Reciprocal exchanges involving virion and complementary sense genes, the LIR and SIR between the two clones of maize streak mastrevirus result in ameliorated symptoms in maize

Abstract

A genetic analysis of maize streak mastrevirus (MSV) virulence was conducted using two infectious DNA clones derived from isolates collected in Kenya. Virulence was tested on the susceptible Pioneer hybrid 3379 using vascular puncture inoculation of kernels. Marked differences in the severity of chlorotic streaks and stunting of seedlings were associated with mild [pMSV-KL (mild)] and severe [pMSVKm( severe)] parental infectious MSV DNA clones. To identify determinants conditioning these differences, chimeric clones were constructed from parental clones employing restriction endonuclease fragments. Clone identities were confirmed by restriction mapping. Putative virulence determinants were identified for genomic fragments encoding the replication initiator (Rep and RepA), movement (MP) and coat (CP) protein ORFs and for those containing non-coding long (LIR) and short intergenic regions (SIR). Recombinant clones containing LIRs plus the 5'–terminus of Rep/RepA (first 189 nt) ORFs from pMSVKm( Severe) and pMSV-KL(Mild) reciprocally exchanged displayed intermediate symptoms. Complementary replacement of the Rep/RepA ORF resulted in symptoms indistinguishable from those of parental clones. The recombinant clone constructed with LIR, Rep and RepA genome sequences from pMSV-Km(Severe) and MP, CP and SIR sequences from pMSV-KL(Mild) showed significantly more severe symptoms and accumulated higher concentrations than pMSV-Km(Severe). In contrast, the reciprocal clone showed significantly milder symptoms and lower viral titers than pMSV-KL(Mild). Viral accumulation was correlated with the intensity of leaf chlorosis and the degree of plant stunting. These data support the hypothesis that MSV virulence is a polymorphic trait involving virion and complementary sense genes (LIR and SIR).