Role of the beet western yellows virus readthrough protein in virus movement in Nicotiana clevelandii

Luteoviruses such as beet western yellows polerovirus (BWYV) are confined t o and multiply within the phloem compartment of their hosts. The readthroug h domain (RTD) of the minor BWYV capsid protein P74 is required for efficie nt virus accumulation in Nicotiana clevelandii. Experiments were carried ou t to determine if the low virus titres observed following agro-inoculation of whole plants with certain RTD mutants are due to a defect in virus multi plication in the nucleate cells of the phloem compartment or to inefficient virus movement to new infection sites. Immuno-localization of wild-type an d an RTD-null mutant virus in thin sections of petioles and in phloem cells of leaf lamina, as well as electron microscopy observations, were all cons istent with the conclusion that the RTD is not essential for efficient viru s multiplication in the nucleate phloem cells but intervenes in virus movem ent to increase the rate at which new infection foci are established and ex pand.