Identification of vectors of rice yellow mottle virus in Tanzania

Rice yellow mottle virus (RYMV) is specific to Africa and has been reported in some countries in East Africa and almost all the countries in West Africa. At present, it is undoubtedly the most important disease of rice in Tanzania. It was first reported in the 1980's. It has spread fast and is now found in almost all the rice growing areas. In view of the increasing incidence and importance of RYMV on rice production in Tanzania, studies on the epidemiology of the disease were initiated in order to find ways of controlling the disease. Transmission studies were carried out on seventy‐seven species of beetles and grasshoppers collected from different rice growing locations to determine vector identity. Four vectors have been identified (three chrysomelids; Dactylispa sp., Chaetocnema sp. and Chaetocnema pulla) and one tetrigid grasshopper. The wide distribution of Chaetocnema spp. in the RYMV endemic areas suggests that the species are the most important vectors responsible for infections in these areas.     

Resistance in rice to Rice yellow mottle virus and evidence of non-seed transmission

Nine rice cultivars were evaluated under screenhouse conditions for resistance to Rice yellow mottle virus (RYMV) and possible seed transmission. Completely randomised design with three replications was used. In Experiment 1, the seedlings were inoculated with the virus at two weeks after planting. In Experiment 2, the seeds collected from Experiment 1 were dried for four weeks before planting. For each genotype, the seeds from healthy plants were planted as a control. Disease incidence and severity (scales 1–9), yield and yield components were recorded. Statistical analyses included Area Under the Disease Progress Curve (AUDPC) and independent t test. The cultivars FARO 37, FARO 52 and Gigante were highly resistant, whereas WAB189-B38HB was resistant. Paddy yield was highest (3.6 g) in FARO 37. There were no symptoms of virus disease in all the plants originating from the seeds of RYMV-infected plants. The differences between the seeds from infected and healthy plants for all the measured traits were not significant (p > 0.05). The number of days to seedling emergence was uniform (5.5 days) in all the cultivars. Plant height, number of tillers per plant, number of days to heading and paddy yield from the seeds of virus-infected plants varied from 54.8 to 68.4 cm, 17 to 21, 85.3 to 96 days and 2.7 to 4 g, respectively. Conversely, a range of 54.9–68.7 cm, 17–22, 83–95 days and 2.8–4.1 g was found in the seeds of healthy plants. Selection and cultivation of high-yielding, resistant and healthy seeds would enhance food security.     

The rice yellow mottle virus P1 protein exhibits dual functions to suppress and activate gene silencing

In plants RNA silencing is a host defense mechanism against viral infection, in which double‐strand RNA is processed into 21–24‐nt short interfering RNA (siRNA). Silencing spreads from cell to cell and systemically through a sequence‐specific signal to limit the propagation of the virus. To counteract this defense mechanism, viruses encode suppressors of silencing. The P1 protein encoded by the rice yellow mottle virus (RYMV) displays suppression activity with variable efficiency, according to the isolates that they originated from. Here, we show that P1 proteins from two RYMV isolates displaying contrasting suppression strength reduced local silencing induced by single‐strand and double‐strand RNA in Nicotiana benthamiana leaves. This suppression was associated with a slight and a severe reduction in 21‐ and 24‐nt siRNA accumulation, respectively. Unexpectedly, cell‐to‐cell movement and systemic propagation of silencing were enhanced in P1‐expressing Nicotiana plants. When transgenically expressed in rice, P1 proteins induced specific deregulation of DCL4‐dependent endogenous siRNA pathways, whereas the other endogenous pathways were not affected. As DCL4‐dependent pathways play a key role in rice development, the expression of P1 viral proteins was associated with the same severe developmental defects in spikelets as in dcl4 mutants. Overall, our results demonstrate that a single viral protein displays multiple effects on both endogenous and exogenous silencing, not only in a suppressive but also in an enhancive manner. This suggests that P1 proteins play a key role in maintaining a subtle equilibrium between defense and counter‐defense mechanisms, to insure efficient virus multiplication and the preservation of host integrity.     

