In situ localization of cacao swollen shoot virus in agroinfected Theobroma cacao

Cacao swollen shoot virus (CSSV) is a small non-enveloped bacilliform virus with a double-stranded DNA genome. A very restricted host range and diffic ulties in transmitting the virus, either mechanically or via its natural ve ctor, have hindered the study of cacao swollen shoot disease. As an alterna tive to the particle-bombardment method previously reported, we investigate d another approach to infect Theobroma cacao. A greater-than-unit length co py (1.2) of the CSSV DNA genome was cloned into the Agrobacterium binary ve ctor pBin19 and was transferred into young plants via Agrobacterium tumefac iens. Typical leaf symptoms and stem swelling were observed seven and eleve n weeks post inoculation, respectively. Viral DNA, CSSV coat protein and vi rions were detected in leaves with symptoms. Agroinfected plants were used to study the in situ localization of CSSV and its histopathologic effects i n planta. In both leaves and petioles, virions were only seen in the cytopl asm of phloem companion cells and of a few xylem parenchyma cells. Light mi croscopy showed that stem swelling results from a proliferation of the xyle m, phloem acid cortex cells.