Field trials with genetically modified poplars shows potential for efficient conversion to sugars but with impact on biomass yield
|Ethanol yield (g/L) for the Belgian and French field trials. Van Acker et al. Click to enlarge.|
The results of field trials with genetically modified poplar trees in Belgium and France shows that the wood of the modified poplar trees—down-regulated for cinnamoyl-CoA reductase (CCR), an enzyme in the lignin biosynthetic pathway—improved saccharification yield—i.e., it can be more efficiently converted into sugars for producing bio-based products such as bio-plastics and bio-ethanol.
However, the study, published as an open access paper in Proceedings of the National Academy of Sciences (PNAS), also found that strong down-regulation of CCR also affected biomass yield. The team, from Belgium, France and the US, led by researchers from VIB and Ghent University, concluded that CCR down-regulation may become a successful strategy to improve biomass processing if the yield penalty can be overcome.
The field trial showed that the suppression of the lignin biosynthesis in the trees is variable. In some trees the suppression—which is visible through a more pronounced red coloration of the wood beneath the bark—is stronger than in other trees. Some branches show almost no red coloration, others a spotty pattern and again other a full red coloration.
|Wild-type poplar wood. Click to enlarge.||GM trees, down-regulated for cinnamoyl-CoA reductase (CCR). Click to enlarge.|
Wood from the most affected transgenic trees—(FAS13 in the French trial)—delivered up to 161% increased ethanol yield over wild type (WT) poplar. Taking the yield penalty for these trees into account, FAS13 still yielded 57% more ethanol than WT.
Pooling all the trees from the French field trial resulted in a 14–26% increase in ethanol yield compared with WT. This value can be considered as the lower bound, the researchers suggested, because, in this experimental design, trees that were less efficiently down-regulated for CCR, were also included in the analyzed pool and not only wood, but complete trees with bark and apical parts. However, the 14–26% increase in ethanol yield per gram of dry wood was outweighed by the overall yield penalty.
Obviously, one of the main future objectives will be to overcome the yield penalty that is often associated with modified lignin. Among the likely causes of the yield penalty are the structural abnormalities of the plant cell wall, such as collapsed xylem, as was previously noticed in greenhouse-grown CCR–down-regulated poplars.—Van Acker et al.
The branches with the highest red coloration give us hope that we will be able to achieve our goal in the future. The biosynthesis of lignin is very complex. We will now search for modifications that provide a strong and uniform suppression of the lignin biosynthesis. Because in the meantime we are also getting a pretty good idea of what causes the growth retardation, we immediately will start to work on poplars that grow normal, but still have a stable suppression of the lignin production. It must be possible to improve the ethanol yield per tree with 50 to 100%.—Prof. Wout Boerjan, VIB and Ghent University, corresponding author
In a new field trial that VIB will start in Wetteren, Belgium, in 2014, trees will be tested in which another enzyme has been suppressed: the ‘CAD-enzyme’. In these trees also a more modern way of suppression of the enzyme has been used. This new trial therefore fits into the search for modifications that provide a more uniform suppression of the lignin biosynthesis.
Rebecca Van Acker, Jean-Charles Leplé, Dirk Aerts, Véronique Storme, Geert Goeminne, Bart Ivens, Frédéric Légée, Catherine Lapierre, Kathleen Piens, Marc C. E. Van Montagu, Nicholas Santoro, Clifton E. Foster, John Ralph, Wim Soetaert, Gilles Pilate, and Wout Boerjan (2013) “Improved saccharification and ethanol yield from field-grown transgenic poplar deficient in cinnamoyl-CoA reductase,” PNAS doi: 10.1073/pnas.1321673111