7.2 Ethylene, abscisic acid, and brassinosteroids
5 min read•august 7, 2024
Plant hormones are crucial for growth and development. , , and play key roles in various processes. These hormones regulate everything from to stress responses, shaping how plants adapt to their environment.
Understanding these hormones is essential for grasping plant physiology. They influence seedling development, stomatal regulation, and . Their interactions and signaling pathways are complex, but vital for plant survival and productivity in changing conditions.
Ethylene and Plant Development
Ethylene's Role in Plant Growth and Development
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Top images from around the web for Ethylene's Role in Plant Growth and Development
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- Ethylene, a gaseous plant hormone, plays a crucial role in various aspects of plant growth and development
- Produced by almost all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seeds
- Ethylene increases during fruit ripening, flower senescence, , and in response to various stresses (mechanical wounding, flooding, chilling, disease)
- Acts at very low concentrations in the plant (less than 1 ppm)
- Ethylene perception involves a family of membrane-associated receptors (ETR1, ETR2, ERS1, ERS2, EIN4) that bind ethylene and initiate
Triple Response and Seedling Development
- , a characteristic set of morphological changes in etiolated seedlings exposed to ethylene, includes inhibition of stem elongation, radial swelling of the stem, and horizontal growth of the seedling
- Observed in dicotyledonous seedlings (pea, bean, Arabidopsis) and some monocotyledonous seedlings (rice, wheat)
- Used as a bioassay to screen for ethylene-insensitive mutants and to study the
- Ethylene promotes the formation of root hairs and adventitious roots, which enhance nutrient and water uptake in seedlings
- Inhibits root and shoot elongation, allowing seedlings to redirect resources towards stress adaptation
Fruit Ripening and Senescence
- Ethylene is known as the "ripening hormone" due to its critical role in the ripening process of climacteric fruits (tomato, banana, apple, avocado)
- Stimulates the expression of genes involved in cell wall softening, color change (chlorophyll degradation and carotenoid accumulation), and aroma development
- Autocatalytic ethylene production occurs during fruit ripening, where ethylene promotes its own synthesis, leading to a rapid increase in ethylene levels
- Ethylene induces the senescence of leaves, flowers, and fruits, characterized by chlorophyll breakdown, nutrient remobilization, and organ abscission
- Exogenous application of ethylene can accelerate senescence (cut flowers), while ethylene inhibitors (1-MCP) can delay senescence and extend shelf life
Abscisic Acid (ABA) Functions
Stomatal Regulation and Water Conservation
- Abscisic acid (ABA), a sesquiterpenoid hormone, regulates various aspects of plant growth and development, particularly in response to abiotic stresses
- Plays a key role in during water deficit conditions, reducing transpirational water loss and maintaining plant water balance
- ABA accumulates in guard cells during , triggering the efflux of potassium ions and the loss of turgor pressure, leading to stomatal closure
- ABA-mediated stomatal regulation involves the activation of guard cell anion channels (SLAC1) and the inhibition of plasma membrane H+-ATPases
- Exogenous application of ABA can induce stomatal closure and enhance drought tolerance in plants (wheat, maize, tomato)
Seed Dormancy and Germination Control
- ABA is a central regulator of , preventing premature germination and ensuring seed survival under unfavorable conditions
- High ABA levels in mature seeds maintain dormancy by inhibiting germination-related processes (embryo growth, reserve mobilization, radicle protrusion)
- ABA biosynthesis and are tightly regulated during seed development and germination, with ABA levels declining during imbibition to allow germination to proceed
- ABA-deficient mutants (aba1, aba2, aba3) exhibit reduced seed dormancy and precocious germination, while ABA-insensitive mutants (abi3, abi4, abi5) show altered seed maturation and storage reserve accumulation
- Exogenous ABA application can induce secondary dormancy in non-dormant seeds, while ABA biosynthesis inhibitors (fluridone) can promote germination
Abiotic Stress Response and Adaptation
- ABA is a key mediator of plant responses to various abiotic stresses, including drought, salinity, and cold
- Accumulates in plant tissues during stress conditions, triggering a cascade of adaptive responses at the molecular, cellular, and physiological levels
- Induces the expression of stress-responsive genes (LEA proteins, dehydrins, osmolytes) that confer stress tolerance and protect cellular structures from damage
- Promotes root growth and root system architecture remodeling to enhance water and nutrient uptake under stress conditions
- ABA-mediated stress signaling involves the activation of SnRK2 protein kinases and the regulation of transcription factors (ABFs, AREBs) that control stress-responsive gene expression
Brassinosteroids
Brassinosteroid Structure and Biosynthesis
- Brassinosteroids, a class of polyhydroxylated steroid hormones, are essential for various aspects of plant growth and development
