How Do Plant Cell Walls Loosen to Let Cells Grow?

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How do plant cell walls loosen to let cells grow? A growing plant cell is held under internal pressure, and its wall has to give way in a controlled way. The short answer is that the wall is loosened in a controlled way, so that turgor pressure can drive expansion. Two parts of the process matter most. The first is a group of proteins called expansins that loosen the wall without cutting it. The second is a drop in pH in the wall that switches those proteins on. This explainer walks through the mechanism step by step, using two open-access review articles from PubMed Central as its sources.

Quick answer

• The growing wall has to resist turgor pressure and still yield in a controlled way.
• Expansins loosen the wall without showing detectable enzymatic activity.
• In growing cells, walls loosen mainly through pH-dependent action of expansins.
• Loosening comes first, and water then enters the cell to drive growth.

The wall has to hold and yield at the same time

A plant cell wall faces two demands that seem to pull against each other. It must be strong enough to resist the force that the cell’s own pressure applies to it. It must also be able to give way under that same force when the cell is supposed to grow. A review on wall loosening describes this directly: the growing cell wall has conflicting requirements, needing to be strong enough to withstand the high tensile forces generated by cell turgor pressure while selectively yielding to those forces to induce wall stress relaxation.

That stress relaxation is the key event. The same source ties it to water uptake and to the movement of wall polymers, which together underlie wall expansion. Relaxation has to happen in specific places, and it has to be controlled by proteins that act on the wall itself.

The structure of the wall explains why loosening is needed at all. In one widely discussed model, cellulose microfibrils are held apart by matrix polysaccharides. The tethered network model proposes that cellulose microfibrils are well separated by matrix polysaccharides, including xyloglucans, which bind to cellulose microfibrils and tether them to form a load-bearing molecular network.

Expansins loosen the wall without cutting it

Expansins are the best-known wall-loosening proteins. One review states that the first group of wall-loosening proteins to be discussed, the expansins, have no detectable enzymatic activity, yet are the clearest examples of endogenous catalysts of plant cell wall loosening. That detail matters. A protein that cut the wall’s bonds would weaken the wall. Expansins are described as doing something else.

The evidence for that point comes from measurements of wall stiffness. The same review reports that the ability of alpha-expansins to induce creep without reducing wall stiffness provided additional evidence that they do not cut cell wall linkages, which would result in reduced cell wall stiffness as well as release of wall polysaccharide fragments. In other words, the wall can creep without losing stiffness, which is the point of the comparison.

The result is called creep. The review describes the action of expansins that results in cell wall creep as loosening. The 2022 review describes creep as an irreversible time-dependent enlargement of the wall. One source notes that the cell wall creep measured in its device is dependent on continuous wall loosening by expansins or other proteins, and thus mimics aspects of cell wall enlargement in living cells.

Expansins are not the only proteins involved. The biomechanical hotspot concept proposes that growing cell walls contain specific, built-in junctions designed for slippage and stress relaxation by the action of expansins and other wall-loosening proteins.

Why enzymes that cut the wall do not explain it alone

Earlier experiments tried to explain loosening with enzymes that cut wall polymers. The results were not simple. One source reports that enzymes that cut only xyloglucan or only cellulose did not induce cell wall creep, whereas endoglucanases able to cut both xyloglucan and cellulose did induce creep. A combination of xyloglucan-specific and cellulose-specific enzymes lacked wall-loosening action, which the same source describes as a puzzling result.

The explanation offered for that puzzle is that walls were loosened only when a relatively inaccessible amalgam containing xyloglucan and cellulose was digested by a single enzyme with both xyloglucanase and cellulase activities. The amalgam was hypothesized to sit inside tight junctions between cellulose microfibrils.

Acid growth: how the wall gets loosened

The loosening depends strongly on pH. A review on expansins summarizes that in growing cells, walls are loosened primarily via pH-dependent action of expansins. The same review says that expansins were shown to mediate pH-dependent cell wall enlargement via a process called cell wall loosening.

Plant hormones help control the pH. The review describes auxin as a trigger for proton release: auxin triggers extrusion of protons into the apoplast, which activates expansins that subsequently loosen the cell wall and allow growth. Acidifying that space is what activates the expansins, according to the review.

The pumps that make the space acidic are enzymes in the cell membrane. According to the review, the most important players in this process are plasma membrane P-type H-ATPases, which pump out protons to the wall matrix, consequently leading to apoplast acidification. The same review adds that the observed cell wall extension required acidic pH, and that extension was also seen when cucumber extracts were applied to cell walls isolated from actively growing tissues of other species.

