as being in the genus Ochrobactrum, five as being in the genus Rhizobium, and one as being in the genus Sinorhizobium. An in vitro pathogenicity test involving inoculation of cucumber cotyledons was developed. All pRi-harboring α-Proteobacteria induced typical root mat symptoms from the cotyledons. Average transformation rates for rhizogenic Ochrobactrum (46%) and Rhizobium (44%) strains were lower than those observed for rhizogenic A. radiobacter strains (64%). However, individual strains from these three genera all had transformation rates comparable to those observed from cotyledons inoculated with a rhizogenic Sinorhizobium strain (75%).     

Yield increase induced by the fungal root endophyte Piriformospora indica in barley grown at low temperature is nutrient limited

The fungal root endophytes Chaetomium globosum, Epicoccum nigrum and Piriformospora indica have value as biocontrol and biofertilising organisms in barley, but have not been well tested at low temperatures. This study assessed the efficacy of the endophytes on barley varieties grown under low temperature stress with variable nutrient input. Seed from three cultivars of spring barley were inoculated with one of the three fungal root endophyte isolates – C. globosum, E. nigrum or P. indica - and grown in low temperature under higher and lower nutrient input regimes. Compared with the control, for P.indica-inoculated plants with the higher nutrient input, flowering was earlier and grain dry weight significantly greater for all barley varieties by a mean of 22 %. The nitrogen and carbon content of the grains did not differ significantly between treatments. Chaetomium globosum and Epicoccum nigrum conferred no significant benefits under either nutrient regime. Piriformospora indica is amenable to axenic culture, sporulates readily and can be multiplied rapidly, suggesting that it could be developed as an effective crop treatment in low temperature stressed barley and may have the potential to increase crop yield in colder growing conditions provided that adequate nutrients are supplied.     

Breeding for resistance to whitefly-transmitted geminiviruses

Geminiviruses comprise a large and diverse family of viruses that infect a wide range of important monocotyledonous and dicotyledonous crop species and cause significant yield losses. The family Geminiviridae is divided into three genera, one of which is Begomovirus. Species of this genus are transmitted by the whitefly Bemisia tabaci in a persistent, circulative manner and infect dicotyledonous plants. Severe population outbreaks of B. tabaci are usually accompanied by a high incidence of begomoviruses. During the last two decades, there has been a worldwide spread of the B biotype of B. tabaci, accompanied by the emergence of whitefly‐transmitted geminiviruses. Control measures in infected regions are based mainly on limitation of vector populations, using chemicals or physical barriers. However, under conditions of severe whitefly attack, none of these control measures has sufficed to prevent virus spread. Thus, the best way to reduce geminivirus damage is by breeding crops resistant or tolerant to the virus, either by classical breeding or by genetic engineering. A number of begomoviruses have been the subject of much investigation, due to their severe economic impact. This review considers the most severe viral diseases of four major crops (tomato, bean, cassava and cotton). The approaches taken to breed for resistance to these viral diseases should provide a perspective of the issues involved in breeding for begomovirus resistance in crop plants.     

香蕉植株内生细菌群落多态性研究

采用平板法对香蕉(Musa nana)植株的内生细菌进行分离纯化,并采用细菌脂肪酸法进行鉴定。结果表明,从香蕉的健康植株和感病植株中共分离得到内生细菌21属24种。从健株分离得到9种内生细菌,其中根、茎和叶分别分离到6种、2种和8种内生细菌。从病株分离得到15属17种内生细菌,其中根、茎和叶分别分离到3种、11种和6种。香蕉健株根部的内生细菌含量最高,达5.195×106cfu g-1,下部叶片内生细菌的含量最低,仅为30 cfu g-1;香蕉病株茎部内生细菌的数量显著高于其他部位,达1.05×107cfu g-1。这说明香蕉在不同生长状态下,其内生细菌的种类和数量存在多样性。