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 difficulties in transmitting the virus, either mechanically or via its natural vector, have hindered the study of cacao swollen shoot disease. As an alternative to the particle-bombardment method previously reported, we investigated another approach to infect Theobroma cacao. A greater-than-unit length copy (1.2) of the CSSV DNA genome was cloned into the Agrobacterium binary vector pBin19 and was transferred into young plants via Agrobacterium tumefaciens. Typical leaf symptoms and stem swelling were observed seven and eleven weeks post inoculation, respectively. Viral DNA, CSSV coat protein and virions were detected in leaves with symptoms. Agroinfected plants were used to study the in situ localization of CSSV and its histopathologic effects in planta. In both leaves and petioles, virions were only seen in the cytoplasm of phloem companion cells and of a few xylem parenchyma cells. Light microscopy showed that stem swelling results from a proliferation of the xylem, phloem acid cortex cells.