Effect of Host Resistance on Pathogenesis of Maize Dwarf Mosaic Virus

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Abstract
The pathogenesis of maize dwarf mosaic virus (MDMV-A) in maize was characterized in locally and systemically infected tissues of susceptible and resistant maize genotypes. Resistant genotypes used in this investigation had been classified by the absence of systemic symptom expression. Because some plants of the resistant genotype occasionally expressed systemic symptoms, we refer to the presence of symptoms as the susceptible phenotype and the absence of symptoms as the resistant phenotype. MDMV capsid protein was detected 2 days after inoculation in the inoculated leaf at the site of inoculation in both susceptible and resistant genotypes. Proximal invasion from the site of inoculation also was detected in both susceptible and resistant genotypes although invasion was delayed 6 days in the resistant genotypes. The resistant genotype, PB3187, expressed both susceptible and resistant phenotypes. When a leaf was inoculated before the emergence of the next three subsequent leaves, systemic symptoms were expressed (susceptible phenotype). However, when a leaf was inoculated after the emergence of the nest three subsequent leaves, the resistant phenotype was expressed. Infectious virus was found within the inoculated leaf, within stalk tissue below the inoculated leaf, and in the roots of PB3187 plants expressing either the susceptible or resistant phenotype. Infectious virus was detected only in tissue above the inoculated leaf in PB3187 plants expressing the susceptible phenotype but not in equivalent leaves of plants expressing the resistant phenotype. This phenomenon of differential phenotype expression was not affected by plant age or temperature. We propose that the resistance mechanism expressed in PB3187 acts at a specific point of systemic virus transport, thereby limiting upward virus transport from the roots to young, developing leaves.