7.1 Classical plant hormones: auxins, cytokinins, gibberellins
3 min read•august 7, 2024
Plant hormones are crucial regulators of growth and development. , , and are three classical plant hormones that play vital roles in various physiological processes, from cell division to .
These hormones work together to control plant growth, often with overlapping or antagonistic effects. Understanding their structures, synthesis, and functions is key to grasping how plants respond to environmental cues and internal signals.
Auxins and Their Effects
Auxin Structure and Synthesis
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- Auxins are a class of plant hormones that play a crucial role in regulating plant growth and development
- The most common naturally occurring auxin is (IAA)
- IAA is synthesized primarily in young leaves, shoot tips, and developing seeds
- serves as a precursor for IAA synthesis through multiple biosynthetic pathways
- Auxins can be synthesized in one part of the plant and transported to other regions where they exert their effects
- Auxin transport occurs in a polar manner, with the hormone moving from the shoot apex towards the base of the plant ()
Physiological Effects of Auxins
- Auxins promote , the phenomenon where the main central stem of a plant is dominant over other side stems
- Auxins produced by the apical meristem suppress the growth of lateral buds, maintaining a single dominant stem ()
- Removal of the apical meristem (decapitation) reduces auxin levels, allowing lateral buds to grow and branch out ()
- Auxins stimulate , particularly in shoots
- They cause cells to take up water and expand, leading to increased plant height
- Auxins induce the expression of genes involved in cell wall loosening () and cell wall synthesis ()
- Auxins play a vital role in fruit development
- They stimulate the growth of ovary walls after fertilization, contributing to fruit enlargement
- Auxins also delay fruit ripening and prevent premature fruit drop ()
- Synthetic auxins (2,4-D) are used as herbicides to selectively kill broadleaf weeds by causing uncontrolled growth
Cytokinins and Their Functions
Cytokinin Structure and Synthesis
- Cytokinins are a class of plant hormones that regulate cell division, shoot initiation, and leaf senescence
- The most common naturally occurring cytokinin is
- Zeatin is synthesized primarily in root tips, young leaves, and developing seeds
- Adenine derivatives serve as precursors for cytokinin synthesis
- Cytokinins can be synthesized in roots and transported to shoots via the xylem
Physiological Effects of Cytokinins
- Cytokinins stimulate cell division (cytokinesis) in plant meristems
- They activate the cell cycle by inducing the expression of () and cyclins
- Cytokinins maintain the meristematic activity of shoot and root apical meristems
- Cytokinins promote shoot initiation and development
- They stimulate the growth of lateral buds and promote branching
- Cytokinins counteract the apical dominance induced by auxins
- Cytokinins delay leaf senescence (aging)
- They maintain and photosynthetic activity in leaves
- Cytokinins upregulate the expression of genes involved in and stress responses
Gibberellins and Plant Growth
Gibberellin Structure and Synthesis
- Gibberellins (GAs) are a class of plant hormones that regulate stem elongation, , and fruit development
- The most biologically active gibberellin is (GA3)
- GA3 is synthesized in young leaves, root tips, and developing seeds
- The precursor for gibberellin synthesis is (GGPP)
- Gibberellins can be synthesized in one part of the plant and transported to other regions where they exert their effects
Physiological Effects of Gibberellins
- Gibberellins promote stem elongation
- They stimulate cell division and cell elongation in the intercalary meristem of monocots (grasses)
- Gibberellins induce the expression of genes involved in cell wall loosening (expansins) and cell wall synthesis ()
- Gibberellins regulate seed germination
- They break seed dormancy by promoting the synthesis of hydrolytic enzymes (α-amylase) that mobilize stored reserves
- Gibberellins counteract the effects of abscisic acid (ABA), a hormone that maintains seed dormancy
- Gibberellins influence fruit development
- They stimulate fruit growth and enlargement
- Gibberellins delay fruit ripening and prevent premature fruit drop (abscission)
- Gibberellin mutants exhibit dwarfism (reduced stem elongation) and delayed flowering
Key Terms to Review (35)
Abscission: Abscission is the process by which plants shed their leaves, flowers, or fruits, allowing for the removal of unnecessary or damaged structures. This natural phenomenon is crucial for plant health and survival, particularly in response to environmental changes and stress. Abscission is regulated by various plant hormones, which play key roles in signaling and facilitating this process.
