We previously demonstrated, in transgenic tobacco plants, that the role of the movement protein (MP) of tobacco mosaic virus is to facilitate the cell-to-cell spread of viral progeny during infection. An analysis of different tissues of these transgenic plants indicated that the MP accumulated in leaf, stem, and root tissue. The highest levels were detected in older leaves. The relative levels of MP in leaf tissue from transgenic plants were equivalent to, or higher than, the levels of MP in tobacco mosaic virus-infected leaf tissue. Results of subcellular fractionation of homogenates of transgenic leaf tissue showed that the MP was most abundant in the cell wall fraction of older leaves and that the protein remained at high levels in the cell wall fraction as the leaves continued to age. Significant levels of the MP were detected in a crude membrane/organelle fraction and a soluble fraction in younger leaves but decreased to low levels in older leaves. These results suggest that the MP accumulates and is stable in cell walls. We have previously shown that the MP modifies the molecular exclusion limit of plasmodesmata, which is consistent with the hypothesis that plant viruses move from cell to cell through altered plasmodesmata. We show here that the ability of the tobacco mosaic virus MP to modify the molecular exclusion limit of plasmodesmata in tobacco depends on the developmental stage of the leaf. The implications of these findings on understanding virus movement and how plasmodesmata function are discussed.The movement of a plant virus from the initial site of infection into adjacent healthy cells is essential for establishing a productive virus infection. In some virus-host interactions, pathogenesis includes systemic movement of virus by means of the vascular tissue. Although little is known about the virus-host interactions necessary for virus spread, recent studies have provided direct evidence that a protein encoded by tobacco mosaic virus (TMV) is required for cell-to-cell movement of the virus. When expressed in transgenic plants, this protein, the movement protein (MP; 32 kDa), complemented the temperature-sensitive defect in movement of the Lsl mutant of the L strain of TMV (1). Using a different approach, Meshi et al. (2) showed that when the single amino acid change that distinguishes the MP gene of the Lsl virus from that of the parental L strain was introduced into the MP gene of the L virus, the mutated L strain showed the same phenotype as the Lsl virus.Plant viruses (or nucleic acids) move from cell to cell through plasmodesmata, channels that extend through cell walls and provide cytoplasmic continuity between adjacent cells. However, on the basis of present knowledge of plasmodesmata architecture (3, 4) and the molecular size exclusion limits of plasmodesmata (5-7), it is generally assumed that the structure must be modified during virus infection for virus progeny to move from cell to cell. In the case of TMV in tobacco, the MP may be responsible for altering plasmodesmata. At p...
A full-length cDNA clone of rice yellow mottle sobemovirus (RYMV) was synthesized and placed adjacent to a bacteriophage T7 RNA polymerase promoter sequence. Capped-RNA transcripts produced in vitro were infectious when mechanically inoculated onto rice plants (Oryza sativa L). Individual full-length clones varied in their degree of infectivity but all were less infectious than native viral RNA. A representative clone, designated RYMV-FL5, caused a disease phenotype identical to that produced by viral RNA except that symptoms were somewhat slower to appear than those induced by viral RNA. The infectivity of RYMV-FL5 was verified by ELISA, Western blot analysis, Northern blot hybridization, RT-PCR, and Southern blot hybridization. Frameshift and deletion mutations introduced into the coat protein cistron demonstrated that the coat protein was dispensable for RNA replication in rice protoplasts. However, the coat protein was required for full infectivity in rice plants, presumably by playing a role in phloem-mediated long-distance movement and possibly in cell-to-cell movement.
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