Untranslatable transcripts of the tobacco etch virus coat protein gene sequence can interfere with tobacco etch virus replication in transgenic plants and protoplasts.

There is now very persuasive evidence that the transmissible agent for spongiform encephalopathies such as scrapie, consists of a modified form of the normal host protein PrPc, devoid of any nucleic acid. On the other hand, because there are many different strains of scrapie agent with distinct phenotypes which can be propagated in animals homozygous for the PrPc gene, it has been suggested that a nucleic acid must be a component of the agent. Can the two views be reconciled?

A 'unified theory' of prion propagation.

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.

Analysis ofthemechanism ofprotection intransgenic plants expressing thepotatovirus X coatprotein or its antisense RNA

Amino acids near the N terminus of the coat protein of tobacco vein mottling virus were deleted or altered by site-directed mutagenesis to determine the effect on aphid transmissibility of the virus. Deletion of a three amino acid sequence Asp-Ala-Gly, which is conserved in aphid-transmissible potyvirus isolates, abolished transmission. The mutation Ala----Thr in this triplet drastically reduced transmission, whereas the mutation Asp----Asn had no effect, and the mutation Asp----Lys consistently reverted to the wild-type residue. The mutation Lys----Glu, in the residue adjacent to the glycine of the triplet, drastically reduced transmission, whereas the mutation Gln----Pro, seven residues downstream from the glycine had no effect. Comparison of the sequences of other potyviruses suggests that the presence of a glycine residue at the third position of the Asp-Ala-Gly triplet is critical for aphid transmissibility and that certain changes in the residues adjacent to this position abolish or greatly reduce aphid transmissibility.

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Amino acid substitutions in the coat protein result in loss of insect transmissibility of a plant virus

The tomato yellow leaf curl virus (TYLCV) gene that encodes the capsid protein (V1) was placed under transcriptional control of the cauliflower mosaic virus 35S promoter and cloned into an Agrobacterium Ti-derived plasmid and used to transform plants from an interspecific tomato hybrid, Lycopersicon esculentum X L. pennellii (F1), sensitive to the TYLCV disease. When transgenic F1 plants, expressing the V1 gene, were inoculated with TYLCV using whiteflies fed on TYLCV-infected plants, they responded either as untransformed tomato or showed expression of delayed disease symptoms and recovery from the disease with increasingly more resistance upon repeated inoculation. Transformed plants that were as sensitive to inoculation as untransformed controls expressed the V1 gene at the RNA level only. All the transformed plants that recovered from disease expressed the TYLCV capsid protein.

Transgenic tomato plants expressing the tomato yellow leaf curl virus capsid protein are resistant to the virus.

The complete nucleotide sequence of the genomic RNAs of strains G2 and G7 of soybean mosaic virus were determined. In both cases, the genome is 9588 nucleotides long, excluding the 3'-terminal poly(A) sequence. A large open reading frame (nucleotides 132 to 9329) encodes a polyprotein of 3066 amino acids with a predicted M(r) of either 349542 (strain G2) or 349741 (strain G7). Based on comparison with the proposed locations of cleavage sites of other potyvirus polyproteins, nine mature proteins are predicted. The mature proteins of the two strains share 94 to 100% amino acid identity, with the greatest variability occurring in the 35K and 42K proteins. Differences in local net charge in portions of these proteins as well as differences in amino acid sequence throughout the genome are discussed in relation to resistance and susceptibility of host plants to strains G2 and G7. Comparison with other potyviruses may be useful for taxonomic clarification of viruses and strains.

Complete nucleotide sequences of two soybean mosaic virus strains differentiated by response of soybean containing the Rsv resistance gene.

Historically, control of plant virus disease has involved numerous strategies which have often been combined to provide effective durable resistance in the field. In recent years, the dramatic advances obtained in plant molecular virology have enhanced our understanding of viral genome organizations and gene functions. Moreover, genetic engineering of plants for virus resistance has recently provided promising additional strategies for control of virus disease. At present, the most promising of these has been the expression of coat-protein coding sequences in plants transformed with a coat protein gene. Other potential methods include the expression of anti-sense viral transcripts in transgenic plants, the application of artificial anti-sense mediated gene regulation to viral systems, and the expression of viral satellite RNAs, RNAs with endoribonuclease activity, antiviral antibody genes, or human interferon genes in plants.

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Genetic engineering of plants for virus resistance.

The nucleotide sequences of the coat protein genes and 3′ non-coding regions of two aphid-transmissible isolates (G2 and G7) of soybean mosaic virus (SMV) were determined. The coat protein of the G2 isolate differs from that of the aphid non-transmissible N isolate by a single amino acid at position 12 (aspartic acid in N, glycine in G2 and G7). The G7 isolate differs from G2 and N at three and four amino acid residues, respectively. The nucleotide sequence similarity of the three isolates in the coat protein coding and 3′ non-coding regions ranges from 93 to 100%, respectively. In contrast, watermelon mosaic virus 2 (WMV 2) is only 77% to 79% similar to the SMV isolates, suggesting that WMV 2 is a distinct virus and not an isolate of SMV as proposed previously.

W. A. Miller

Papers

Complete nucleotide sequences of two soybean mosaic virus strains differentiated by response of soybean containing the Rsv resistance gene.

Genetic engineering of plants for virus resistance.

Nucleotide sequences of the coat protein genes of two aphid-transmissible strains of soybean mosaic virus.