Mechanisms of resistance. Expression of coat protein.

Expression of viral CP genes in transgenic plants can lead to virus resistance by interference of either the transcript or the protein with virus infection. Dependence of resistance on CP accumulation can be most convincingly shown by comparison of plants that accumulate CP with plants that accumulate a nontranslatable CP transcript. Even in cases in which CP accumulation is required, the degree of resistance does not always correlate with CP levels in transgenic plants. In cases in which CPMR can be overcome by inoculation with viral RNA instead of virions, interference with virion disassembly is the likely cause of resistance. Classical crossprotection can also sometimes be overcome by RNA inoculation, and, in this case, appears to work by a similar mechanism. There is no evidence yet that CPMR is caused by a nonspecific plant defense response that might be triggered by the accumulating CP. Measurement of virus accumulation in protoplasts prepared from transgenic plants was used to show interference with early events of virus infection. There is no clear evidence yet for inhibition of local virus spread in transgenic plants. A reduced rate of virus accumulation in inoculated leaves can usually also be explained with reduced rate of replication. However, in the case of CPMR to CMV, it appears that early events, as well as systemic spread, are affected. Reduced vector transmission of virus infection from inoculated transgenic plants to nontransgenic plants has been observed. It is not known whether this is just a consequence of lower virus levels in the transgenic plants or whether direct interference with acquisition and transmission by the vector is also involved. In addition to virion formation, CP can function in different ways in plant virus infections. Replication, long-distance spread, and vector transmission can also depend on the presence of CP. Expression of genes encoding nonfunctional CPs in transgenic plants can be tried in order to interfere with normal CP function. Knowledge of CP function(s) in a particular plant-virus interaction will be useful to design gene constructs. Since CP accumulation levels in transgenic plants do not always correlate with resistance, newly generated transgenic plant lines are now frequently tested for virus resistance before further characterization. However, if the objective of a transformation experiment is also to study the mechanism of CPMR, it is necessary to determine transcript and protein levels in the transgenic plants. Gene constructs encoding nontranslatable and antisense CP transcripts should be included in the experiment. If possible, transgenic plants should be inoculated with virions and viral nucleic acid, and replication in isolated protoplasts should be determined.