Plants rendered herbicide-susceptible by cauliflower mosaic virus-elicitedsuppression of a 35S promoter-regulated transgene

Crop plants genetically modified for herbicide tolerance were some of the f irst to be released into the environment, Frequently, the cauliflower mosai c virus (CaMV) 35S promoter is used to drive expression of the herbicide to lerance transgene. We analyzed the response to CaMV infection of a transgen ic oilseed rape line containing the bialaphos tolerance gene (BAR) from Str eptomyces hygroscopicus, regulated by the 35S promoter. Oilseed rape is sus ceptible to CaMV, but plants recover from infection. CaMV infection altered the expression of the herbicide tolerance gene such that plants became sus ceptible to the herbicide, The effect on transgene expression differed in i nfections with viral pathogenic variants typical of those found in natural situations worldwide. Susceptibility to the herbicide was most likely a res ult of transcriptional gene silencing of the transgene, Our results show th at transgene phenotypes can be modified by pathogen invasion.