Tsitokiniinid

Tsitokiniinid, known in English as cytokinins, are a class of plant hormones derived from adenine that regulate cell division and a range of developmental processes. They occur naturally in plants and can be produced synthetically. Common natural cytokinins include trans-zeatin, isopentenyladenine, zeatin, and kinetin.

Biosynthesis and metabolism occur through a pathway starting with isopentenyl transferase enzymes that produce cytokinin nucleotides, which are later activated by LOG enzymes to free base forms. Cytokinins are degraded by cytokinin oxidases/dehydrogenases (CKX), providing a mechanism to control hormonal levels within tissues.

Transport and distribution are important for cytokinin action. They are mainly synthesized in root tissues and transported via the xylem to shoots, enabling long-distance signaling. Transport proteins such as ABCG14 facilitate this movement, while other transporters mediate cellular uptake and distribution of cytokinin ribosides.

Signaling involves a multistep phosphorelay. Cytokinins are perceived by membrane-bound sensor histidine kinases (AHK2, AHK3, AHK4/CRE1). The signal is transferred through histidine phosphotransfer proteins (AHPs) to nuclear response regulators, including type-B ARRs that activate transcription and type-A ARRs that provide negative feedback.

Functions and interactions are diverse. Cytokinins promote cell division, shoot initiation, leaf expansion, and chloroplast development, and they delay senescence. They influence apical dominance, shoot branching, and root-to-shoot balance, often interacting with auxins and nutrient signals to coordinate growth and development.

Applications include use in plant tissue culture to induce shoot formation from explants and in agriculture to modulate growth, delay aging processes, or improve yield, typically in combination with other hormones to achieve desired organ development. In Finnish contexts, tsitokiniinid refer to this same hormone class.