Application of a Simplified Molecular Protocol to Reveal Mixed Infections with Begomoviruses in Cassava

Samples of cassava leaves exhibiting severe symptoms of cassava mosaic disease (CMD) were collected with the PhytoPASS kit in fields surrounding the city of Bujumbura (Burundi). These materials were then sent to Belgium for polymerase chain reaction determination of the CMD begomoviruses inducing the observed symptoms. Different pairs of specific primers were used to amplify DNA sequences specific to African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Malawi virus (EACMMV), East African cassava mosaic Zanzibar virus (EACMZV), the Uganda variant of East African cassava mosaic virus (EACMV-UG) and South African cassava mosaic virus (SACMV). It was revealed that mixed infections were prevailing in the analyzed materials. Most of the samples submitted to this analysis were found to be co-infected by three different begomoviruses (ACMV + EACMV + EACMV-UG). The so revealed mixed infections could explain the high severity of CMD symptoms noticed on cassava in the region of Bujumbura while the diversity within the CMD causal agents illustrates the importance to take this parameter into consideration for a successful use of plant genetic resistance to control the disease.     

Comparative Analysis of Tissue Tropism of Bipartite Geminiviruses

Abutilon mosaic virus (AbMV), a bipartite geminivirus of the genus Begomovirus, has been vegetatively propagated for many years in Abutilon sellovianum in which it is strictly phloem-restricted. Using in situ hybridization and immunological analyses, the tissue tropism of AbMV in the laboratory host Nicotiana benthamiana was compared with that of two other bipartite begomoviruses, African cassava mosaic virus (ACMV) and tomato golden mosaic virus (TGMV). Analysis of the first systemically infected leaves and longitudinal sections of axillary and flower buds revealed that all three viruses are initially confined to the vascular traces, although both ACMV and TGMV are later detectable in nearly all tissue types. In contrast, AbMV remained strictly phloem-limited in this host throughout the course of infection. The ability of ACMV and TGMV to move out of N. benthamiana phloem tissues is correlated with the development of severe symptoms in comparison with the mild symptoms associated with AbMV infection. It was also demonstrated that Sida micrantha mosaic virus, a virus that is closely related to AbMV, is phloem-limited in Malva parviflora even though it induces severe leaf curl, stunting and necrosis in this host. The present data demonstrate that bipartite begomoviruses can exhibit strikingly different patterns of tissue tropism.     

Particle purification and properties of cassava Ivorian bacilliform virus

A virus found in cassava from the north-west of the Ivory Coast was transmitted by inoculation with sap extracts to herbaceous species in six plant families. Chenopodium quinoa was used as a propagation host and C. murale was used for local lesion assays. The virus particles are bacilliform, c. 18 nm in diameter, with predominant lengths of 42,49 and 76 nm and a structure apparently similar to that found in alfalfa mosaic virus. Purified preparations of virus particles had A₂₆₀/A₂₈₀ of 1.7 ±0.05, contained one protein of M_rc. 22 000, and yielded three species of RNA with M_r (× 10^-6) of c. 0.7, 0.8 and 1.2. Although the virus particles were poorly immunogenic, an antiserum was produced and the virus was detected by enzyme-linked immunosorbent assay (DAS-ELISA) in leaf extracts at concentrations down to c. 6 ng/ml. Four other field isolates were also detected, including a strain which caused only mild systemic symptoms in C. quinoa instead of necrosis. The naturally infected cassava source plants were also infected with African cassava mosaic virus (ACMV) but when the new virus was cultured in Nicotiana benthamiana, either separately or together with ACMV, its concentration was the same. The new virus did not react with antisera to several plant viruses with small bacilliform or quasi-bacilliform particles, and alfalfa mosaic virus reacted only weakly and inconsistently with antiserum to the cassava virus. The new virus, for which the name cassava Ivorian bacilliform virus is proposed, is tentatively classified as the second member of the alfalfa mosaic virus group.     

Isolation and characterisation of a new Venezuelan strain of cassava common mosaic virus

A new strain of cassava common mosaic virus, designated C_sCMV-ve, was isolated from cassava (Manihot esculenta Crantz) plants growing in Venezuela. The 520 nm long semi-flexuous virus particles sedimented in sucrose gradients as one component and contained a single coat protein molecule of c. 28 kDa, and a ssRNA of c. 2.1 times 10⁶. A single dsRNA species of c. 4.2 times 10⁶kDA was isolated from virus-infected cassava leaves. In double antibody sandwich enzyme-linked immunosorbent assay, C_sCMV-ve reacted strongly with antisera to C_sCMV and potato virus X, but not with those to other typical members of the potexvirus group. Ultrastructural studies of C_sCMV-ve infected cells showed the presence of large bundles of inclusion bodies in the cytoplasm, a proliferation of the rough endoplasmic reticulum, and large crystals in the peroxisomes. C_sCMV-ve in crude buffered sap or in purified preparations was not mechanically transmitted to known hosts of the Chaya and Brazilian strains of C_sCMV. Isolated virus RNA also failed to infect a range of test-plant species. This is the first report of the presence of C_sCMV in Venezuela.     

Can Salicylic Acid Affect the Intercellular Transport of the Tobacco Mosaic Virus by Changing Plasmodesmal Permeability?

We studied the effects of salicylic acid (SA) on the plasmodesmal permeability as evaluated by the tobacco mosaic virus (TMV) spreading in tobacco Nicotiana glutinosaleaves, where TMV induces necrotic lesions. When leaves were treated with SA simultaneously with their viral inoculation, SA retarded the development of necrotic lesions and reduced their number. When inoculated leaves were kept on the SA solution at an elevated temperature (31°C) for a short period of time, the size of the necrotic lesions, which developed after leaf transfer to room temperature, was decreased. SA stimulated the formation of rapid callose involved in the control of the plasmodesmal permeability, which was assessed from fluorescence after tissue staining with Aniline Blue. On the basis of these data, we suggest that SA suppressed TMV spreading in the inoculated tobacco leaves by reducing the plasmodesmal permeability.     

斯里兰卡木薯花叶病毒在亚洲的暴发、传播与防控

木薯是重要的经济与粮食作物,由于其出色的耐旱、耐贫瘠、易种植等特性,在全球热带和亚热带地区广泛种植。木薯在生长过程中会受到多种病虫害的威胁,其中分布最广、危害最严重的是木薯花叶病毒(Cassava mosaic virus,CMV)。CMV主要通过带毒茎秆和媒介昆虫烟粉虱传播,可以造成木薯叶片出现斑驳花叶、卷曲甚至枯萎,植株整体长势衰弱,导致木薯产量下降,严重威胁木薯生产。我国和东南亚木薯产区此前并没有CMV发生和危害的报道。可近年来,斯里兰卡木薯花叶病毒(Sri Lanka cassava mosaic virus,SLCMV)在东南亚地区广泛传播,已经成为该地区木薯生产的主要病害,对我国的木薯生产也构成了重大威胁。本文围绕SLCMV在亚洲的发生、传播机制和防治策略展开了讨论,以期为防控SLCMV在我国的危害提供参考。