Environmental Engineering Reference
In-Depth Information
and RNA interference (RNAi), and is therefore a requirement for a modern model system. Plans for
the development of Brachypodium as a model to accelerate the domestication of grasses for use as
biomass crops (e.g., switchgrass and Miscanthus) were spelled out in a U.S. Department of Energy
report on the research needed to establish a domestic biofuel industry (DOE 2006). As a result,
JGI approved a proposal to sequence the Brachypodium genome with a whole-genome shotgun
sequencing strategy through their Community Sequencing Program for 2007. In May 2009, JGI
released the final 8× sequence and the version 1.0 annotation of the Brachypodium genome. The
assembly incorporates all of the mapping and BAC sequence resources and has been shown to be
of very high quality: Gaps in the assembly represent only approximately 0.4% of the genome, and
Illumina EST data support more than 92% of the predicted protein-coding genes (International
Brachypodium Initiative 2010). Through the Community Sequencing Program for 2009 (http://
www.jgi.doe.gov/sequencing/cspseqplans2009.html), JGI has approved the resequencing of six
additional Brachypodium lines using next-generation sequencing platforms. The information
gained through this project will permit comparisons of the genetic diversity of phenotypically
distinct Brachypodium lines and assist in identifying the genetic basis for phenotypes critical to the
improvement of bioenergy and cereal crops as well as many other research interests.
23.5 BrachyPodIum as a model For BIoenerGy Grasses
In the development of biofuel feedstocks, two key considerations are biomass quantity and biomass
quality (Carpita and McCann 2008). Biomass quantity refers to the amount of feedstock produced
per unit of land. Biomass quality reflects the efficiency with which the feedstock can be converted
to the desired biofuel. Biomass quality encompasses feedstock recalcitrance and the feedstock's
suitability for the conversion process used. Thus, plant materials that exhibit less resistance to
degradation or contain larger relative amounts of easily fermentable sugars are of higher “quality.”
Brachypodium's typical grass characteristics, together with the many resources available for its
study, make Brachypodium a suitable system for investigating the factors that contribute to the
quantity and quality of feedstocks.
23.5.1 f
actorS
i
influEncing
B
iomaSS
q
uantity
: p
lant
a
rchitEcturE
Genetically influenced traits such as growth habit, stem density, and plant height affect the quantity
of biomass that can be harvested. The small size and ease of growth of Brachypodium will facilitate
the application of forward and reverse genetic approaches to study these traits. In addition, natural
variation in these traits has been demonstrated in recently cataloged collections of Brachypodium
accessions, providing researchers with another tool to investigate phenotypes of interest for
increasing biomass (Vogel et al. 2009). For example, some accessions have an erect growth habit,
whereas others exhibit extensive spreading under the same environmental conditions (Figure 23.3).
This trait is relevant to biomass quantity; more erect plants can be planted at higher densities and
may be less subject to crop losses due to lodging.
Plant height is another major contributor to biomass yield. Assuming there are no accompanying
deleterious characteristics, taller plants will have more useable biomass. For example, a
comprehensive study of biomass traits in sorghum recombinant inbred lines found a positive and
highly significant correlation between plant height and biomass yield (Murray et al. 2008). Pathways
associated with the growth-promoting hormones gibberellins and brassinosteroids are potential
targets for increasing plant height (Fernandez et al. 2009). The idea of substantially improving yield
by altering phytohormone pathways has a historical precedent: Semi-dwarf rice and wheat varieties
were key components of the “Green Revolution” of the 1960s and 1970s, during which the grain
yields of cereal crops in developing countries increased dramatically (Evans 1993; Conway 1997;
Hedden 2003). These dwarf varieties contained a loss-of-function mutation in a gibberellin (GA)
biosynthetic gene in rice (Sasaki et al. 2002; Spielmeyer et al. 2002) and a gain-of-function mutation
