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July 18, 2005

NC State Scientists Discover Tree-Specific Regulatory Genes

FOR IMMEDIATE RELEASE

Dr. Vincent Chiang

Researchers at North Carolina State University have discovered tree-specific genetic “on/off switches” that could lead to better understanding of how trees form wood and resist stress.

MicroRNA, or miRNA, is a short genetic strand that “targets” a specific gene within an organism. The miRNA acts as an on/off switch for certain genetic traits by bonding to a certain portion of the longer target gene and then “cleaving” the gene, effectively shutting down that gene’s function. Discovery of miRNAs is leading to novel insights into the control of gene expression – the physical traits governed by genes – during development in animals and plants.

Current research into plant miRNAs had centered on Arabidopsis, an herbaceous plant with a fully sequenced genome believed to be a good basis for all plant genetic research, including tree research.

In an article published in the July 1 online edition of The Plant Cell, Dr. Vincent Chiang, professor of forestry at NC State, Dr. Shanfa Lu and other NC State researchers confirm the discovery of miRNAs that are specific to trees, and not present in Arabidopsis. The team also validated the targets of the miRNAs – the genes that these fragments control.

Working with tissues from Populus trichocarpa, part of the family of trees that includes aspen and cottonwoods, Lu and his colleagues isolated 21 miRNA gene families, 11 of which were tree-specific, meaning that they are not present in Arabidopsis.

In addition, the team discovered the targets of the miRNAs, which all involved the ways trees react to mechanical stress. “Trees suffer mechanical stress all the time, simply by having to remain upright and endure wind, rain and snow,” Chiang said.

“Persistent wood formation is one way that trees combat this stress. Long-term mechanical fitness and wood growth are normally not required by Arabidopsis. By discovering miRNAs that control a tree’s response to mechanical stress, we hope to be able to understand how and why trees form wood, which may lead to the development of hardier, wood-rich trees better suited to the forest products industry. A more abundant and higher quality supply of raw materials to both the solid wood and pulp and paper industries reduces the demand on natural stands and offers benefits all the way down the line from the grower to the consumer.”

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Note to editors: An abstract of the paper follows.

“Novel and Mechanical Stress-Responsive MicroRNAs in Populas trichocarpa that are Absent from Arabidopsis”

Authors: Shanfa Lu, Ying-Hsuan Sun, Rui Shi, Catherine Clark, Laigeng Li, and Vincent L. Chiang, Forest Biotechnology Group, Department of Forestry and Environmental Resources, College of Natural Resources, North Carolina State University, Raleigh, North Carolina
Published: July 1, 2005, in The Plant Cell

Abstract: MicroRNAs (miRNAs) are small, noncoding RNAs that can play crucial regulatory roles in eukaryotes by targeting mRNAs for silencing. To test whether miRNAs play roles in the regulation of wood development in tree species, we isolated small RNAs from the developing xylem of Populus trichocarpa stems and cloned 22 miRNAs. They are the founding members of 21 miRNA gene families for 48 miRNA sequences, represented by 98 loci in the Populus genome. A majority of these miRNAs were predicted to target developmental- and stress/defense-related genes and possible functions associated with the biosynthesis of cell wall metabolites. Of the 21 P. trichocarpa miRNA families, 11 have sequence conservation in Arabidopsis thaliana but exhibited species-specific developmental expression patterns, suggesting that even conserved miRNAs may have different regulatory roles in different species. Most unexpectedly, the remaining 10 miRNAs, for which 17 predicted targets were experimentally validated in vivo, are absent from the Arabidopsis genome, suggesting possible roles in tree specific processes. In fact, the expression of a majority of the cloned miRNAs was upregulated or downregulated in woody stems in a manner consistent with tree-specific corrective growth against tension and compression stresses, two constant mechanical loads in trees. Our results show that plant miRNAs can be induced by mechanical stress and may function in one of the most critical defense systems for structural and mechanical fitness.