"We started using the live imaging of the sepals to gather data to make a hypothesis about the patterning," Roeder explains. "Then we ran that hypothesis as a model in the computer, to see whether it would give us the patterns we were seeing in the imaging."
At first, the computer model was unable to produce the patterns found in the actual sepals. So the team tweaked the model until it independently produced the range of cell sizes the team had seen in the living organ.
They found the sepal generates its epidermal cell-size pattern based not on an organ-wide control mechanism, but on when or whether each individual cell decides to divide and on the length of its cell cycle. This sort of random, probabilistic development process results in sepal patterns that not only differ from flower to flower, but from sepal to sepal within an individual flower.
"This is so contrary to our normal way of thinking," says Roeder, "in which we assume that there's always something dictating exactly what each cell is going to do."
Cells in the sepal can undergo one of two growth cycles. The first is normal cell division, in which the cell duplicates its chromosomes and then splits into two smaller cells. The other is a specialized type of cell cycle called endoreduplication, in which the cell duplicates its chromosomes but does not split in two; instead, it simply continues to grow ever larger.
The team's original hypothesis was that "the earlier a cell decides to endoreduplicate, the longer it will have to grow," says Roeder. "And the more endocycles it goes through, the bigger it will get."
For a cell to become a giant cell, she explains, it will generally need to endoreduplicate during its first cell cycle. If it waits a cycle or two to s
|Contact: Lori Oliwenstein|
California Institute of Technology