Mechanisms underlying systemic invasion of pea plants by pea enation mosaic virus

Leaves of Pisum sativum L.cv. 'Perfected Wales' were inoculated with pea enation mosaic virus, at the 3rd and 4th nodes as early as size permitted. Systemic virus spread was then measured over a period of approximately 9 days by assaying the infectivity of 1-cm stem pieces above the 4th and below the 3rd node. The presence of virus antigen and double-stranded (ds) RNA in these stem pieces was measured by radioimmunoassay over the same period. Relative infectivity and amounts of dsRNA in infected tissue were lowest approximately 3 days after inoculation and sharply increased thereafter. A transient plasmodesmatal abnormality was observed 4-6 days after inoculation. The transient nature of the abnormality, which was preceded (24 h) by low infectivity and dsRNA concentrations, may indicate a plant defense reaction that is subsequently overcome. The rise in dsRNA concentration, starting the 4th day after inoculation, coincided with the increase of vesicular material (cytopathological structures) in the phloem of infected plants. Vesicular material was found in sieve elements that were mature at the time of inoculation. Their presence in this tissue can only be explained by transport of the vesicles from infected sites.