Genetic modifications for improvement of specific traits or the addition of new
traits to economically important plants is a major objective worldwide. Not only is
cellulose a constituent of all plants, a number of plants (such as cotton and forest
trees) are grown specifically for their cellulose content. In general, the objective of
genetic manipulation of the cellulose synthesizing capacity in these plants is to
either increase the amount of cellulose or modify the physical properties of the
cellulose during synthesis. For example, the secondary cell wall in cotton fibers
determines the fiber properties. Considering that the secondary cell wall in cotton
fibers is approximately 95% cellulose, the properties of the cotton fiber are dependent
not only on the amount of cellulose deposited, but also on other features such
as the structure and orientation of the cellulose microfibrils and the degree of polymerization of the glucan chains. Additionally, manipulation of cellulose
synthesis in a number of crop plants may be important for improving specific
agronomic traits. As an example, stalk lodging in maize results in significant yield
losses, and an increase in the cellulose content in the cells in the stalk may allow
improvements in stalk strength and harvest index (Appenzeller et al.
, 2004). Apart
from its importance in the growth and development of plants, cellulose is also an
abundant renewal energy resource that is present in the biomass obtained from
agricultural residues of major crops. Corn stover is the most abundant agriculture
residue in the United States and it can be used for various applications including
bioethanol production (Kadam and Mcmillan, 2003). Increasing the content of
cellulose and reducing the lignin content of corn plants is therefore considered to
be beneficial for ethanol production.
Cellulose biosynthesis in plants can be modified by manipulation of the
cellulose synthase (CesA
) genes or other genes that influence cellulose production. CesA
genes have been identified in most plants, and as a result they are prime
targets for directly modifying cellulose synthesis by genetic manipulation. CesA
genes are part of a gene family, and as a result a number of features of these genes
will have to be analyzed before they can be manipulated usefully. Some of these
features may include understanding of the expression of the different CesA
the redundant nature of each gene in a specific cell type, and the phenotype that is
generated when each gene is mutated or overexpressed (Holland et al.
In corn, the majority of the cellulose in the stalk is in the vascular bundles. Based
on their expression patterns, 3 of the 12 CesA
genes in corn appear to be involved
in cellulose synthesis during secondary wall formation and their promoter
sequences have been identified (Appenzeller et al.
, 2004). These promoters can
now be used for expression of CesA
genes in specific cell types for increasing their
Direct modification of cellulose content by manipulation of the cellulose
synthase genes has been performed in only a few cases so far. To improve fiber
quality of cotton fibers, the A. xylinum
acsA and acsB genes were transferred to
cotton (Li et al.
, 2004). The fiber strength and length of fibers were found to be
greater in the transformed plants, as well as the cellulose content was found to be
higher in the transformed plants as compared to the control plants. In potato,
cellulose content was modified in the tuber using sense and antisense expression
of the full length StCesA
3 and class-specific regions (CSR) of the four potato CesA
cDNAs (Oomen et al.
The antisense and sense StCesA
demonstrated that the cellulose content could be decreased to 43% and increased
to 200% of the wild type, respectively, by modifying the RNA expression levels
(Oomen et al.
, 2004). Interestingly, the increase in cellulose content by increasing
expression of a single CesA
gene was found to be remarkable considering that
multiple copies of different CesA
s are believed to be required for assembly of
cellulose-synthesizing complexes. The utility of antisense transgenic lines in
generating a range of phenotypes is suggested to be particularly useful, especially
where null mutations are potentially lethal (Oomen et al.
, 2004). In Arabidopsis
the transgenic approach using antisense expression exhibited a slightly
different phenotype as compared to a mutation in the corresponding gene (Burn et al.
, 2002a). The modulation of CesA
RNA expression levels and concomitantly
cellulose content has also been demonstrated in tobacco plants using virusinduced
silencing of a cellulose
synthase gene (Burton et al.
, 2000). Apart from the CesA
genes, genes with an indirect role in cellulose biosynthesis, such as the
sucrose synthase, have been manipulated in the cotton fiber using suppression
constructs. A 70% or more suppression of the sucrose synthase activity in the
ovule led to a fiberless phenotype suggesting that this enzyme has a rate-limiting
role in the initiation and elongation of fibers (Ruan et al.
, 2003). In other instances,
while some researchers have shown an increase in cellulose accumulation following
manipulation of genes for reduced lignin synthesis in aspen trees (Hu et al.
1999; Li et al.
, 2003a), other researchers did not find any evidence in support of
enhanced cellulose synthesis upon severe downregulation of lignin biosynthetic
genes (Anterola and Lewis, 2002). It is believed that the synthesis of cellulose
is interconnected with the synthesis of other components of the plant cell
wall, and manipulation of a number of genes would therefore affect cellulose
production. However, not much is known as to how the different pathways are
interconnected, but a systems view of these interactions is beginning to emerge
(Somerville et al.