An Agrobacterium-mediated transient expression assay has been described for in vivo analysis of constitutive or inducible gene expression in Arabidopsis plants. By simple infiltration of Agrobacterium cells carrying appropriate gene constructs into Arabidopsis leaves, transient expression assays can be performed within 3 d without using expensive instruments or complicated procedures. Two days after agroinfiltration, Arabidopsis plants can be treated with chemical inducers such as dexamethasone to induce transgene expression. The expression of the beta-glucoronidase (GUS) reporter gene can be determined by histochemical staining. In addition, RNA and protein can be extracted from agroinfected leaves and used for reverse transcription-polymerase chain reaction (RT-PCR), Northern blot, Western blot, immunoprecipitation, and enzyme assays.