Phenyl-Adenine, Identified in a LIGHT-DEPENDENT SHORT HYPOCOTYLS4-Assisted Chemical Screen, Is a Potent Compound for Shoot Regeneration through the Inhibition of CYTOKININ OXIDASE/DEHYDROGENASE Activity

In vitro shoot regeneration is implemented in basic plant research and commercial plant production, but for some plant species, it is still difficult to achieve by means of the currently available cytokinins and auxins. To identify novel compounds that promote shoot regeneration, we screened a library of 10,000 small molecules. The bioassay consisted of a two-step regeneration protocol adjusted and optimized for high-throughput manipulations of root explants of Arabidopsis (Arabidopsis thaliana) carrying the shoot regeneration marker LIGHT-DEPENDENT SHORT HYPOCOTYLS4. The screen revealed a single compound, the cytokinin-like phenyl-adenine (Phe-Ade), as a potent inducer of adventitious shoots. Although Phe-Ade triggered diverse cytokinin-dependent phenotypical responses, it did not inhibit shoot growth and was not cytotoxic at high concentrations. Transcript profiling of cytokinin-related genes revealed that Phe-Ade treatment established a typical cytokinin response. Moreover, Phe-Ade activated the cytokinin receptors ARABIDOPSIS HISTIDINE KINASE3 and ARABIDOPSIS HISTIDINE KINASE4 in a bacterial receptor assay, albeit at relatively high concentrations, illustrating that it exerts genuine but weak cytokinin activity. In addition, we demonstrated that Phe-Ade is a strong competitive inhibitor of CYTOKININ OXIDASE/DEHYDROGENASE enzymes, leading to an accumulation of endogenous cytokinins. Collectively, Phe-Ade exhibits a dual mode of action that results in a strong shoot-inducing activity.