32002:

Resistance to viral infection by transgenic plants expressing a truncated viral replicase transgene correlates with the stability of the transgene protein.

Authors:	Hou, Yu-Ming(A)Bao, Yiming(A)Nelson, Richard, S.(A)
Affiliations:	(A): Plant Biology Division, Samuel Noble Foundation, Ardmore, OK 73402
Presenter:	Hou, Yu-Ming , ymhou@noble.org

The expression of replication-associated viral genes in transgenic plants has been evaluated as a resistance strategy against virus infection. Resistance is generally observed in plants expressing a defective or truncated replicase gene. It has been proposed that this resistance may be due to competition for viral and host factors by the defective replicase, thus preventing virus replication (i.e. a dominant negative inhibition). Donson et al. (MPMI 6: 635-642, 1993) have generated transgenic tobacco plants containing a full-length replicase and part of the movement protein genes from tobacco mosaic tobamovirus (TMV). One transgenic line, Rep 21-8, has a transposon-like element inserted within the helicase domain of the replicase gene, resulting in the potential expression of a truncated 106 kDa transgene protein rather than full-length 126 and 183 kDa replication-associated proteins. Rep 21-8 plants were resistant to infection by TMV and various distantly related tobamoviruses; a characteristic that differs from those associated with resistance by gene silencing. We have determined that the resistance to TMV infection by Rep 21-8 plants was dependent on environmental conditions; that is, resistance was observed in the greenhouse but not in the growth chamber. The environmental effects were examined by maintaining the TMV-inoculated homozygous Rep 21-8 plants at different light intensities and temperatures. Results of this study showed that only slightly higher temperatures in the growth chamber could trigger resistance breakdown. Although the 106 kDa protein was present in all Rep 21-8 plants prior to virus challenge and temperature treatment and in resistant plants at low temperature after TMV inoculation, it was not detected in diseased plants at high temperatures. Thus, there is a correlation between the resistance phenotype and the steady-state presence of the 106 kDa transgene protein. This observation will be further addressed in regard to the effect of temperature on the relative steady-state levels of the 106 kDa protein and host proteins. Using TMV mutants altered in the 126 kDa protein sequence, the relationship of stable 126 kDa protein expression and TMV replication ability was investigated. Mutant viruses expressing unstable 126 kDa proteins were, at best, attenuated in accumulation. The importance of a stable 126 kDa protein for viral replication and a stable 106 kDa protein for dominant negative inhibition will be discussed.