Analysis of the mechanism of protection in transgenic plants expressing the potato virus X coat protein or its antisense RNA

Abstract

Transgenic tobacco plants engineered to express either the potato virus X (PVX) coat protein (CP+) or the antisense coat protein transcript (CP‐antisense) were protected from infection by PVX, as indicated by reduced lesion numbers on inoculated leaves, delay or absence of systemic symptom development and reduction in virus accumulation in both inoculated and systemic leaves. The extent of protection observed in CP+ plants primarily depended upon the level of expression of the coat protein. Plants expressing antisense RNA were protected only at low inoculum concentrations. The extent of this protection was even lower than that observed in plants expressing low levels of CP. In contrast to previous reports for plants expressing tobacco mosaic virus or alfalfa mosaic virus CP, inoculation of plants expressing high levels of PVX CP with PVX RNA did not overcome the protection. Specifically, lesion numbers on inoculated leaves and PVX levels on inoculated and systemtic leaves of the CP+ plants were reduced to a similar extent in both virus and RNA inoculated plants. Although these results do not rule out that CP‐mediated protection involves inhibition of uncoating of the challenge virus, they suggest that PVX CP (or its RNA) can moderate early events in RNA infection by a different mechanism.

Transgenic Nicotiana debneyii expressing viral coat protein are resistant to potato virus S infection

“…This result is similar to those obtained using transgenic plants expressing PVX coat protein (Hemenway et al, 1988).…”

“…It is likely that the mechanisms involved in each case are different and may in fact vary according to the particular virus involved. The fact that transgenic plants expressing PVS coat protein are protected from infection by PVS RNA, together with similar results obtained with PVX coat protein-expressing plants (Hemenway et al, 1988), indicates that some event other than or in addition to virus uncoating is being inhibited. In contrast to TMV, particle assembly of which proceeds in a 3' to 5' direction from an origin of assembly (OAS) located 925 nucleotides upstream of the 3' terminus of the RNA (Zimmern, 1977), the assembly of PVX is believed, by analogy with papaya mosaic virus (Lok & AbouHaidar, 1986), to occur in the 5' to 3' direction from an OAS located near the extreme 5' terminus of the RNA.…”

“…This form of coat proteinmediated protection has now been demonstrated for a number of different viruses including alfalfa mosaic virus (A1MV; Van Dun et al, 1987;Tumer et al, 1987;Loesch-Fries et al, 1987), cucumber mosaic virus (CMV; Cuozzo et al, 1988), tobacco rattle virus , tobacco streak virus and PVX (Hemenway et al, 1988;Hoekema et al, 1989). More recently, simultaneous resistance to two viruses, PVX and potato virus Y, has been reported in transgenic potato plants expressing the nucleocapsid proteins from each of these viruses (Lawson et al, 1990).…”

“…In contrast to TMV, particle assembly of which proceeds in a 3' to 5' direction from an origin of assembly (OAS) located 925 nucleotides upstream of the 3' terminus of the RNA (Zimmern, 1977), the assembly of PVX is believed, by analogy with papaya mosaic virus (Lok & AbouHaidar, 1986), to occur in the 5' to 3' direction from an OAS located near the extreme 5' terminus of the RNA. It has been proposed by Hemenway et al (1988) that PVX coat protein in transgenic plants may bind at or near this Y-proximal OAS of the challenge viral RNA and subsequently prevent translation of the replicase and/or interfere with replication. Such interactions may result in the at least partial recoating of infectious PVX RNA and would be consistent with the observation that TMV coat protein does not protect against inoculation with TMV RNA because, in the latter case, binding of coat protein subunits to the OAS site would not be expected to interfere with replicase translation or RNA replication, as indicated by cotranslational disassembly experiments (Wilson, 1984(Wilson, , 1986.…”

Stem-loop structure in the 5′ region of potato virus X genome required for plus-strand RNA accumulation

“…Cockerham (1955) classi®ed different isolates of PVX into four groups based on phenotype to two endogenous resistance genes in potato. PVX-CP (group 2; Orman et al, 1990), PVX-X3 and PVX-UK3 (group 3;Huisman et al, 1988;Kavanagh et al, 1992), PVX-HB (group 4; Querci et al, 1993), and a Russian isolate that we have designated PVX-R (Skryabin et al, 1988) were compared to our isolate (Hemenway et al, 1988) denoted as PVX-WA. PVX-WA is likely to be a group 3 strain based on amino acid similarities, but has not been tested on the appropriate host plants.…”