Activation of the pathogen-inducible Gst1 promoter of potato after elicitation by Venturia inaequalis and Erwinia amylovora in transgenic apple (Malus x domestica).

Rather than using a constitutive promoter to drive transgenes for resistance against fungal and bacterial diseases in genetic engineering of apple (Malus x domestica) cultivars, a promoter induced only after infection was preferred. The ability of the Pgst1 promoter from potato (Solanum tuberosum L.) to drive expression of the gusA reporter gene was determined in two genotypes of apple: the fruit cultivar Royal Gala and the M.26 rootstock. beta-Glucuronidase activity in the transgenic lines grown in a growth chamber was determined quantitatively using fluorometric assays and compared to the activity in Cauliflower Mosaic Virus (CaMV) 35S promoter-driven transgenic lines. In both apple genotypes, the Pgst1 promoter exhibited a low level of expression after bacterial and fungal inoculation compared to the level obtained with the PCaMV35S promoter (15% and 8% respectively). The Pgst1 promoter was systematically activated in apple at the site of infection with a fungal pathogen. It was also activated after treatment with salicylic acid, but not after wounding. Taken together, these data show that, although the Pgst1 promoter is less active than the PCaMV35S promoter in apple, its pathogen responsiveness could be useful in driving the expression of transgenes to promote bacterial and fungal disease resistance.