The genetics of virulence of the Maize Streak Mastrevirus (MSV)

A study of the genetics of virulence was conducted for Maize Streak Virus (MSV), genus Mastrevirus, family Geminiviridae. The genomes of four clones were sequenced and their virulence determined using the susceptible maize hybrid pioneer 3379 inoculated by vascular puncture inoculation. Virulence of clones (in parentheses) were mild (pMSV-KL), moderately mild (pMSV-Z), moderate (pMSV-KH), and severe (pMSV-Km). Virulence determinants were mapped using chimeric clones constructed from parental clones pMSV-KL (mild) and pMSV-Km (severe) employing restriction endonuclease fragments. Clone identities were confirmed by restriction mapping. Virulence determinants were identified for genomic fragments encoding the replication initiator (Rep and RepA) and the coat (CP) proteins and those containing the noncoding long (LIR) and small (SIR) intergenic regions. Recombinant clones containing the LIR plus N-terminus of the Rep/RepA (first 189 nt) gene from pMSV-Km (severe) and pMSV-KL (mild) reciprocally exchanged displayed an intermediate symptom phenotype. Complementary replacement of the Rep/RepA gene (donor) resulted in symptoms indistinguishable from those of the donor parental clone. The recombinant clone with LIR, Rep and RepA derived from pMSV-Km (severe) and CP, movement protein and SIR derived from pMSV-KL (Mild) incited significantly more severe symptoms and accumulated to higher concentrations than pMSV-Km (Severe), whereas the reciprocal clone showed significantly milder symptoms and lower viral titers than pMSV-KL (Mild). Viral accumulation was generally correlated with intensity of leaf chlorosis and degree of plant stunting. These data support the hypothesis that MSV virulence is a polymorphic trait involving virion and complementary sense genes, the LIR and SIR.