Engineered poplar for bioproduction of the triterpene squalene

Building sustainable platforms to produce biofuels and specialty chemicals has become an increasingly important strategy to supplement and replace fossil fuels and petrochemical-derived products. Terpenoids are the most diverse class of natural products which have many commercial roles as specialty chemicals. Poplar is a fast growing, biomass dense bioenergy crop with many species known to produce large amounts of the hemiterpene isoprene, suggesting an inherent capacity to produce large amounts of other terpenes. Here we aimed to engineer poplar with optimized pathways to produce squalene, a triterpene commonly used in cosmetic oils, a potential biofuel candidate, and the precursor to the further diversified classes of triterpenoids and sterols. The squalene production pathways were either re-targeted from the cytosol to plastids or co-produced with lipid droplets in the cytosol. Squalene and lipid droplet co-production appeared to be toxic, which we hypothesize to be due to disruption of adventitious root formation, suggesting a need for tissue specific production. Plastidial squalene production enabled up to 0.63 mg/g fresh weight in leaf tissue, which also resulted in reductions in isoprene emission and photosynthesis. These results were also studied through a technoeconomic analysis, providing further insight into developing poplar as a production host.

构建可持续平台以生产生物燃料和专用化学品，已成为补充乃至替代化石燃料及石油化工衍生产品的日益重要的战略方向。萜类化合物（Terpenoids）是种类最为多样的天然产物类群，作为专用化学品拥有诸多商业应用场景。杨树（Poplar）是速生、高生物量密度的能源作物，多个已知树种可产生大量半萜异戊二烯（hemiterpene isoprene），这暗示其具备合成大量其他萜类物质的内在潜力。本研究旨在通过优化代谢途径对杨树进行工程化改造，以生产角鲨烯（squalene）——一种常用于化妆品用油的三萜类物质，同时也是潜在的生物燃料原料，更是多种三萜类化合物及甾醇的前体分子。研究中，角鲨烯合成途径被分为两种策略：一是从细胞质基质（cytosol）重新靶向至质体（plastids），二是在细胞质基质中与脂滴（lipid droplets）协同生产。结果显示，脂滴协同生产策略会产生毒性，我们推测其原因是不定根形成过程受到干扰，这表明需要采用组织特异性的生产方式。采用质体途径生产角鲨烯时，叶片组织中的角鲨烯产量可达0.63 mg/g鲜重，同时会导致异戊二烯排放及光合作用水平下降。本研究还通过技术经济分析（technoeconomic analysis）对上述结果进行了探究，为开发杨树作为生产宿主提供了更深入的理论参考。