Synthesis of cytokinin glucuronides for the selection of transgenic plant cells

Glucuronide derivatives of cytokinins have been synthesized for use as agents for the selection of plant cells transformed with a β-glucuronidase (GUS) gene. In this selection system, the GUS gene functions as both a selectable, as well as a screenable gene. GUS liberates active cytokinin from inactive cytokinin glucuronides which then stimulates growth and regeneration of the transformed cells. The frequently used cytokinin N6-benzyladenine was conjugated to glucuronic acid at N-3 or at N-9 but only the former was a substrate for GUS. The glucuronide of isopentenyladenine was also made, by coupling at the N-3. This compound was readily hydrolysed by GUS.