Systemic Spread and Propagation of a Plant-Pathogenic Virus in European Honeybees, Apis mellifera

Li, Ji Lian; Cornman, R. Scott; Evans, Jay D.; Pettis, Jeffery S.; Zhao, Yan; Murphy, Charles; Peng, Wen Jun; Wu, Jie; Hamilton, Michele; Boncristiani, Humberto F.; Zhou, Liang; Hammond, John; Chen, Yan Ping

TRSV is a type species of the genus Nepovirus within the family Secoviridae (16) . TRSV infects a wide range of herbaceous crops and woody plants, some of considerable economic importance. The infected plants show discoloration, malformation, and stunted growth, accompanied by reduced seed yield or almost total seed loss due to flower and pod abortion. Of a number of plant diseases caused by TRSV, bud blight disease of soybean (Glycine max L.) is the most severe. It is characterized by necrotic ring spots on the foliage, curving of the terminal bud, and rapid wilting and eventual death of the entire plant, resulting in a yield loss of 25 to 100% (17) . Like other members of the genus, TRSV has a bipartite genome of positive-sense, single-stranded polyadenylated RNA molecules, RNA-1 and RNA-2, which are encapsidated in separate virions of similar size. Both RNA molecules possess a genome-linked protein (Vpg) covalently bound at their 5= ends. RNA-1 encodes a large polyprotein precursor that is proteolytically processed into protease cofactor (P1A), putative ATPdependent helicase (Hel), picornain 3C-like protease (Pro), and RNA-directed RNA polymerase (Pol). RNA2 encodes a virion capsid protein (CP), a putative movement protein (MP), and an N-terminal domain involved in RNA-2 replication (P2A). Proteins encoded by RNA-1 are required for RNA replication, while proteins encoded by RNA-2 function in cell-to-cell movement and viral RNA encapsulation. RNA-1 is capable of replication independently of RNA-2, but both are required for systemic infection. Transmission of TRSV can occur in several ways. The numerous vectors include a dagger nematode (18) , aphids, thrips, grasshoppers, and tobacco flea beetle (19) (20) (21) ; however, vertical transmission through seeds is important for long-distance dispersal of the virus (22) . It has also been shown that honeybees transmit TRSV when they move between flowers and transfer virusborne pollen from infected plants to healthy ones (23) (24) (25) (26) . It was, however, unknown prior to our study whether honeybees could become infected by plant viruses they physically encounter or consume.