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Populus trichocarpa x deltoides

Poplar: an experimental model of economic importance

Introduction

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Photo Marc Villar

About 30 species of aspen and poplars colonize the immense territories of the Northern Hemisphere - from the boreal forest to the banks of the valley of the Loire - and their inter-American (Populus trichocarpa x deltoides) and Euroamerican (P. deltoides x nigra) hybrids constitute large planted areas in Europe, Asia and North America. In France, poplars are the second most important deciduous species for the production of logs. As a raw material for the veneer industry, they also constitute a crucial factor in the maintenance of the diversity of the ripysilves and are a large sink for carbon dioxide because of their sequestration of a large quantity of photoassimilates in their wood. This importance for industrial and environmental forestry explains why biologists and forestry geneticists have shown so much interest in poplars in the last few decades. In biology, the poplar owes its notoriety to the fact that is has been chosen as a model for the study of the development and function of trees. Its economic value, the existence of protocols for vegetative multiplication and efficient genetic transformation, the construction of several genetic maps, together with rapid growth, have raised the poplar to the rank of model organism for forest biochemistry, physiology and genetics in only a few years. The relatively small size of its genome, 520 Mb, is a supplementary asset. This ensemble of virtues has logically led to the sequencing of its genome. The research activities developed within the framework of the ForEST project aim to identify the transcriptional mechanisms involved in the development of tree-fungus associations and in the regulation of the formation of wood in response to environmental constraints.

Interactions between the poplar and biotrophic fungi

This involves:

  • the identification of the genes leading to the formation of symbiotic (ectomycorrhiza) or pathogenic (foliar rust agent) biotrophic structures and
  • to compare the gene networks involved in these two types of interactions.

Within the ForEST project, the comparative analysis of the transcript profiles (through EST sequencing) during the colonization of poplar by the phytopathogenic rust fungus, Melampsora larici-populina, and the ectomycorhizal fungi, Pisolithus tinctorius and Laccaria bicolor, allowed the identification of differentially-expressed genes in these different biotrophic interactions. The cDNA libraries analysed corresponded to different stages of development of the infection structures.

Assembly of the different sets of ESTs has generated 526 tentative consensi (TC) of rust-infected leaves, 2,570 TC of germlings from Melampsora larici-populina, 3,591 TC of Populus/Pisolithus ectomycorrhizas, and 1,786 TC of Populus/Laccaria ectomycorrhizas. In addition, we have analyzed 2,020 TC from poplar roots incubated in 50 ┬ÁM auxin (IAA), a signaling molecule involved in the ectomycorrhizal symbiosis formation. Finally, 2,183 TC of drought-stress roots of poplar were generated.

Wood formation in response to environmental constraints

During their long lifespan, trees must confront numerous environmental constraints. In order to better support these constraints, they have developed adaptations; some of these are linked to the formation of wood. It is well established that the relative proportion of the constituents of the cell wall as well as the size and arrangement of cells of the xylem determines some of the technological properties of wood. In fact, the cell wall can be compared to a composite material which consists of a matrix of lignins, hemicelluloses and amorphous cellulose, reinforced by long packets of crystalline cellulose microfibrils. The models derived from the mechanics of composite materials have shown the importance of the angle of the microfibrils, the degree of crystalinity of the cellulose and the mechanical properties of the matrix in explaining the variations in the properties of the wood. Thus it is probable that the genes implicated in the construction of the cell wall play a primordial role in the definition of the properties of the wood.

Furthermore, there are very delicate mechanisms which permit the tree to regulate the differentiation of the wood cells in response to different environmental constraints. When deciduous trees are subjected to a mechanical constraint provoked by the leaning of their trunk, they respond by producing a specific type of wood, tension wood, which permits them to reorient their axes in fine. Compared with normal wood, tension wood is characterized by a reduced number of vessels and more numerous fibers, and by cell walls which are much thicker. Likewise, when submitted to a moderated water stress, the tree will react at the level of wood differentiation : vessels will be smaller but more numerous whereas there will be less fibres. Our objective is to identify genes implicated in the formation of wood and which are important for the adaptation of trees to their environment. These candidate genes will be useful for creating improved forest varieties adapted to a changing environment and which will produce quality wood.

Last update on 15 January 2008

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