Short and long distance spread of potato leafroll luteovirus: effects of host genes and transgenes conferring resistance to virus accumulation in potato.

Replicase mediated resistance against Potato Leafroll Virus in potato Desirée plants

“…On the other hand, the increase in the virus titer observed from 40 dpi on may result from the spread of viral particles to cells that are in close proximity with vascular tissues. This idea is based on the fact that even when PLRV is normally restricted to vascular tissues (Kojima et al, 1969;Derrick and Barker, 1997), viral particles have been shown to spread out of the vascular tissues and infect adjacent cells (Derrick and Barker, 1997). If this were the case in our RolA1 plants, it could be concluded that the cellular mechanisms responsible for the initial resistance observed were not present in the adjacent cells to prevent the replication of the viral particles.…”

“…In contrast, when the promoter 35S from the Cauliflower Mosaic Virus (CaMV) was used, replicase transcripts were detected in all tissues examined (Figure 3). In naturally infected plants, the viral components of PLRV and other Luteovirus are found exclusively in sieve elements, companion cells, and phloem parenchymatic cells (Kojima et al, 1969;Derrick and Barker, 1997). We therefore aimed to develop resistant plants in which the expression of the PLRV transgenes was restricted to the tissues susceptible to this virus.…”

Spread Throughout the Plant: Systemic Transport of Viruses

“…Avelar allows downward movement of PepMoV-FL through the external phloem, but restricts upward movement through the internal phloem, resulting in young stem tissues that are virus-free (Guerini et al 1999). Finally, differential involvement of the internal and external phloem in viral systemic movement was also shown using PLRV-resistant potato plants in which virus is restricted to the internal phloem, whereas both internal and external phloem display PLRV accumulation in the susceptible potato plants (Barker et al 1986;Derrick et al 1992Derrick et al , 1997. Thus, plant viruses may move to the roots, downward from the inoculated leaf, through the abaxial phloem of leaves and external phloem of petioles and stems, but utilize the adaxial leaf phloem and internal petiole and stem phloem for their upward movement to the sink leaves.…”

“…Generally, local movement is a relatively slow process (e.g., 5-15 µm/h, see Gibbs 1976), presumably restricted by the rate of viral replication. In contrast, long-distance movement through the vascular system is rather rapid (e.g., 50-80 mm/h (see Gibbs 1976), as it occurs with the flow of photoassimilates and, in some Barker et al 1986;Derrick et al 1992Derrick et al , 1997 PepMoV-FL Resistant pepper strain Guerini et al 1999 a In susceptible hosts, the indicated viruses, except for PLRV, move from non-vascular ME to different vascular-associated and vascular cell types (BS, VP, CC, SE) of the inoculated leaves and, through SE, to systemic leaves, which they first enter through the vasculature and then unload into ME. In restrictive hosts, viral movement is arrested at different cellular boundaries, which usually are specific for a host-viral strain combination as shown in this Table. Arrows mark the cell types between which the indicated viral strain can move in the corresponding restrictive host, and solid vertical lines indicate the cell boundaries at which the viral movement is arrested Spread Throughout the Plant: Systemic Transport of Viruses 87 cases, does not require viral replication (Susi et al 1999;Wintermantel et al 1997).…”

“…4, long-distance transport proceeds at different rates and in two directions: upward movement is faster, and downward movement is slower (Andrianifahanana et al 1997;Cheng et al 2000). Tracing the movement of TMV, Pepper mottle potyvirus (PepMoV), and PLRV demonstrated that these two movement modes occur through structurally different types of phloem -external and internal (Barker et al 1986;Cheng et al 2000;Derrick et al 1992Derrick et al , 1997Guerini et al 1999). The external and internal phloem in the transport veins of petioles and stems of such plant families as Solanaceae, Cucurbitaceae and others derive from the abaxial (facing away from the axis of the plant and located on the underside of the leaf) and adaxial (facing toward the axis of the plant and located on the upper side of the leaf) phloem, respectively, of the major leaf veins (Cheng et al 2000;Turgeon 1989).…”