Cytokinin
Introduction
Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. They are involved in various processes of plant growth and development, including cell division, apical dominance, and leaf senescence. Cytokinins are also known to delay the aging of tissues, a process known as senescence.
History
The discovery of cytokinins dates back to the 1940s when scientists were studying the rapid division of cells in vascular tissues. The term "cytokinin" was coined in 1964 by the Czech scientist Folke K. Skoog, who along with his colleagues, discovered the effects of cytokinins on cell division in plants.
Biosynthesis and Metabolism
Cytokinins are synthesized in the roots of the plant and are transported to other parts through the xylem. The biosynthesis of cytokinins involves the conversion of adenosine triphosphate (ATP) or adenosine diphosphate (ADP) into isopentenyladenine-type cytokinins. This process is catalyzed by the enzyme isopentenyl transferase.
The metabolism of cytokinins involves several steps, including oxidation, reduction, methylation, and conjugation. The major pathway for cytokinin degradation is the oxidative cleavage of the unsaturated side chain, catalyzed by cytokinin oxidase/dehydrogenase.
Types of Cytokinins
There are several types of cytokinins, including adenine-type cytokinins, phenylurea-type cytokinins, and aromatic cytokinins. Adenine-type cytokinins include zeatin, kinetin, and benzylaminopurine. Phenylurea-type cytokinins include diphenylurea and thidiazuron. Aromatic cytokinins include topolins and their derivatives.
Functions
Cytokinins play a crucial role in various aspects of plant growth and development. They promote cell division and differentiation, stimulate chloroplast development, delay senescence, and regulate nutrient uptake. They also play a role in the plant's response to light and are involved in the process of photomorphogenesis.
Cytokinins in Agriculture
In agriculture, cytokinins are used to promote growth and yield of crops. They are also used in tissue culture to induce cell division and to prevent leaf senescence. Additionally, cytokinins are used in the production of seedless fruits and in the delay of fruit ripening.