Apical dominance: Apical dominance is the phenomenon where the main central stem of a plant grows more strongly than the other side stems, primarily due to the influence of plant hormones. This growth pattern allows plants to focus their energy on upward growth towards light, optimizing photosynthesis and resource acquisition while suppressing the growth of lateral buds.
Auxins: Auxins are a class of plant hormones that play a crucial role in coordinating various growth and behavioral processes in plants, including cell elongation, apical dominance, and responses to light and gravity. They influence several physiological functions, connecting processes like nutrient uptake, transport, and growth regulation throughout the plant.
Basipetal transport: Basipetal transport refers to the movement of substances, particularly hormones, from the tips of plant structures, such as leaves or stems, towards the base or lower parts of the plant. This type of transport is essential for regulating growth and developmental processes, facilitating the distribution of nutrients and signals throughout the plant.
Biosynthesis: Biosynthesis is the process by which living organisms produce complex molecules from simpler ones, often using energy derived from metabolic processes. This is critical for the production of vital compounds like hormones, which play key roles in regulating growth, development, and responses to environmental stimuli in plants.
Cdks: Cyclin-dependent kinases (cdks) are a family of protein kinases that play a crucial role in regulating the cell cycle and various cellular processes by phosphorylating specific target proteins. These enzymes are activated when bound to cyclins, which are regulatory proteins whose levels fluctuate throughout the cell cycle, enabling cdks to drive key events like cell division, growth, and differentiation in plants.
Cell elongation: Cell elongation is the process by which plant cells increase in size, leading to the growth and expansion of plant tissues. This mechanism is crucial for various plant developmental processes, including organ growth, phototropism, and response to environmental stimuli, and is heavily influenced by plant hormones.
Cellulose synthase: Cellulose synthase is an enzyme responsible for the biosynthesis of cellulose, a crucial polysaccharide that forms the primary structural component of plant cell walls. This enzyme plays a key role in determining the strength and rigidity of plant tissues, influencing overall plant growth and development. It operates at the plasma membrane, catalyzing the polymerization of glucose units into long chains that form cellulose microfibrils.
Chlorophyll Levels: Chlorophyll levels refer to the concentration of chlorophyll pigments present in plant tissues, which play a crucial role in photosynthesis by absorbing light energy. These levels can significantly impact a plant's ability to produce food and grow, influencing its overall health and productivity. Factors such as light availability, water supply, nutrient status, and hormonal influences can affect chlorophyll levels in plants.
Cross-talk between hormones: Cross-talk between hormones refers to the complex interactions and regulatory mechanisms that occur among various plant hormones, influencing each other's signaling pathways and overall physiological responses. This interplay allows plants to integrate multiple signals, enabling them to adapt to environmental changes, regulate growth and development, and respond to stressors efficiently. Understanding this concept is crucial for comprehending how auxins, cytokinins, and gibberellins work together in plant physiology.
Cyclin-dependent kinases: Cyclin-dependent kinases (CDKs) are a family of protein kinases that play a crucial role in regulating the cell cycle by controlling the transition between different phases. These enzymes need to bind to specific proteins called cyclins to become active, forming complexes that phosphorylate target proteins and trigger various cellular processes, including cell division and growth. The interaction between CDKs and cyclins is essential for proper cell cycle progression, ensuring that the cell divides accurately and efficiently.
Cytokinins: Cytokinins are a class of plant hormones that promote cell division and growth, playing crucial roles in various plant processes such as shoot and root development, as well as nutrient allocation. They work in conjunction with other hormones, influencing the balance of growth and differentiation in plants, which is essential for mineral uptake, transport, and overall physiological health.
