Which Of The Following Is A Plant Hormone

The world of plant hormones is a fascinating realm, influencing everything from growth and development to responses to environmental stimuli. Plant hormones, also known as phytohormones, are signal molecules produced within plants, occurring in extremely low concentrations. These substances regulate cellular processes in targeted cells locally or at distant locations. Unlike animal hormones, plant hormones are not produced in specific tissues or glands and can be synthesized in various parts of the plant.

Key Plant Hormones

Identifying which of the following is a plant hormone requires understanding the major classes of these chemical messengers. Let's explore the primary types:

Auxins: Promote cell elongation, apical dominance, and root development.
Cytokinins: Stimulate cell division, delay senescence, and promote shoot formation.
Gibberellins (GAs): Enhance stem elongation, germination, and flowering.
Abscisic Acid (ABA): Inhibits growth, induces dormancy, and helps plants respond to stress.
Ethylene: Promotes fruit ripening, senescence, and abscission.
Brassinosteroids (BRs): Regulate cell elongation, vascular development, and stress responses.
Jasmonates (JAs): Mediate plant defense responses and regulate growth and development.
Salicylic Acid (SA): Involved in plant defense against pathogens.
Strigolactones (SLs): Inhibit shoot branching and regulate symbiotic interactions.

These hormones interact in complex ways, often synergistically or antagonistically, to fine-tune plant growth and development.

Auxins: The Growth Promoters

Auxins are among the most well-known plant hormones, primarily responsible for cell elongation and apical dominance.

Discovery and History

The discovery of auxins dates back to Charles Darwin's experiments with phototropism in the late 19th century. Darwin observed that grass coleoptiles bend towards light and hypothesized that a signal transmitted from the tip of the coleoptile caused this bending. Later, scientists isolated and identified indole-3-acetic acid (IAA) as the primary auxin.
Functions
- Cell Elongation: Auxins stimulate cell elongation in shoots. They increase cell wall plasticity, allowing cells to expand under turgor pressure.
- Apical Dominance: Auxins produced in the apical bud inhibit the growth of lateral buds, a phenomenon known as apical dominance. This ensures that the plant grows taller rather than bushier.
- Root Development: At low concentrations, auxins promote root initiation and development. They are often used in rooting powders to encourage root formation in plant cuttings.
- Tropic Responses: Auxins mediate plant responses to environmental stimuli such as light (phototropism) and gravity (gravitropism).
Synthesis and Transport

Auxins are primarily synthesized in young leaves, developing seeds, and apical meristems. They are transported from cell to cell via polar auxin transport, which involves the coordinated action of influx and efflux carriers.

Cytokinins: The Cell Division Stimulators

Cytokinins are plant hormones that promote cell division, delay senescence, and play a role in shoot formation.

Discovery and History

Cytokinins were discovered during research aimed at identifying factors that stimulate cell division in plant tissue culture. Scientists found that adding certain substances, such as kinetin, could induce cell division in callus tissue.
Functions
- Cell Division: Cytokinins promote cell division (cytokinesis) in plant tissues. They stimulate the progression of the cell cycle and are essential for plant growth and development.
- Delay of Senescence: Cytokinins can delay the senescence (aging) of leaves and other plant organs. They help maintain protein synthesis and prevent the breakdown of chlorophyll.
- Shoot Formation: In tissue culture, cytokinins promote the formation of shoots from callus tissue. They interact with auxins to regulate shoot and root development.
- Nutrient Mobilization: Cytokinins can mobilize nutrients from older tissues to younger, actively growing tissues.
Synthesis and Transport

Cytokinins are synthesized primarily in roots and transported to other parts of the plant via the xylem. They are also produced in developing seeds and fruits.

Gibberellins: The Elongation Enhancers

Gibberellins (GAs) are a class of plant hormones that promote stem elongation, germination, and flowering.

