Cytokinins, the Cinderella of plant growth regulators

Cytokinins (CKs) are a complex group of compounds. The complexity of CKs is not just present at the level of chemical structures, but as a family of compounds. CKs occur in plants as nucleotides, nucleosides (ribosides) that are produced during de novo biosynthesis, free bases, and conjugates that are in stored/inactivated forms. Almost all organisms make cytokinins. CKs are structural components of the tRNA, and they are located next to the anticodon loop beginning with U of a subset of tRNAs in most eukaryotes and bacteria. The biosynthesis of CKs uses several pathways, and there is evidence that its biosynthesis is also regulated by other growth regulators such as auxins, strigolactones and abscisic acid. Its signaling pathway involves a phosphotransfer signal cascade. Despite advances made so far in the knowledge of CKs, there is still a long way to go. In this review, we summarize the most up-to-date knowledge on the biosynthesis of CKs and the signaling pathway that leads to the response to the presence of CKs in plant tissue, and we identify areas that require more research to complete our understanding of the role of the CKs in plants.