A novel mite-transmitted virus with a divided RNA genome closely associated with pigeonpea sterility mosaic disease

The agent of sterility mosaic, a disease that is a major constraint on pigeon pea (Cajanus cajan) production in the Indian subcontinent, is transmitted by the eriophyid mite, Aceria cajani. This agent has remained elusive for decades despite intensive efforts but we report the isolation of highly flexuous filamentous virus-like particles (VLPs) of 3 to 10 nm in width and of undefined lengths from sterility mosaic disease (SMD)-affected pigeon pea plants. Purified VLP preparations from virus-infected pigeon pea and Nicotiana benthamiana had a buoyant density in caesium chloride of 1.22-1.23 g/cm3 and contained a major virus-specific protein species of ~32 kDa and 5-7 RNA species of ~6.8 to 1.1 kb. The sequence of some complementary DNA clones to RNA from purified VLP preparations had no significant matches in database searches. Two oligonucleotide primers derived from one such sequence, when used in reverse transcriptase-polymerase chain reaction assays, amplified a product of 321 bp specifically from SMD-affected pigeon pea plants. Purified VLP preparations were used to produce polyclonal antibodies that, in infected plants, detected the virus using enzyme-linked immunosorbent assay (ELISA) and the virus-specific 32-kDa protein in western immunoblotting (WIB). In such assays, the virus was detected consistently in all SMD-affected pigeon pea plant samples from several different locations in India, but not in samples from symptom-free pigeon pea plants from the same locations. In experimental studies, all pigeon pea plants inoculated with viruliferous A. cajani and those plants graft-inoculated with SMD-affected tissue were infected with the virus as assessed by ELISA and WIB, but not any uninfected pigeon pea plants. This virus, tentatively named Pigeon pea sterility mosaic virus (PPSMV), has some properties similar to virus species in the genera Tospovirus and Tenuivirus and with the eriophyid mite-transmitted High plains virus (HPV) but is distinct from these and from all other characterized viruses. The combination of novel properties shown by PPSMV and HPV suggest that they may constitute species in a new genus of plant viruses.