1International Crops Research Institute for the Semi-Arid Tropics, Patancheru-502324, India.
2Scottish Crop Research Institute, Invergowrie DD2 5DA, Dundee, Scotland, UK.
Abstracts of Research Papers Presented during the National Symposium of Indian Virological Society at Unit of Plant Virology, Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi-110 012, October 14–1.
Sterility mosaic (SMD) is the major disease of pigeonpea responsible for annual yield losses of worth >US$ 300 million. This disease first described in 1931, is endemic in the Indian subcontinent and recognized only in pigeonpea-growing countries of Asia. The SMD-affected plants show mosaic symptoms and greatly reduced or complete cessation of flowering. The SMD causal agent transmitted by the eriophyid mite, Aceria cajani (Acari: Arthropoda), has remained elusive to identification and characterization over many decades. Studies demonstrated that it is not a fungus, bacterium or phytoplasma-like agent or is caused by mite toxemia. In the absence of other likely causes and on the basis of symptoms and transmission by mites, the SMD agent was assumed to be a virus but, despite several attempts, the causal virus has remained elusive.
Application of a new purification procedure resulted in preparations containing aggregates of highly flexuous, filamentous virus-like particles (VLPs) of 8 to 11 nm diameter, and they contained a major protein of 32 kDa and up to 6 segmented RNA species of size 6.8–1.1 kb. Such particles were isolated consistently from all SMD-affected plant samples collected from different locations of peninsular India and from SMD-affected pigeonpea samples infected by graft inoculation, and by infective mites. Because of this very close association, the virus was named, pigeonpea sterility mosaic virus (PPSMV) and was the first evidence of a causal agent for SMD, ending seven decades of quest for its etiology.
The purified PPSMV preparations were not infective to plants, but the virus was transmitted experimentally by mechanical inoculation of fresh leaf sap extracts of SMD-affected pigeonpea to Nicotiana benthamiana and N. clevelandii, but not to pigeonpea. Using viruliferous mites PPSMV was transmitted to most wild relatives of pigeonpea, and also to Phaseolus vulgaris. Polyclonal antibodies to PPSMV preparations produced in a rabbit detected PPSMV in plant tissues by ELISA. The nucleotide sequences of several cDNA clones derived from viral RNA have no significant homology with other reported viruses. An RT-PCR assay for PPSMV detection was developed with oligonucleotide primers sequences derived from the RNA-5. The transmission efficiency of PPSMV to pigeonpea by single A. cajani was up to 53% and it was 100% when >5 mites per plant were used. A. cajani acquired PPSMV after a minimum acquisition access period of 15 min, and transmitted the virus after a minimum inoculation access period of 90 min. No latent period was observed or evidence of transovarial transmission. Electron microscopy of ultrathin sections of the infected plants detected three types of inclusions; quasi-spherical membrane bound-bodies (MBBs) of 100–150 nm diameter which were immuno-gold labelled with PPSMV antiserum, electron dense material (EDM) and fibrous inclusions (FIs). The MBBs 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. These agents, together with PPSMV and HPV, probably represent species in a new genus of plant viruses.
PPSMV isolates with varying degrees of severity and physico-chemical properties occur at various locations in the subcontinent, which are contributing to the breakdown in SMD-resistance. Characterization of PPSMV isolates and determination of their geographic distribution is essential to select and deploy appropriate resistant varieties for sustainable SMD management.