Expansins: Expansins are a group of plant proteins that play a crucial role in cell wall loosening and expansion. They facilitate the growth of plant cells by disrupting hydrogen bonds in the cell wall, allowing for elongation and growth in response to various stimuli, such as hormonal signals, particularly from auxins and gibberellins.
Frits Went: Frits Went was a Dutch botanist known for his pioneering work in plant physiology, particularly in the discovery and study of plant hormones. He is best remembered for his research on auxins, a class of hormones that regulate various aspects of plant growth and development, including cell elongation, fruit development, and responses to light and gravity. His work laid the foundation for our understanding of how plants use hormones to communicate internally and adapt to their environment.
Fruit development: Fruit development refers to the process by which flowering plants produce and mature their fruits, which are essential for seed dispersal and reproduction. This process is regulated by various hormones and involves multiple stages, including fertilization, ovary development, and ripening, all of which contribute to the overall success of plant reproduction.
Gene expression regulation: Gene expression regulation is the process by which cells control the timing and amount of gene expression, ensuring that genes are turned on or off as needed. This regulation is crucial for plant development and responses to environmental stimuli, including the influence of hormones and various signaling pathways that affect growth and physiological processes.
Geranylgeranyl diphosphate: Geranylgeranyl diphosphate (GGPP) is a key intermediate in the biosynthesis of isoprenoids, which are essential components in the growth and development of plants. This compound plays a crucial role in the synthesis of various plant hormones and secondary metabolites that influence plant physiology, including auxins, cytokinins, and gibberellins.
Gibberellic acid: Gibberellic acid is a plant hormone belonging to the gibberellin family, which plays a crucial role in regulating various growth and developmental processes in plants. It promotes stem elongation, seed germination, and flowering, and is essential for breaking dormancy in seeds. Gibberellic acid works in conjunction with other plant hormones such as auxins and cytokinins to coordinate growth responses in plants.
Gibberellins: Gibberellins are a group of plant hormones that play critical roles in regulating growth and development. They influence processes such as stem elongation, seed germination, and flowering, making them essential for various stages of a plant's life cycle. Their interactions with other hormones highlight their importance in the complex signaling networks that control plant physiology.
Gravitropism: Gravitropism is the growth response of a plant to gravity, allowing it to orient itself properly in its environment. This response enables roots to grow downward (positive gravitropism) and stems to grow upward (negative gravitropism), which is essential for optimizing light capture and water/nutrient absorption. Understanding gravitropism involves exploring how hormones regulate this process and how signal transduction pathways facilitate communication within the plant.
Hormonal Balance: Hormonal balance refers to the precise regulation and interaction of plant hormones to ensure proper growth, development, and response to environmental stimuli. It is crucial for maintaining physiological functions in plants, as imbalances can lead to abnormal growth patterns, developmental disorders, or failure to adapt to changing conditions. This equilibrium among hormones like auxins, cytokinins, and gibberellins orchestrates complex processes such as cell division, elongation, and differentiation.
Indole-3-acetic acid: Indole-3-acetic acid (IAA) is a natural plant hormone, specifically an auxin, that plays a crucial role in various growth and developmental processes in plants. It influences cell elongation, apical dominance, and the formation of roots and flowers, making it essential for plant growth regulation. As a major auxin, IAA is integral to coordinating plant responses to environmental stimuli and internal developmental cues.
Monopodial growth: Monopodial growth is a type of plant growth characterized by a single main stem that continues to elongate while producing lateral branches. This growth pattern allows the plant to reach upward effectively, maximizing light capture and competing successfully for resources. The process is closely regulated by various hormones, which influence how and when these stems and branches develop.
