Constitutive nitrate reductase: a dominant conditional marker for plant genetics

Summary We describe the development of a counter-selection system based on the use of an engineered nitrate reductase (NR) gene. The engineered gene consists of an NR cDNA placed under the control of the CaMV 35S promoter (35S-NR). Seedlings and cells derived from transgenic Nicotiana plumbaginifolia plants transformed with 35S-NR are efficiently killed by the selective agent chlorate on medium containing ammonium as the sole nitrogen source. Under these nitrate-free conditions wild-type plants are not affected by chlorate because the endogenous wild-type Niagene is not expressed.