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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: 603-650-1939
E-mail: victor.ambros@dartmouth.edu
Prof. Kathryn Barton
Carnegie Institution, USA
Phone: 650-325-1521 x224
E-mail: barton@andrew2.stanford.edu
Prof. Bonnie Bartel
Rice University, USA
Phone: 713-348-5602
E-mail: bartel@bioc.rice.edu
Prof. David Baulcombe
John Innes Centre, UK
Phone: 44-160-345-0420
E-mail: david.baulcombe@bbsrc.ac.uk
The study, entitled "Control of leaf morphogenesis by microRNAs", was funded by the National Institutes of Health, the National Science Foundation of the US and the Max Planck Society, Germany. Original work:
Javier F. Palatnik, Edwards Allen, Xuelin Wu, Carla Schommer, Rebecca Schwab, James Carrington & Detlef Weigel Control of leaf morphogenesis by microRNAs Nature, Advance Online Publication, 20. August 2003, DOI: 10.1038/nature01958 (http://dx.doi.org/10.1038/nature01958)
Journal
Nature