Nutrient remobilization: Nutrient remobilization is the process by which plants relocate nutrients from older, senescing tissues to newer, actively growing parts of the plant. This mechanism is vital for maximizing nutrient use efficiency, especially under conditions of nutrient limitation, and plays a significant role in plant development and response to environmental stresses.
Peter J. Davies: Peter J. Davies is a prominent plant physiologist known for his significant contributions to the understanding of plant hormones, specifically auxins, cytokinins, and gibberellins. His research has helped to elucidate the complex roles these hormones play in plant growth and development, influencing various physiological processes such as cell division, elongation, and differentiation.
Phototropism: Phototropism is the growth response of a plant in which it bends or moves toward light sources. This process is essential for optimizing light capture for photosynthesis, ultimately influencing plant growth and development. It involves hormone signaling pathways that mediate the plant's directional growth and showcase how environmental cues can impact physiological processes.
Plant tissue culture: Plant tissue culture is a technique used to grow plant cells, tissues, or organs in a controlled, sterile environment on a nutrient medium. This method allows for the propagation of plants in vitro, enabling researchers and horticulturists to produce large numbers of identical plants efficiently. By manipulating growth conditions and adding specific hormones, the growth and development of the plant tissues can be directed for various purposes, including cloning, genetic modification, and disease elimination.
Rooting hormone: Rooting hormone is a plant growth regulator that promotes the development of roots in plant cuttings or grafts. This hormone helps enhance root formation by increasing cell division and elongation, thereby accelerating the process of rooting in various plant species. It's particularly useful in horticulture and agriculture for propagating plants from cuttings or improving the success rate of transplanting.
Seed germination: Seed germination is the process by which a seed develops into a new plant, initiating growth after a period of dormancy. This crucial stage involves a series of physiological and biochemical changes, allowing the embryo within the seed to emerge and begin to grow into a seedling. The process is highly influenced by environmental factors and internal hormonal signals that regulate growth and development.
Signal Transduction: Signal transduction is the process by which cells convert external signals into a functional response, often involving a series of molecular events that alter cellular activities. This process is crucial for how cells communicate with their environment, respond to hormones, and interact with other organisms, which are vital for growth, development, and adaptation.
Sympodial growth: Sympodial growth is a type of plant growth where the main axis of the plant is replaced by a series of lateral branches, creating a zigzag or branched appearance. This growth pattern is often seen in certain species of plants, allowing them to adapt to their environment, enhance light capture, and optimize resource allocation. By developing multiple stems or branches from a central point, plants can maximize their survival and reproductive success in varying habitats.
Transport pathways: Transport pathways refer to the various routes and mechanisms through which substances such as water, nutrients, and hormones move within plants. These pathways are essential for maintaining physiological functions, supporting growth, and facilitating communication between different parts of the plant. In the context of plant hormones, understanding these pathways helps explain how hormones like auxins, cytokinins, and gibberellins influence processes such as growth regulation and response to environmental stimuli.
Tryptophan: Tryptophan is an essential amino acid that plays a critical role in the biosynthesis of several important molecules in plants, including the plant hormone auxin. As a precursor for the synthesis of auxins, tryptophan influences various growth processes and physiological responses, making it fundamental to plant development and environmental adaptation. Its involvement extends to other plant hormones, linking it to broader signaling pathways in plants.
Xyloglucan endotransglycosylases: Xyloglucan endotransglycosylases (XETs) are enzymes that play a crucial role in plant cell wall modification by cleaving xyloglucan chains and facilitating the rearrangement of these components, effectively altering cell wall structure and properties. This enzyme activity is particularly influenced by classical plant hormones, which modulate growth and development processes such as cell elongation, division, and differentiation.
Zeatin: Zeatin is a type of cytokinin, a class of plant hormones that play essential roles in various growth and developmental processes. As a naturally occurring compound, zeatin promotes cell division, influences shoot and root development, and is involved in delaying leaf senescence. Its importance in plant physiology is underscored by its ability to interact with other hormones, contributing to the overall growth and response of plants to environmental stimuli.