They showed that, to make a flat leaf, microRNAs need to target and destroy the TCP target genes at the right time and at the right place in the plant as it develops. If Arabidopsis makes too much microRNA JAW, then leaves are crinkled and wrinkly. This is because too much microRNA overloads the normal balance of TCP gene expression, which then causes too much cell division in growing leaves. The result is too many cells to crowd into a relatively flat space.

"Think about trying to lay a carpet in a room that is too small," Carrington said. "The only way to fit it all in is to introduce bulges and ripples. This is what happens when microRNA JAW does not properly regulate the TCP genes. There are too many cells that are forced into the leaf plane, resulting in the introduction of improper curvature."

"This is among the first demonstrations that microRNAs control a specific developmental process, and the work opens the possibility of an entirely new layer of controlling plant morphogenesis," said Weigel.

According to the researchers, the "flatness" of a plant leaf is of considerable biological importance.

"Plants evolved flat leaves for important functional reasons," Carrington said. "A flat surface captures more light and energy from the sun. Plants also have cells on their top surfaces that are specially designed for that purpose. On the underside, leaves are more specialized for gas exchange. The whole process is remarkably efficient, and that is due in part to formation of leaves with the proper shape."

As more and more microRNAs are discovered and their role in plant growth and development becomes clear, the entire process of genetic manipulation of pl
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Contact: James Carrington
carrington@orst.edu
541-737-3347
Oregon State University
20-Aug-2003