Serological and molecular characterization of begomoviruses infecting cassava (Manihot esculenta Crantz) in Africa
Loading...
Date
2001
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Sokoine University of Agriculture
Abstract
The outbreak of an extremely severe form of cassava mosaic virus disease (CMD) that is sweeping across East and Central Africa and causing food shortages and consequently famine related deaths provided the incentive for this study. A diagnostic survey of the disease was conducted in cassava growing areas of Kenya, to identify the viruses associated with the disease and to study the presence
of whitefly transmitted begomoviruses namely, African cassava mosaic virus(ACMV), East African cassava mosaic virus (EACMV) and the recombinant Uganda variant virus (UgV). This “new" UgV virus was of particular interest, since it was this virus type which was found persistently associated with the severe form of CMD. In addition, cassava samples were obtained from many other cassava growing regions in sub-Sahara Africa, to gain an overview of the identities and distribution of viruses causing CMD in Africa.A total cf 230 leaf samples and 185 hardwood stem cuttings were collected and analyzed. Disease severity data were collected and viruses were typed and characterized using ELISA and PCR. In Kenya, the disease incidence was highest (80-100%) in the western regions neighbouring Uganda and lowest (25-50%) at the coastal region of the country. In Western and Nyanza provinces, 52.2% of the samples tested were infected with UgV, 21.7% with ACMV and 17.4% with mixed infections of ACMV and UgV. All samples from the coast province were infected with EACMV only and no other begomovirus was found in this cassava growing
region. However, in about 15% of the cassava samples, filamentous viruses were found, which were not detected in samples from other regions. When cassava samples obtained from other major cassava cultivating regions of Africa were analyzed, ACMV was the only virus found in West Africa, with very few exceptions of EACMV that was detected in samples from Nigeria and Guinea. Outside Kenya, UgV was detected in samples from Uganda and the Democratic Republic of the Congo, D.R.C. This indicates that UgV is rapidly spreading into western regions of the continent thereby endangering the cassava production in West Africa. Symptoms of begomovirus diseases in cassava were indistinguishable, however,the most distinct and outstanding feature of UgV infections was the almost complete reduction of tuber formation and often very severe fol'ar symptoms which were also induced by double infections with both ACMV and UgV. Symptoms in Nif-C'Hana bentham'ana plants infected with an ACMV isolate Corn Kenya, ACMV-KE, could be distinguished from an ACMV isolate from Nigeria, ACMV-NG, by the bright yellow blotches which are typically induced by ACMV-KE on the inoculated leaves. Symptoms induced by EACMV in N. benthamiana were similar to those induced by UgV and considerably milder than those induced by either ACMV-KE or ACMV-NG. Severely affected N. benthamiana plants were mostly doubly infected with ACMV and UgV. The complete nucleotide (nt) sequences of DNA-A (2781 - 2801 nt) and DNA-B (2724 - 2726 nt) genomic components of selected virus types were obtained. The • sequences of DNA-A genomic components shared a high identity with those of
either ACMV, EACMV, or UgV already described, implying that DNA-A was highly conserved among the different cassava begomoviruses. The “new", recombinant UgV was verified by the characteristic and almost invariable recombination sequences present in the coat protein genes of all isolates sequenced. This sequence originates from ACMV, while all other DNA-A regions reflect typical sequences of EACMV. Since the recombination comprised only a short segment of the coat protein, the virus was verified as a strain of EACMV and hence named EACMV-UgV, which is used synonymously with UgV. Analysis of the DNA-B sequence of a Kenyan isolate of EACMV-UgV revealed a DNA-B component identical to those of ACMV isolates described. Furthermore, for all virus isolates with characteristic EACMV-UgV-like DNA-A genomic components, only ACMV-like DNA-B genomic components were amplified. Full-length clones of DNA-A and DNA-B genomic components were constructed and attempts were made to introduce homologous(ACMV/ACMV) and heterologous (EACMV/ACMV; EACMV-UgV/ACMV) combinations of DNA-A and DNA-B genomic components into N. benthamiana plants. Inoculation of linearized constructs containing full-length clones or of partial head-to-tail dimers of DNA-A
and DNA-B genomic components resulted in the introduction of DNA-A genomic components into the host plant. However, in heterologous combinations, only DNA- A genomic component was detected in inoculated plants which showed weak
Description
Dissertation
Keywords
Cassava mosaic virus disease, African cassava mosaic virus, Cassava production, Begomoviruses infecting cassava (Manihot esculenta Crantz)