Nopaline synthase is an enzyme which catalyses the synthesis of nopaline, an opine which is formed as the result of the condensation of the amino acid arginine and alpha-ketoglutaric acid. When wild-type A. tumefaciens infects a host plant, the opine synthase gene present on the T-DNA region of the Ti plasmid of the bacterium directs infected host cells to synthesize an opine, such as nopaline. The type of opine produced is specific to the particular strain of A. tumefaciens. Opines are metabolized as a source of carbon and nitrogen only by a bacterium possessing a Ti-plasmid and the gene specific for catabolism of the particular opine.

This trait is introduced to permit the identification of transformed plant embryos.