Mutations in the eIF(iso)4G translation initiation factor confer high resistance of rice to Rice yellow mottle virus

We report here evidence of the role that the isoform of the eukaryotic translation initiation factor 4G (eIF(iso)4G) plays in naturally occurring resistance in plant/virus interactions. A genetic and physical mapping approach was developed to isolate the Rymv1 locus controlling the high recessive resistance to Rice yellow mottle virus (RYMV) in the rice (Oryza sativa) variety Gigante. The locus was mapped to a 160-kb interval containing a gene from the eIF(iso)4G family. The stable transformation of a resistant line with the cDNA of this gene, derived from a susceptible variety, resulted in the loss of resistance in transgenic plants. The allelic variability of this gene was analysed in three resistant and 17 susceptible varieties from different cultivated rice species or subspecies. Compared with susceptible varieties, resistant varieties present specific alleles, characterized by either amino acid substitutions or short amino-acid deletions in the middle domain of the protein. The structure of this domain was modelled and showed that the substitutions were clustered on a small surface patch. This suggests that this domain may be involved in an interaction with the virus.     

Occurrence of symptomless source of Piper yellow mottle virus in black pepper (Piper nigrum L.) varieties and a wild Piper species

Symptomless nature of Piper yellow mottle virus (PYMoV) infection in three varieties of black pepper (Piper nigrum) (Panniyur 1, Panniyur 5 and Panchami) and a wild species of Piper (Piper colubrinum) was confirmed by polymerase chain reaction (PCR) using PYMoV specific primers. The virus could be transmitted from these PYMoV-infected symptomless plants onto symptom producing black pepper cv. Karimunda through mealybug vector, Ferrisia virgata and by graft transmission. About 20–50% seedlings showed typical symptoms of the PYMoV at 30 days after mealybug inoculations while it was 75–94% at 90 days after inoculation. PCR test of the inoculated seedlings confirmed the presence of PYMoV in 50–64%, 76–100% and 80–100% of plants in 30, 60 and 90 days after inoculation, respectively. Similarly, 50–66%, 91–100% and 100% of graft-transmitted plants showed typical symptoms of the disease at 30, 60 and 90 days after grafting. PCR test of the graft-transmitted plants showed 100% PYMoV infection at 60 days after grafting. The results clearly demonstrated the existence of PYMoV-infected symptomless plants that can act as source for secondary spread of the virus in the field.     

Genetic basis and mapping of the resistance to rice yellow mottle virus. II. Evidence of a complementary epistasis between two QTLs

 The genetic basis of resistance to rice yellow mottle virus (RYMV) was studied in a doubled-haploid (DH) population derived from a cross between the very susceptible indica variety ‘IR64’ and the resistant upland japonica variety Azucena. As a quantitative trait locus (QTL) involved in virus content estimated with an ELISA test has been previously identified on chromosome 12, we performed a wide search for interactions between this QTL and the rest of the genome, and between this QTL and morphological traits segregating in the population. Multiple regression with all identified genetic factors was used to validate the interactions. Significant epistasis accounting for a major part of the total genetic variation was observed. A complementary epistasis between the QTL located on chromosome 12 and a QTL located on chromosome 7 could be the major genetic factor controlling the virus content. Resistance was also affected by a morphology-dependent mechanism since tillering was interfering with the resistance mechanism conditioned by the epistasis between the two QTLs. Marker-assisted backcross breeding was developed to introgress the QTLs of chromosome 7 and chromosome 12 in the susceptible ‘IR64’ genetic background. First results confirmed that if both QTLs do not segregate in a backcross-derived F₂ population, then the QTL of chromosome 12 cannot explain differences in virus content. A near-isogenic line (NIL) approach is currently being developed to confirm the proposed genetic model of resistance to RYMV. Received: 20 April 1990 / Accepted: 30 April 1998     

Evaluation of maize/pepper intercropping model in the management of pepper veinal mottle virus,genus Potyvirus,family Potyviridae on cultivated pepper (Capsicum annuum L.) in Nigeria

