A Dexamethasone-Inducible Gene Expression System Is Active in Citrus Plant

Inducible gene expression systems, in which the level or timing of activity of a gene of interest can be controlled exogenously, are an effective means to assess gene function.  However, such systems have not been widely employed in crop plants.  Here we show that the glucocorticoid receptor (GR)-based inducible gene expression system functions in Citrange (Citrus sinensis x C. trifoliata) plants.  We generated transgenic Citrange plants containing a two component 35S::LhGR/ pOp6::b-glucuronidase (GUS) system (Wielopolska et al., 2005), in which the synthetic transcription factor, LhGR, is glucocorticoid-inducible and can activate the pOp6 promoter driving expression of the b-glucuronidase (GUS) reporter gene.  We describe a method for inducing LhGR activity using the synthetic glucocorticoid dexamethasone (DEX) in transgenic Citrange. We are also testing other promoters to drive LhGR expression, such as UBQ10, which has been reported in Arabidopsis for example to show a more moderate expression pattern than 35S. With the advent of transgenic approaches to engineer new traits in Citrus, control of transgene activity will be essential to realize the full potential of such manipulations. The method we describe here is likely to be of general use in many Citrus cultivars to temporally control the activity of introduced transgenes.