Prospects for Genetic Engineering of Cellulose Biosynthesis in Plants

Manipulation of Cellulose Biosynthesis in Plants
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 genes, 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., 2000). 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 cellulose content.

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 StCesA3 and class-specific regions (CSR) of the four potato CesA cDNAs (Oomen et al., 2004). The antisense and sense StCesA3 transformants 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 CesAs 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., 2004).