Three pepper cultivars obtained from National Horticultural Research Institute (NIHORT) Idi-Ishin, Ibadan were intercropped with maize for two planting seasons between March and September in each year. These pepper cultivars were NHV1-D96, and NHV1-E96 and NHV1-F96. A 90-day maturing maize variety (DMSR-1) was used as the intercropping companion plant. The pepper seedlings were raised in a greenhouse. A randomised complete block design was used for this experiment. Each variety was intercropped with maize and replicated three times including the sole plot. The results obtained for each year were not significantly different from each other. There was a significant difference in pepper veinal mottle virus (PVMV) disease incidence and severity at a probability of less than 5% in the treatment used. PVMV disease incidence and severity was relatively higher in the sole pepper crop compared with pepper intercropped with maize. In the three varieties of peppers intercropped with maize, less than 17% disease incidence and 15% disease severity were recorded in all the varieties with a minimum yield of 4 tons per hectare compared with the sole pepper cropping of the same variety that recorded as high as 75% disease incidence and 72% disease severity with a maximum yield of 3.3 tons per hectare. There was a significant negative correlation at probability less than 0.05 between disease incidence, severity and the fruit yield of pepper. Variety NHV1-F96 in the maize intercrop recorded the highest yield of 15.99 tons/ha with a land equivalent ratio of 2.4 tons/ha. The success of the PVMV disease management evaluated in this study was judged by the extent of reduction in number of diseased plants and by an increase in vigor of the cultivated pepper crop, with an increase in fruit yield and quality. This signifies that for devising effective viral disease management for any crop it is important that the vectors of the virus present in the particular area are exactly controlled from having contact with the target plant. The reduction of pest incidence with intercropping of non-host plants should be carefully considered.     

First Report of a Strain of Alternanthera yellow vein virus Infecting Eclipta prostrate (L.) L. (Compositae) in China

In 2005, Eclipta prostrata plants exhibiting yellow vein symptoms were observed in Guangzhou, Guangdong province, China. A virus isolate G8 was cloned from a symptomatic plant. The complete nucleotide sequence of G8 DNA-A was determined to be 2745 nucleotides, which had typical characteristics of Begomovirus genome organization. The comparison of complete nucleotide sequence of DNA-A showed that isolate G8 shared the highest sequence identity with Alternanthera yellow vein virus (AlYVV) isolates G38 and Hn51 at 95.9% and 94.3%, respectively. These results show that G8 infecting E. prostrata in Guangdong is a strain of AlYVV.     

Genetic basis and mapping of the resistance to Rice yellow mottle virus. III. Analysis of QTL efficiency in introgressed progenies confirmed the hypothesis of complementary epistasis between two resistance QTLs

Our previous studies have hypothesised that a complementary epistasis between a QTL located on chromosome 12 and a QTL located on chromosome 7 was one of the major genetic factors controlling partial resistance to Rice yellow mottle virus (RYMV). We report research undertaken to verify this hypothesis and to introgress the resistant allele of these two QTLs from an upland resistant japonica variety, Azucena, into a lowland susceptible indica variety IR64. Three cycles of molecular marker-assisted back cross breeding were performed using RFLP and microsatellite markers. Resistance to RYMV was evaluated in F₂ and F₃ offspring of the BC₁ and BC₂ generations. Marker-assisted introgression (MAI) was very efficient: in the selected BC₃ progeny the proportion of the recipient genome was close to 95% for the ten non-carrier chromosomes, and the length of the donor chromosome segment surrounding the two QTLs was less than 20 cM. The relevancy of the complementary epistasis genetic model proposed previously was confirmed experimentally: in BC₁ and BC₂ generations only F3 lines having the allele of the resistant parent on QTL₁₂ and QTL₇ show partial resistance to RYMV. Comparison of our experimental process of MAI with the recommendations of analytic and simulation studies pointed out the methodological flexibility of MAI. Our results also confirmed the widely admitted, but rarely verified, assumption that QTL-alleles detected in segregating populations could be treated as units of Mendelian inheritance and that the incorporation of these alleles into elite lines would result in an enhanced performance. The next step will be the design of tools for the routine use of molecular markers in breeding for partial resistance to RYMV and the development of material for the analysis of resistance mechanisms and the structure of a virus resistance gene in rice. Received: 11 August 2000 / Accepted: 20 March 2001     

