Native β‐glucuronidase activity in sugarbeet (Beta vulgaris)

β‐Glucuronidase (EC 3.2.1.31) activity, initially thought absent from plants, has been found in a number of plant families. During an analysis of Agrobacterium‐mediated transformation of sugarbeet (Beta vulgaris L.), significant glucuronidase activity was observed in control (non‐transformed) tissues when the fluorogenic substrates 4‐methylumbelliferyl‐β‐d‐glucuronic acid, resorufin glucuronic acid and 3‐carboxyum‐belliferyl‐β‐d‐glucuronic acid were used to quantify β‐glucuronidase activity under standard protocol conditions. Similarly, the colorigenic substrate p‐nitrophenyl‐β‐d‐glucuronide was hydrolyzed by this sugarbeet‐derived glucuronidase. Biochemical and immunological data are presented to indicate significant differences between sugarbeet‐derived glucuronidase and that from Escherichia coli (EC 3.2.1.31) encoded by gusA. These differences provide means of distinguishing between the two activities in extracts that contain a mixture of both. Use of X‐glue, the substrate utilized in histochemical localizations of glucuronidase activity, gave no reaction product (i.e., indigo precipitate) at pH 7.0. However, at pH 3.0, 4.0 and 5.0 formation of the indigo precipitate was evident within 1 h at 37°C in sugarbeet callus and by 4 h in leaves and petioles. The specific activity of sugarbeet glucuronidase was observed to be strongly pH dependent, with an optimum near pH 4.0. The use of various β‐glucuronidase assay techniques as applied to transformation of sugarbeet is discussed.