Detectable ,-glucuronidase activity in transgenic cranberry is affected by endogenous inhibitors and plant development.

Extracts of cranberry, rich in flavonols and proanthocyanins, inactivated β-glucuronidase (GUS) in assays using either purified bacterial GUS or preparations of transgenic tobacco (Nicotiana tabacum L.) or transgenic cranberry (Vaccinium macrocarpon Ait.) expressing the gusA gene. Histochemical GUS assays produced random and generally unpredictable staining. The addition of polyvinylpolypyrrolidone (PVPP) during the preparation of transgenic cranberry leaf extracts increased the detectable GUS activity in fluorogenic assays more than 200-fold. Detectable GUS activity varied among transclones and also within a transclone depending on the developmental and physiological state of the tissue, as well as the growth environment. Use of PVPP altered the relative ranking of plants based on their total transgenic enzyme activity and resulted in different conclusions as to the effects of genotype or growth environment on transgene expression.