Cytokinins are a class of plant growth hormones that regulate various aspects of plant growth and development. The term “cytokinin” was first coined by Folke Skoog and his colleagues in the 1950s to describe compounds that promote cell division in plant tissue cultures.
The first naturally occurring cytokinin, kinetin, was isolated from autoclaved herring sperm DNA in 1955. Over the years, numerous other cytokinins have been identified from various plant sources. All naturally occurring cytokinins are adenine derivatives with either an isoprenoid or aromatic side chain. Some common examples include zeatin, dihydrozeatin, and benzyladenine.
Cytokinins are involved in many physiological processes in plants, including cell division and differentiation, apical dominance, nutrient mobilization, and delay of senescence. They interact with other plant hormones like auxins and gibberellins to regulate plant growth and development. The levels, transport, and signaling of cytokinins are all carefully controlled in plants.
Cytokinin Structure
The basic chemical structure of cytokinins consists of an adenine moiety attached to a side chain. The adenine part is similar across all cytokinins and confers the hormonal activity. The side chain varies and determines the specific cytokinin identity and activity.
Naturally occurring cytokinins have an isoprenoid side chain derived from isopentenyl diphosphate. Zeatin and kinetin are two examples. On the other hand, synthetic cytokinins have an aromatic side chain, such as benzyladenine and phenylurea cytokinins.
While naturally occurring cytokinins are integral plant growth regulators, synthetic cytokinins are used commercially to promote plant growth and productivity. They are applied as sprays or added to tissue culture media. Common synthetic cytokinins include benzyladenine, kinetin, and thidiazuron.
The adenine moiety is the key pharmacophore that activates cytokinin responses in plants. Substitutions to the adenine or removal of the side chain reduces or abolishes cytokinin activity. The composition of the side chain modulates the stability and potency of the cytokinin.
Cytokinin Synthesis
Cytokinins are synthesized throughout the plant body, but the major sites of synthesis are the root apical meristem, young leaves, and developing seeds. There are two main pathways for cytokinin biosynthesis in plants:
De Novo Biosynthesis Pathways
The main de novo biosynthesis pathway starts with adenine nucleotides as the precursor. The adenine is prenylated by dimethylallyl diphosphate (DMAPP) in a reaction catalyzed by the enzyme isopentenyl transferase (IPT). This reaction produces isopentenyladenine ribonucleotides (iPRMP). These iPRMPs are then hydroxylated by cytochrome P450 monooxygenases to produce trans-zeatin ribonucleotides (tZRMP).
The iPRMPs and tZRMPs are converted to their respective nucleobase cytokinins, isopentenyladenine (iP) and trans-zeatin (tZ), by the enzyme LONELY GUY (LOG). The LOG enzymes are key regulators of cytokinin activation in plants.
The iP and tZ cytokinins can then be interconverted between their active free base and inactive nucleoside forms as needed for biological activity and transport.
Synthesis Locations
The root apical meristem is a major site of cytokinin synthesis, especially the trans-zeatin forms. Root-synthesized cytokinins are transported up the xylem to the shoot to regulate bud growth and shoot branching.
Young developing leaves also produce significant amounts of cytokinins, predominately in the form of isopentenyladenine. The cytokinins produced in young leaves act locally to promote chloroplast development and leaf growth.
Developing seeds are another key site of cytokinin production, where the cytokinins stimulate cell division and growth of the endosperm and embryo. The cytokinins produced by the developing seeds help regulate seed and fruit development.
Cytokinin Transport
Cytokinins are transported through plants via the xylem and phloem vascular tissues. The xylem transports water and nutrients from the roots up to the leaves while the phloem transports sugars and other molecules throughout the plant.
Cytokinins can move passively through the transpiration stream in the xylem but they are also actively transported by specific carrier proteins. The purine permease (PUP) family of transporters play an important role in directed cytokinin transport and help regulate cytokinin levels in different parts of the plant.
In the roots, cytokinins are transported from the vascular cylinder into the root tip primarily through the symplast pathway. From the root tip, they are loaded into the xylem for acropetal transport to the shoot. The rate of xylem transport is controlled by the cytokinin concentration gradient between roots and shoots.
In the shoot, cytokinins unload from the xylem into target tissues. The phloem allows for basipetal transport of cytokinins from source leaves to sink tissues such as roots, fruits and developing shoots. Therefore, the vascular transport system allows cytokinins to coordinate growth and development throughout the plant.
Cytokinin Function
Cytokinins play several key roles in plant growth and development. Some of the main physiological functions of cytokinins include:
- Promoting cell division – Cytokinins stimulate cell division, especially in lateral buds and shoots. This leads to the growth of new leaves, branches, and stems. Cytokinins counteract the growth-inhibiting effects of auxins to regulate shoot initiation and growth.
- Delaying senescence – Cytokinins can delay the aging and death of plant tissues, including leaves and flowers. They help prolong the active life and function of plant organs by preventing chlorophyll degradation and protein loss.
- Regulating shoot and root growth – Cytokinins produced in the roots help stimulate shoot growth. Meanwhile, cytokinins made in the shoots inhibit root growth. This systemic regulation coordinates overall plant growth patterns.
