The N gene in Nicotiana sp. confers hypersensitive resistance to all strains of tobacco mosaic virus (TMV) and limits the rate of virus spread in infected leaves. To examine the role of the movement protein (MP) of TMV in the hypersensitive reaction (HR), transgenic Nicotiana tabacum cv. Xanthi-nc (genotype NN) plants that express the MP gene were produced and the molecular size exclusion limit of plasmodesmata in leaf mesophyll cells was monitored. At the HR-permissive temperature (24°) movement from cell to cell of fluorescein isothiocyanate-labeled dextran of molecular mass 3.9 kDa was detected while 9.4-kDa molecules failed to move. At the HR-non permissive temperature (33°) the 9.4-kDa probe moved readily from cell to cell. In contrast, in transgenic Xanthi (genotype nn) which express the MP gene the 9.4-kDa probe moved from cell to cell at 24 and 33°. These results suggest that the N gene may modify the ability of the MP to alter plasmodesmata molecular exclusion limits, although expression of the TMV-MP gene alone did not induce the HR. Furthermore, when MP(+) Xanthi-nc tobacco lines were inoculated with a TMV that lacked a MP gene the HR was induced, and the concentration of MP in the transgenic lines was correlated with the degree of the HR.
Bibliographical noteFunding Information:
We thank M . Dyer and S . Leltner for maintenance of transgenlc plants at Washington University and Dr . B . talk at the University of California, Davis, for the use of greenhouse space . This work was supported by grantsfrorn the National Science Foundation to R .N .B . (DMB8717012) and W .J .L . (DMB8703624) and by the Monsanto Company to R .N .B .