Plant Host Range and Leafhopper Transmission of Maize fine streak virus

Maize fine streak virus (MFSV), an emerging Rhabdovirus sp. in the genus Nucleorhabdovirus, is persistently transmitted by the black-faced leafhopper, Graminella nigrifrons (Forbes). MFSV was transmitted to maize, wheat, oat, rye, barley, foxtail, annual ryegrass, and quackgrass by G. nigrifrons. Parameters affecting efficiency of MFSV acquisition (infection) and transmission (inoculation) to maize were evaluated using single-leafhopper inoculations and enzyme-linked immunosorbent assay. MFSV was detected in ≈20% of leafhoppers that fed on infected plants but <10% of insects transmitted the virus. Nymphs became infected earlier and supported higher viral titers than adults but developmental stage at aquisition did not affect the rate of MFSV transmission. Viral titer and transmission also increased with longer post-first access to diseased periods (PADPs) (the sum of the intervals from the beginning of the acquisition access period to the end of the inoculation access period). Length of the acquisition access period was more important for virus accumulation in adults, whereas length of the interval between acquisition access and inoculation access was more important in nymphs. A threshold viral titer was needed for transmission but no transmission occurred, irrespective of titer, with a PADP of <4 weeks. MFSV was first detected by immunofluorescence confocal laser scanning microscopy at 2-week PADPs in midgut cells, hemocytes, and neural tissues; 3-week PADPs in tracheal cells; and 4-week PADPs in salivary glands, coinciding with the time of transmission to plants.

Complete Genome Sequence and In Planta Subcellular Localization of Maize Fine Streak Virus Proteins

ABSTRACT The genome of the nucleorhabdovirus maize fine streak virus (MFSV) consists of 13,782 nucleotides of nonsegmented, negative-sense, single-stranded RNA. The antigenomic strand consisted of seven open reading frames (ORFs), and transcripts of all ORFs were detected in infected plants. ORF1, ORF6, and ORF7 had significant similarities to the nucleocapsid protein (N), glycoprotein (G), and polymerase (L) genes of other rhabdoviruses, respectively, whereas the ORF2, ORF3, ORF4, and ORF5 proteins had no significant similarities. The N (ORF1), ORF4, and ORF5 proteins localized to nuclei, consistent with the presence of nuclear localization signals (NLSs) in these proteins. ORF5 likely encodes the matrix protein (M), based on its size, the position of its NLS, and the localization of fluorescent protein fusions to the nucleus. ORF2 probably encodes the phosphoprotein (P) because, like the P protein of Sonchus yellow net virus (SYNV), it was spread throughout the cell when expressed alone but was relocalized to a subnuclear locus when coexpressed with the MFSV N protein. Unexpectedly, coexpression of the MFSV N and P proteins, but not the orthologous proteins of SYNV, resulted in accumulations of both proteins in the nucleolus. The N and P protein relocalization was specific to cognate proteins of each virus. The subcellular localizations of the MFSV ORF3 and ORF4 proteins were distinct from that of the SYNV sc4 protein, suggesting different functions. To our knowledge, this is the first comparative study of the cellular localizations of plant rhabdoviral proteins. This study indicated that plant rhabdoviruses are diverse in genome sequence and viral protein interactions.

Maize fine streak virus, a New Leafhopper-Transmitted Rhabdovirus

A previously uncharacterized virus was isolated from fall-planted sweet corn (Zea mays L., Syngenta GSS 0966) leaves showing fine chlorotic streaks. Symptomatic plants were negative in enzyme-linked immunosorbent assay against many maize viruses, but reacted weakly with antisera to Sorghum stunt mosaic virus suggesting a distant relationship between the viruses. The virus was readily transmitted by vascular puncture inoculation (VPI), but not by leaf-rub inoculation. Symptoms on maize included dwarfing and fine chlorotic streaks along intermediate and small veins that developed 12 to 17 days post-VPI. The isolated virus was bacilliform (231 ± 5 nm long and 71 ± 2 nm wide), with a knobby surface, and obvious helical structure typical of rhabdovirus morphology. Nucleorhabdovirus virions were observed by transmission electron microscopy of infected maize leaf tissue sections. Proteins unique to infected plants were observed in extracts of infected leaves, and the isolated virion contained three proteins with molecular masses 82 ± 2, 50 ± 3, and 32 ± 2 kDa. Preliminary sequence analysis indicated the virus had similarity to members of the family Rhabdoviridae. The virus was transmitted by Graminella nigrifrons under persistent conditions. The data indicate the virus, provisionally designated Maize fine streak virus, is a new species in the genus Nucleorhabdovirus.