Ultrastructural Studies on Potato Phloem Cells Infected with Potato Leaf Roll Virus—Comparison of Two Potato Varieties

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Abstract
The sequence of cytopathological changes in the phloem of two potato cultivars, Osa and Apta, after infection by potato leaf roll virus (PLRV), was investigated. Osa is susceptible but tolerant to the virus, whereas Apta is resistant but sensitive. The cytopathological differences between the two potato varieties concern mainly the range of the apoplast changes in the parts of the phloem that are involved in the systemic spread of the infection in the plant. The sieve tubes of Apta, into which the virus particles are translocated, respond by intensive formation of callose, which results in their necrosis and inhibition of virus spread. In the case of Osa the callose reaction of the sieve tube was less intensive. It is assumed, on the basis of the present investigation, that the ability of the potato plant to establish apoplastic defence against the spread of the virus particles varies depending on the genotype of the cultivar.