New Study Reveals PXY and ER Receptors Team Up to Control Cambium Wood Formation

Wood gives buildings their strength and, by some estimates, stores more carbon than nearly any other living thing on land. That entire process begins in a narrow, living strip of cells sitting just under the bark.

Researchers believed they more or less understood how this strip regulates itself. New work led by a team in England suggests a crucial extra layer was missing.

Cells beneath the bark

That strip is the cambium: a sheath of stem cells encircling the stem, continually renewing itself as the plant grows.

On the inside, it produces xylem - the woody tissue - while the outside generates the conduits that distribute sugars around the plant.

Previous studies characterised the cambium as a genuine stem-cell region, directed by a small set of organising cells. Whether an individual cell continues dividing or instead matures into wood depends on the signals it receives.

Dr Peter Etchell is a plant biologist at Durham University in north-east England. His group has spent years examining how the cambium determines when to produce wood, yet one persistent puzzle remained.

A surprising partnership

Plant cells monitor their environment using receptor proteins - sensors embedded in the outer membrane that bind to particular molecules passing by.

According to the standard view, each receptor recognises its matching signal and passes the instruction into the cell, with receptors assumed to act on their own.

Two receptors stood out to the researchers. One, PXY, promotes division in cambium cells. The other, ER, appeared in locations where wood was being laid down.

A broad screen of plant receptors had suggested PXY and ER might physically interact, so the team set out to check directly.

They found the two receptors join together at the cell surface to form a single complex. Until now, these receptor families had not been shown to cooperate in this way.

“Plants are real masters of detecting and responding to signals,” said Etchells.

Catching proteins together

Demonstrating that two proteins make contact inside a living cell requires careful experimental design.

The team labelled PXY and ER with tags that only fluoresce when the proteins come close enough to touch. Inside the cells, the signal appeared where both were present.

In a follow-up test, they extracted one protein from the cellular mixture to see what travelled with it. When PXY was pulled out, ER came along as well, tightly bound.

The same result was seen with ERL2, a close relative of ER - but a third related receptor did not join. That specificity argued the interaction was genuine rather than an artefact of chemistry.

Even so, these experiments could not show how forming a complex alters the messages each receptor sends inside the cell, which the study leaves unresolved.

When the signal goes into overdrive

Confirming contact between the proteins was only part of the story; the team also needed to show the partnership influences wood formation.

PXY responds to a signalling molecule called TDIF, so the researchers genetically engineered thale cress (Arabidopsis thaliana), a small model plant, to saturate its wood-forming region with additional TDIF.

With the excess signal, the cambium became disorganised. Cells began dividing in multiple directions.

Cell files that usually form orderly lines became short and warped, and only a small amount of normal wood developed.

Next, the researchers removed ER in those same plants. The disorder eased.

The cambium returned to a well-defined ring, and the neatly aligned cell files grew to more than twice their previous length - which the team interpreted as evidence that PXY and ER act as a pair rather than as separate, independent receptors.

Wood in the wrong places

This receptor partnership appears to do more than boost division. It also helps stop cambium cells from becoming wood prematurely. Those cells must stay pliable; if they harden too soon, the stem-cell supply is used up.

When both receptor families were disabled at the same time, that restraint broke down.

Rigid, hollow vessel cells - the type that normally transport water through mature wood - were found scattered through regions where the cambium should still have been actively dividing.

Taken together, the findings revise the picture of how wood is produced. ER had been linked to the cambium for years, but its function there was unclear.

The study’s explanation is that ER operates via PXY: locked together in one complex, they promote division while keeping wood formation confined to the correct locations.

Why the discovery matters

Wood is one of the planet’s standout carbon stores, holding more than almost anything else living on land, according to one study’s estimate. It is also the fundamental material behind timber and paper.

Any mechanism that controls how much wood a plant makes has implications beyond laboratory research.

If scientists can determine how this receptor complex directs growth, it may be possible to adjust it - encouraging plants to produce more wood, with potential downstream benefits for tougher crops and increased carbon capture.

The team is now exploring how the two receptors modify one another’s internal signalling, and whether the same pairing also occurs in trees.

“So we already knew that signalling in plants was complex, but it has been fantastic to uncover another layer of complexity,” said Etchells.

If this kind of receptor pairing can occur here, comparable partnerships could be shaping growth in other parts of the plant as well, still undiscovered.