From Plant Cell Walls to Biofuels—Arabidopsis thaliana Model - Bioenergy Crop Plants - page 82

in lignin biosynthesis in inflorescence of A. thaliana by providing coniferyl and sinapyl alcohols
(Sibout et al. 2005). These facts are supported by a limp floral stem phenotype exhibited by the
mature Arabidopsis CAD double mutant ( cad-4 cad-5 ), and these mutants show a modified pattern
of lignin staining. These results suggest that AtCAD4 and AtCAD5 genes do possess a developmen-
tal role in A. thaliana . In a recent gene functional analysis study, AtCAD1 was been shown to play
a role in the lignification of elongating stems in Arabidopsis (Eudes et al. 2006).
5.2.4 c ElluloSE m icrofiBrilS
Secondary xylem or wood accounts for most of the biomass in trees, 42-50% of which is cellulose.
Other main components of wood are hemicellulose (25-30%) and lignin (20-25%) (Suzuki et al.
2006). This biomass is one of the major carbon sinks on Earth and provides renewable material
that can be used for manufacturing or as an energy source. The latter, in the context of conversion
to biofuels, has recently received increased attention, with cellulose being the most abundant and
valuable material for conversion processes. Therefore, understanding the biosynthesis of cellulose
along with the other cell wall components is very important.
Cellulose is composed of linear chains of β-1,4-linked glucans that are synthesized by cellulose
synthase (CesA) complexes at the plasma membrane. Parallel glucan chains can form extensive
hydrogen bonds between each other, which gives rise to cellulose microfibrils. Each microfibril
is predicted to contain 36 crystalline glucan chains (Mutwil et al. 2008; Taylor 2008). Cellulose
microfibril orientation in the cell wall is critical to the strength of the walls (Somerville 2006).
5.2.4.1 cellulose synthase Genes
Cellulose synthase genes ( CesAs ) are presumed to encode catalytic subunits of cellulose synthase,
which is part of the enzyme complex that is responsible for the synthesis of cellulose. Freeze-
fracture electron microscopy of plasma membranes allows visualization of cellulose synthases
within rosettes of six intramembrane protein aggregates (Kimura et al. 1999). Each subunit synthe-
sizes β-1,4-glucan chains, which then crystallize to form cellulose microfibrils. These microfibrils
provide the main strengthening components of plant cell walls, helping to maintain turgor pressure
and controlling the extension direction of a plant cell (Somerville 2006).
The first plant CesA gene was identified in cotton ( Gossypium hirsutum ) fiber by subdomain
similarity to a bacterial gene with low sequence similarity, and CesA proteins were predicted to be
membrane-bound proteins (Pear et al. 1996). Availability of a genomic database and mutant col-
lections in Arabidopsis allowed further genetic identification and characterization of CesA genes in
Arabidopsis (Arioli et al. 1998; Somerville et al. 2004). Arabidopsis has 10 CesA genes (Richmond
2000), some of which are required for primary wall cellulose synthesis ( AtCesA1 , AtCesA3 , and
AtCesA6 ) and some of which are required for secondary wall cellulose synthesis ( AtCesA4 , AtCesA7 ,
AtCesA8 ) (Doblin et al. 2002). Although the evidence to date strongly suggests that the CesA pro-
teins are β-1,4-glucosyl transferases, the precise biochemical function and activity of CesA genes in
what may be a multistep cellulose synthetic process still remain to be unambiguously determined
(Doblin et al. 2002).
5.2.4.2 effects of mutations on cesA Genes
Mutational analysis and recent advancements have given us clues about how cellulose synthesis
is regulated. Null mutants of AtCesA1 and AtCesA3 are lethal to embryos, indicating they are not
redundant with one another (Persson et al. 2007b). Null mutations for AtCesA6 showed reduced
cellulose synthesis and anisotropic cell swelling (MacKinnon et al. 2006). AtCesA2, AtCesA5, and
AtCesA9 are partially redundant with AtCesA6 during the different stages of cellular development
(Desprez et al. 2007). Together, these data provide evidence that CesA1 and CesA3 are invariable
components of the primary wall cellulose synthase complexes where other CesAs (CesA2, 5, 6, 9)
may substitute for each other in the complex formation (Desprez et al. 2007).