Beyond expectations: the development and biological activity of cytokinin oxidase/dehydrogenase inhibitors

Cytokinins are one of the main groups of plant hormones that regulate growth and development of plants. Cytokinin oxidase/dehydrogenase (CKX) is an enzyme that rapidly and irreversibly degrades cytokinins and thus directly affects their concentration and physiological effect. Genetically modified plants with reduced CKX activity in the shoot, i.e. with a higher concentration of cytokinins, showed e.g. increased tolerance to drought stress, formed larger inflorescences and had higher grain yield. For these reasons, chemical compounds capable of inhibiting the CKX activity (CKX inhibitors) were sought. First, they were identified among strong synthetic cytokinins, but their inhibitory activity was low. The trend has been to develop potent CKX inhibitors with minimal intrinsic cytokinin activity in the hope of avoiding the negative effect of cytokinins on root growth. Cloning CKX, production of key recombinant enzymes from Arabidopsis (AtCKX2) and maize (ZmCKX1 and ZmCKX4a), development of screening bioassays and progress in X-ray crystallography and synthetic organic chemistry led to extensive progress in the development of these compounds. Currently, the most suitable CKX inhibitors are seeking their application in research and the commercial sphere in two main areas — plant tissue cultures and agriculture. The key milestones that preceded it are summarized in this review.