Expression of a hyperthermophilic endoglucanase in hybrid poplar modifies the plant cell wall and enhances digestibility

Acidothermus cellulolyticus E1 endoglucanase expressed in planta undergoes extensive hydroxyproline-O-glycosylation and exhibits enhanced impact on biomass digestibility

KEY MESSAGE

E1 holoenzyme was extensively Hyp-O-glycosylated at the proline rich linker region in plants, which substantially increased the molecular size and improved the enzymatic digestibility of the biomass of transgenic plants. Thermophilic E1 endo-1,4-β-glucanase derived from Acidothermus cellulolyticus has been frequently expressed in planta to reconstruct the plant cell wall to overcome biomass recalcitrance. However, the expressed holoenzyme exhibited a larger molecular size (~ 100 kDa) than the theoretical one (57 kDa), possibly due to posttranslational modifications in the recombinant enzyme within plant cells. This study investigates the glycosylation of the E1 holoenzyme expressed in tobacco plants and determines its impact on enzyme activity and biomass digestibility. The E1 holoenzyme, E1 catalytic domain (E1cd) and E1 linker (E1Lk) were each expressed in tobacco plants and suspension cells. The accumulation of holoenzyme was 2.0- to 2.3- times higher than that of E1cd. The proline-rich E1Lk region was extensively hydroxyproline-O-glycosylated with arabinogalactan polysaccharides. Compared with E1cd, the holoenzyme displayed a broader optimal temperature range (70 to 85 ºC). When grown in greenhouse, the expression of E1 holoenzyme induced notable phenotypic changes in plants, including delayed flowering and leaf variegation post-flowering. However, the final yield of plant biomass was not significantly affected. Finally, plant biomass engineering with E1 holoenzyme showed 1.7- to 1.8-fold higher saccharification efficiency than the E1cd lines and 2.4- to 2.7-fold higher than the wild-type lines, which was ascribed to the synergetic action of the E1Lk and cellulose binding module in reducing cell wall recalcitrance.

Transcriptome profiling reveal key hub genes in co-expression networks involved in Iridoid glycosides biosynthetic machinery in Picrorhiza kurroa

Picrorhiza kurroa is a medicinal herb rich in hepatoprotective iridoid glycosides, picroside-I (P-I) and picroside-II (P-II). The biosynthetic machinery of picrosides is poorly understood, therefore, 'no-direction' gene co-expression networks were used to extract linked/closed and separated interactions in terpenoid glycosides-specific sub-networks. Transcriptomes generated from different organs, varying for P-I and P-II contents such as shoots grown at 15 and 25 °C and nursery-grown shoots, stolons, and roots resulted in 47,726, 44,958, 40,117, 66,979, and 55,578 annotated transcripts, respectively. Occurrence of 2810 ± 136 nodes and 15,626 ± 696 edges in these networks indicated intense, co-expressed, closed loop interactions. Either deregulation/inhibition of abscisic acid (ABA) biosynthesis/signaling or constitutive degradation of ABA resulted in organ-specific accumulation of P-I and P-II. Biosynthesis, condensation and glucosylation of isoprene units may occur in shoots, roots or stolons; but addition of phenylpropanoid moiety and further modification/s of the iridoid backbone occurs mainly inside vacuoles in roots.

Targeted genome editing of plants and plant cells for biomanufacturing

Plants have provided humans with useful products since antiquity, but in the last 30 years they have also been developed as production platforms for small molecules and recombinant proteins. This initially niche area has blossomed with the growth of the global bioeconomy, and now includes chemical building blocks, polymers and renewable energy. All these applications can be described as “plant molecular farming” (PMF). Despite its potential to increase the sustainability of biologics manufacturing, PMF has yet to be embraced broadly by industry. This reflects a combination of regulatory uncertainty, limited information on process cost structures, and the absence of trained staff and suitable manufacturing capacity. However, the limited adaptation of plants and plant cells to the requirements of industry-scale manufacturing is an equally important hurdle. For example, the targeted genetic manipulation of yeast has been common practice since the 1980s, whereas reliable site-directed mutagenesis in most plants has only become available with the advent of CRISPR/Cas9 and similar genome editing technologies since around 2010. Here we summarize the applications of new genetic engineering technologies to improve plants as biomanufacturing platforms. We start by identifying current bottlenecks in manufacturing, then illustrate the progress that has already been made and discuss the potential for improvement at the molecular, cellular and organism levels. We discuss the effects of metabolic optimization, adaptation of the endomembrane system, modified glycosylation profiles, programmable growth and senescence, protease inactivation, and the expression of enzymes that promote biodegradation. We outline strategies to achieve these modifications by targeted gene modification, considering case-by-case examples of individual improvements and the combined modifications needed to generate a new general-purpose “chassis” for PMF.

