Cytokinins

Skoog and Miller (1965) defined cytokinins as compounds promoting cytokinesis (cell division) in plant cells, derived from the nitrogen-containing compound adenine. An example is 6-furfurylaminopurine (kinetin). 

1.0Discovery of cytokinins

• The discovery of cytokinins originated from the observation that plant cells in culture were not dividing. In 1913, Haberlandt demonstrated that phloem sap could induce nondividing potato tuber tissue to re-enter an actively dividing state. In the 1940s and 1950s, F. Skoog's team found that extracts like coconut milk and yeast stimulated cell division in the presence of auxin. 
• C. O. Miller identified the active material as adenine, leading to the isolation of kinetin from herring sperm DNA in 1956. Although kinetin is not naturally found in plants, it spurred the search for naturally occurring cytokinins. 
• In the early 1960s, zeatin, a naturally occurring cytokinin, was isolated from maize seed and plum fruitlets, leading to the discovery of various other cytokinins.

2.0Biosynthesis of cytokinins

The biosynthesis of cytokinins, essential plant hormones, involves a relatively straightforward process. It begins with the precursor adenine, a nitrogen-containing compound and specifically isopentenyl diphosphate (IPP) with the help of Enzyme often called isopentenyl transferases (IPTs) N⁶(Δ²-isopentenyl)-adenosine-5'-phosphate is formed. 

This step results in the formation of various cytokinin forms, including isopentenyladenine (iP), trans-zeatin (tZ), cis-zeatin (cZ), and dihydrozeatin (DHZ).

3.0Types of cytokinins

Cytokinins are plant hormones that can be naturally occurring or synthetically produced: 

Natural Cytokinins:

• Zeatins
• Isopentenyladenine (iP)
• Kinetin
• Dihydrozeatins

Synthetic Cytokinins

• Benzyladenine (BA) or 6-Benzylaminopurine (BAP):
• Meta-Topolins
• Forchlorfenuron (CPPU)
• Thidiazuron (TDZ)

4.0Types of cytokinins (Natural & Synthetic Cytokinins)

Physiological effects

Cell division: 

• Kinins promote cell division, and the application of a cytokinin and auxin mix to undifferentiated cells initiates differentiation. 

• A high cytokinin-to-auxin ratio fosters the growth of shoots, buds, and leaves, while a low ratio encourages root formation. An equal proportion of both leads to callus formation. 

Cell elongation:

• Notable cell enlargement has been observed in various plant tissues, including leaf discs from etiolated Phaseolus vulgaris leaves, pumpkin cotyledons, tobacco pith cultures, cortical cells of tobacco roots, and excised Jerusalem artichoke tissue, following kinetin treatment. 

• While cytokinins, including kinetin, have been found to promote cell expansion in leafy cotyledons of certain plants like mustard, sunflower, cucumber, and radish this effect is not demonstrated by auxin or gibberellin, as they do not stimulate cell expansion in cotyledons.

Overcome apical dominance :

• Cytokinins counteract the effects of auxins in regulating axillary bud growth, disrupting apical dominance. Application of cytokinins to the shoot apex or directly to axillary buds in many species releases them from inhibition. Tomato mutants with strong apical dominance have lower cytokinin levels. 

• Recent studies with transgenic plants confirm that elevated auxin levels increase apical dominance, while excess cytokinin diminishes it. Intriguingly, applying cytokinin to suppressed apical buds in auxin-overproducing plants releases them from dominance. 

Breaking dormancy of seeds:

• Cytokinins play a role in breaking seed dormancy in various plants, including lettuce, tobacco, white clover, and carpet grass. 
• In the case of parasitic seeds like Striga asiatica, germination typically requires the presence of a host plant. However, when treated with kinetin, these seeds can germinate even in the absence of their host.

Delay of senescence (The Richmond-Lang Effect):

• Richmond and Lang (1957) demonstrated that senescence in detached leaves of Xanthium can be significantly delayed by kinetin treatment. This phenomenon, known as the Richmond-Lang effect, prolongs the aging process for many days. 
• It's important to note that the cytokinin-induced delay in leaf senescence is observed specifically in detached leaves, and cytokinins have minimal or no impact on senescence in attached organs.

Additionally, the formation of adventitious roots contributes to the delay in leaf senescence. As roots are rich in cytokinins, the transport of these hormones from roots to leaves could explain the observed postponement of senescence.

Role in abscission:

• Cytokinins can accelerate as well as retard the process of abscission in leaf petioles depending on the site of their application 

Promotion of chloroplast development :

• Cytokinins play a significant role in promoting the transformation of etioplasts into chloroplasts. When etiolated seedlings are treated with cytokinins and subsequently exposed to light, there is a remarkable enhancement in this conversion process. 
• The resulting chloroplasts exhibit extensive grana formation and increased chlorophyll content. Moreover, the rate of synthesis of photosynthetic enzymes is notably higher compared to etiolated seedlings that are illuminated without prior cytokinin treatment.