Short-rotation woody crops for bioenergy and biofuels applications

Abstract

Purpose-grown trees will be part of the bioenergy solution in the United States, especially in the Southeast where plantation forestry is prevalent and economically important. Trees provide a "living biomass inventory" with existing end-use markets and associated infrastructure, unlike other biomass species such as perennial grasses. The economic feasibility of utilizing tree biomass is improved by increasing productivity through alternative silvicultural systems, improved breeding and biotechnology. Traditional breeding and selection, as well as the introduction of genes for improved growth and stress tolerance, have enabled high growth rates and improved site adaptability in trees grown for industrial applications. An example is the biotechnology-aided improvement of a highly productive tropical Eucalyptus hybrid, Eucalyptus grandis x Eucalyptus urophylla. This tree has acquired freeze tolerance by the introduction of a plant transcription factor that up-regulates the cold-response pathways and makes possible commercial plantings in the Southeastern United States. Transgenic trees with reduced lignin, modified lignin, or increased cellulose and hemicellulose will improve the efficiency of feedstock conversion into biofuels. Reduced lignin trees have been shown to improve efficiency in the pre-treatment step utilized in fermentation systems for biofuels production from lignocellulosics. For systems in which thermochemical or gasification approaches are utilized, increased density will be an important trait, while increased lignin might be a desired trait for direct firing or co-firing of wood for energy. Trees developed through biotechnology, like all transgenic plants, need to go through the regulatory process, which involves biosafety and risk assessment analyses prior to commercialization.

Figure 1.

A Eucalyptus hybrid, with or without the addition of a freeze tolerance gene, after a typical winter in the Southeast United States. (a) photograph of control Eucalyptus after winter temperatures of 16°F in South Carolina, (b): rd29a::CBF2 transgenic EH1, photograph taken from the same field trial and time as the tree in photograph (a). Photograph (c) an aerial photograph of block plots of different lines of rd29a::CBF2 Eucalyptus in a field trial in Alabama after winter temperatures of 19°F. A control tree block is marked with a “*”, and all other similar blocks are also control blocks.

Figure 2.

Bags containing male cone clusters of untransformed control and transgenic lines of P. taeda (ArborGen unpublished data). Yellow-colored pollen is clearly visible inside bags containing untransformed male cones (left), while no pollen was found inside the bags containing male cones of lines transformed with genes for pollen ablation (right).