- Structurally similar to animal steroid hormones (testosterone, estradiol) but with distinct functions in plants
- Brassinolide, the most bioactive brassinosteroid, was first isolated from Brassica napus (rapeseed) pollen
- occurs via the mevalonate pathway and involves a series of oxidation and reduction steps catalyzed by cytochrome P450 enzymes (DWF4, CPD, ROT3)
- Brassinosteroid-deficient mutants (dwf1, dwf4, cpd) exhibit dwarfism, reduced fertility, and altered leaf morphology, highlighting the importance of brassinosteroids in plant development
Cell Elongation and Plant Growth Regulation
- Brassinosteroids promote cell elongation and expansion, particularly in young tissues (hypocotyls, petioles, leaves)
- Stimulate cell wall loosening by inducing the expression of cell wall-modifying enzymes (expansins, xyloglucan endotransglycosylases)
- Interact with other plant hormones (auxins, gibberellins) to regulate cell elongation and plant growth
- Exogenous application of brassinosteroids can increase plant height, leaf size, and biomass accumulation in various crop species (rice, tomato, soybean)
- Brassinosteroid-insensitive mutants (bri1, bin2) exhibit reduced cell elongation and compact plant architecture
Stress Tolerance and Defense Responses
- Brassinosteroids enhance plant tolerance to a wide range of abiotic stresses, including drought, salinity, extreme temperatures, and heavy metals
- Modulate the expression of stress-responsive genes and the accumulation of protective compounds (proline, glycine betaine, polyamines)
- Interact with other stress hormones (ABA, ethylene) to fine-tune plant responses to environmental challenges
- Brassinosteroids also participate in plant defense against biotic stresses, such as pathogen infection and insect herbivory
- Enhance the accumulation of defense-related compounds (phytoalexins, glucosinolates) and the activation of defense signaling pathways (salicylic acid, jasmonic acid)
Key Terms to Review (26)
Abiotic stress response: The abiotic stress response refers to the mechanisms and processes that plants use to cope with non-living environmental factors that can negatively impact their growth and survival, such as drought, salinity, temperature extremes, and nutrient deficiencies. This response is essential for plant resilience and involves a complex interplay of signaling pathways, hormones, and adaptive changes at the cellular and physiological levels.
Abscisic acid: Abscisic acid (ABA) is a plant hormone that plays a crucial role in regulating various physiological processes, particularly during stress responses. It helps plants manage drought conditions, seed dormancy, and influences growth by acting as a signaling molecule that coordinates developmental processes.
Abscisic acid signaling pathway: The abscisic acid signaling pathway is a crucial hormonal mechanism in plants that regulates various physiological processes, particularly in response to stress conditions like drought or salinity. This pathway involves the perception of abscisic acid (ABA) by receptors, leading to a series of biochemical events that ultimately result in physiological responses such as stomatal closure, seed dormancy, and stress tolerance. Understanding this pathway highlights the interplay between plant hormones, environmental signals, and adaptive responses.
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.
Brassinosteroid biosynthesis: Brassinosteroid biosynthesis refers to the complex series of biochemical processes through which brassinosteroids, a class of plant hormones, are synthesized in plants. These hormones play a crucial role in various plant growth and development processes, including cell elongation, vascular differentiation, and stress responses. The pathway involves several key enzymes and intermediates, showcasing the intricate regulation and signaling mechanisms that link these hormones to other hormonal pathways like ethylene and abscisic acid.
Brassinosteroids: Brassinosteroids are a class of polyhydroxysteroids that play a crucial role in plant growth and development. They are involved in various physiological processes, including cell elongation, division, and differentiation, and interact with other plant hormones like ethylene and abscisic acid to regulate stress responses and overall plant health.
Catabolism: Catabolism is the metabolic process that breaks down complex molecules into simpler ones, releasing energy in the process. This process is crucial for maintaining energy balance within cells, especially during growth, development, and stress responses. Catabolism connects to various physiological processes, helping organisms convert stored energy into usable forms and manage their energy needs efficiently.
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.
Drought stress: Drought stress refers to the physiological condition in plants caused by a lack of water, leading to adverse effects on growth, metabolism, and overall health. It significantly influences various processes in plants, such as water uptake, nutrient transport, and photosynthesis, which are crucial for plant survival and productivity.
Ethylene: Ethylene is a plant hormone that plays a crucial role in regulating various physiological processes, including growth, development, and responses to environmental stimuli. It is known for its involvement in fruit ripening, leaf senescence, and stress responses, making it an essential factor in plant physiology.
