How Does Auxin Control Plant Growth?
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How does auxin control plant growth? Auxin is a plant hormone that controls how cells grow, divide and specialize. Two mechanisms are described in the research reviews used for this article. One is physical: auxin makes cell walls looser so that cells can expand. The other is genetic: auxin changes which genes are switched on, through a receptor complex that marks repressor proteins for destruction. This article explains both mechanisms in plain language and notes where the research is still open.
Quick answer
- Auxin controls several processes. One review says “The plant hormone auxin controls diverse aspects of plant growth and development by regulating the fundamental cellular processes of expansion, division, and differentiation.”
- It loosens cell walls. A second review says “The plant hormone auxin is identified as a stimulator of cell elongation, as it increases cell wall extensibility.”
- It works through a receptor. The TIR1/AFB receptor family mediates “different transcriptional responses,” according to the second review.
- Some details are still open. The first review says “how auxin controls cell expansion is only now attaining molecular-level definition.”
What auxin is
The first review describes the early work that identified the chemical: “subsequent biochemical studies exploiting Went’s bioassay identified auxin isolated from human urine, Rhizopus exudates, and eventually plant material, as indole-3-acetic acid (IAA).” The review uses the abbreviation IAA for that compound.
The first review says auxin “controls diverse aspects of plant growth and development.” The rest of this article covers the two main ways the reviews describe its action.
The first mechanism: cell expansion
The second review explains how auxin contributes: “The plant hormone auxin is well known to stimulate cell elongation via increasing wall extensibility.”
The same review gives the mechanism at the level of the wall. It says “Specifically, auxin regulates cell wall properties by initiating wall loosening.”
The first review describes a classic model for how this loosening happens. It says “At its core, the acid growth theory posits that auxin activates plasma membrane (PM) H + -ATPases, resulting in proton efflux.” Proton efflux means the cell pumps hydrogen ions out of the cell. The first review calls the acid growth theory “the decades-old acid growth theory” and says it “is still useful.” The same review notes that the theory was “First proposed nearly 50 years ago.”
The second review adds more steps to the same picture. It says “Auxin activates the expression of cell wall-related genes and stimulates the synthesis of proton pumps, which leads to apoplast acidification.” It then states: “In an acidic environment, wall-loosening proteins are active and cause wall enlargement.” Read together, the two statements describe a chain in which auxin changes gene expression, the cell becomes more acidic at the wall, and the wall loosens and enlarges. The chain is a description from the review, and the review does not say how each step is timed.
The second mechanism: gene switching
Auxin also changes gene activity. The first review says “Auxin regulates numerous growth processes via the control of gene transcription by SCF TIR1/AFB -Aux/IAA-ARF nuclear signaling modules.” That sentence names three groups of proteins: the TIR1/AFB receptors, the Aux/IAA repressors, and the ARF transcription factors. Each one has a role in turning genes on or off.
The second review explains how these parts fit together. It says “Auxin acts through the TRANSPORT INHIBITOR RESISTANT 1/AUXIN SIGNALING F-BOX (TIR1/AFB) nuclear auxin receptor family, the degradation of the transcriptional regulators AUXIN/INDOLE-3-ACETIC ACID (AUX/IAAs) and the AUXIN RESPONSE FACTORs (ARFs), which mediate different transcriptional responses.” In plain terms, the receptor and the repressors are the switch, and the ARFs carry out the response.
The sequence is described in some detail. The second review says “TIR1/AFBs are part of the Skp1/Cullin/F-box (SCF) complex, which promotes degradation of AUX/IAAs, which otherwise repress auxin-mediated transcription.” The repressors keep genes off in the absence of auxin. When auxin is present, the review says “the hormone mediates the linkage of TIR1/AFBs with AUX/IAAs and the degradation of the latter through proteasomal activity.” Removing the repressor frees the ARFs to act, and the genes they control can then be expressed.
The first review uses similar language. It says “Upon an auxin stimulus, auxin promotes the interaction between Aux/IAA and TIR1/AFB proteins, acting as a molecular glue between these two components of the auxin coreceptor.” The phrase “molecular glue” describes the step where auxin holds the receptor and repressor together, which is the trigger for the degradation.
How the two mechanisms connect to growth
The second review uses the Arabidopsis seedling as a model: “Arabidopsis seedling hypocotyls elongate exclusively by cell expansion, making this organ a model system in which to investigate the contribution of auxin signaling to cell elongation.” In this model, growth depends on cell expansion, which makes it a clear place to test auxin’s effect.
From that work, the second review concludes that “auxin induces cell expansion through the degradation of AUX/IAAs.” This is a gene-level explanation of the growth response.
The first review also describes the broader scope of the signaling. It says that “the mechanisms underlying auxin perception and signaling to regulate transcription have largely been elucidated.” That statement covers the gene side of the picture. The unresolved part is how the cell expansion itself is controlled at the molecular level.
For a practical garden setting, our guide on how to grow tomatoes from seed to harvest covers the plant side of growth from the start of the season. Our guide on how to prune tomato plants covers cutting back growth on the same crop. Both guides are separate from the research discussed here.
How we chose
We chose two non-retail research reviews from the PubMed Central archive: “Auxin-Mediated Cell Expansion” and “Auxin in Cell Wall Expansion.” Both were fetched directly with curl and checked for the quoted statements in this article. Both are technical papers, so the article uses their definitions and summaries rather than their experimental details. We did not use retail pages.
What we could not verify
The reviews describe the mechanisms, but they do not give a single hormone concentration that produces a given growth response. They also do not give a timeline for how fast a cell responds to auxin, or a general rule for how much growth a given amount of auxin produces. The first review says that some observations “do not fit into a single, comprehensive explanation” of cell expansion.
What the sources do not say
The first review lists unresolved observations, including “near-instantaneous effects on ion transport and root growth.” It says these effects do not fit one comprehensive explanation, and it calls for more research. The reviews do not say how this gap will be closed. They also do not give a practical guide for growing any particular plant with auxin. Where a number or a practice is not in the quoted text, this article leaves it out.
Frequently asked questions
What is auxin in simple terms?
Auxin is a plant hormone that regulates growth. The first review says it “controls diverse aspects of plant growth and development by regulating the fundamental cellular processes of expansion, division, and differentiation.” The best-known form is indole-3-acetic acid, abbreviated IAA.
How does auxin make cells grow larger?
According to the second review, auxin stimulates cell elongation “via increasing wall extensibility,” and it does this by “initiating wall loosening.” A looser cell wall lets the cell take in water and stretch.
What is the TIR1/AFB receptor?
It is the receptor family the second review names as the auxin sensor in the nucleus. The review describes TIR1/AFB as “part of the Skp1/Cullin/F-box (SCF) complex, which promotes degradation of AUX/IAAs.”
Does auxin turn genes on or off?
Auxin changes gene activity. The second review says that the repressor proteins “otherwise repress auxin-mediated transcription,” and that when auxin is present, the repressors are degraded. The released transcription factors then drive the response.
Is the cell wall part of the explanation?
Yes, according to the second review. It says “auxin regulates cell wall properties by initiating wall loosening.” The first review adds that the acid growth theory, which involves proton pumps in the plasma membrane, “is still useful” as an explanation for part of this process.