Discovery and History

Gibberellins were first discovered in Japan during studies of bakanae disease in rice. The disease caused rice seedlings to grow excessively tall and spindly. Scientists isolated gibberellic acid (GA3) from the fungus Gibberella fujikuroi, which caused the disease.
Functions
- Stem Elongation: Gibberellins promote stem elongation, particularly in dwarf varieties of plants. They stimulate cell division and cell elongation in the stem.
- Germination: Gibberellins promote seed germination by breaking seed dormancy and stimulating the synthesis of enzymes that mobilize stored nutrients.
- Flowering: In some plants, gibberellins induce flowering. They can trigger the transition from vegetative growth to reproductive growth.
- Fruit Development: Gibberellins can promote fruit set and fruit growth. They are sometimes used in agriculture to increase fruit size and yield.
Synthesis and Transport

Gibberellins are synthesized in young leaves, developing seeds, and apical meristems. They are transported throughout the plant via the xylem and phloem.

Abscisic Acid: The Stress Responders

Abscisic acid (ABA) is a plant hormone that inhibits growth, induces dormancy, and helps plants respond to stress, particularly drought.

Discovery and History

Abscisic acid was discovered independently by two groups of researchers in the 1960s. One group studied its role in inducing dormancy in buds, while the other investigated its involvement in abscission (shedding) of leaves.
Functions
- Dormancy: Abscisic acid induces dormancy in buds and seeds, helping plants survive unfavorable conditions such as winter or drought.
- Stress Response: ABA mediates plant responses to various environmental stresses, including drought, salinity, and cold. It promotes the closure of stomata, reducing water loss from leaves.
- Growth Inhibition: ABA inhibits growth in stems and roots. It counteracts the effects of auxins and gibberellins.
Synthesis and Transport

Abscisic acid is synthesized in leaves, roots, and developing seeds. It is transported throughout the plant via the xylem and phloem.

Ethylene: The Ripening Agent

Ethylene is a gaseous plant hormone that promotes fruit ripening, senescence, and abscission.

Discovery and History

The effects of ethylene on plant growth and development have been known for centuries. In the early 20th century, scientists identified ethylene as the active agent responsible for these effects.
Functions
- Fruit Ripening: Ethylene promotes the ripening of many fruits, including bananas, tomatoes, and apples. It stimulates the breakdown of chlorophyll, the softening of cell walls, and the production of volatile compounds that contribute to fruit flavor and aroma.
- Senescence: Ethylene promotes the senescence (aging) of leaves, flowers, and other plant organs. It triggers the breakdown of chlorophyll and other cellular components.
- Abscission: Ethylene promotes the abscission (shedding) of leaves, flowers, and fruits. It stimulates the formation of an abscission layer at the base of the organ.
- Stress Response: Ethylene mediates plant responses to various environmental stresses, including wounding, flooding, and pathogen attack.
Synthesis and Transport

Ethylene is synthesized in many plant tissues, particularly in ripening fruits, senescing leaves, and stressed tissues. As a gas, it can diffuse readily throughout the plant.

Brassinosteroids: The Multifaceted Regulators

Brassinosteroids (BRs) are a class of plant hormones that regulate cell elongation, vascular development, and stress responses.

Discovery and History

Brassinosteroids were first isolated from Brassica napus (rapeseed) pollen in the 1970s. Scientists identified brassinolide as the most active brassinosteroid.
Functions
- Cell Elongation: Brassinosteroids promote cell elongation, particularly in stems and roots. They interact with auxins to regulate cell expansion.
- Vascular Development: BRs play a role in the development of vascular tissues (xylem and phloem). They promote the differentiation of vascular cells.
- Stress Response: Brassinosteroids mediate plant responses to various environmental stresses, including drought, salinity, and temperature extremes.
Synthesis and Transport

Brassinosteroids are synthesized in many plant tissues, including leaves, stems, and roots. They are transported throughout the plant via the xylem and phloem.

Jasmonates: The Defense Signalers

Jasmonates (JAs) are a class of plant hormones that mediate plant defense responses and regulate growth and development.

Discovery and History

Jasmonates were first identified as signaling molecules involved in plant defense against herbivores and pathogens. Jasmonic acid (JA) is the primary jasmonate.
Functions
- Defense Response: Jasmonates play a key role in plant defense against insects, pathogens, and other threats. They induce the synthesis of defensive proteins and secondary metabolites.
- Growth and Development: JAs regulate various aspects of plant growth and development, including root growth, leaf senescence, and flower development.
- Wound Response: Jasmonates mediate plant responses to wounding. They promote the synthesis of proteins involved in wound healing.
Synthesis and Transport

Jasmonates are synthesized in response to various stimuli, including herbivore attack, pathogen infection, and wounding. They are transported throughout the plant via the xylem and phloem.