Engineering hydroxyproline-O-glycosylated biopolymers to reconstruct the plant cell wall for improved biomass processability

Reconstructing the chemical and structural characteristics of the plant cell wall represents a promising solution to overcoming lignocellulosic biomass recalcitrance to biochemical deconstruction. This study aims to leverage hydroxyproline (Hyp)-O-glycosylation, a process unique to plant cell wall glycoproteins, as an innovative technology for de novo design and engineering in planta of Hyp-O-glycosylated biopolymers (HypGP) that facilitate plant cell wall reconstruction. HypGP consisting of 18 tandem repeats of "Ser-Hyp-Hyp-Hyp-Hyp" motif or (SP4)18 was designed and engineered into tobacco plants as a fusion peptide with either a reporter protein enhanced green fluorescence protein or the catalytic domain of a thermophilic E1 endoglucanase (E1cd) from Acidothermus cellulolyticus. The engineered (SP4)18 module was extensively Hyp-O-glycosylated with arabino-oligosaccharides, which facilitated the deposition of the fused protein/enzyme in the cell wall matrix and improved the accumulation of the protein/enzyme in planta by 1.5-11-fold. The enzyme activity of the recombinant E1cd was not affected by the fused (SP4)18 module, showing an optimal temperature of 80°C and optimal pH between 5 and 8. The plant biomass engineered with the (SP4)18 -tagged protein/enzyme increased the biomass saccharification efficiency by up to 3.5-fold without having adverse impact on the plant growth.

Xylan in the Middle: Understanding Xylan Biosynthesis and Its Metabolic Dependencies Toward Improving Wood Fiber for Industrial Processing

Lignocellulosic biomass, encompassing cellulose, lignin and hemicellulose in plant secondary cell walls (SCWs), is the most abundant source of renewable materials on earth. Currently, fast-growing woody dicots such as Eucalyptus and Populus trees are major lignocellulosic (wood fiber) feedstocks for bioproducts such as pulp, paper, cellulose, textiles, bioplastics and other biomaterials. Processing wood for these products entails separating the biomass into its three main components as efficiently as possible without compromising yield. Glucuronoxylan (xylan), the main hemicellulose present in the SCWs of hardwood trees carries chemical modifications that are associated with SCW composition and ultrastructure, and affect the recalcitrance of woody biomass to industrial processing. In this review we highlight the importance of xylan properties for industrial wood fiber processing and how gaining a greater understanding of xylan biosynthesis, specifically xylan modification, could yield novel biotechnology approaches to reduce recalcitrance or introduce novel processing traits. Altering xylan modification patterns has recently become a focus of plant SCW studies due to early findings that altered modification patterns can yield beneficial biomass processing traits. Additionally, it has been noted that plants with altered xylan composition display metabolic differences linked to changes in precursor usage. We explore the possibility of using systems biology and systems genetics approaches to gain insight into the coordination of SCW formation with other interdependent biological processes. Acetyl-CoA, s-adenosylmethionine and nucleotide sugars are precursors needed for xylan modification, however, the pathways which produce metabolic pools during different stages of fiber cell wall formation still have to be identified and their co-regulation during SCW formation elucidated. The crucial dependence on precursor metabolism provides an opportunity to alter xylan modification patterns through metabolic engineering of one or more of these interdependent pathways. The complexity of xylan biosynthesis and modification is currently a stumbling point, but it may provide new avenues for woody biomass engineering that are not possible for other biopolymers.