- Nutrient mobilization – Cytokinins facilitate the export of nutrients from source tissues like leaves to sink tissues like fruits and seeds. This aids in fruit development.
- Responding to stresses – Cytokinins play a role in plant stress responses and adaptation. Their levels change in response to stresses like drought, flooding, wounding, and pathogens. They interact with other hormones in complex stress response pathways.
Overall, cytokinins are crucial for many developmental and physiological processes in plants. Their promotion of cell division and regulation of source-sink transport enable plant growth, branching, and fruit production. Their anti-aging effects help extend the functioning of plant organs. Cytokinins also facilitate adaptation to changing environmental conditions and stressors.
Cytokinin Effects
Cytokinins have several important effects on plant growth and development. Some of the key effects include:
- Cell division – Cytokinins stimulate cell division, especially in combination with auxins. This contributes to growth of plant organs and tissues.
- Apical dominance – Cytokinins produced in the roots counteract auxins produced in the shoot apical meristem. This reduces apical dominance and allows lateral buds to grow into branches.
- Leaf senescence – Cytokinins delay leaf senescence by maintaining chlorophyll levels and protein synthesis in aging leaves.
- Nutrient mobilization – Cytokinins regulate the breakdown of nutrients in aging tissues and their transport to actively growing parts of the plant.
- Seed germination – Cytokinins break seed dormancy and promote germination, acting antagonistically to abscisic acid.
- Flower and fruit development – Cytokinins play a key role in flower formation, fruit set, and seed development.
- Stress response – Cytokinins interact with other plant hormones to regulate plant responses to stresses such as drought, extreme temperatures, and pathogen attack.
The diverse effects of cytokinins are mediated through complex interactions with other plant hormones, especially auxins. The relative levels and ratios of cytokinin and auxin regulate many growth and developmental processes. Overall, cytokinins and auxins have both complementary and antagonistic effects that allow for precise control over plant growth and development.
Cytokinin Applications
Cytokinins have many important applications, especially in agriculture and horticulture. As plant growth regulators, cytokinins can be used to promote plant growth, delay senescence, stimulate axillary bud growth, and alter sink-source relationships. Some key agricultural uses include:
- Increasing crop yields – Foliar sprays containing cytokinins can increase grain or fruit yields in crops like wheat, rice, barley, sugarcane, grapes, and apples. The cytokinins stimulate cell division, delay senescence, and alter sink-source balances to favor reproductive growth.
- Promoting branching – Treating plants with cytokinins can cause rapid growth of lateral buds and stimulate axillary branching. This is useful for promoting bushier, more compact growth in ornamental plants and increasing bud break in fruit trees.
- Delaying leaf senescence – Since cytokinins counteract leaf aging, they can be used to prolong leaf life and photosynthetic capacity later into the season. This helps plants stay greener and active for longer.
- Breaking dormancy – Soaking seeds or tubers in cytokinin solutions can help break dormancy and promote faster, more uniform germination. This technique is used for crops like potatoes and onions.
- Increasing flower production – Cytokinin application increases flower bud formation and flowering in many plants. They are sometimes sprayed on seed crops to maximize seed yields.
Cytokinins like kinetin and benzyladenine are also essential components of tissue culture media. They stimulate cell division and shoot proliferation in explants, allowing rapid micropropagation of plants from small amounts of starting material. Their ability to induce callus growth and somatic embryogenesis is also invaluable for propagating many species in vitro.
Cytokinin Research
Cytokinin research is an active area in plant biology and agriculture. Scientists are working to better understand the biochemical pathways, gene regulation, and physiological effects of these important plant hormones. Some key areas of current research include:
Elucidating cytokinin signaling and perception – Significant progress has been made in identifying cytokinin receptors, such as the Arabidopsis histidine kinases (AHKs), and downstream transcription factors that mediate cytokinin responses. However, there is still more to learn about the molecular mechanisms of cytokinin perception and signal transduction.
Examining crosstalk with other hormones – Plant hormones often interact, and there is evidence that cytokinins communicate with auxin, abscisic acid, gibberellins, and ethylene. Understanding the molecular basis of this crosstalk will provide insight into how plant growth and development are coordinated.
Investigating cytokinin transport and degradation – The levels and locations of cytokinins are tightly regulated in plants. Active areas of study include identifying transporters that move cytokinins between tissues and characterizing enzymes that irreversibly degrade cytokinins.
Exploring cytokinin functions in plant-microbe interactions – Some plant-associated bacteria and fungi produce cytokinins. Research is ongoing into how microbial-derived cytokinins influence the plant immune system, nodulation, and mycorrhizal associations.
Developing cytokinin applications in agriculture – There is interest in using cytokinins to improve crop yields, stress tolerance, and nutritional quality. Challenges include optimizing cytokinin types, amounts, and delivery methods to achieve desired effects without unintended consequences.
Overall, cytokinin research will continue advancing our understanding of plant hormone biology while working to translate findings into agricultural innovations. Key open questions remain regarding cytokinin receptors, transport, metabolic pathways, and mechanisms of action in different plant tissues and processes. Exciting discoveries lie ahead as researchers employ emerging techniques in cytokinin studies.