Resistance to Long-Distance Movement Inside Plant Cells

In susceptible hosts, plant viruses that do not show tissue restrictions move from the mesophyll via bundle sheath cells, phloem parenchyma, and companion cells into phloem sieve elements (SE) where they are translocated, then unloaded at a remote site from which further infection will occur. This pathway is typically part of an elaborate symplastic network in plants through which viruses establish systemic infection. Plasmodesmata, elaborate and highly regulated structures with which viruses interact for both cell-to-cell and long-distance movement, provide symplastic connectivity between the epidermal/mesophyll cells and cells within the vasculature, including sieve elements. Entry into the SE-companion cell complex is currently thought to be the most significant barrier to long-distance movement. Once present in a companion cell, a virus potentially has direct access to the sieve tube, the conducting element of the phloem that serves as the pathway for both nutrient and virus transport throughout the plant. Virus particles loaded in the phloem apparently follow the same pathway as photoassimilates and other solutes, albeit not necessarily via strictly passive processes.

Most plant viruses require CP for long-distance movement, independent of any requirement for CP in cell-to-cell movement. Analysis of CP mutants for a number of viruses including TMV and TEV suggests that CP is essential for entry into and/or spread through sieve elements. Some DNA viruses also require CP for long-distance movement, although other white fly–transmitted geminiviruses do not require CP for systemic infection. Phloem-limited viruses, e.g., Luteovirus, are typically limited to phloem parenchyma, companion cells, and SE, and apparently lack the ability to exit phloem tissue or possibly to infect non-phloem tissues. A few viruses, most notably members of the Sobemovirus genus, use xylem for long-distance movement. The mechanisms of viral interaction with xylem are largely unknown. Because systemic movement is more difficult to study than cell-to-cell movement, relatively few host factors that are essential for this process thereby defining potential R gene candidates have been identified to date. Down-regulation of pectin methylesterase, shown to interact with TMV MP, resulted in impaired movement of TMV, probably by blocking virion exit from phloem. This finding is consistent with the hypothesis that phloem loading and unloading of virus involve distinct factors.

Some examples of natural virus resistance appear to involve mechanisms that negatively affect systemic movement. For instance, the V20 strain of tobacco exhibits a strain-specific defect in supporting systemic infection by TEV. Using a TEV clone that expressed a reporter protein, β-glucuronidase (GUS), genome amplification, cell-to-cell and long-distance movement were measured in V20 tobacco and a susceptible line. Long-distance movement from leaf to leaf was markedly restricted in V20, associated with reduced entry into and exit from SE. This trait was attributed to the interaction of two unlinked, unidentified recessive genes. These data support the hypothesis that long-distance movement requires a set of host functions distinct from those involved in cell-to-cell movement. In another case, Cowpea chlorotic mottle virus (Bromoviridae) infects and moves cell-to-cell through inoculated leaves of soybeans homozygous for two recessive genes but entry into vascular tissue is restricted. In potato, the recessive ra allele, when homozygous, completely blocks vascular transport of Potato virus A (PVA) in graft-inoculated plants. Given the degree of conservation observed for some basic functions in plants, fundamental knowledge about the structure and function of plant vasculature will likely be relevant as efforts to identify these genes proceed. In some cases, systemic movement is not prevented but delayed and reduced.

In Capsicum genotypes homozygous for the resistance allele pvr3, Pepper mottle virus (PepMoV-FL) accumulated in inoculated leaves and moved into the stem but did not enter internal phloem for systemic movement to young tissues. Infection by a second virus, CMV, alleviated this restriction, which suggests that CMV was able to compensate for the defect in the host, either by providing a factor that facilitates movement of both viruses or alleviating the restriction by an unknown mechanism. A similar type of resistance was described for CMV whereby virus remained localized to the lower portions of the plant. Dufour and coworkers showed that CMV accumulated in external but not internal phloem in the petiole of the inoculated leaf and the lower stem of the resistant genotype. Derrick & Barke evaluated potato lines resistant to Potato leafroll virus (PLRV) and showed that the resistance was associated with an exclusion of virus from external phloem bundles, whereas virus occurred in both internal and external phloem in the susceptible line. Again, the identity of these genes in the host and their role in viral infection are unknown. Relatively few dominant genes are known for resistance to systemic movement of plant viruses.

The Arabidopsis RTM system is one exception. Many A. thaliana ecotypes support TEV replication and cell-to-cell movement in inoculated leaves but do not allow systemic movement. The loci RTM1, RTM2, and RTM3 are required for restriction of long-distance movement of TEV. Resistance mediated by the RTM genes is specific to TEV and does not involve a hypersensitive response or induction of SAR. RTM1 and RTM2 were isolated by map-based cloning. The deduced RTM1 protein is similar to the Artocarpus integrifolia lectin, jacalin. Jacalin belongs to a family of proteins with members that are implicated in defense against insects and fungi. The deduced RTM2 protein contains several domains including an N-terminal region with similarity to plant small heat shock proteins. Both these genes are expressed in phloem, specifically SEs, but the mechanism by which TEV movement in this system is restricted is not understood.

The study of plant resistance genes (R genes), namely, plant genes in which genetic variability occurs that alters the plant’s suitability as a host, also raises many fundamental questions regarding the molecular, biochemical, cellular, and physiological mechanisms involved in the plant-virus interaction and the evolution of these interactions in natural and agricultural ecosystems. Over the past decade, the cloning and analysis of numerous plant R genes have stimulated attempts to develop unifying theories about mechanisms of resistance and susceptibility, and coevolution of plant pathogens and their hosts. The focus has been mainly on monogenic dominant resistance to fungal and bacterial pathogens; however, there is clear evidence that common mechanisms can be involved in virus resistance. Considerable progress is evident in the areas of R gene structure, identification of molecular interactions important in plant viral infection, and elucidation of mechanisms of resistance and viral evolution since the last Annual Review of plant virus resistance genes was published in 1990 (64). For this review, we emphasize the current status of R genes that have been characterized at a molecular level, possible connections to down-stream host responses, and factors that may influence durability of resistance in agricultural ecosystems.