Occurrence of latent virus infection in visually-rated cowpea (Vigna unguiculata L. Walp) seedlings

Abstract

The presence of latent infections was studied in five cowpeas varieties. Seeds of the varieties were planted and the seedlings inoculated with antigens from Cucumber mosaic virus (CMV) genus Cucumovirus, Bean common mosaic virus (BCMV) genus Potyvirus (Blackeye cowpea mosaic virus strain), Southern bean mosaic virus (SBMV) genus Sobemovirus and Cowpea mottle virus (CPMoV) genus Carmovirus seven days after planting. Seedlings expressing symptoms were rouged at two weeks after inoculation, while asymptomatic ones were subjected to serological indexing to detect the presence/absence of latent infection. Protein A-sandwich enzyme-linked immunosorbent assay (PAS ELISA) was employed for the serological detection of CMV, SBMV and CPMoV, while antigen-coated plate (ACP) ELISA was used to detect BCMV in the asymptomatic plants. Cowpea seedlings without virus symptoms but with positive serological reactions were considered as being latently infected. All of the inoculated TVu 1272 and SuVita-2 plants showed symptoms consistent with CMV and CPMoV infections, respectively. The rate of CMV latent infection was high in TVu 1179 (14.5%), low in SuVita-2 (1.3%) but not recorded in TVu 1272.     

Factors affecting seedling recruitment in an invasive grass (Pennisetum setaceum) and a native grass (Heteropogon contortus) in the Hawaiian Islands

In the Hawaiian Islands, native Heteropogon contortus (pili grass) is being replaced by alien grasses, one of which is Pennisetum setaceum (fountain grass). Both grasses depend on seeds for population growth. To help understand factors promoting the spread of the alien and decline of the native, we investigated the effects of physical disturbance, nutrient addition, and seed supplementation on seedling recruitment in experimental field plots. In the first year, our field site experienced an unusual drought, and seedling recruitment was greater for H. contortus than for P. setaceum under all treatments. Disturbance increased recruitment of H. contortus seedlings during some sampling periods. Recruitment was not significantly increased by seed additions for either species despite our finding of only 49 and 4 seeds m^–2 in the seed bank for H. contortus and P. setaceum, respectively. In the first year, most P. setaceum seedlings died between monthly surveys. We resurveyed our field plots in a second, wetter year and found the pattern was reversed: recruitment of P. setaceum seedlings was greater than H. contortus seedlings in most treatments. Greenhouse comparisons of seedling survival under three drought regimes (water every 5,7 and 10 days) revealed that H. contortus seedlings tolerate drought better than P. setaceum seedlings. Seedling recruitment for these species in the leeward Hawaiian Islands appears to be primarily dependent on water availability, with the alien having the advantage in wetter years. Once seedlings of the long-lived alien become established, the alien seems capable of maintaining its dominance over H. contortus, even during periods of drought.     

Chickpea chlorotic dwarf virus,a new leafhopper-transmitted geminivirus of chickpea in India1

A disease of chickpea in India, characterised by chlorosis, severe stunting and phloem browning, was shown to be caused by a geminivirus. This virus was transmitted by the leafhopper Orosius orientalis from chickpea to chickpea and several other plant species. A method for purification of this virus was devised and a polyclonal antiserum produced. The majority of the purified particles were geminate. The size of the coat protein was shown to be 32 kD and the nucleic acid was shown to be circular ssDNA of 2900 nucleotides. By immunosorbent electron microscopy this virus was shown to be unrelated to the leafhopper-transmitted geminiviruses known to infect dicotyledons such as beet curly top, bean summer death and tobacco yellow dwarf viruses. On the basis of particle morphology, leafhopper transmission, host range and serology this virus was considered to be a new, hitherto undescribed, geminivirus and was named chickpea chlorotic dwarf virus.     

