Environmental Engineering Reference
In-Depth Information
species being transformed. Metabolic engineering requires knowledge of the existing pathways
within the plant, which are best defined at the genomic level. The use of techniques such as zinc-
finger nucleases may allow specific modification of plants to produce superior biofuel crops by the
addition of new genes (Shukla et al. 2009) or modification of existing ones (Townsend et al. 2009).
Thus, genomics has accelerated the discovery of genes that might be manipulated to produce supe-
rior bioenergy crops.
2.11 Promoters and control oF exPressIon
Knowledge of regulatory systems at the genome level will be important for breeding and selecting
superior crop varieties and especially for the precise control of important transgenes. The manipu-
lation of regulatory genes may be an important path to significant gene improvements in energy
crops. The production of transgenic bioenergy crops with useful transgenes will require the use of
gene promoters that direct transgene expression, at appropriate levels, in the required tissue and at
the necessary time during plant development. The species specificity of these processes (Furtado
et al. 2008) will require detailed analysis and understanding of regulatory processes controlling
gene expression in target species. Exploiting our growing knowledge (Held et al. 2008) of the role of
small RNA molecules in the regulation of plant performance and composition may also be impor-
tant in the development of bioenergy crops.
2.12 model BIoenerGy croPs
Several crops for which genomics tools are available are model systems for analyses of the processes
required to adapt plants to be better energy sources. Although not an energy crop,
Arabidopsis
(a general plant model) and rice (a grass species model) models are very useful for research on
energy crops. The rice genome is now very well documented and characterized. Sequencing of many
genotypes and of related species in the genus is increasing genomic knowledge of rice. However, the
sorghum plant is a much better model for other C
4
bioenergy species such as sugarcane and maize
than are C
3
plants like rice. Maize has been proposed as a useful model genome for energy crops
in the grasses (Lawrence and Walbort 2007).
2.13 GenomIcs oF sPecIFIc BIoenerGy sPecIes
Plant genomes are generally large compared with other organisms (Table 2.2). However, recent
advances in DNA sequencing technology have greatly increased the rate at which plant genomes
are being characterized.
2.13.1 S
orghum
Sorghum is a very efficient C
4
plant with good biomass potential and tolerance to hot and dry
environments. Sorghum was domesticated as a food crop and is used today mainly as a source of
animal feed. The development of sorghum as an energy crop will require selection for increased
plant biomass rather than grain yield. This could lead to a significant change in plant architecture.
The genome sequence of sorghum has been reported by Paterson et al. (2009). Sweet sorghum has
been identified as a source of biomass for first-generation biofuel production. However, the develop-
ment of high-biomass sorghum genotypes may provide an important source of biomass for biofuel
production in the environments to which sorghum is adapted. Sorghum can be produced in locations
without the water that might be required for other energy crops such as maize or sugarcane. The
availability of the genome sequence and other genomic resources in sorghum should facilitate the
rapid development of these improved types.
