CMV protection in transgenic cucumber plants with an introduced CMV-O cp gene

We introduced the CMV-O coat-protein gene into cucumber plants, using a Ti-Agrobacterium-mediated transformation system, with the aim of producing cucumber plants with CMV resistance. The RNA transcripts from the CaMV 35s-cp gene could be detected in the leaves of the R0 transgenic cucumber plants, as well as in the epicotyls containing two cotyledons of transgenic progeny plants, by Northern-blot analysis; but the presence of coat protein originating from the CaMV 35s-cp gene could not be detected in the cotyledons or leaves of R0 and transgenic progeny plants by Westernblot analysis. The progenies of a cross between cv "Sharp 1" and transgenic plants (pure line "1021") possessing the cp gene displayed strong resistance to inoculation of the CMV-Y strain, although both the control cv "Sharp 1" and segregated cp- plants displayed many spotted disease symptons on their leaves 5-6 days after CMV-Y inoculation on the cotyledons. The control "1021" had a slight tolerance toward CMV-Y inoculation. The transgenic cucumber plants displayed the absence of resistance to ZYMV. However, transgenic plants showed a reduced degree of disease symptom development following a double inoculation of CMV and ZYMV. The CMV resistance of the present transgenic cucumber plants seems to be due to the synergism of the slight CMV tolerance in the pure line "1021" and the protection against CMV afforded by the introduction of the CMV cp gene. This leads to the possibility of producing cucumber plants with the agronomic characteristics of very strong CMV resistance by the combination of genotypes of cucumbers and the CMV cp gene. The transgenic plants possessing the cp gene should thus be useful as a genetic source for producing cucumber plants with the agronomic characteristic of CMV resistance.