Zytokinin

Zytokinin, more commonly called cytokinins, is a class of plant hormones that promote cell division and influence various aspects of growth and development. They are adenine derivatives; natural cytokinins include zeatin, isopentenyladenine (iP), and kinetin, and synthetic analogs such as benzyladenine (BA) and 6-benzylaminopurine (BAP). Cytokinins work together with auxins to regulate shoot and root formation and many other growth processes.

Biosynthesis and transport: The main biosynthetic steps are catalyzed by isopentenyl transferases (IPTs), which attach isopentenyl groups to adenine nucleotides. Zeatin-type cytokinins are produced by hydroxylation via cytochrome P450 enzymes (CYP735A). Cytokinins are transported through the plant, primarily in the xylem from roots to shoots, and can also move via the phloem. Inactivation and degradation are carried out by cytokinin oxidases/dehydrogenases (CKX).

Signaling and regulation: Cytokinin perception occurs through membrane-bound receptor histidine kinases (AHKs). A phosphorelay involving histidine phosphotransfer proteins (AHPs) transmits the signal to response regulators: type-B ARRs activate transcription of cytokinin-responsive genes, while type-A ARRs provide negative feedback to modulate sensitivity.

Functions and interactions: Cytokinins stimulate cell division, influence meristem activity, promote shoot initiation, delay leaf senescence, and affect nutrient mobilization and chloroplast development. Their effects are modulated by the balance with auxin; the relative cytokinin-to-auxin ratio helps determine whether tissues form shoots or roots.

Applications: In plant biology and agriculture, cytokinins are used to enhance tissue culture propagation, delay senescence in leaves and fruits, and influence yield and architecture. They are a key part of strategies for crop improvement and controlled in vitro propagation.