1. The Emergence of Auxin: The First Plant Growth Regulator
Early Observations and Experiments: The concept of auxin, the first identified plant growth regulator, originated from studies on plant phototropism in the late 19th and early 20th centuries. Charles Darwin and his son (1880) discovered that the tip of the oat coleoptile is the light-sensitive part, but bending occurs below the tip, suggesting the existence of a transmissible “influence”.
Experiments by Boysen-Jensen (1913) and Paál (1919): They respectively proved that this “influence” is a diffusible chemical substance with polarity (transport from the apex to the base), laying the foundation for the identification of auxin as a plant growth regulator.
Went’s Classic Experiment (1926): He cut off the tip of an oat coleoptile and placed it on an agar block. After a period of time, he placed the agar block on one side of a decapitated coleoptile. Even without light, the coleoptile bent to the opposite side of the agar block. This directly proved that the tip produces a diffusible, growth-promoting chemical substance, which Went named “auxin” — the first recognized plant growth regulator. He also established the oat coleoptile bending bioassay for quantifying auxin activity.
Chemical Identification: In 1934, Kögl et al. isolated this active substance from human urine, Rhizopus cultures, and corn oil, and identified it as indole-3-acetic acid (IAA), the core natural plant growth regulator in the auxin family.
2. Major Auxin: Indole-3-Acetic Acid (IAA) as a Plant Growth Regulator
Chemical Nature: Indole-3-acetic acid is the most important and biologically active natural auxin, serving as a key plant growth regulator in plants.
Other Natural Auxins: There are other compounds with auxin activity in plants, such as 4-chloroindoleacetic acid (present in legume seeds), phenylacetic acid, and indolebutyric acid, but IAA is the core form of auxin-type plant growth regulators.
Synthetic Analogs: Due to the instability of IAA in vivo and its easy oxidation and decomposition, a variety of synthetic auxin-like plant growth regulators with stable properties and strong activity have been developed, which are widely used in agricultural production and research, such as:
- Naphthaleneacetic acid (NAA) — a common synthetic plant growth regulator
- 2,4-Dichlorophenoxyacetic acid (2,4-D) — a widely used auxin-type plant growth regulator
- Indolebutyric acid (IBA, also used as a rooting agent) — a practical plant growth regulator
3. Biosynthesis and Transport of Auxin
3.1 Biosynthesis of the Plant Growth Regulator Auxin
There are two main pathways for the biosynthesis of auxin, a key plant growth regulator:
Tryptophan-Dependent Pathway: This is the main pathway for IAA biosynthesis. Tryptophan is converted to IAA through different branches:
Indole-3-pyruvate pathway: The main pathway. Tryptophan is converted to indole-3-pyruvate under the catalysis of tryptophan aminotransferase, and then oxidized and decarboxylated to IAA under the catalysis of flavin monooxygenase.
Other branches: Including the indoleacetamide pathway, tryptamine pathway, and indoleacetaldoxime pathway, whose importance varies in different species and tissues for the synthesis of this plant growth regulator.
Tryptophan-Independent Pathway: Exists but is not the main pathway for auxin (plant growth regulator) biosynthesis.
3.2 Transport of the Plant Growth Regulator Auxin
The transport of auxin, a critical plant growth regulator, is crucial for its regulatory function, with polarity and activity.
Polar Transport: Refers to the unidirectional transport of auxin (mainly IAA) from the morphological apex to the base of the plant (e.g., from the shoot tip to the root), which is a unique transport method in plants with a speed of about 5-20 mm/h, ensuring the proper function of this plant growth regulator.
Transport Mechanism: Relies on specific input and output carrier proteins distributed on the cell membrane, which is essential for the accurate delivery of the plant growth regulator auxin.
Input Carriers: AUX1/LAX family proteins, responsible for transporting extracellular IAA⁻ (anionic form) or IAAH (protonated form) into the cell, ensuring the intracellular accumulation of the plant growth regulator.
Output Carriers:
PIN protein family: The key determinant of the directionality of auxin transport. The asymmetric (polar) localization of PIN proteins on the plasma membrane determines the direction of auxin efflux, regulating the distribution of this plant growth regulator. For example, in the root tip, the polar distribution of PIN proteins guides auxin to flow to the root cap and then back to the elongation zone, forming the “reverse fountain” model.
