Cytopathology of Pigeonpea sterility mosaic virus in pigeonpea and Nicotiana benthamiana: similarities with those of eriophyid mite-borne agents of undefined aetiology

Pigeonpea sterility mosaic virus (PPSMV) is transmitted by the eriophyid mite, Aceria cajani, and is very closely associated with sterility mosaic disease (SMD) of pigeonpea (Cajanus cajan) in the Indian subcontinent. Antiserum produced to purified PPSMV preparations detected a virus-specific 32 kDa protein in sap of SMD-affected pigeonpea plants by ELISA and Western blotting. PPSMV was transmitted mechanically in sap of SMD-affected pigeonpea leaves to Nicotiana benthamiana. Ultrastructural studies of symptom-bearing leaves of two pigeonpea cultivars, (ICP8863 and ICP2376) and N. benthamiana infected with PPSMV, detected quasi-spherical, membrane bound bodies (MBBs) of c. 100-150 nm and amorphous electron-dense material (EDM). These structures were distributed singly or in groups, in the cytoplasm of all cells, except those in conductive tissues. Fibrous inclusions (FIs), composed of randomly dispersed fibrils with electron lucent areas, were present in the cytoplasm of palisade cells and rarely in mesophyll cells of the two pigeonpea cultivars but were not detected in infected N. benthamiana plants. In the PPSMV-infected pigeonpea cultivars and N. benthamiana, immuno-gold labelling, using antiserum to PPSMV, specifically labelled the MBBs and associated EDM, but not the FIs. The MBBs and associated inclusions are similar in appearance to those reported for plants infected with the eriophyid mite-transmitted High Plains virus and the agents of unidentified aetiology associated with rose rosette, fig mosaic, thistle mosaic, wheat spot chlorosis and yellow ringspot of budwood. The nature of these different inclusions is discussed.