Effect of environmental conditions and leafhopper gender on Maize chlorotic dwarf virus transmission by Graminella nigrifrons (Homoptera: Cicadellidae)

Empoasca papayae (Hemiptera: Cicadellidae)-Mediated Transmission of Papaya Meleira Virus-Mexican Variant in Mexico

Papaya meleira virus (PMeV) causes sticky disease in Carica papaya in Brazil and Mexico. Despite its economic importance and the need for effective phytosanitary control, it remains unknown whether any insect is the vector of this virus. The aim of this work was to identify potential insect vectors of the PMeV-Mexican variant (PMeV-Mx) and determine whether these potential vectors are capable of transmitting the virus. Adult insects were collected in papaya fields in the south-southeast region of Mexico and were identified morphologically and molecularly. Their abundance and frequency were determined, and quantitative reverse transcription polymerase chain reaction was performed to establish if they carried PMeV-Mx. The Cicadellidae family (Hemiptera) was the most diverse and abundant, and Empoasca papayae was the most abundant species and had the highest virus titers. PMeV-Mx transmission assays were conducted under controlled conditions using E. papayae on C. papaya 'Maradol'. E. papayae was a carrier of PMeV-Mx at 6 h after exposure, and its viral titer increased with time, peaking at 2.125 pg/μl of PMeV-Mx RNA from 20 ng/µl of cDNA, 5 days after exposure (dae). From 14 days after plants were exposed to insects, PMeV-Mx was detected and quantified in 100% of the evaluated papaya plants, whose viral RNA titer increased from 0.06 (21 dae) to 26.6 pg/μl of PMeV-Mx RNA (60 dae) from 20 ng/µl of cDNA. Three months later, these plants developed sticky disease symptoms, demonstrating that E. papayae is capable of transmitting PMeV-Mx to C. papaya 'Maradol'.

Genome Sequence of a Novel Iflavirus from the Leafhopper Graminella nigrifrons

A novel iflavirus was discovered from the transcriptome of the leafhopper Graminella nigrifrons. The assembled virus genome has 9,700 nucleotides (nt) and encodes a 3,035-amino-acid polyprotein. Sanger sequencing was used to confirm the sequence encoding the polyprotein and indicated a genome length of 9,617 nt with a polyadenylated tail.

Genetic insights into Graminella nigrifrons Competence for maize fine streak virus infection and transmission

BACKGROUND

Most plant-infecting rhabdoviruses are transmitted by one or a few closely related insect species. Additionally, intraspecific differences in transmission efficacy often exist among races/biotypes within vector species and among strains within a virus species. The black-faced leafhopper, Graminella nigrifrons, is the only known vector of the persistent propagative rhabdovirus Maize fine streak virus (MFSV). Only a small percentage of leafhoppers are capable of transmitting the virus, although the mechanisms underlying vector competence are not well understood.

METHODOLOGY

RNA-Seq was carried out to explore transcript expression changes and sequence variation in G. nigrifrons and MFSV that may be associated with the ability of the vector to acquire and transmit the virus. RT-qPCR assays were used to validate differential transcript accumulation.

RESULTS/SIGNIFICANCE

Feeding on MFSV-infected maize elicited a considerable transcriptional response in G. nigrifrons, with increased expression of cytoskeleton organization and immunity transcripts in infected leafhoppers. Differences between leafhoppers capable of transmitting MFSV, relative to non-transmitting but infected leafhoppers were more limited, which may reflect difficulties discerning between the two groups and/or the likelihood that the transmitter phenotype results from one or a few genetic differences. The ability of infected leafhoppers to transmit MFSV did not appear associated with virus transcript accumulation in the infected leafhoppers or sequence polymorphisms in the viral genome. However, the non-structural MFSV 3 gene was expressed at unexpectedly high levels in infected leafhoppers, suggesting it plays an active role in the infection of the insect host. The results of this study begin to define the functional roles of specific G. nigrifrons and MFSV genes in the viral transmission process.

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.