Currently, most tree-improvement programs are based on the management of genetic resources, including the selection of superior clones from existing forests, the conservation of genetic variability, partially controlled propagation and classical breeding for desired traits. Although molecular breeding is routine in agriculture, and numerous agrobiotechnological companies are producing many new genetically engineered field crops, vegetables and ornamental plants, forest tree species have been left far behind (Tzfira et al. 1998). Plant transformation techniques, gene isolation and characterization are no longer serious obstacles, and so forest trees are becoming an attractive target for genetic engineering and molecular breeding in the twenty-first century, even though various constraints need to be overcome. Different tools are now available to transform plants genetically, and the most commonly used - Agrobacterium and particle bombardment - have been extensively reviewed (Teichmann and Polle 2006). Early reports on the genetic transformation of forest trees focused on Populus species (Leplé et al. 1992). Even today, Populus remains the principal genetically transformed tree species, both as a model system and for practical reasons. Transformation of other economically relevant tree species such as pine, spruce or Eucalyptus has been described but is still confined to specialized laboratories. In this area technical advance is required.

Furthermore, the ecological consequences of transformation need to be taken into account. During their perennial life cycle, forest trees must adapt to seasonal climatic changes and to a wide range of pests and abiotic stresses. Consequently, tree populations exhibit high diversity, reflected in many eco-types; there is, therefore, a need to transform different ecotypes for stable expression of the transgenes through cycles of environmental changes. Additionally, the long life cycle of forest trees calls for stability of the transgenes over several years. As the common constitutive promoters are silenced in many annual transgenic plants, unique constructs with more suitable promoters, preferably of tree origin, are likely to be required for the long-term expression of foreign genes in forest trees. Nevertheless, it should be noted that homologous promoters could also be silenced in transgenic trees by positional effects or other epigenetic processes.

Since stress tolerance is a multigenic trait, the genetic engineering for drought tolerance requires a deeper understanding of the key mechanisms involved in drought resistance. To advance this knowledge integrated approaches studying drought tolerance with ecophysiological, molecular, metabolomic, and proteomic methods are urgently needed. To unravel the mechanisms of drought tolerance in trees beyond common mechanisms also present in herbaceous plants, it will be necessary to devise experiments addressing the unique characteristics of tree species. As trees are perennial and exist for decades, different stages of their life cycle can be distinguished. The first, and probably most vulnerable phase with respect to drought, is the establishment of the seedlings. An important question is whether seedlings and mature trees show similar or different regulation of their responses to drought. The development of certain morphological structures is also important for drought adaptation. Many trees develop a specific root and xylem system, which favours drought resistance, e.g., roots that explore deep soil zones and a xylem resistant to cavitation. Experiments addressing such adaptive traits are missing to date.

The knowledge of how trees adapt to drought is of interest not only for academic but also for various practical reasons. One is that large-scale deforestation results in changes in the microclimatic conditions, generally causing decreased water availability and, thus, requires afforestation with trees adapted to severe environmental stress. Another important reason is that the demand of renewable resources including wood will increase because of the increasing world population and, thus, requires an extension of areas used for silviculture. Since fertile soils amenable for agriculture will not be available for this purpose, the breeding of tree species able to persist under arid or semi-arid conditions is necessary. Furthermore, significant changes in precipitation patterns are predicted in temperate regions with currently high precipitation during the growth phase. Current forest management and cultivation strategies are not in agreement with the expected future climatic changes, and thus require the introduction of more stress tolerant ecotypes or novel engineered tree species to minimise the risk.

Acknowledgements. We are grateful to the German Science foundation for supporting Poplar Research Group Germany (PRG), the European communities for funding ESTABLISH (QLK5-CT-2000-01377), and the Bundesministerium für Verbraucherschutz, Ernährung und Landwirtschaft (BMVEL) to provide a travel grant to AP.

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