Pumping protons also changes the cell’s electrical state. One source states that pumping protons causes plasma membrane hyperpolarisation and also activates potassium channels, which together maintain a low intracellular osmotic potential, thus allowing sustained water uptake and turgor pressure forcing the cell wall to extend. For a related look at how hormones steer growth, see how auxin controls plant growth.

Why loosening comes before water enters

The order of events is one of the more useful points in the sources. In growing cells, walls are loosened, which means that the load-bearing part of the wall is relaxed, releasing the tensile stress and simultaneously reducing cell turgor. Once that happens, water can move in. Consequently, water flows into the cell, expanding the wall and restoring turgor and wall stress, together driving cell growth.

The review states the sequence plainly. Cell expansion starts with cell wall loosening or relaxation, followed by a decrease and a subsequent increase of cell turgor, not the other way round. This means turgor does not open the wall first. The wall loosens, turgor decreases, and then turgor increases again.

The sources also point out that the spatial pattern matters. Altogether, the spatiotemporal specificity of expansin expression and protein localization, along with the spatial-specific control over proton fluxes that change apoplastic pH, are important factors controlling expansin-mediated cell expansion. The spatial control is the point of that sentence: where expansins are present and where protons change apoplastic pH both matter.

Two models of wall structure

Researchers describe the wall with more than one model. The sources compare two contemporary models of primary cell wall structure that differ in how cellulose microfibrils are mechanically connected. In the tethered network model, xyloglucans bind extensively to cellulose surfaces and form the sole load-bearing links between microfibrils. In the biomechanical hotspot model, the load-bearing points are limited junctions where cellulose meets a xyloglucan-cellulose amalgam.

The hotspot model describes limited cellulose-cellulose junctions, bonded together by a xyloglucan-cellulose amalgam, that have limited enzymatic accessibility. That is the structural feature the model places the load on.

For readers interested in what happens after a cell expands, the transport of water in the plant is a useful next step. See how xylem moves water for the pathway that carries the water toward growing tissue.

How we chose

This explainer uses two open-access review articles from PubMed Central. Both pages were checked directly and returned article text before any quote was used. One review covers catalysts of plant cell wall loosening and the enzymes and structural models behind it. The other covers expansin-mediated cell wall biomechanics, including the role of acidification in controlling expansin activity. Together they cover the question from the structural side and the signaling side.

Every factual sentence in the body is tied to a verbatim quote recorded in the source notes. Sentences that could not be tied to a quote were removed. The article does not add numbers, prices or product claims that the sources do not state.

What we could not verify

The two sources describe the mechanism in general terms across several plant species and tissues. They do not settle every detail of the process, and the points below are left open rather than filled in with guesses.

What the sources do not say

The sources describe pH-dependent loosening, but this article does not give one pH value that applies to every plant or every tissue. The sources also describe the proton pump pathway in several species, and this article does not claim that the same pathway operates in every plant tissue. Finally, the structural models are described as competing models, so this article does not declare one of them the final answer.

Frequently asked questions

What does turgor pressure have to do with cell growth?

Turgor pressure pushes outward on the wall. The sources say the growing wall must be strong enough to withstand the tensile forces this pressure creates, while still yielding in a controlled way so that the cell can expand.

Are expansins enzymes that cut the cell wall?

No. One review states that expansins have no detectable enzymatic activity. Another source explains that the ability of expansins to induce creep without reducing wall stiffness is evidence that they do not cut cell wall linkages.

What role does pH play in wall loosening?

Walls are loosened primarily through pH-dependent action of expansins. Auxin triggers proton release into the apoplast, which activates expansins, and the sources describe acidic pH as required for the observed wall extension.

Does the wall loosen before water enters the cell?

Yes, according to the review. Cell expansion starts with wall loosening or relaxation, followed by a decrease and then an increase of cell turgor, which means loosening comes before the water-driven rise in pressure.

Which model of the wall is correct?

The sources compare two models, the tethered network model and the biomechanical hotspot model. They describe them as contemporary models that differ in how cellulose microfibrils are connected, and this article does not choose between them.

Sources

• Catalysts of plant cell wall loosening, F1000Research, 2016: https://pmc.ncbi.nlm.nih.gov/articles/PMC4755413/
• Expansin-mediated developmental and adaptive responses: A matter of cell wall biomechanics?, Quantitative Plant Biology, 2022: https://pmc.ncbi.nlm.nih.gov/articles/PMC10095946/