What can soap bubbles tell us about cell division? More than you might think: in 1886 Leo Errera noted that bubbles in soap resembled dividing cells. He thought the shape of the bubbles could predict where plant cells would divide, and by 1888 he had formulated a rule saying that the cell plate—the structure that splits a dividing plant cell in two—adopts the same area-minimizing geometry as a soap film. Jacques Dumais and Sébastien Besson at Harvard have now done some bubbly experiments of their own, extending Errera’s rule and accounting for exceptions to it.
At cell division, cells assemble complex molecular structures to ensure that chromosomes get separated into different daughter cells as the parent cell splits in two. But how does the molecular machinery “know” where to build a division plane? “It’s a really long-standing problem,” Dumais says. While “when” has been answered by studying cell-cycle genes, “where” was “still an open question.”
There are many examples of dividing plant cells that do not follow Errera’s simple rule: for example, the cells in developing stomata (the microscopic pores in plant leaves), in healing wounds, and where identically shaped cells form a honeycomb. Here, cells dividing in the same place would weaken the plant in the same way that a wall with the bricks lined up, and not staggered, is weak.
Besson and Dumais fixed soap films in place with pins, and moved the boundary separating two bubbles around with a glass needle. They noted that no matter how many different ways they drew the glass needle, the bubble would pop back to one of a fairly limited number of shapes. They compared these shapes with the cells of trichomes—glandular structures on the leaves of a Venus flytrap—and realized that the cells divided into the shapes predicted by the bubbles. “A dividing cell has a few options that it can choose,” Dumais says, and using the bubble experiments, “we have found the probability distribution for all the different options.”
The shape of bubbles is determined by surface tension. The tension that gives plant cells their shape, and therefore specifies where they will divide, comes from actin fibers. Clive Lloyd at the John Innes Centre in Norwich, UK, says that knowing how this tension is resolved in individual cells is important for understanding how actin molecules drive tissue organization, something Dumais now wants to explore by relating the choice of division plane to the shape of the growing tip of plants.