Auxin Biosynthesis: Definition, Functions, & Uses
What Is Auxin?
Auxins are a class of chemical compounds responsible for regulating the developmental, growing, and long-lasting processes, including the reproductive process of plants. They are formed in nearly every part of the plant and transported through different parts to express their action. This competence of these hormones to act in synergy or alone implies that their functions can be synergistic or even antagonistic.
Auxin Mechanism Of Action
The main sites of auxin production are the apical meristems of shoots, young leaves, and seeds.
Auxin transport is polar and goes only in one direction: basipetal from its site of production.
In this way, the polar transport creates an auxin concentration gradient, which induces specific responses.
The movements of auxin outside the cell are mediated by auxin-specific transport proteins in the plasma membrane.
Plant hormones work through signal transduction and bring about more than one type of cellular response.
Auxin binds to the enzyme-linked receptors, thereby catalyzing a reaction.
In brief, when auxin binds to a receptor, a repressor protein for some genes binds to ubiquitin which causes degradation of the repressor protein, allowing the transcription of the auxin response genes leading to cellular growth and development.
Auxin Biosynthesis:
In auxin biosynthesis, tryptophan (Trp) is converted to indole-3-pyruvic acid and finally to indole-3-acetic acid. This pathway is considered to be the major route of auxin production in plants.
Auxin Biosynthesis Regulation
Auxin biosynthesis is a highly regulated process by development, tissue type, and environmental signals.
Through these components, the genes involved in encoding the enzymes responsible for auxin biosynthesis are modulated, resulting in a rise or reduction of auxin.
Auxin Transport And Distribution
After being synthesised, auxin flows through the plant via two major pathways: the polar auxin transport system and the non-polar auxin transport system.
PAT is known to contribute to the long-distance, basipetal, directional flow of auxin from primarily shoot apical meristem to root tips.
This occurs via auxin influx and efflux carriers at the plasma membrane: AUX1/LAX PIN and ABCB, respectively.
Leaning towards the non-polar auxin transport system, auxin moves through the phloem, allowing distribution to different plant parts.
Auxin Homeostasis And Metabolism
Plants maintain auxin homeostasis through the harmony of biosynthesis, transport, and metabolism. The levels of auxin are regulated by :
Conjugation: Auxin gets inactivated or stored when conjugated with amino acids sugars or other compounds.
Auxin gets oxidized by enzymes called auxin oxidases and becomes inactivated.
Catabolism: Multiple catabolic pathways contribute to decreasing the level of auxin by mediating degradation.
Auxin Perception And Signal Transduction
Plants perceive the signal generated by auxin through an elaborate mechanism of receptors and following cascades.
The primary receptors of auxins are the TIR1/AFB proteins, comprising Transport Inhibitor Response 1/Auxin Signaling F-Box, which is part of the SCF complex formed by Skp1-Cullin-F-box type of ubiquitin ligase.
Upon auxin binding, TIR1/AFB receptors interact with the Aux/IAA repressor proteins and thus target these for degradation via the 26S proteasome.
Thereby, the auxin response factors are released, now able to activate or repress the expression of auxin-responsive genes.
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Frequently Asked Questions (FAQs)
Transcription factors including DORNRÖSCHEN and DORNRÖSCHEN-LIKE, phytohormones such as cytokinins, and brassinosteroids, with light signals are known modulators of auxin biosynthesis.
The major precursor of auxin biosynthesis in plants is indole-3-pyruvic acid, itself a derivative of the amino acid tryptophan.
The plasma membrane-localized auxin influx and efflux carriers that mediate polar auxin transport regulate auxin transport in plants.
Plants maintain homeostasis of auxin by a balance between biosynthesis with conjugation to amino acids and oxidation and catabolism.
Auxin signals are perceived via the TIR1/AFB receptor proteins, which interact with the Aux/IAA repressor proteins, targeting them for degradation and thereby releasing auxin response factors to either activate or repress the expression of auxin-responsive genes.