Passing on fundamental life lessons from parent to offspring is not unique to humans and animals. Ferns do it too. Not with words, but through pressure. By applying force at precisely the right locations, a fern tells its embryo what is up and what is down, and therefore where roots and leaves should develop. This was discovered by PhD candidate Sjoerd Woudenberg in his research on the fern Ceratopteris richardii. He defended his doctoral thesis today at Wageningen University & Research.

We tend to think of ferns as large, feathery fronds in the forest; relics from the age of the dinosaurs. Sjoerd Woudenberg’s research at the Biochemistry chair group, instead focuses on a far less visible stage of their life cycle: the very first cell division of the fertilised egg. This division always occurs in exactly the same direction. That turns out to be crucial, because it lays the foundation for all further development. “The plant immediately establishes an axis,” Woudenberg explains, “with one side oriented towards capturing sunlight and the other able to develop into roots.”
The orientation of this first division is not chosen by the cell itself, but enforced by the surrounding ‘maternal tissue’, the prothallus: a small, heart-shaped plant that lies on the soil and serves as a nursery for the young fern.

Stress as a compass
Initially, Woudenberg suspected that plant hormones or other chemical signals in the cell controlled this decisive first division. But when he exposed the plant embryos to the plant hormone auxin or to chemicals known to disrupt cell division, surprisingly little happened. “Later in development, I did see clear abnormalities,” he says, “but that very first cell division stubbornly continued to behave normally.” It was as if the young embryo ignored chemical instructions.

That left only a handful of alternative explanations, including mechanics: pressure, tension and physical forces from the surrounding tissue acting as a compass. To test this hypothesis, Woudenberg collaborated with colleagues of the Physical Chemistry & Soft Matter chair group (WUR). Together, they mapped the entire network of cells surrounding the embryo and fed these data into a computer model, allowing them to calculate where mechanical stress accumulates within the tissue.

“Every cell is under pressure: this we call turgor,” Woudenberg explains. “That creates more stress in some places than in others.” Compare it to a balloon: once inflated, the pressure on the rubber is equal everywhere. But if you pull on it from two sides, the tension concentrates at specific points. The same principle applies to cells. The analysis revealed that this tension consistently builds up at the same location around the embryo: exactly adjacent to where the fertilised egg forms its dividing wall during the first cell division.

A link with microtubules
How the fertilised egg ultimately translates this mechanical stress into a specific division plane is not yet fully understood. Woudenberg suspects that microtubules may play an important role. These are dynamic, hollow tubes that act as intracellular highways, transporting essential building blocks for cell growth. Scientists have long known that microtubules play a role in cell division and that they are sensitive to mechanical stress. “So the link is quite plausible,” Woudenberg says. Whether this is indeed how ferns orient their development will need to be determined by further research. The researcher also does not entirely rule out the involvement of other, yet untested, chemical signals.

For now, Woudenberg’s fascination with ferns shows no sign of waning. He will continue his research as a postdoctoral researcher in Ghent, where he will focus on the development of vascular tissue in the same fern species, Ceratopteris richardii.