"MicroRNAs" control plant shape and structure


CORVALLIS, Ore. - New discoveries about tiny genetic components called microRNAs explain why plant leaves are flat.

The study may be a first step, researchers say, in revolutionizing our understanding of how plants control their morphology, or shape. A plant's ability to grow structures with a specific shape is critical to its normal function of capturing energy from the sun and producing products like grain and fiber.

As such, these findings could ultimately have profound implications for advances in agriculture.

The research was published online today in Nature, a scientific journal, by scientists from Oregon State University, the Max Planck Institute in Germany and the Salk Institute in California.

Understanding the genetic basis of plant shape is just one of the first outgrowths of research done with microRNAs, tiny bits of genetic material with powerful abilities to control gene "expression." Careful regulation of large sets of genes allows plants to specify which cells turn into leaf, root or other types of cells.

MicroRNAs work like a digital radar system to hone in on target genes. The target gene messenger RNA, which is the critical molecule that communicates normal gene functions, is either destroyed or inactivated through molecular processes that are directed by the microRNA. It is this type of negative regulation, or turning off expression of specific genes, that triggers development of plant parts with the proper shape.

"In this study we've demonstrated the real life consequences of a microRNA, showing how regulated destruction of a set of messenger RNA targets controls the shape of a plant's leaf," said James Carrington, professor and director of the Center for Gene Research and Biotechnology at OSU.

Collaborative research teams led by Detlef Weigel at the Max Planck Institute and Salk Institute, and Carrington at OSU, identified a microRNA they called "JAW," and five specific "TCP" target genes that collectively control cellular division in plant leaves and other organs. The work was done using Arabidopsis, a small leafy plant in the mustard family.

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 plants for useful purposes may become far more precise and vast new opportunities may open up to produce more efficient or productive plants.

"We will probably discover microRNAs that function in most aspects of plant growth and development, including flowering, root structure and seed production," Carrington said. "The potential impacts of this could be quite large."