"Each cell starts out with a chance to become a giant cell," Roeder says. "It's a probabilistic thing; each cell has a certain probability of making that decision. Once it makes the decision, however, its fate is determined."
But endoreduplication isn't the only thing that decides the ultimate size of a sepal cell, the research team found. A cell that endoreduplicates early can grow to be an even larger giant if its cell cycles are longer than average, giving it plenty of growing time.
To prove their point, the team performed a series of experiments in which they altered the levels of cell-cycle inhibitors in the sepal cells. When they decreased the inhibitorincreasing the frequency with which the cell divides, and thus reducing the length of the cell cyclethe sepal cells were unable to grow into giant cells.
"These findings back up our hypothesis," says Roeder. "And when you change the parameters in the computer model, as if you were reducing the level of a cell-cycle inhibitor, the model shows the same pattern."
Understanding exactly how sepal cells decide whether to grow big or small could some day lead to practical applications, Roeder notes. For instance, the utility of various crops as biofuels depends on how much cellulose they contain. A sepal with a large number of giant cells has much less cell-wall surface area than a sepal with lots of smaller cells; since the cell wall is where cellulose is found, giant-cell-laden sepals would be less useful as biofuel.
"This work gives us ideas about how growth happens in these plants," says Roeder. "And once we better understand plant growth and cell division, we can better manipulate them."
|Contact: Lori Oliwenstein|
California Institute of Technology