GERMPLASM CONSERVATION OF COCOA (THEOBROMA CACAO L.) AND VIRUS ELIMINATION THROUGH TISSUE CULTURE

Abstract

The tree Theobroma cacao also known as cocoa or cacao is a tropical crop with origins in the Amazon basin dominating the economies of most West African countries. Cocoa swollen shoot virus disease (CSSVD) is a major problem threatening the cocoa industry in West Africa and hence, the economies of producing countries. Since 1940, contradictory views have been expressed that the CSSV was not transmitted through cocoa seeds. Research carried out using PCR, real-time PCR and fragment analysis to test this claim indicated that the CSSV DNA could be found in every component part of the cocoa seeds and was also present in seedlings from pods harvested from CSSV -infected Amelonado cocoa trees. Control measures in the field have to date only resulted in the partial elimination of CSSV infected plants. Investigations were performed into the use of somatic embryogenesis to generate disease free clonal plant materials from CSSV -infected Amelonado trees. A series of virus indexing techniques including conventional PCR, real-time PCR and fragment analysis indicated that secondary somatic embryos generated from CSSV-infected cocoa trees were generally free ofCSSV. CSSV -infected budwood grafted to cocoa trees was used to demonstrate the movement of the virus to new flush leaves. Visual inspection of leaves was found to be inadequate for the detection of latent infections. PCR is a sensitive approach for CSSV detection and can highlight the disease in advance of the appearance of visual symptoms. Agarose-based analysis of PCR products was prone to false 'negatives' in the early stages of infection and took longer than capillary electrophoresis to detect the virus in the leaf samples from grafted plants. Long term conservation of cocoa using encapsulation-dehydration techniques indicated that somatic embryos from individual genotypes did not show any significant difference in survival in respect to LN storage duration. The survival response of the genotypes after rewarming at 35°C for 5 min was consistent across all LN storage durations. These results demonstrate the utility of encapsulation/dehydration as a means of establishing cryopreserved collections of a crop for which seed storage is not currently an option.