Early signs of infection in Cacao swollen shoot virus (CSSV) inoculated cocoa seeds and the discovery of the cotyledons of the resultant plants as rich sources of CSSV

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.