Effects of turnip mosaic virus and turnip yellow mosaic virus on the survival, growth and reproduction of wild cabbage (Brassica oleracea)

Wild plants of Brassica oleracea (wild cabbage) are commonly infected with turnip mosaic poty virus (TuMV), turnip yellow mosaic tymovirus (TYMV) and several other viruses. A field experiment in which plants were inoculated either with TuMV or TYMV showed that virus infection significantly reduced survival, growth and reproduction. Relative to water inoculated-controls, plants infected with TYMV had greater mortality, were shorter, had a smaller leaf area and number, showed a greater amount of damage from herbivory and chlorosis, were less likely to flower and produced fewer pods and lower total seed output. Plants infected with TuMV did not appear to be adversely affected at first; however, mortality after 18 months was higher than control plants. Although TuMV infection had no effect on the number of plants flowering, the infected plants did produce fewer pods and a lower total seed output. We conclude that both viruses can significantly affect vegetative and reproductive performance of wild cabbage and hence that introgression of virus resistance (particularly when conferred by a major gene or a transgene) from a crop might increase plant fitness in natural populations of this species. Ecological risk assessments of virus resistance transgenes must do more than survey adult plants in natural populations for the presence of the target virus. Failure to detect the virus could be due to high mortality on infection with the virus.     

Mapping and introgression of a tomato yellow leaf curl virus tolerance gene,TY-1

The whitefly-transmitted tomato yellow-leaf curl gemini-virus (TYLCV) is a major pathogen of tomatoes. The wild tomato species Lycopersicon chilense, which is resistant to the virus, was crossed to the cultivated tomato, L. esculentum. The backcross-1 selfed (BC1S1) generation was inoculated and a symptomless plant was selected. This plant was analyzed using 61 molecular markers, which span the tomato genome, to determine which L. chilense chromosome segments were introgressed. A BC2S1 population was cage-inoculated with viroliferous whiteflies (Bemisia tabaci), the natural insect vector of the virus, and subjected to RFLP analysis. Markers on chromosomes 3 and 6 were significantly associated with the level of tolerance; the association of chromosome-6 markers was further substantiated in two additional BC2S1 populations. A tolerant BC2S1 plant which was homozygous for L. chilense introgressions in chromosomes 3, 6 and 7 was crossed to generate a BC3S1 population which was planted in an infested field. A TYLCV-tolerance gene with partial dominance, TY-1, was mapped to chromosome 6; two modifier genes were mapped to chromosomes 3 and 7. Field and whitefly-mediated cage inoculations of nearly-isogenic lines in BC3S3 supported our conclusion that TY-1 is the major TYLCV-tolerance locus.     

新疆地区烟粉虱生物型的区域分布及其携带的番茄黄化曲叶病毒检测

【目的】明确不同烟粉虱Bemisia tabaci (Gennadius)生物型在新疆地区的区域分布及其携带番茄黄化曲叶病毒的情况。【方法】本文利用烟粉虱mtDNA COⅠ基因片段作标记以及番茄黄化曲叶病毒(tomato yellow leaf curl virus, TYLCV)特异性引物, 鉴定新疆不同地区烟粉虱生物型, 并检测其携带TYLCV情况。【结果】石河子市、 博乐市、 岳普湖县、 泽普县、 叶城县、 莎车县、 墨玉县、 和田市与和田县烟粉虱属于B型烟粉虱, 且720 bp COⅠ基因序列一致性为100%; 阿克苏市、 哈密市、 鄯善县、 乌鲁木齐市、 克拉玛依市与伊宁市烟粉虱属于Q型烟粉虱, 720 bp COⅠ基因序列一致性为99.9%~100%; 吐鲁番市与库尔勒市兼有B型烟粉虱和Q型烟粉虱。伊宁市、 乌鲁木齐市、 鄯善县、 哈密市、 阿克苏市、 库尔勒市、 莎车县、 和田县和墨玉县的烟粉虱样本携带TYLCV, 除2011年底已经暴发危害的莎车县外, 其他区域均为番茄黄化曲叶病毒病发生的高风险区域。【结论】本研究为新疆地区烟粉虱及番茄黄化曲叶病毒病的科学防控提供依据。     