Salicylic Acid: The Immunity Booster

Salicylic acid (SA) is a plant hormone involved in plant defense against pathogens, particularly biotrophic pathogens (pathogens that obtain nutrients from living host tissue).

Discovery and History

Salicylic acid was first identified as a signaling molecule involved in systemic acquired resistance (SAR), a long-lasting defense response that protects the entire plant against a wide range of pathogens.
Functions
- Defense Response: Salicylic acid plays a key role in plant defense against pathogens. It induces the synthesis of pathogenesis-related (PR) proteins, which have antimicrobial activity.
- Systemic Acquired Resistance (SAR): SA mediates the establishment of SAR, a systemic defense response that provides long-lasting protection against pathogens.
- Thermotolerance: Salicylic acid can enhance plant tolerance to high temperatures.
Synthesis and Transport

Salicylic acid is synthesized in response to pathogen infection. It is transported throughout the plant via the phloem.

Strigolactones: The Branching Regulators

Strigolactones (SLs) are a class of plant hormones that inhibit shoot branching and regulate symbiotic interactions.

Discovery and History

Strigolactones were first identified as signaling molecules involved in the interaction between plants and arbuscular mycorrhizal fungi. They were later found to inhibit shoot branching.
Functions
- Shoot Branching Inhibition: Strigolactones inhibit the growth of axillary buds, reducing shoot branching. They help maintain apical dominance.
- Symbiotic Interactions: SLs promote the colonization of roots by arbuscular mycorrhizal fungi, which enhance nutrient uptake.
- Stress Response: Strigolactones mediate plant responses to nutrient deficiency and drought.
Synthesis and Transport

Strigolactones are synthesized in roots and transported to shoots via the xylem. Their synthesis is increased under nutrient-deficient conditions.

Interactions and Cross-Talk

Plant hormones do not act in isolation. They interact with each other in complex ways, often synergistically or antagonistically, to fine-tune plant growth and development. This interaction is often referred to as hormone cross-talk.

Auxin and Cytokinin Interaction: Auxins and cytokinins have opposing effects on shoot and root development. High auxin-to-cytokinin ratios promote root formation, while low ratios promote shoot formation.
Gibberellin and Abscisic Acid Interaction: Gibberellins and abscisic acid have antagonistic effects on seed germination and dormancy. GAs promote germination, while ABA induces dormancy.
Ethylene and Auxin Interaction: Ethylene and auxin interact to regulate leaf abscission. Ethylene promotes the formation of the abscission layer, while auxin inhibits it.
Jasmonate and Salicylic Acid Interaction: Jasmonates and salicylic acid interact to regulate plant defense responses. In general, JAs are more effective against herbivores and necrotrophic pathogens (pathogens that kill host tissue), while SA is more effective against biotrophic pathogens.

Practical Applications

Understanding plant hormones has numerous practical applications in agriculture and horticulture.

Crop Improvement: Plant hormones can be used to improve crop yield and quality. For example, gibberellins can be used to increase fruit size, while cytokinins can be used to delay leaf senescence.
Weed Control: Synthetic auxins, such as 2,4-D, are used as herbicides to kill broadleaf weeds.
Plant Propagation: Auxins are used in rooting powders to promote root formation in plant cuttings.
Fruit Ripening: Ethylene is used to promote the ripening of fruits such as bananas and tomatoes.
Stress Tolerance: Plant hormones can be used to enhance plant tolerance to environmental stresses such as drought, salinity, and cold.

Conclusion

Plant hormones are essential regulators of plant growth, development, and stress responses. Understanding the functions and interactions of these hormones is crucial for advancing our knowledge of plant biology and for developing new strategies to improve crop production and sustainability. From auxins promoting cell elongation to abscisic acid helping plants survive drought, each hormone plays a unique and vital role in the life of a plant. This intricate system of chemical signals ensures that plants can adapt to their environment and thrive.