How to stop

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How to stop

To grow or not to grow, that is the question. It influences the form of an organ and organism, such as a  plant. Therefore, the question when to start growing and when to stop is an important one. It is also the question that occupies plant root researchers from Germany.

Root growth is divided in several parts organised in different regions along the root. Al the way in the tip of the root are the quickly dividing cells. Because plant cells, due to their cell wall, are connected to each other and not be able to move around, over time the cells get further away from the root tip. How further away from the root tip, how slower the cells divide. Then comes a moment they are too far from the root tip and division stops, and they start stretching. Even further from the root tip they also stop doing that, they are now adult root cells, ready to specialise .

It was already known that accumulation of the hormone auxin in the root tip gets the division in these cells starting. How further away from the root tip, how less influence auxin has on the cell. Cell division slows down. Not only auxin, but also the hormones cytokinin, brassinosteroids and ethylene influence root growth. So, tries cytokinin to reduce the influence of auxin. Cytokinin kind of convinces the cells to stop dividing and start stretching.

Now German researchers found that cytokinin also has a role in stopping this cell stretching. When they added cytokinin to the growth medium, the researchers noticed that the roots stopped sooner with stretching. While in plants, through a mutation, blind for cytokinin the root cells stretched for longer. Is cytokinin calling for the stretching cells to grow up and become adults?

Having a less flexible cell wall is a characteristic of adult root cells. Keeping this in mind the researchers studied the effect of cytokinin on cell wall flexibility. They noticed that when cytokinin was present, the cell wall is stiffening quicker than when there is no cytokinin in the growth medium. It appears that root cells stop stretching because their cell wall becomes less flexible. The researchers did not stop there, they also noticed that root cells from plants that are not able to transport auxin properly actually have a more flexible cell wall, even when cytokinin is present.

Is auxin influencing here the action of cytokinin? Hinders auxin here the stretching effect of cytokinin? Are auxin and cytokinin actually working together to turn growing root cells into adults? Or is there another hormone that is taking charge here? These are questions that still need to be answered. Clear is though, the cell wall needs to stiffen up for a cell to stop growing. How quickly this happens depends on how much cytokinin and auxin is available. But for now, stopping is not so simple as it looks.

Literature

Shanda Liu, Sören Strauss, Milad Adibi, Gabriella Mosca, Saiko Yoshida, Raffaele Dello Ioio, Adam Runions, Tonni Grube Andersen, Guido Grossmann, Peter Huijser, Richard S. Smith, and Miltos Tsiantis (2022) Cytokinin promotes growth cessation in the Arabidopsis root. Current Biology 32, 1–12 doi: 10.1016/j.cub.2022.03.019