烟粉虱携带番茄黄化曲叶病毒检测及其传入山东省的考证

【背景】番茄黄化曲叶病毒（TYLCV）是由媒介昆虫烟粉虱传播的一种双生病毒,对蔬菜及烟草等经济作物造成严重危害。前人资料表明,该病毒于2006年传人我国南方地区,2007年传人山东省,2008年后在山东各地逐渐蔓延扩散。【方法】为了考证TYLcV传人山东省的时间,本研究利用mtCOI基因对于2005和2006年7—8月份在山东省不同地区作物上共采集的15份烟粉虱样品进行了生物型鉴定,并进一步检测了烟粉虱携带TYLCV情况,同时对PCR扩增产物进行了测序分析。【结果】2005年的4份样品烟粉虱生物型均为B型,均不携带TYLCV。2006年的11份烟粉虱样品为B型与Q型混合样品,其中,2份烟粉虱样品检测到TYLCV,进一步证实该病毒为TYLCV。【结论与意义】本研究首次证实了TYLCV早在2006年就已经传入山东省。研究结果不仅对于防控该病毒具有重要指导意义,而且对于其入侵生物学研究也具有重要参考价值。     

Barley yellow dwarf virus: effects on carbohydrate metabolism in oat (Avena sativa) during cold hardening

Barley yellow dwarf virus (BYDV) causes significant losses in yield and in overwintering ability of winter cereals. Mechanisms by which the physiology of plants is affected by the virus are not clear. To see how carbohydrates in the crown of winter cereals were affected by BYDV, fructan isomers of degree of polymerization (DP) 3–5, fructan DP>6 and the simple sugars, glucose, fructose and sucrose, were measured before and during cold hardening in three oat ( Avena sativa L.) cultivars, 'Wintok', 'Coast Black' and 'Fulghum'. On a fresh weight basis fructan DP>6 decreased by 50% in infected 'Wintok' and 'Coast Black' and by 25% in 'Fulghum'. Two DP3, one DP4 and one DP5 isomer were significantly higher than non-infected controls. The percentages of simple sugars in infected crowns were significantly higher than controls in all three cultivars in every week except the first week of hardening. Crude enzyme extracts from BYDV infected plants incubated with sucrose suggested higher invertase and lower sucrose-sucrosyl transferase activity. When incubated with 1-kestose and neokestin, no significant difference was found in fructose fructosyl transferase or in hydrolase activity. The activity of unidentified enzymes catalysing the synthesis of larger (DP>5) fructan was altered by BYDV. The decrease of carbohydrates in the crown induced indirectly by BYDV may alter the plant's capacity to regenerate tillers in the spring. The ability of plants to prevent or tolerate carbohydrate fluctuations induced by BYDV infection may be an important genetically regulated characteristic for developing virus-resistant cultivars.     

Characterisation of a variant of zucchini yellow fleck virus (ZYFV), a potyvirus causing a wilt disease of melons in Israel*

A flexuous filamentous virus 750 nm in length was isolated from field-grown melons (Cucumis melo L.) and squirting cucumber (Ecballium elaterium L.). Infection of melon seedlings of most commercial varieties resulted in a lethal wilting, but late infections gave dieback. The virus infected systemically all economically important cucurbit crops. Its coat protein consists of a single polypeptide, 34 500 D in size, encapsidating a single stranded RNA genome of 10.1 kb. Antiserum against the Italian zucchini yellow fleck virus (ZYFV) reacted specifically with the Israeli isolate in both ISEM and Western blot. The virus was aphid transmissible in a non-persistent manner. Based on host range, host reaction and serological data, it is suggested that the virus described is an Israeli variant of ZYFV.