Transcriptome of the Maize Leafhopper ( Dalbulus maidis ) and Its Transcriptional Response to Maize Rayado Fino Virus (MRFV), Which It Transmits in a Persistent, Propagative Manner

The transcriptome of the corn leafhopper, D. maidis , revealed conserved biochemical pathways for immunity and discovered transcripts responsive to MRFV-infected plants at two time points, providing a basis for functional identification of genes that either limit or promote the virus-vector interaction. Compared to other hopper species and the propagative plant viruses they transmit, D. maidis shared 15 responsive transcripts with S. furcifera (to southern rice black-streaked dwarf virus [SRBSDV]), one with G. nigrifrons (to maize fine streak virus [MFSV]), and one with P. maidis (to maize mosaic virus [MMV]), but no virus-responsive transcripts identified were shared among all four hopper vector species.

Infectious Maize rayado fino virus from Cloned cDNA

A full-length cDNA clone was produced from a U.S. isolate of Maize rayado fino virus (MRFV), the type member of the genus Marafivirus within the family Tymoviridae. Infectivity of transcripts derived from cDNA clones was demonstrated by infection of maize plants and protoplasts, as well as by transmission via the known leafhopper vectors Dalbulus maidis and Graminella nigrifrons that transmit the virus in a persistent-propagative manner. Infection of maize plants through vascular puncture inoculation of seed with transcript RNA resulted in the induction of fine stipple stripe symptoms typical of those produced by wild-type MRFV and a frequency of infection comparable with that of the wild type. Northern and Western blotting confirmed the production of MRFV-specific RNAs and proteins in infected plants and protoplasts. An unanticipated increase in subgenomic RNA synthesis over levels in infected plants was observed in protoplasts infected with either wild-type or cloned virus. A conserved cleavage site motif previously demonstrated to function in both Oat blue dwarf virus capsid protein and tymoviral nonstructural protein processing was identified near the amino terminus of the MRFV replicase polyprotein, suggesting that cleavage at this site also may occur.

Identification of Resistance to Maize rayado fino virus in Maize Inbred Lines

Maize rayado fino virus (MRFV) causes one of the most important virus diseases of maize in America. Severe yield losses, ranging from 10 to 50% in landraces to nearly 100% in contemporary cultivars, have been reported. Resistance has been reported in maize populations, but few resistant inbred lines have been identified. Maize inbred lines representing the range of diversity in the cultivated types and selected lines known to be resistant to other viruses were evaluated to identify novel sources of resistance to MRFV. The virus was transmitted to maize seedlings using the vector Dalbulus maidis, and disease incidence and severity were evaluated beginning 7 days postinoculation. Most of the 36 lines tested were susceptible to MRFV, with mean disease incidence ranging from 21 to 96%, and severity from 1.0 to 4.3 (using a 0 to 5 severity scale). A few genotypes, including CML333 and Ki11, showed intermediate levels of resistance, with 14 and 10% incidence, respectively. Novel sources of resistance, with incidence of less than 5% and severity ratings of 0.4 or less, included the inbred lines Oh1VI, CML287, and Cuba. In Oh1VI, resistance appeared to be dominant, and segregation of resistance in F2 plants was consistent with one or two resistance genes. The discovery of novel sources of resistance in maize inbred lines will facilitate the identification of virus resistance genes and their incorporation into breeding programs.