Brachypodium - Bioenergy Crop Plants - page 612

monocots, including the grasses, have type II primary cell walls. Dicots, such as Arabidopsis , and
noncommelinoid monocots, including orchids and lilies, have type I primary cell walls. Cell wall
composition varies from species to species and even among cell types within a single plant (Knox
2008; Popper 2008). Although the primary and secondary walls of both grasses and dicots contain
cellulose, the major hemicellulose is glucuronoarabinoxylan (GAX) in grasses and xyloglucan in
dicots. Grass cell walls also contain ferulic acid and ρ-coumaric acid, hydroxycinnamate compounds
that are only minor components of most dicot walls. Ferulate-mediated cross-linking of GAX and
lignin decreases the digestibility of monocot walls (Grabber 2005) with direct implications for
biofuel production. In addition to the differences in hemicellulose and hydroxycinnamates mentioned
above, grass primary walls, unlike their dicot counterparts, incorporate mixed linkage (β1,3- and
β1,4-linked) glucans and contain few structural proteins and little pectin compared with the large
amounts of pectin—up to 35% dry weight—found in type I walls (Vogel 2008). Supporting the use
of Brachypodium as a model for grass cell walls is the finding that the monosaccharide profiles of
Brachypodium, wheat, barley, and Miscanthus cell walls are similar to each other but different from
the profile of Arabidopsis (Gomez et al. 2008).
Cell wall composition is critical in the context of the biofuel production process. The conversion
of lignocellulosic biomass into liquid fuels generally proceeds via four steps: (1) mechanical and/or
thermochemical pretreatment of feedstocks to make the cell wall components more accessible, (2)
enzymatic hydrolysis of the pretreated biomass to release sugars from the carbohydrate polymers
of the wall, (3) microbial fermentation of the released sugars to produce liquid fuels, and (4)
recovery of the biofuels from the fermentation medium (for example, by distillation) (Himmel et al.
2007; Wyman 2007; Kumar et al. 2008). The first two steps—pretreatment and hydrolysis—are
necessary to overcome the recalcitrance of plant materials (i.e., their resistance to degradation).
Recalcitrance is arguably the largest obstacle to the economical, efficient, and environmentally
friendly production of biofuels (Himmel et al. 2007; Wyman 2007). There are various pretreatment
options, including treatment with dilute acid, washing with large volumes of hot water, and ammonia
fiber expansion (Wyman et al. 2005). Although each pretreatment method has distinct advantages
and disadvantages, pretreatments in general require substantial inputs of energy, chemicals, or
other resources (Wyman et al. 2005). Also, the production and use of hydrolytic enzymes can
be both expensive and limiting (Wyman 2007; Kumar et al. 2008). Additional considerations
include the possibility that pretreatments can release or produce inhibitors of fermentation (Himmel
et al. 2007) and that the most commonly used fermentative microbes—primarily the budding
yeast Saccharomyces cerevisiae , but also species such as the anaerobic bacterium Clostridium
thermocellum —use only six-carbon sugars such as the glucose monomers of cellulose, not five-
carbon sugars such as the xylose found in hemicelluloses (Demain et al. 2005; van Maris et al.
2006). From a feedstock development perspective, biomass recalcitrance might be decreased in
a number of ways, e.g., by altering the structure or cell-cell-adhesion properties of plant tissues
to allow greater access by chemicals and hydrolytic enzymes or by reducing the degree of cross-
linking of the cell wall components, the crystallinity of cellulose, or the amount of lignin. Increasing
the amount of cellulose or the percentage of hexoses in other cell wall polymers could also improve
production efficiency by providing more substrates for microbial fermentation.
The resources available for Brachypodium will be useful for addressing these issues. Because
cell walls support and protect plants, there is always the concern that modifying the wall will
negatively affect plant fitness. In this respect, the natural variation documented for Brachypodium
could prove especially valuable. If an accession with improved digestibility and/or fermentability
can be found, its cell wall characteristics could reveal new avenues for altering wall composition and
structure without severely decreasing viability. In a complementary approach, the effects of targeted
modifications or randomly induced mutations can be analyzed more rapidly in Brachypodium
than in grasses with longer generation times. However, to identify naturally occurring or mutant
plants with desirable cell wall traits, new screening procedures must be implemented. As a first
step, Gomez et al. (2008) showed that treatments with hot, dilute sulfuric acid resulted in limited