Cytokinins are a class of plant hormones that play an important role in plant growth and development. They help regulate cell division, shoot and root growth, and other developmental processes. An important question in plant biology is how cytokinins are transported throughout the plant body. This article will examine whether the plant’s vascular system carries cytokinins from one part of the plant to another.
What are cytokinins?
Cytokinins are a class of plant hormones that were discovered in the 1950s. The first cytokinin to be identified was kinetin, which was isolated from autoclaved herring sperm DNA. Since then, over 50 naturally occurring cytokinins have been identified in plants. All cytokinins are derivatives of adenine with an attached isoprenoid side chain.
The most common naturally occurring cytokinins are zeatin, dihydrozeatin, and isopentenyladenine. Zeatin and dihydrozeatin are synthesized directly in plant cells, while isopentenyladenine is synthesized indirectly from tRNA degradation. In addition to natural cytokinins, there are also synthetic cytokinins such as kinetin and benzyladenine that have biological activity.
Cytokinins help regulate many aspects of plant growth and development, including:
- Cell division – Cytokinins stimulate cell division by promoting the transition from G2 to M phase of the cell cycle.
- Shoot growth – Cytokinins promote shoot growth by stimulating shoot apical meristem activity and lateral bud growth.
- Root growth – High cytokinin levels inhibit root growth while low levels promote it.
- Leaf senescence – Cytokinins delay leaf senescence by maintaining chlorophyll levels.
- Seed germination – Cytokinins help break seed dormancy and promote germination.
- Nutrient mobilization – Cytokinins regulate mobilization and distribution of nutrients in the plant.
Through these various roles, cytokinins help coordinate plant development in response to environmental and hormonal signals.
How are cytokinins transported in plants?
For cytokinins to exert their effects on distant parts of the plant, they must be transported from sites of synthesis to target tissues. Cytokinins are synthesized in actively growing tissues such as root tips, developing shoots, young leaves, and fruits. The vascular system provides the routes for cytokinin transport.
There are two major long-distance transport pathways in plants:
- Xylem – transports water and minerals from the roots upwards.
- Phloem – transports sugars, amino acids, and hormones between source and sink tissues.
Cytokinins can be transported through both xylem and phloem, but phloem transport appears to be predominant. Radio-labeling studies have shown that cytokinins applied to leaves are rapidly transported through the phloem to other parts of the plant. Cytokinin levels are much higher in phloem sap compared to xylem sap, further indicating the importance of phloem transport.
Evidence for cytokinin transport through phloem
There are several key lines of evidence indicating that the phloem is the primary route for cytokinin transport:
- Higher cytokinin concentrations in phloem sap than xylem sap
- Rapid basipetal (shoot to root) transport of radiolabeled cytokinins
- Impaired root growth and shoot branching when phloem transport is blocked
- Detection of cytokinin biosynthesis enzymes and tRNA in phloem tissues
Studies using radiolabeled cytokinins clearly demonstrate their ability to move rapidly through the phloem in a source-to-sink manner. For example, when radiolabeled zeatin is applied to a leaf, it can be detected in the roots within hours. Physical blockage of the phloem inhibits this downward translocation and alters growth patterns.
Additionally, all the enzymes needed for cytokinin biosynthesis, as well as their tRNA substrates, have been found in phloem tissues. This suggests that cytokinins may be actively synthesized within the phloem as they are transported.
Mechanism of phloem transport
Cytokinins are transported in the phloem along with photoassimilates and other macromolecules. The mechanism of phloem transport relies on bulk flow driven by hydrostatic pressure gradients between source and sink tissues. Phloem transport occurs through cylindrical conduits called sieve tube elements.
Cytokinins likely move symplastically between cells through plasmodesmata until reaching the sieve elements. The exact molecular details of how cytokinins enter and exit the phloem remains unclear. Transport is likely facilitated by plasma membrane transport proteins, which may actively load and unload cytokinins into the phloem stream.
Does xylem play any role?
While phloem appears to be the major long-distance transport pathway, the xylem may also contribute to local cytokinin movement. Although xylem sap generally has lower cytokinin content, xylem transport does occur. For example, root-applied cytokinin can move upward in the xylem to shoots.
Xylem transport may facilitate upward cytokinin movement from root tips to regulate shoot growth. Downward xylem transport likely depends on diffusion from other tissues into the xylem stream. In some cases, cytokinins deposited in xylem tissue can be remobilized to influence lateral shoot growth.
Thus, xylem plays a secondary role, primarily moving cytokinins over short distances. Phloem is the principal long-distance route, but bi-directional movement between xylem and phloem probably occurs to facilitate local cytokinin exchange between tissues.
How is cytokinin transport regulated?
For optimal growth and development, cytokinin levels and transport must be finely regulated. Cytokinin homeostasis is maintained through metabolic control of biosynthesis, degradation, and transport. Several key mechanisms help regulate cytokinin transport:
- Local biosynthesis – Cytokinin synthesis in source tissues helps regulate the amount of cytokinin entering transport pathways.
- Metabolic degradation – Cytokinin oxidase enzymes can degrade cytokinins and reduce transport to sinks.
- Reversible glycosylation – Conjugation to glucose inactivates cytokinins and may inhibit transport.
- Transporter proteins – Active transport proteins likely mediate phloem loading and unloading of cytokinins.
- Feedback regulation – Transport may be self-regulated based on demands of sink tissues.
Environmental factors like drought, light, and nutrient levels can influence cytokinin biosynthesis and transport. Hormonal signals like auxin also regulate cytokinin activity and transport. Overall, a complex signaling network allows plants to coordinate cytokinin movement with environmental cues and developmental requirements.
Conclusion
In summary, the vascular system, particularly the phloem, serves as the primary long-distance transport route for cytokinins in plants. Cytokinins synthesized in source tissues enter the phloem translocation stream and are delivered to sinks to regulate growth and development. While xylem plays a minor role, phloem transport is the principal mechanism for cytokinin movement.
Tight metabolic and environmental control helps plants adjust cytokinin transport to integrate external signals with endogenous hormonal cues. This enables optimal coordination of plant growth patterns in dynamic environments. Further research on the molecular details of phloem loading, transport, and unloading of cytokinins will shed more light on this important hormonal regulation system.