Ethylene signaling pathway: The ethylene signaling pathway is a molecular mechanism in plants that mediates the response to the plant hormone ethylene, influencing various physiological processes such as fruit ripening, leaf abscission, and stress responses. This pathway involves a series of proteins and gene expressions that are activated upon ethylene binding to its receptors, ultimately leading to the regulation of target genes that govern key developmental and stress-related responses.
Fruit ripening: Fruit ripening is the process through which fruits mature, becoming sweeter, softer, and more palatable, often accompanied by changes in color and aroma. This transformation is primarily regulated by plant hormones, particularly ethylene, which triggers a cascade of biochemical changes that prepare the fruit for dispersal and consumption by animals. The interplay between various hormones and environmental factors orchestrates this process, highlighting the significance of hormonal signal transduction and cross-talk.
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.
Germination control: Germination control refers to the regulatory mechanisms that govern the process of seed germination, ensuring that seeds sprout under optimal conditions for growth. This control is crucial for plants as it helps them time their germination to coincide with favorable environmental conditions, which can be influenced by various hormones and environmental signals.
Hormonal crosstalk: Hormonal crosstalk refers to the interactions and overlapping functions of different plant hormones that influence various physiological processes. These hormonal interactions can enhance or inhibit each other's effects, leading to a complex network of signaling pathways that regulate growth, stress responses, and developmental processes in plants. Understanding hormonal crosstalk is crucial for grasping how plants adapt to their environment and maintain homeostasis.
Leaf abscission: Leaf abscission is the process by which leaves detach from a plant, often as a response to environmental cues or hormonal signals. This process is crucial for plants to conserve resources and manage stress, particularly during unfavorable conditions like drought or changing seasons. Hormones like ethylene and abscisic acid play significant roles in regulating leaf abscission, signaling the plant to initiate this energy-saving mechanism.
Photomorphogenesis: Photomorphogenesis is the process by which plants and some fungi respond to light cues to regulate their growth and development. This phenomenon allows organisms to adapt to their light environment, optimizing processes like seed germination, stem elongation, and leaf expansion. Light acts as a crucial signal, influencing gene expression and hormone levels, which ultimately affect plant morphology.
Plant growth regulation: Plant growth regulation refers to the physiological processes and hormonal actions that control the growth and development of plants. This includes mechanisms that respond to environmental stimuli and manage various growth phases, such as seed germination, flowering, and fruit ripening. Key hormones like ethylene, abscisic acid, and brassinosteroids play significant roles in regulating these processes, ensuring that plants adapt and thrive in changing conditions.
Seed dormancy: Seed dormancy is a survival strategy that prevents seeds from germinating until environmental conditions are favorable for growth. This process is crucial for ensuring that seeds do not sprout during unfavorable seasons, such as winter or dry periods, which can lead to seedling death. Understanding the mechanisms of seed dormancy involves looking at various hormonal influences, particularly ethylene, abscisic acid, and brassinosteroids, which play key roles in regulating this phenomenon.
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.
Stomatal Closure: Stomatal closure is the process by which plants reduce or completely close the stomata, small openings on the leaf surface, to limit water loss and control gas exchange. This response is critical for plant survival, especially under stress conditions such as drought or high salinity, and is influenced by various hormonal signals and environmental factors.
Stress Response: Stress response refers to the physiological and biochemical changes that occur in plants when they encounter unfavorable environmental conditions, such as drought, extreme temperatures, or high salinity. These responses enable plants to adapt and survive by altering their growth patterns, metabolic processes, and hormonal balance, often mediated by specific plant hormones.
Synergistic effects: Synergistic effects refer to the phenomenon where the combined action of two or more factors leads to a greater outcome than the sum of their individual effects. In plant physiology, this concept is crucial as it helps explain how different hormones, like ethylene, abscisic acid, and brassinosteroids, can work together to regulate various processes such as growth, stress responses, and development.
Temperature response: Temperature response refers to how plants adjust their physiological processes and growth patterns in reaction to changes in temperature. This is crucial for various plant hormones, including ethylene, abscisic acid, and brassinosteroids, which mediate responses to temperature fluctuations, enabling plants to adapt to their environment for optimal survival and reproduction.
Triple response: The triple response is a physiological process in plants triggered by ethylene when they encounter obstacles during growth. This response includes three main changes: stem elongation, increased lateral growth, and upward curvature. These changes help the plant navigate around barriers, ensuring its growth continues towards optimal light and resources.
Tropism: Tropism is the directional growth response of plants to external stimuli, allowing them to adapt to their environment. This growth can be towards or away from stimuli such as light, gravity, or water, and plays a crucial role in a plant's ability to thrive. By responding to these signals, plants can optimize their growth and survival strategies in varying conditions.