Engineering custom morpho- and chemotypes of Populus for sustainable production of biofuels, bioproducts, and biomaterials

Abstract

Humans have been modifying plant traits for thousands of years, first through selection (i.e., domestication) then modern breeding, and in the last 30 years, through biotechnology. These modifications have resulted in increased yield, more efficient agronomic practices, and enhanced quality traits. Precision knowledge of gene regulation and function through high-resolution single-cell omics technologies, coupled with the ability to engineer plant genomes at the DNA sequence, chromatin accessibility, and gene expression levels, can enable engineering of complex and complementary traits at the biosystem level. Populus spp., the primary genetic model system for woody perennials, are among the fastest growing trees in temperate zones and are important for both carbon sequestration and global carbon cycling. Ample genomic and transcriptomic resources for poplar are available including emerging single-cell omics datasets. To expand use of poplar outside of valorization of woody biomass, chassis with novel morphotypes in which stem branching and tree height are modified can be fabricated thereby leading to trees with altered leaf to wood ratios. These morphotypes can then be engineered into customized chemotypes that produce high value biofuels, bioproducts, and biomaterials not only in specific organs but also in a cell-type-specific manner. For example, the recent discovery of triterpene production in poplar leaf trichomes can be exploited using cell-type specific regulatory sequences to synthesize high value terpenes such as the jet fuel precursor bisabolene specifically in the trichomes. By spatially and temporally controlling expression, not only can pools of abundant precursors be exploited but engineered molecules can be sequestered in discrete cell structures in the leaf. The structural diversity of the hemicellulose xylan is a barrier to fully utilizing lignocellulose in biomaterial production and by leveraging cell-type-specific omics data, cell wall composition can be modified in a tailored and targeted specific manner to generate poplar wood with novel chemical features that are amenable for processing or advanced manufacturing. Precision engineering poplar as a multi-purpose sustainable feedstock highlights how genome engineering can be used to re-imagine a crop species.

Keywords: bioproducts; cell-type specificity; genome engineering; morphotype; poplar.

Figure 1

Modified poplar morphotypes can improve management practices, increase photosynthetic capture, and serve as a sustainable chassis for biofuel, biomaterial, and/or bioproduct production. Three morphotypes are depicted (dwarf bushy, columnar bushy, tall spindle) along with a leaf with increased trichome density. Morphotype images generated using Biorender.com. Leaf photograph credit: W. Patrick Bewg.

Figure 2

Manipulation of plum branch angle by altering known regulatory genes. (A) Representative control, (B) silencing of TAC1 (Hollender et al., 2018b), (C) over-expression of Arabidopsis TAC1, (D) silencing of WEEP (Hollender et al., 2018a).

Figure 3

Xylan: A major component of woody cell walls. (A) Schematic representation O-acetylglucuronoxylan present in the secondary cell walls of poplar xylan highlighting abundant substituents and the characteristic reducing end sequence. The symbol nomenclature for glycans (www.ncbi.nlm.nih.gov/glycans/snfg.html) was used. (B) Use of synthetic biology to engineer custom cell wall chemotypes in poplar for downstream functionalization into bio-sourced materials.