Cytokinins are a group of plant hormones that play a crucial role in regulating cellular processes and plant development. These hormones, characterized by their adenine ring structure, are pivotal in promoting cell division, stimulating bud formation, and influencing various other physiological functions. The discovery and understanding of cytokinins have significantly advanced our knowledge of plant biology and have applications in agriculture and horticulture.
In 1948, U.S. Skog and China Cui Cheng observed that adenine in tobacco tissue cultures could trigger differentiation and budding in tobacco pith tissue. This finding laid the groundwork for further research into substances that could promote cell division. In 1955, Carlos Miller identified a compound from the degradation products of yeast and herring sperm DNA that could stimulate cell division. This substance, named kinetin, has a chemical structure known as furan methyl adenine or furfuryl gland purine.
The quest to identify naturally occurring cytokinins in higher plants led to a breakthrough in 1963 when Letham isolated zeatin from fertilized corn seeds between 11-16 days old. Since then, several adenine derivatives have been identified in higher plants, including dihydrozeatin, zeatin riboside, and isopentenyl adenine. The development of synthetic cytokinins, such as benzyl adenine and tetrahydropyranyl benzyl adenine, has expanded the range of these influential compounds.
Cytokinins are synthesized most actively in the root meristem (apical) and are transported through the xylem from the roots to the stems. They can also be formed in leaves, buds, young fruits, and developing seeds, with zeatin being the first cytokinin obtained from maize immature seeds. These hormones can be produced by cleavage of transfer RNA or synthesized from precursors such as medroxyprogesterone valerate and adenine.
Promotion of Cell Division: Cytokinins primarily stimulate cell division, working in conjunction with auxins and gibberellins, each playing a distinct role. While auxins promote nuclear division, cytokinins mainly affect the division of the cytoplasm.
Stimulation of Bud Formation: Cytokinins are essential for bud differentiation. For instance, when detached leaves are treated with cytokinins, buds can form at the leaf margins and basal veins.
Enhancement of Cell Expansion: These hormones can cause the expansion of cotyledons or leaf discs in dicotyledonous plants, such as beans and radishes, by promoting cell lateral thickening.
Development of Lateral Buds: Cytokinins can override apical dominance imposed by growth hormones, encouraging the growth and development of lateral buds.
Delaying Leaf Senescence: Cytokinins have anti-aging properties that can delay the yellowing and aging of leaves. This effect is also utilized to keep fruits and flowers fresh and to prevent fruit drop.
Breaking Seed Dormancy: Cytokinins can substitute for light in breaking the dormancy of seeds that require light for germination, such as lettuce and tobacco, thus promoting germination in the dark.
Cytokinins have been the subject of numerous studies, revealing their potential to enhance crop yields and improve plant health. For example, cytokinin treatment in young citrus fruits can significantly reduce fruit drop and result in larger, darker green fruits compared to untreated controls. Additionally, the global market for cytokinins is projected to grow, reflecting their increasing use in agriculture. According to a report by MarketsandMarkets, the plant growth regulators market size is estimated to be valued at USD 2.9 billion in 2020 and projected to reach USD 4.2 billion by 2025, growing at a CAGR of 6.7% during the forecast period.
For more detailed information on cytokinins and their applications, authoritative sources such as the American Society of Plant Biologists and ScienceDirect offer extensive research and reviews on the topic.
The discovery and subsequent research into cytokinins have revolutionized our understanding of plant growth and development. These hormones are not only fundamental to plant physiology but also offer practical benefits in agriculture, horticulture, and beyond. As research continues, the potential applications of cytokinins are likely to expand, further cementing their importance in the science of plant biology.
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