Decreasing Biomass Recalcitrance by Targeted Modification of Plant Cell Walls

Overview

The goal of the plant cell wall polysaccharide synthesis and structure group is to use targeted modification of plant cell walls (biomass) to facilitate improved conversion of biomass to fermentable sugars.

Our studies will emphasis wall synthesis and structure in:

Populus trichocarpa, a model woody crop for biomass and bioenergy production (Tuskan 1998), which has extensive genetic and genomic resources available as a result of sequencing the P. trichocarpa genome (Tuskan et al. 2006).

Switchgrass (Panicum virgatum L.), a model herbaceaous crop because of its high yields, drought tolerance, and ability to grow well on marginal land (McLaughlin and Kszos 2005).

To address recalcitrance we will:

          Develop systems biology models for cell wall biosynthesis pathways.

          Modify hemicellulose biosynthesis and identify new wall biosynthesis enzymes.

          Use chemical, immunological, and spectroscopic methods to relate recalcitrance to wall structure/architecture.

Related BESC studies will:

          Identify factors that regulate cellulose biosynthesis (ORNL).

          Reduce recalcitrance by modifying lignin biosynthesis (Noble Foundation).

          Generate Populus with less recalcitrant cell walls (ORNL).

          Generate switchgrass with less recalcitrant cell walls (UGA, Noble Foundation).

Why target Plant Cell Walls? (a glossary of terms is provided here)

Some of the structural features of lignocellulosic biomass (plant cell walls) that make it difficult to enzymically deconstruct are:

          The crystalline nature of cellulose

          The presence of hemicelluloses that coat cellulose fibers

          The presence of lignin

          The multi-layered architecture of cell walls

Thus, to reduce or eliminated biomass recalcitrance requires understanding how polysaccharides and lignin are synthesized and then assembled into a functional cell wall.

Plant cells form two types of wall that differ in function and composition:

Primary walls (see Figure 1A) . These walls surround growing and dividing plants cells. They provide mechanical strength but must expand to allow the cell to grow and divide. Dicot (e.g. Populus) primary walls contain approximately equal amounts cellulose, hemicellulose, and pectin. Hemicellulose is more abundant than pectin in grass (e.g switchgrass) primary walls.

Secondary walls (see Figure 1B) . The much thicker and stronger secondary walls, which account for most of the polysaccharides in biomass, are deposited after the cells cease to grow. The secondary walls of xylem fibers, tracheids and sclereids are further strengthened and made even more recalcitrant by the incorporation of lignin. Dicot and grass secondary walls are composed predominantly of cellulose, hemicellulose, and lignin.

At least 2000 genes are believed to be required for cell wall synthesis and assembly. Less than 5% of these genes have been functionally characterized.

Cell wall polysaccharides are synthesized by glycan and glycosyl transferases using diverse nucleotide sugars (NDP-sugars, see Figure 1).

Cellulose is synthesized at the plasma membrane and then inserted directly into the cell wall. Hemicellulose and pectin are synthesized in the Golgi complex, transported in intracellular vesicles that fuse with the plasma membrane and than inserted into the cell wall (Fig.1).

Genes/proteins involved in nucleotide sugar (Ridley et al. 2001; Harper and Bar-Peled, 2002), cellulose, hemicellulose (Farrokhi et al. 2006) and pectin (Sterling et al. 2006) synthesis have been identified, but most remain to be functionally characterized. Little is known about how polysaccharide synthesis is coordinated to allow the formation of functional cell walls or how cell wall synthesis differs in the approximately 40 different cell types present in plants.

Our knowledge of lignin formation is more advanced, but not complete (Boerjan, et al. 2003; Chen et al 2006). Monolignol precursors are synthesized in the cytoplasm and transported to the cell wall where they are oxidized by peroxidases and laccases and then polymerized to form lignin.

There are numerous genes encoding transcription factors that regulate suites of cell wall biosynthetic genes, as well as genes involved in the transport and assembly of polymers into the cell wall and in remodeling wall architecture and cross-linking of wall polymers.

Modifying the activity of any of these genes or genes involved in polysaccharide biosynthesis may lead to walls with altered structure/architecture and reduced or even eliminate recalcitrance.