Eugenol specialty chemical production in transgenic poplar (Populus tremula X P. alba) field trials: lessons learned and future directions

Da Lu

A foundational study assessed biochemical pathway introduction into poplar to produce eugenol, chavicol, p-anol, and/or isoeugenol, as well as their sequestered storage products, from potentially available substrates, coniferyl and p-coumaryl alcohols. While poplar accumulates many Phe-derived phenylpropanoids, such as flavonoids, it was unknown whether significant carbon flux to monolignols vs other phenylpropanoid (acetate) pathway metabolites would be kinetically favored. Various transgenic poplar lines generated eugenol and chavicol glucosides in ca. 0.45 % (~0.35 and ~0.1 %, respectively) in dry weight foliage tissue in field trials, as well as traces of aglycones. By contrast, there were only traces of them in branch tissues, even after ~4 year field trials. Bioproduct accumulation in foliage plateaued, even at the lowest introduced gene expression levels, suggesting limited substrate monolignol availability. Nevertheless, this allows foliage collection for platform chemical production, with the remaining (stem) biomass available for wood, pulp and paper and bioenergy product purposes. Increased gene expression did not increase targeted bioproduct accumulation. However, the exciting possibility now exists of significantly increasing amounts via numerous "omics" technologies, together with systems biology, synthetic biology and metabolic modeling approaches. These approaches offer a very attractive means to identify and overcome current metabolic blocks/restricted carbon flux to the target bioproducts in foliage tissues, as well as developing approaches for their accumulation in stems and branches. However, already the harvesting and processing of eugenol glucoside in large scale poplar plantations offers a viable entry point into displacing the current 3500-4000 tonnes worldwide production of eugenol bioproducts from clove (oil) via dedicated poplar plantations. Some transformed lines displayed unexpected precocious flowering after 4 years field trial growth. This necessitated terminating (felling) these particular plants, as USDA APHIS prohibits possibility of their interacting (cross-pollination, etc.) with wild type (native lines). In future, there are biotechnological approaches available (e.g. gene editing) to produce sterile plant lines. This would be a very attractive approach for field production, as numerous (sterile) plant lines are readily re-generated from sterile parent plant lines stem segments. Multiple harvesting could be achieved (2-3 times/2 years, for example) through coppicing.

Eugenol specialty chemical production in transgenic poplar (Populus tremula X P. alba) field trials: lessons learned and future directions

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