CELLULAR RESISTANCE TO PLANT VIRUSES

Resistance at the single cell level may be characterized as a state where virus replication does not occur, or occurs at essentially undetectable levels in inoculated cells. This type of resistance has been termed “extreme resistance” (ER), “cellular resistance,” or “immunity”. A classical example of this type of resistance is observed when Vigna unguiculata ‘Arlington’ is challenged with the Comovirus Cowpea mosaic virus (CPMV). A protease inhibitor that prevents CPMV polyprotein processing was proposed as a candidate for the mechanism by which replication was prevented, but this has not been confirmed. For plant viruses with both RNA- and DNA-encoded genomes, diverse host factors that are involved in or required for completion of the viral infection cycle have been identified. Most of these factors were identified through the analysis of large experimental collections of mutagenized hosts. For example, Arabidopsis mutants homozygous for the tom1 and tom2A lesions do not support TMV accumulation in single cells. Tom1 encodes a transmembrane protein localized in the tonoplast that interacts with the helicase domain of the tobamovirus-encoded replicase protein. Tom2A also encodes a transmembrane protein that interacts with Tom1; both proteins define important components for tobamoviral replication complex.

Another illustration of this approach from the Arabidopsis model was the identification of the allele lsp1, the result of a mutation at this locus that encodes a homolog of the eukaryotic translation factor eIF(iso)4E. When homozygous, this defect resulted in plants that did not support infection by Tobacco etch virus (TEV), a result presaging later observations from pepper, lettuce, pea, and tomato that implicate host translation factors in resistance to potyviruses and CMV.

The second type of mechanism that can result in resistance at the single-cell level involves an active resistant response to virus infection that occurs rapidly enough to limit virus replication before cell-to-cell movement occurs. Plants with this response may show no symptoms or extremely limited necrosis (pinpoint lesions). Well-known examples of this response include resistance controlled by Tm-1 for TMV in tomato, the R gene against CPMV in cowpea, Nx and Rx for PVX and Ry for Potato virus Y (PVY) in potato, Sw5 in tomato, and Rsv1 in soybean. This response has been studied in some detail using the Ry gene for ER in potato as a model. Plants carrying the Ry gene do not show any visible symptoms when challenged with PVY. Virus accumulation is not detected in the inoculated leaves by either RNA hybridization or ELISA. Furthermore, protoplasts isolated from resistant genotypes do not support viral replication. Because HR was not evident, it was postulated that these genes might encode inhibitors of virus accumulation. However, there may be no mechanistic distinction between reactions previously categorized as ER and HR.

When each of the PVY-encoded proteins was expressed in leaves of PVY-resistant plants, the nuclear inclusion of a protease (NIaPro) induced HR, demonstrating that the HR mechanism may be a component of the ER response. The same trends hold true for Rx/PVX-CP in potato, Sw5 in tomato, and Rsvin soybean. For elicitation of Ry-mediated resistance, the protease domain of PVY NIaPro, specifically the integrity of the protease active site, is required. Mutant analysis of NIaPro, however, demonstrated that NIa protease activity is not sufficient for elicitation of resistance because elicitor-defective mutants still retained a high level of protease activity. The location of Ry in a genomic region containing many NBS-LRR sequences is consistent with the possibility that Ry may encode a NBS-LRR-type protein typical of genes controlling HR observations from pepper, lettuce, pea, and tomato that implicate host translation factors in resistance to potyviruses and CMV.

The second type of mechanism that can result in resistance at the single-cell level involves an active resistant response to virus infection that occurs rapidly enough to limit virus replication before cell-to-cell movement occurs. Plants with this response may show no symptoms or extremely limited necrosis (pinpoint lesions). Well-known examples of this response include resistance controlled by Tm-1 for TMV in tomato, the R gene against CPMV in cowpea, Nx and Rx for PVX and Ry for Potato virus Y (PVY) in potato, Sw5 in tomato, and Rsv1 in soybean. This response has been studied in some detail using the Ry gene for ER in potato as a model. Plants carrying the Ry gene do not show any visible symptoms when challenged with PVY. Virus accumulation is not detected in the inoculated leaves by either RNA hybridization or ELISA.  Furthermore, protoplasts isolated from resistant genotypes do not support viral replication. Because HR was not evident, it was postulated that these genes might encode inhibitors of virus accumulation. However, there may be no mechanistic distinction between reactions previously categorized as ER and HR. When each of the PVY-encoded proteins was expressed in leaves of PVY-resistant plants, the nuclear inclusion of a protease (NIaPro) induced HR, demonstrating that the HR mechanism may be a component of the ER response.

 The same trends hold true for Rx/PVX-CP in potato, Sw5 in tomato, and Rsvin soybean. For elicitation of Ry-mediated resistance, the protease domain of PVY NIaPro, specifically the integrity of the protease active site, is required. Mutant analysis of NIaPro, however, demonstrated that NIa protease activity is not sufficient for elicitation of resistance because elicitor-defective mutants still retained a high level of protease activity. The location of Ry in a genomic region containing many NBS-LRR sequences is consistent with the possibility that Ry may encode a NBS-LRR-type protein typical of genes controlling HR.