Agronomic performance of Populus deltoides trees engineered for biofuel production

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Background: One of the major barriers to the development of lignocellulosic feedstocks is the recalcitrance of plant cell walls to deconstruction and saccharifcation. Recalcitrance can be reduced by targeting genes involved in cell wall biosynthesis, but this can have unintended consequences that compromise the agronomic performance of the trees under feld conditions. Here we report the results of a feld trial of fourteen distinct transgenic Populus deltoides lines that had previously demonstrated reduced recalcitrance without yield penalties under greenhouse conditions. Results: Survival and productivity of the trial were excellent in the frst year, and there was little evidence for reduced performance of the transgenic lines with modifed target gene expression. Surprisingly, the most striking phenotypic efects in this trial were for two empty-vector control lines that had modifed bud set and bud fush. This is most likely due to somaclonal variation or insertional mutagenesis. Traits related to yield, crown architecture, herbivory, patho‑ gen response, and frost damage showed few signifcant diferences between target gene transgenics and empty vector controls. However, there were a few interesting exceptions. Lines overexpressing the DUF231 gene, a putative O-acetyltransferase, showed early bud fush and marginally increased height growth. Lines overexpressing the DUF266 gene, a putative glycosyltransferase, had signifcantly decreased stem internode length and slightly higher volume index. Finally, lines overexpressing the PFD2 gene, a putative member of the prefoldin complex, had a slightly reduced volume index. Conclusions: This feld trial demonstrates that these cell wall modifcations, which decreased cell wall recalcitrance under laboratory conditions, did not seriously compromise frst-year performance in the feld, despite substantial challenges, including an outbreak of a stem boring insect (Gypsonoma haimbachiana), attack by a leaf rust pathogen (Melampsora spp.), and a late frost event. This bodes well for the potential utility of these lines as advanced biofuels feedstocks.