Cytokinin receptors from structural, evolutionary, and functional perspectives: a review

Cytokinins are a major class of plant hormones that have been recognized as key regulators of plant growth and development since their biological activity was first described in the 1950s. However, cytokinin receptors were identified only about twenty-five years ago, opening a new field of research into their molecular and physiological functions. Since then, extensive studies have examined receptor ligand-binding properties, evolution, distribution, subcellular localization, hormone cross-talk, and biological roles, including their involvement in plant-microbe interactions. Because of their central importance in plant biology, cytokinin receptors have also become targets for the development of selective agonists and antagonists, and numerous receptor mutants with diverse phenotypes have been described. By contrast, structural characterization has progressed more slowly, leaving major gaps in our understanding of how receptor architecture determines function. In this review, we synthesize current knowledge of cytokinin receptors, with particular emphasis on linking structural features to functional outcomes.