Cytokinin, first identified in the 1950s as a regulator of cell division, plays a crucial role in plant growth, development, and responses to environmental stresses. Recent studies have elucidated the biosynthesis, metabolism, transport, and signaling pathways of cytokinins, revealing their ancient origin from two-component signaling systems in bacteria. Cytokinin biosynthesis begins with the modification of adenosine to form isopentenyladenine (iP), which is then converted to trans-zeatin riboside through hydroxylation. Cytokinin signaling involves histidine kinases (HKs) and response regulators (RRs), with HKs detecting cytokinins and RRs mediating downstream responses. The signaling pathway includes phosphotransfer proteins (AHPs) that relay signals from HKs to RRs, and type-B RRs act as transcription factors, while type-A RRs function as negative regulators. Cytokinin transport occurs via various transporters, including EQUILIBRATIVE NUCLEOSIDE TRANSPORTERS (ENTs), ATP-BINDING CASSETTE TRANSPORTER G (ABCGs), and PURINE PERMEASES (PUPs). Cytokinin levels are regulated by conjugation to sugars and degradation by cytokinin oxidases (CKXs). Cytokinin signaling is crucial for cell division, meristem activity, and stress responses, with its effects on plant development and stress tolerance being well-documented. Cytokinin modulation has been used to improve crop traits such as drought tolerance and yield. The complex interplay between cytokinin signaling and other hormones like auxin is essential for proper plant development and adaptation to environmental conditions.Cytokinin, first identified in the 1950s as a regulator of cell division, plays a crucial role in plant growth, development, and responses to environmental stresses. Recent studies have elucidated the biosynthesis, metabolism, transport, and signaling pathways of cytokinins, revealing their ancient origin from two-component signaling systems in bacteria. Cytokinin biosynthesis begins with the modification of adenosine to form isopentenyladenine (iP), which is then converted to trans-zeatin riboside through hydroxylation. Cytokinin signaling involves histidine kinases (HKs) and response regulators (RRs), with HKs detecting cytokinins and RRs mediating downstream responses. The signaling pathway includes phosphotransfer proteins (AHPs) that relay signals from HKs to RRs, and type-B RRs act as transcription factors, while type-A RRs function as negative regulators. Cytokinin transport occurs via various transporters, including EQUILIBRATIVE NUCLEOSIDE TRANSPORTERS (ENTs), ATP-BINDING CASSETTE TRANSPORTER G (ABCGs), and PURINE PERMEASES (PUPs). Cytokinin levels are regulated by conjugation to sugars and degradation by cytokinin oxidases (CKXs). Cytokinin signaling is crucial for cell division, meristem activity, and stress responses, with its effects on plant development and stress tolerance being well-documented. Cytokinin modulation has been used to improve crop traits such as drought tolerance and yield. The complex interplay between cytokinin signaling and other hormones like auxin is essential for proper plant development and adaptation to environmental conditions.