Brachypodium - Bioenergy Crop Plants - page 594

23.1 the need For a model Grass
In a 2005 feasibility study, the U.S. Departments of Energy and Agriculture predicted that within
four decades, the United States could sustainably produce over 1 billion dry tons of plant biomass
annually for the generation of energy and other products (DOE and USDA 2005). Crop and other
residues from grasses, including 256 million tons of corn stover and 52 million tons of wheat straw,
represent approximately 348 million tons of this total. An additional 368 million tons is expected to
come from the cultivation of perennial grasses and trees as dedicated bioenergy crops (DOE and USDA
2005). Switchgrass ( Panicum virgatum ) and Miscanthus ( Miscanthus × giganteus ) have emerged as
particularly attractive potential energy crops (DOE 2007; Dohleman and Long 2009). Given that
the grasses being considered as energy crops are essentially undomesticated wild selections, there is
considerable potential for improving them. In addition, with the exception of forest trees, many of the
traits desirable in an energy crop (e.g., thicker stems, more cell walls) have not been selected for in
traditional crops, in which, for the most part, breeding has focused on reproductive organs or digestible
leaves, tubers, or stems. Unfortunately, breeding the grasses proposed as energy crops is complicated
by their reproductive strategy (self-incompatibility or sterility) which prevents the development of
inbred lines, selfing, etc. Basic research on the biology of grasses could be used to design rational
approaches to breeding superior energy crops and to accelerate the domestication of these new crops.
The most rapid way to gain this basic knowledge is through the use of an appropriate model system.
Arabidopsis thaliana serves as an extremely powerful generalized plant model; however, it is
not suitable to study many aspects of grass biology because of the biological differences that have
arisen between dicots and monocots in the 150 million years since they last shared a common
ancestor. One example that is particularly relevant to the need for a model for bioenergy crops is
the dramatic difference between grass and dicot primary cell walls in terms of the major structural
polysaccharides present, how those polysaccharides are linked together, and the abundance and
importance of pectins, proteins, and phenolic compounds (Carpita 1996; Vogel 2008). A partial
list of additional areas in which Arabidopsis is not an appropriate model for the study of grasses
includes mycorrhizal associations, architecture of the grass plant, grain properties, intercalary
meristems, and grass development.
The tremendous importance of grasses as food, feed, and, increasingly, as fuel argues strongly for
the development of a truly tractable grass model system. Rice, with its sequenced genome and large
research community, at first would seem to fill this need. Upon closer examination, the demanding
growing conditions, large size, and long generation time of rice greatly increase the difficulty
and expense of conducting high-throughput functional genomic experiments. Furthermore, the
semiaquatic and tropical nature of rice limits the applicability of rice as a model for temperate grasses,
especially in areas such as freezing tolerance and vernalization. Brachypodium distachyon (hereafter
referred to as Brachypodium) is well suited to meet the need for an experimentally tractable model
for the grasses. In this chapter, we provide a general introduction to Brachypodium, details about
using Brachypodium as a model, a summary of genomic resources available for Brachypodium, and
examples of how Brachypodium can be applied to the development of grasses as energy crops.
23.2 IntroductIon to B. diStAchyon
The utility of Brachypodium as a model system for the study of grasses was discussed in a 2001
paper that indicated that Brachypodium possesses the biological, physical, and genomic attributes
required for use as a model system (Draper et al. 2001). The small size and rapid generation time of
Brachypodium enable high-throughput studies. Large numbers of plants (1000 plants/m 2 ) can easily
be grown in growth chambers or greenhouses, allowing studies to be conducted under controlled
environmental conditions. For comparison, the same space accommodates only 50 wheat plants,
36 rice plants, 13 sorghum plants, 6 maize plants, 6 switchgrass plants, or 2 Miscanthus plants
(Rayburn et al. 2009) (Table 23.1 and Figure 23.1, a-c). As a group, the grasses are notorious for very