Abstract
Despite the huge efforts and funds expended to try and bring Cacao swollen shoot virus (CSSV) disease under control, the disease has remained largely intractable. Inspired by the need to inject new ideas into the current breeding programmes for CSSV resistance, the interactions between cocoa and the virus in CSSV-infected plants from 3 days post inoculation (dpi) to about 30 dpi were studied using both microscopic and molecular methods. Cellular modifications such as nucleic acid-rich inclusion bodies, apoptosis, and the in situ localisation of the virus in novel tissues that is cotyledons and hypocotyl, were discerned by microscopic examinations. These cellular modifications were observed in cotyledons of seedlings as young as 8 dpi. By validating these microscopic results using PCR it was also discovered that the cotyledons of the inoculated plants were virus-rich. CSSV was amplified or detected in total DNAs extracted from all 4 CSSV-isolates studied, and in some cases the detection was from as early as 3dpi plants. Compared to cocoa leaves, the traditional source from which CSSV or its DNA is usually extracted, the cotyledons were much easier to process and analyse. The significance of these findings to the CSSV-resistant breeding programmes, and to CSSV research in general is discussed. Key words: CSSV, apoptosis, in situ localisation, nucleic acid-rich inclusion bodies, microscopic, examination.
Highlights
Cacao swollen shoot virus (CSSV) causes a devastating disease of cocoa (Theobroma cacao L.), the main economic crop of some West African countries, causing significant economic loss to those countries
The isolates were selected based on the uniqueness of the symptoms they induced in an infected plant (Table 1): CSSV 1A induces red-vein banding that transforms into chlorosis as the leaf matures, and stem and root swellings in an infected plant
Some of the cells from CSSV-infected cotyledons were shown to contain spherical inclusion bodies, which stained for nucleic acids using the periodic acid Schiff toluidine blue (PASCT) and azure bromide blue staining methods
Summary
Cacao swollen shoot virus (CSSV) causes a devastating disease of cocoa (Theobroma cacao L.), the main economic crop of some West African countries, causing significant economic loss to those countries. To control CSSV in the field, either the mealybug or the virus has to be targeted but attempts to target these two agents have so far not yielded the desired results. Breeding for CSSV resistance, a virus targeted strategy, has been recognised as the most sustainable among the known control measures-at least in Ghana (Dzahini-Obiatey et al, 2006; Posnette, 1981). Lack of rapid and reliable screening methods is hampering progress. Crucial information on the interaction between the virus and cocoa, which can feed into the development of such rapid screening methods are lacking
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