Environmental Engineering Reference
In-Depth Information
27.5 a tree For lIGnocellulosIc ethanol
Fossil fuels have been the dominant energy resource for the modern world. The major limitations of
solid biomass fuels are the difficulty of handling and the lack of portability for mobile engines. To
address these issues, research is being conducted to convert solid biomass into liquid and gaseous
fuels. Biological (fermentation) and chemical means (pyrolysis and gasification) can be used to
produce fluid biomass fuels. Ethanol for automotive fuels is currently produced from starch biomass
in a two-step process: starch is enzymatically hydrolyzed into glucose, and then yeast is used to
convert the glucose into ethanol. The first four aliphatic alcohols (methanol, ethanol, propanol,
and butanol) are of interest as fuels because they can be synthesized biologically and they have
characteristics that allow them to be used in current engines. Ethanol is nontoxic, water-soluble, and
quickly biodegradable. Blending ethanol in gasoline dramatically reduces carbon monoxide tailpipe
emissions. Carbon monoxide emissions are responsible for as much as 20% of smog formation.
27.5.1 B
iomaSS
p
roduction
and
m
olEcular
B
iology
of
c
ElluloSE
Trees constitute most lignocellulosic biomass existing on our planet. Trees also serve as important
feedstock materials for various industrial products. Wood from forest trees modified for more
cellulose or hemicelluloses could be a major feedstock for fuel ethanol. Xylan and glucomannan
are the two major hemicelluloses in wood of angiosperms. However, little is known about the genes
and gene products involved in the synthesis of these wood polysaccharides. Further, much research
at present is being directed to understand regulatory mechanisms of cellulose synthase (CesA)
genes of trees. In the production of cellulose fiber materials, it is highly desirable to engineer trees
with more cellulose and controllable cellulose properties such as degree of polymerization and
crystallinity. However, little is known about the genes controlling wood cellulose formation. The
discovery of differential expression of three secondary cell-wall-related CesA genes in response to
tension stress and the identification of an mechanical stress-responsive element (MSRE) containing
a DNA fragment in the EgraCesA3 promoter provide an important clue for the future improvement
of cellulosic material production in trees (Lu et al. 2008).
Genetic improvement of cellulose biosynthesis in woody trees is one of the major goals of tree
biotechnology research. However, progress in this field has been slow because of (1) unavailability of
key genes from tree genomes; (2) the inability to isolate active and intact CesA complexes; and (3) the
limited understanding of the mechanistic processes involved in the wood cellulose development.
Recent advances in molecular genetics of CesA from aspen trees suggest that two different types
of CesA are involved in cellulose deposition in primary and secondary walls in xylem (Joshi 2003).
The three distinct secondary CesA from aspen—PtrCesA1, PtrCesA2, and PtrCesA3—appear
to be aspen homologs of
Arabidopsis
secondary CesAs, AtCesA8, AtCesA7, and AtCesA4,
respectively, on the basis of their high identity/similarity (>80%). These aspen CesA proteins share
the transmembrane domain (TMD) structure that is typical of all known “true” CesA proteins: two
TMDs toward the N-terminal and six TMDs toward the C-terminal. The putative catalytic domain
is present between TMDs 2 and 3. All signature motifs of processive glycosyltransferases are also
present in this catalytic domain. In a phylogenetic tree based on various predicted CesA proteins
from
Arabidopsis
and aspen, aspen CesAs fall into families similar to those seen with
Arabidopsis
CesAs, suggesting their functional similarity. The coordinate expression of three aspen secondary
CesAs in xylem and phloem fibers and their simultaneous tension stress-responsive upregulation
suggest that these three CesA may play a pivotal role in the biosynthesis of better quality cellulose
in the secondary cell walls of plants. These results are likely to have a direct effect on the genetic
manipulation of trees in the future.
Further, because lignin limits the use of wood for fiber, chemical, and energy production,
strategies for its downregulation are of considerable interest. Transgenic aspen trees, in which
expression of a lignin biosynthetic pathway gene
Pt4CL1
encoding 4-coumarate:coenzyme A ligase
