Cell Wall Engineering Group
Currently, biofuels, such as ethanol are produced largely from starch that comes from grains, but it represents only a little proportion of sugar polymer availability on Earth. Large quantities of sugar are present in plant cell wall since the main constituents (70-95%) are cellulose and hemicellulose polymers. Therefore they represent extensive resources of fermentable sugars for BioEnergy production. The rest of the plant cell walls (5-30%) is mainly composed of lignin which is a very strong aromatic polymer embedding cell wall polysaccharides. It confers a strong recalcitrance to biomass and inhibits efficient extraction and hydrolysis of cell wall polysaccharides, thus it prevents cost-effective lignocellulosic biofuels production. Unfortunately, lignin provides compressive resistance to plant cells and cannot simply be genetically removed without incurring deleterious consequences on plant productivity. Lignin gives a strong structural support to plants and also protects them against some of biological, chemical and physical stresses. Therefore, it is important to develop tools and strategies that will be used to control spatially and temporarily cell wall modifications.
Our mission at JBEI is to create several “universal” approaches that can be used to engineer various energy crops belonging to Monocot and Dicot plant classes with the aims to reduce their cell wall recalcitrance to pretreatments and increase sugar yields without impacting plant growth and development. We use and develop synthetic biology tools and test them on model plant species such as Arabidopsis and Rice prior implementing them on energy crops.
Redesigning the lignin distribution in the plant biomass to improve sugar recovery from plant cell wall without impacting plant growth and development.
During plant development, vessels have a key role and allow water and nutrients to be transported from the root system to aboveground organs. Several secondary cell wall mutants have been characterized as irregular xylem mutants caused by collapse vessels (Figure 1) and regularly show growth defects. Therefore we designing strategies and to modify lignin composition or and composition in target tissues (Figure 2).
Figure 1: Impact of lignin reduction. A, Picture of wild type and lignin mutant (Arabidopsis plant) showing the impact of lignin reduction on plant development, B Arabidopsis stem cross-sections showing collapsed vessels in lignin mutant.
Figure 2: Modification of lignin composition/content in interfascicular fibers. Arabidopsis stem cross-sections from wild type and a lignin re-engineered line
Rewiring regulatory network of secondary cell wall to enhance polysaccharide deposition without enhancing lignin deposition
Regulatory networks are like electrical wiring and plant secondary cell wall deposition are key switches, represented by master transcription factors that control cellulose, hemicellulose and lignin biosynthesis. Unfortunately, this network is not fully understood and is rather complicated since some of these key switches have commune downstream biosynthetic pathways but regulate them in specific tissues. Therefore we are developing approaches to disconnect some of the metabolic pathways from key transcription factors to manipulate directly or indirectly cell wall composition and content in target tissues (Figure 3).
Figure 3: Rewiring of regulatory networks. Strategies used to re-engineer secondary cell wall biosynthesis of bioenergy crops. The drawing represents the disconnection of a pathway from its native regulatory network and the integration of new regulatory route as well as the grafting of a new biosynthetic pathway for the production of a new component.
Engineering the biosynthesis pathways to produce “novel” lignin monomers in order to modify lignin composition and its biochemical and biophysical properties to reduce its recalcitrance to pretreatment
Most of the strategies aiming to reduce lignin content using silencing approaches to reduce enzymatic steps within the phenylpropanoid biosynthetic pathway (also called lignin pathway) have been associated with undesired phenotypes affecting plant growth, biomass yield and stress tolerance. Therefore, instead of reducing the lignin content, we are developing alternative strategies to decrease cell wall recalcitrance by modifying lignin composition. We are testing approached to modify the polymerization degree of lignin by incorporating specific lignin monomers and to incorporate weak bond into the lignin polymers in order to facilitate its fragmentation during pretreatments.
Identification of transporters mediating export of monolignols or/and other lignin monomers in order to modify the composition and distribution of lignin as well as to optimize lignin engineering strategies.
Lignin polymers are formed in the apoplast and are the result of the condensation of monolignols (H, G and S units) at 90%, the rest being part of unusual monomers. Monolignols derives from the phenylpropanoid biosynthetic pathway, which occurs in the cytosol, and get released into the apoplast via an unknown export mechanism. Since the monomeric composition of the lignin plays an important role for the cell wall recalcitrance and since some of our lignin engineering strategies focus on the modification of its composition, we are currently trying to identify plant proteins that mediate lignin monomer export. We are using a yeast complementation approach and are screening several transporters that could potentially transport secondary metabolites.
Fig 4: Yeast complementation. The drawing represents a yeast screen on the presence of toxic level of a selected lignin monomer. Yeast cells expressing a transporter able to export the toxic compound out of it cytosol allow the growth on the media containing it.
