CELL-TO-CELL MOVEMENT OF POTATO-VIRUS-X IS ASSOCIATED WITH A CHANGE IN THE SIZE-EXCLUSION LIMIT OF PLASMODESMATA IN TRICHOME CELLS OF NICOTIANA-CLEVELANDII

To study the effect of cell-to-cell movement of potato virus X (PVX) o n the size-exclusion limit of plasmodesmata we have microinjected viru s particles and fluorescently labelled dextrans (F-dextrans) with aver age molecular mass of 4.4, 10, and 20 kDa into Nicotiana clevelandii t richome cells. A vector construct of PVX expressing GUS (PVX.GUS) move d from the injected cell and modified the plasmodesmata to allow passa ge of the 4.4- and 10-kDa F-dextrans but not the 20-kDa F-dextran. A m utant PVX with a deletion in the gene for the 25-kDa protein accumulat ed in inoculated protoplasts to the same level as the wild-type PVX. T his mutant virus did not move from the injected cell and did not modif y the plasmodesmata to allow passage of the 10-kDa F-dextran. However, the 4.4-kDa F-dextran moved from the cells infected with the mutant v irus. These data therefore illustrate that movement of PVX is associat ed with modification of plasmodesmata. The full plasmodesmatal modific ation requires a functional 25-kDa protein but partial modification oc curs when this protein is present in a mutant form. (C) 1996 Academic Press, Inc.