Higher copy numbers of the potato RB transgene correspond to enhanced transcript and late blight resistance level.

Foliar late blight disease of potato ranks among the costliest of crop disease worldwide. Genetic resistance is an environmentally friendly and cost effective alternative to chemical controls and wild potato species are rich sources of resistance genes. In this study, we examined the effects of the late blight resistance gene RB, isolated from the wild potato Solanum bulbocastanum and under the control of its endogenous promoter, in transgenic cultivated potato. In multi-year tests, the RB transgene provided effective disease resistance in a variety of genetic backgrounds, including commercially prominent potato cultivars, in the absence of fungicides. Application of RB transgene-specific assays allowed estimation of RB copy numbers and transcript levels. A clear trend of enhanced resistance with increasing copy numbers and increasing transcript levels emerged. Our results indicate that RB, like other disease resistance genes, is constitutively transcribed at low levels. Significantly, two independent transgenic lines, each with an estimated 15 copies of the RB transgene, maintain high RB transcript levels and are ranked among the most resistant of 57 lines surveyed. Thus even in these ultra-high copy number lines, innate RNA silencing mechanisms have not been fully activated. Previous proposals that transgene transcript levels must surpass a threshold in order to trigger RNA silencing may therefore apply to disease resistance genes as well. Strategies for the deployment of RB are discussed in light of the current research.