ABCB/PGP protein family: Some members are also involved in auxin efflux transport, assisting in the distribution of the plant growth regulator.
Non-Polar Transport: Auxin, as a plant growth regulator, can also be transported rapidly and non-polarly over long distances through vascular tissues (phloem).
4. Signaling Transduction Pathway of Auxin
The core of auxin signaling transduction — a key plant growth regulator — is transcriptional regulation mediated by the ubiquitin-proteasome pathway, and its core components include:
1. Receptors
ABP1: Located on the plasma membrane and endoplasmic reticulum, it may be involved in some rapid non-genomic responses of the plant growth regulator auxin, but its core signaling function is controversial.
2. Inhibitors: AUX/IAA Protein Family
In the absence or low concentration of auxin (plant growth regulator), AUX/IAA proteins bind to auxin response factors (ARFs) and inhibit their transcriptional activation activity.
3. Transcription Factors: ARF Protein Family
They can bind to specific sequences in the promoters of auxin-responsive genes and regulate their transcription, mediating the regulatory effect of the plant growth regulator auxin.
4. Signaling Transduction Process
When the intracellular auxin (plant growth regulator) concentration increases, IAA acts as a “molecular glue” to promote the binding of TIR1/AFB receptors to AUX/IAA inhibitors.
This binding leads to polyubiquitination of AUX/IAA proteins.
The released ARF transcription factors can activate (or inhibit) the expression of downstream auxin-responsive genes, ultimately triggering specific physiological responses regulated by this plant growth regulator.
5. Role of Auxin: Cell Elongation
The most classic role of auxin, a core plant growth regulator, is to promote cell elongation in organs such as stems, hypocotyls, and coleoptiles, but it usually inhibits root elongation. Its mechanism of promoting cell elongation involves rapid and long-term effects:
1. Acid Growth Theory (Rapid Effect)
Phenomenon: Auxin (plant growth regulator) treatment can cause rapid cell elongation within 10-15 minutes.
Mechanism:
Auxin activates H⁺-ATPase on the plasma membrane.
Proton pumps pump H⁺ into the cell wall space, leading to acidification of the cell wall environment.
Low pH activates wall-loosening proteins such as expansins in the cell wall.
Expansins break the hydrogen bonds between cell wall polysaccharides, making the cell wall “loose”.
Due to the turgor pressure inside the cell, water enters the cell, allowing the cell to elongate.
2. Regulation of Gene Expression (Long-Term Effect)
Through the above-mentioned TIR1/AFB-AUX/IAA-ARF signaling pathway, auxin (plant growth regulator) regulates the expression of a large number of genes.
These gene products are involved in the synthesis of new cell wall substances, the synthesis of more membrane proteins, and the regulation of the cell cycle, providing a material basis for sustained growth mediated by this plant growth regulator.
3. Inhibition of Root Elongation
Unlike stems, high concentrations of auxin (plant growth regulator) inhibit root cell elongation.
The mechanism may involve inducing ethylene synthesis in roots, activating different signaling pathways, or being mediated by tissue-specific proteins.
6. Role of Auxin: Plant Tropisms
Tropism is the directional growth response of plant organs to environmental stimuli. The asymmetric distribution of auxin, a key plant growth regulator, is the chemical basis of tropic responses.
1. Phototropism
Phenomenon: Stems (or coleoptiles) bend and grow toward the light source, regulated by the plant growth regulator auxin.
Cholodny-Went Model:
Unilateral light irradiation causes lateral transport of auxin (plant growth regulator) in the tip to the shaded side.
The auxin concentration on the shaded side is higher than that on the light side.
Auxin is transported downward to the elongation zone.
Cells on the shaded side elongate faster, leading to the organ bending toward the light.
Modern Revision: Phototropism is mainly mediated by phototropins. Blue light activates phototropins, which regulate the polar localization of auxin output carriers such as PIN3, establishing a lateral gradient of the plant growth regulator auxin at the organ tip.
2. Gravitropism
Phenomenon: Roots grow toward the center of the earth (positive gravitropism), and stems grow away from the center of the earth (negative gravitropism), regulated by the plant growth regulator auxin.
