Sytokiinit

Sytokiinit is a term used in some non-English sources to denote cytokinins, a class of plant hormones involved in regulating cell division and various developmental processes. The standard English term is cytokinin. Cytokinins are N6-substituted adenine derivatives, with examples including zeatin, isopentenyladenine (iP), kinetin, and dihydrozeatin.

Biosynthesis and distribution: Cytokinins are primarily synthesized in root and shoot meristems by adenylate IPT enzymes, with activation steps carried out by LOG enzymes. They are transported through the xylem to distant tissues where they influence meristem activity and organ formation.

Mechanism of action: Cytokinin perception occurs via membrane-bound histidine kinase receptors such as AHK2, AHK3, and AHK4/CRE1. Perception triggers a multistep phosphorelay to response regulators, notably type-B transcription factors that activate gene expression. Type-A ARRs provide negative feedback to modulate signaling.

Functions: Cytokinins promote cell division and shoot initiation, influence leaf expansion and chloroplast development, and delay leaf senescence. They also regulate nutrient mobilization, shoot branching, and the balance with other hormones such as auxin to control organogenesis and growth patterns.

Regulation and interactions: Cytokinin levels are controlled by biosynthesis and degradation, including cytokinin oxidases/dehydrogenases (CKX) that break down cytokinins. They interact with auxin, gibberellins, and other hormonal pathways to shape development and responses to environmental conditions. In agriculture, altering cytokinin levels can impact crop yield, storage life, and stress tolerance.

Occurrence and history: Cytokinins are produced by plants and, in some contexts, by certain microbes. They were identified mid-20th century and have since been a central focus of plant hormone research. Sytokiinit, when used, generally refers to the same hormone class described as cytokinins.

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