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and lead to broad systemic alterations to the biology of the organisms. Due to this complexity they cannot be
easily engineered into either
Saccharomyces cerevisiae or
Escherichia coli (Montague et al. 2012).
Another important species that may serve as platform cell factory is
Corynebacterium glutamicum (Becker et
al. 2013). It has been suggested as a model organism for synthetic biology as it is capable of producing a
variety of valuable chemicals and materials, and is already applied in the industrial production of amino acids
(Woo and Park 2014). A number of approaches to optimise
Corynebacterium glutamicum, including the
development of standard DNA parts, DNA/RNA parts, and devices (e.g. to sense metabolites) are discussed by
Woo and Park (2014). In general, strain development is an important application for synthetic biology in
microbes (as for example the generation of optimised
Corynebacterium glutamicum strains for the production
of amino acids or even the establishment of multi-use platform strains) (Wendisch 2014).
The following potential environmental applications of designed microorganisms have been identified in CBD
(2014); the expected benefits are that they could provide less toxic and more effective tools for
bioremediation, which would positively impact local biodiversity (CBD 2014):
enhance mining metal recovery and to aid in acid mine drainage bioremediation (Brune and Bayer
2012).
design whole-cell biosensors that will indicate the presence of a target, such as arsenic in drinking
water.
design an arsenic biosensor that would be suitable for field use in developing countries, using freeze-
dried transformed
Escherichia coli that changed colour in the presence of arsenic (French et al. 2011).
engineering
Escherichia coli to secrete auxin, a plant hormone intended to promote root growth (French
et al. 2011; WWICS 2013b).
pre-coating seeds with the bacteria, to be planted in areas at risk from desertification (French et al.
2011; WWICS 2013b).
Degradation of herbicides by “reprogrammed”
Escherichia coli.
4.2
Synthetic biology in other species
There are also attempts to use hosts beyond bacteria and yeast, e.g. to produce spider silk (dragline silk
protein) in the milk of transgenic mice (Xu et al. 2007). The animal host was more efficient as compared to
microbial ones in expression and homogeneity.
Recently, synthetic biology has also been suggested as providing new strategies for therapeutic applications
(Ye and Fussenegger 2014). For this, for example gene circuits are assembled into biosensing devices. The
designed circuits monitor, quantify, and treat diseases by sensing disease signals, and producing and releasing
tailor-made therapeutic molecules. Another recent development aims at regulating gene drives that influence
reproductive capacity (Oye et al. 2014). Potential applications are the elimination of mosquito-borne diseases
like malaria and dengue, reversing the development of pesticide and herbicide resistance, and the local
eradication of invasive species. However, it has been acknowledged that this development has substantial
implications concerning environmental and security aspects, and risk management (Oye et al. 2014).
4.3
Synthetic biology in plants
Synthetic metabolic pathways may be designed for the expression in heterologous hosts, in particular
microorganisms. However, for a range of applications plants will be better suited, e.g. for the large scale
production of compounds or evidently when targeting cell walls for more efficient production of biomass.
Concomitant with recent advances in plant biotechnology, the field benefits from methods for synthetic
biology that have already been successfully applied by the microbial biotechnology community. In particular
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DNA assembly techniques are also adopted by plant biologists (Patron 2014). However, the progress in
synthetic biology in plants is slower compared to that in microbial systems (O'Connor and Brutnell 2014).
In the plant field,
Arabidopsis thaliana is the model organism, and a plethora of information on gene function
or metabolic and regulatory pathways has been created by basic research (Kliebenstein 2014). Thus, it may be
seen as the
plant equivalent to Escherichia coli or yeast, offering the possibility to use it
as a platform organism
(Nielsen and Keasling 2011).
To date, there is scarce knowledge on plant genes in general (Rhee and Mutwil 2014), and consequently also
on those involved in plant biosynthetic pathways. Thus, their manipulation is largely limited to selected, well
characterised target genes. In addition, stable overexpression of multiple transgenes limits reconstruction of
complex biochemical pathways (Giuliano 2014). It is therefore important to understand the enzymes that
determine chemical features, and factors influencing functional properties (Zhang et al. 2014). Steps towards
successful synthetic biology approaches include transcript profiling of selected plants to select candidate genes
(
i.e. parts) that will be used to assemble synthetic metabolic pathways (Facchini et al. 2012). Only upon
deciphering the regulatory complexity the ultimate of synthetic biology,
i.e. engineering of changes beyond
the limits of natural variation, may be reached. Reports of synthetic biology approaches in plants are currently
limited to synthetic regulatory elements and switches for the spatiotemporal manipulation of gene expression
and engineering of signalling networks (Cabello et al. 2014). Despite some success, multi-gene transfer is still
difficult (Jirschitzka et al. 2013).
Important challenges that in particular apply to the plant field include the predictability of chosen approaches
by establishing the number of genes to be manipulated; also potential interactions and modes of interaction
remain to be determined (Kliebenstein 2014). Decomposition of systems by using reverse engineering is a way
to investigate these questions. The genetic background plays a determinant role by controlling the outcome of
genetic manipulation, e.g. by influencing epigenetic stability, but also the effect the introduced sequences
have on the host organism. To date, these effects become obvious whenever a transgene is introduced into an
organism, and have to be considered when attempting synthetic biology approaches. Questions to be
answered also include which parts of the genomic background (nuclear and organellar genomes) have to be
manipulated to optimise the expression of a pathway, but also the fate and effects of the engineered
metabolite. In some cases there is the potential for toxicity, or the metabolite may cause disruption of the
metabolic network. In any case, transcriptional and post-transcriptional regulatory changes have to be
monitored. Designing a pathway may also include the necessity to remove the plants perception of the
introduced metabolite or its precursors. The challenge in synthetic biology in plants thus includes to create
integrative models to facilitate predictive engineering. For this, a key component is to identify how the plant
regulatory networks react to changes.
Cabello et al. (2014) highlighted advances in plant synthetic biology that include the development of synthetic
regulatory elements, switches for the spatiotemporal manipulation of gene expression and engineering of
signalling networks, the successful introduction of a synthetic pathway for the production of halogenated
alkaloids in
Catharanthus roseus, and the relocation of an entire cytochrome P450 monooxygenase pathway
from the endoplasmatic reticulum to the chloroplast. Further reports describe the development of an auxin
biosensor, or the design of a synthetic signal transduction pathway, allowing linking the detection of a
metabolite to the expression of target genes.
4.3.1
Assembly of plant pathways in heterologous hosts
Heterologous production should facilitate rapid and robust access to the desired compound. Adjusting
expression parameters for the new host and further codon optimisation are the prerequisites to achieve this