What is Cytokinins ?

 Cytokinins are a class of plant hormones that play a vital role in regulating various physiological processes in plants. They were first discovered in the early 20th century and have since been recognized for their diverse effects on plant growth, development, and response to environmental stimuli. Cytokinins act in concert with other plant hormones, such as auxins and gibberellins, to maintain proper growth and development, coordinate cell division and differentiation, and respond to changing environmental conditions. In this article, we will delve into the functions, biosynthesis, and signaling pathways of cytokinins, shedding light on their essential contributions to plant life.

Functions of Cytokinins:

• Cell Division and Growth: One of the primary functions of cytokinins is to stimulate cell division and promote overall growth in plants. Cytokinins promote cell division by activating a signaling pathway that leads to the synthesis of proteins and enzymes involved in the cell cycle. This effect is particularly critical in plant meristems, regions of actively dividing cells, where cytokinins contribute to the formation of new tissues and organs.
• Delaying Senescence: Cytokinins also play a role in delaying senescence, the process of aging and deterioration in plant tissues and organs. By inhibiting the breakdown of chlorophyll and other cellular components, cytokinins help maintain the green color and physiological function of leaves and other plant parts. This delay in senescence contributes to the extended lifespan and productivity of plants.
• Apical Dominance and Lateral Bud Activation: Cytokinins help regulate apical dominance, the phenomenon where the terminal bud at the top of the plant suppresses the growth of lateral buds along the stem. When cytokinin levels are high, lateral buds are activated, leading to branching and the development of side shoots. Conversely, a decrease in cytokinin levels promotes apical dominance, directing the plant's energy and resources toward the growth of the main shoot.
• Delaying Leaf Senescence: Cytokinins delay the senescence of leaves by inhibiting the breakdown of chlorophyll and other cellular components, leading to the retention of green color and physiological function in mature leaves.
• Regulation of Root Growth: Cytokinins influence root growth by promoting lateral root development and inhibiting primary root elongation. This balance between primary and lateral root growth is crucial for optimizing nutrient uptake and responding to changes in the soil environment.

Biosynthesis of Cytokinins:

Cytokinins are synthesized in various plant tissues, including roots, shoots, and developing seeds. The biosynthetic pathway of cytokinins involves several steps and is influenced by various factors, including light, nutrient availability, and developmental stage. The main precursors for cytokinin biosynthesis are isopentenyladenine (iP) and adenosine triphosphate (ATP).

The first step in cytokinin biosynthesis is the conversion of ATP to isopentenyladenosine monophosphate (iPMP) by the enzyme adenylate isopentenyltransferase (IPT). This reaction occurs in the cytosol. iPMP is then transported to the endoplasmic reticulum, where it is further converted to isopentenyladenosine (iPA) by the enzyme cytochrome P450 monooxygenase. iPA can serve as a bioactive cytokinin or be further modified to produce other active forms.

Active Cytokinin Signaling Pathway:

Cytokinins exert their effects by binding to specific receptors, which trigger a signaling cascade that leads to various cellular responses. The primary receptors for cytokinins are histidine kinases known as cytokinin receptors or CRE1/AHKs (Cytokinin Response 1/Arabidopsis Histidine Kinases). These receptors are localized in the plasma membrane of plant cells.

When a cytokinin molecule binds to a cytokinin receptor, it activates the receptor's kinase domain, leading to the autophosphorylation of the receptor on a specific histidine residue. This phosphorylation event initiates a signaling cascade that involves a series of downstream components, including histidine phosphotransfer proteins (AHPs) and response regulators (ARRs).

Once activated, the AHPs transfer the phosphoryl group from the receptor to the ARRs, a family of transcription factors. The phosphorylation of ARRs changes their conformation, enabling them to act as transcription factors that regulate the expression of specific target genes. These target genes, in turn, mediate the various physiological responses associated with cytokinin signaling, such as cell division, shoot branching, and delay of senescence.

Interplay with Other Plant Hormones:

Cytokinins do not act alone; they interact with other plant hormones, such as auxins and gibberellins, to orchestrate the complex processes that underlie plant growth and development. One of the most well-known interactions is the balance between cytokinins and auxins, often referred to as the cytokinin-auxin balance.

Auxins promote cell elongation and inhibit lateral bud development, while cytokinins have the opposite effect, promoting cell division and activating lateral buds. The interplay between these two hormones helps determine the pattern of growth and branching in plants.

For example, in apical meristems, where auxin levels are high and cytokinin levels are low, auxins inhibit lateral bud activation, leading to apical dominance. However, when the balance shifts due to changes in cytokinin levels or sensitivity, lateral buds are released from apical dominance, leading to branching and side shoot formation.

Environmental Regulation of Cytokinin Levels:

The levels of cytokinins in plants are not constant and can be influenced by various environmental factors. For example, exposure to light and darkness can affect cytokinin biosynthesis and degradation. Light promotes the biosynthesis of cytokinins, especially in young leaves, and also influences the distribution and transport of cytokinins within the plant.

Environmental stresses, such as drought, salinity, and pathogen attacks, can also influence cytokinin levels. In response to these stresses, plants may alter their cytokinin signaling to adjust growth and development, allocate resources more efficiently, and initiate defense responses.

In conclusion, cytokinins are essential plant hormones that regulate various aspects of plant growth, development, and responses to environmental cues. Their functions in cell division, apical dominance, lateral bud activation, leaf senescence, and root growth make them critical players in shaping the architecture and productivity of plants. Understanding the biosynthesis and signaling pathways of cytokinins provides valuable insights into the complexities of plant growth regulation and can contribute to the development of innovative strategies to enhance crop yield, stress tolerance, and overall plant performance.