Starch-Starch Statolith Hypothesis:
Perception: Columella cells in the root cap and endodermal cells in the stem contain specialized amyloplasts. Changes in gravity cause amyloplasts to settle to the bottom of the cell.
Signal Conversion: Amyloplast sedimentation triggers intracellular signals.
Auxin Redistribution: The signal causes changes in the polar localization of proteins such as PIN3/PIN7 in root cap columella cells, leading to increased transport of the plant growth regulator auxin to the lower side of the root.
Gradient Formation and Differential Growth: In the elongation zone of the root, the auxin (plant growth regulator) concentration on the lower side is high, inhibiting elongation; the concentration on the upper side is low, leading to faster elongation, resulting in the root bending downward. The mechanism in stems is the opposite.
“Reverse Fountain” Model: In roots, auxin (plant growth regulator) is transported from the root tip to the root cap and then back to the elongation zone, forming a cycle. Gravitational stimulation changes the lateral distribution of this plant growth regulator in this cycle.
3. Other Tropisms
Hydrotropism, thigmotropism, etc., also involve the asymmetric distribution and signal integration of the plant growth regulator auxin.
7. The Impact of Auxin on Plant Growth and Development
Auxin, as a key plant growth regulator, is involved in the regulation of almost all growth and development processes of plants, and its role has concentration effects and tissue specificity.
1. Apical Dominance
Phenomenon: The growth of the apical bud inhibits the growth of lateral buds, regulated by the plant growth regulator auxin.
Role of Auxin: Auxin (plant growth regulator) synthesized by the apical bud is transported downward through polar transport, and the high concentration of auxin accumulated at the lateral buds inhibits the germination and growth of lateral buds. Removing the apical bud can release the inhibition.
2. Organogenesis
Lateral and Adventitious Root Formation: Auxin, a core plant growth regulator, can promote the division of pericycle cells to form lateral roots, and also promote the production of adventitious roots in parts such as stems and leaves.
Leaf and Vascular Tissue Development: Auxin (plant growth regulator) guides leaf vein pattern formation through the “canalization” hypothesis. High concentrations of auxin flow can induce cells along its transport path to differentiate into vascular tissue.
3. Embryonic Development and Polarity Establishment
In the early stage of embryonic development, polar transport of auxin (plant growth regulator) mediated by PIN1 is crucial for establishing the apical-basal axis. The position of the auxin maximum determines the formation of the root meristem.
4. Fruit Development
Fruit Set and Early Development: After pollination, auxin (plant growth regulator) produced by seeds stimulates the ovary to develop into a fruit.
Ripening Regulation: Auxin (plant growth regulator) usually inhibits fruit ripening and senescence. Exogenous auxin treatment can delay the ripening and softening of certain fruits.
5. Interaction with Other Hormones and Plant Growth Regulators
The role of auxin, a key plant growth regulator, is often synergistic or antagonistic with other hormones and plant growth regulators to jointly finely regulate plant growth:
- The ratio with cytokinin (another plant growth regulator) regulates organ differentiation.
- Can induce the biosynthesis of ethylene, which interacts with this plant growth regulator.
- Has a synergistic effect with gibberellin (a plant growth regulator) in promoting stem internode elongation.
- Has complex interactions with abscisic acid in processes such as stomatal movement and seed dormancy, coordinating with other plant growth regulators.
8. Summary: Auxin as a Core Plant Growth Regulator
As the first discovered plant hormone and a core plant growth regulator, the core characteristics of auxin are:
- Synthesis Sites: Mainly in the shoot apical meristem, young leaves, and developing seeds, ensuring the continuous supply of this plant growth regulator.
- Transport Characteristics: Unique polar transport, which is the basis for establishing concentration gradients and transmitting positional information of the plant growth regulator.
- Mode of Action: Exerts core functions through transcriptional regulation mediated by the nuclear receptor TIR1/AFB, realizing the regulatory role of this plant growth regulator.
- Action Characteristics: Has dual concentration effects and pleiotropy, regulating almost all growth and development processes as a key plant growth regulator.
- Core Function: By establishing local concentration maxima and asymmetric distribution, it acts as a positional signal to coordinate cell division, elongation, and differentiation, and is a key plant growth regulator for plants to adapt to the environment and achieve plastic growth.
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