Functional and chemical characterization of XAF

Plants rely on stiff cells walls for support. But these same cell walls have to be flexible to grow when required. Nguyen-Phan and Fry examine a chemical that regulates how this happens.

Plant cell walls have many demands put upon them. As plants lack skeletons, the job of supporting the plant rests on their rigidity. However, plants also need to be able to move, despite having no muscles. The same plant cell walls, therefore, also need flexibility.

A common problem cell walls face is water. When a plant needs to flex, it can fill its vacuoles up with water. This is a bit like filling a balloon. The walls have to be able to give without bursting and killing the cell. Looking closely at the cell wall, you would see a matrix of polysaccharides, particularly of xyloglucan and cellulose fibrils. These fibrils are tiny fibres that come together to make the cell wall.

The proteins that can pull apart these fibril matrices are xyloglucan endotransglucosylase/hydrolases. They’re known as XTHs for short. These XTHs are able to cut ‘n’ paste xyloglucan chains together, making these fibrils longer. XETs (xyloglucan endotransglucosylases) involved in a lot of processes that require cell expansion, like ripening.

However, people have also found evidence for a role of XTHs in wall assembly. Expression of AtXTH22, a touch-inducible protein, was rapidly upregulated by hormones and by touch, darkness, heat shock and cold shock, leading to alterations in plant elongation.

What Nguyen-Phan and Fry wanted to do was examine the biochemistry of how XTHs worked as a plant grew to reshape cell walls on demand. They performed a series of biochemical analyses of compounds found in cell walls known as CHP (cold-water-extractable, heat-stable polymer), or XAF (XTH-activating factor).

Their experiments showed that cauliflower floret CHP was able to mobilise XTHs most effectively. It didn’t have a similar effect on other wall enzymes like peroxidase, β-glucosidase or phosphatase. This is the clue that XAF has a role in the restructuring of xyloglucan in cell walls.