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Cre recombinase gene from bacteriophage P1 was transiently expressed by a Potato Virus X (PVX)-based vector in transgenic lox-target Nicotiana benthamiana plants to remove the selectable marker gene. The target construct consisted of two directly oriented lox sites flanking a bar gene located between a gfp coding region and an upstream CaMV 35S promoter. The Cre-mediated excision of intervening sequence placed the gfp coding region under the transcriptional control of the CaMV 35S promoter. GFP activity was observed in PVX-Cre systemically infected leaves, regenerants from PVX-Cre infected explants and T 1 progeny of these regenerants. PVX-Cre was removed efficiently from the regenerants by adding the nucleoside analogue ribavirin to the culture medium. Molecular data proved a correlation between gfp expression and precise site-specific excision of the bar gene in all examined transgenic lines. The frequency of recombination expressed as a percentage of regenerated plants exhibiting marker gene excision varied from 48% to 82%. These results demonstrate that a plant virus vector can be used efficiently to express cre recombinase in vivo providing an alternative method for the production of transgenic plants without marker genes.

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