MicroRNAs - Tiny molecules shape up plants

A team of German and US scientists demonstrates that development and growth of plants is controlled by tiny pieces of RNAs

Small RNA molecules have big effects on plant shape, according to a report published in the online edition of the journal Nature (Advance Online Publication, August 20, 2003). These findings were the result of a transatlantic collaboration between two teams, one led by Detlef Weigel, Director at the Max Planck Institute for Developmental Biology in Germany, and the other by Jim Carrington, Director of the Center for Gene Research and Biotechnology at Oregon State University. Using the model plant Arabidopsis, the new study shows for the first time how very short RNAs called microRNAs control plant shape by guiding destruction of much larger messenger RNAs. MicroRNAs work like a digital radar to hone in on the messenger RNAs of target genes. The messenger RNA, which is the critical molecule that communicates normal gene function, is either destroyed or inactivated through molecular processes that are directed by the microRNA. Turning off the activity of specific genes is very important to prevent abnormal plant development, just as it is in preventing cancerous growth in animals, according to Weigel, who is also an Adjunct Professor at the Salk Institute in California, where this research was started. The study may be a first step, researchers say, in revolutionizing our understanding of how plants control their shape and growth. A plants ability to grow structures with a specific shape is critical to its ability of capturing energy from the sun and turning it into products such as grain and fiber. As such, these findings could ultimately have profound implications for advances in agriculture and forestry.

Since the DNA double helix was discovered 50 years ago, biologists have focused on the role of DNA in controlling gene activity. Only recently have scientists begun to appreciate the importance of microRNAs in keeping gene activity in check, a discovery that was hailed as the breakthrough of 2002 by Science magazine (Small RNAs make big splash, Vol. 298, page 2296). Just a year ago, several groups, including the one led by Carrington, discovered that plants are full of microRNAs. These tiny RNAs are in turn products of much larger RNAs. "Since plants that do not produce enough microRNAs were quite sick, we knew that we were onto something important, but we couldn't quite trace the exact cause for the many abnormalities we observed," said Carrington.

In their collaborative study, Weigel and Carrington now have pinpointed, for the first time, the precise interplay between a particular microRNA, called "Jaw", and its targets. "For several years, we had a mutant plant that overproduced a large RNA of unknown function. Surprisingly, this RNA did not seem to make a protein, like most other RNAs do. After we read Carringtons publications, we suddenly realized that this RNA was instead chopped up into microRNAs," said Weigel.

With DNA chips, scientists can measure all messenger RNAs in a plant at once. Using such DNA chips, Weigel and Carrington discovered that the Jaw microRNA specifically caused the coordinated destruction of several TCP messenger RNAs. The TCP genes prevent excess cell division in leaves. Without them, there is too much cell division and the leaves buckle instead of staying flat. This was exactly what the mutant plants with too much Jaw microRNA looked like. Next, the researchers changed the TCP genes so that their messenger RNAs could no longer be recognized by microRNAs. Plants harboring the altered TCP genes grew no further than the seedling stage, proving that microRNA targeting of messenger RNAs is very important for normal plant development.

Both the Jaw microRNA and its TCP targets are found in all flowering plants examined, including cereals. "This tells us that this mode of controlling leaf shape is not only used in Arabidopsis, our favorite plant model in the lab, but also in plants of agronomic importance such as corn or soy bean," said Weigel.

As more and more microRNAs are discovered and their role in plant growth becomes clearer, new opportunities may open up to breed more efficient or productive plants. "We will probably discover microRNAs that function in most aspects of plant development, including flowering, root growth and seed production," Carrington said. Weigel agrees: "Indeed, in unpublished work we have found that other microRNAs and their targets seem to have important roles in helping the plant to decide when to produce flowers. The potential impact of this could be quite large, since it provides a new way of adapting plants to their environment."

Also participating in the study were: Javier F. Palatnik, Max Planck Institute for Developmental Biology and The Salk Institute; Edwards Allen, Oregon State University; Xuelin Wu, The Salk Institute; and Carla Schommer and Rebecca Schwab from Max Planck Institute for Developmental Biology.

Experts who can comment on this research:

Prof. Victor Ambros

Dartmouth College, USA
Phone: +1 (603) 650 - 1939
E-mail: victor.ambros@dartmouth.edu

Prof. Kathryn Barton
Carnegie Institution, USA
Phone: +1 (650) 325-1521 x224
E-mail: barton@andrew2.stanford.edu

Prof. Bonnie Bartel
Rice University, USA
Phone: +1 (713) 348-5602
E-mail: bartel@bioc.rice.edu

Prof. David Baulcombe
John Innes Centre, UK
Phone: +44 (0) 1603 450420
E-mail: david.baulcombe@bbsrc.ac.uk