the US$250 billion polymer market by the year 2015 (Nielsen 2005; Endres et al.
2007). As a matter of fact, traditional chemical industries are already shifting from
chemical to biological processes and new opportunities are continuously emerging
in the pharmaceutical, food, and biomedical areas. As a consequence, using sugarcane to manufacture plastics has several potential advantages over the traditional
methods of production, including higher yields, greater purity, lower energy use,
and less waste production (Nielsen 2005).
Importantly, by using bioplastics it is likely to reduce toxic emissions in the
environment and also diminish loads of industrial wastes on landfills (Fritz et al.
2001; Seinbüchel 2003). Besides, when plants are used as biofactories, the major
limitations of organic synthesis, namely long product lead time and expensive
plant design to handle toxic compounds at high pressure and temperature, are
overcome (Nielsen 2005). As a consequence, various research groups worldwide
have made important progress in the development of sugarcane as a novel
biofactory in the last decade.
For instance, Brumbley et al. (2004) engineered the genetic pathway for poly-3hydroxybutyrate (PHB) in sugarcane. In general, polyhydroxyalkanoates (PHA)
which include PHB, have thermoplastic properties and are biodegradable. In a
subsequent transgenic approach, McQualter et al. (2005a) reported that transgenic
sugarcane plants harboring a chloroplast-targeted version of Escherichia coli
chorismate pyruvate-lyase (CPL) (Siebert et al. 1996) and a 4-hydroxycinnamoylCoA hydratase/lyase from Pseudomonas fluorescens (HCHL) (Gasson et al. 1998)
(both enzymes providing a one-enzyme pathway from a naturally occurring plant
intermediate), were able to synthetize p-hydroxybenzoic acid (pHBA, an aromatic
hydroxiacid which constitutes monomers of liquid crystal polymers used in the
electrical and optical industries),which was quantitatively converted to glucose
conjugates by endogenous uridine diphosphate (UDP)-glucosyl transferases and
apparently stored in the vacuole. The largest amounts detected in leaf and stem
tissue were 7.3 and 1.5 % dry weight, respectively, while there were no evident
phenotypic defects. However, as a result of diverting carbon away from the
phenylpropanoid pathway, there was a severe reduction in leaf chlorogenic acid,
subtle changes in lignin composition, and an apparent compensatory upregulation
of phenylalanine ammonia-lyase (McQualter et al. 2005b).
Brumbley et al. (2007) transformed sugarcane with three genes from the bacterium Ralstonia eutropha that encode the genetic pathway for the biosynthesis of
PHB. In the best transformed line, PHB accumulated at 2.5 % of leaf dry weight.
Furthermore, transgenic plants were evaluated as a production platform for pHBA
using two different bacterial genes, one from Escherichia coli and the other from
Pseudomonas fluorescens. Each of these genes modifies a different existing biochemical pathway in sugarcane. In the best line, a glycosylated form of pHBA
accumulated in the leaf and stem tissue at 7.3 and 1.5 % dry weight, respectively.
Purnell et al. (2007) demonstrated that several transgenic sugarcane lines
accumulating the bacterial PHB exhibited a vertical PHB concentration gradient,
while the polymer concentration showed the lowest level in the youngest leaves
and increased with leaf age. In addition, there was a horizontal gradient along the
134
F. C. Gómez-Merino et al.
2007). As a matter of fact, traditional chemical industries are already shifting from
chemical to biological processes and new opportunities are continuously emerging
in the pharmaceutical, food, and biomedical areas. As a consequence, using sugarcane to manufacture plastics has several potential advantages over the traditional
methods of production, including higher yields, greater purity, lower energy use,
and less waste production (Nielsen 2005).
Importantly, by using bioplastics it is likely to reduce toxic emissions in the
environment and also diminish loads of industrial wastes on landfills (Fritz et al.
2001; Seinbüchel 2003). Besides, when plants are used as biofactories, the major
limitations of organic synthesis, namely long product lead time and expensive
plant design to handle toxic compounds at high pressure and temperature, are
overcome (Nielsen 2005). As a consequence, various research groups worldwide
have made important progress in the development of sugarcane as a novel
biofactory in the last decade.
For instance, Brumbley et al. (2004) engineered the genetic pathway for poly-3hydroxybutyrate (PHB) in sugarcane. In general, polyhydroxyalkanoates (PHA)
which include PHB, have thermoplastic properties and are biodegradable. In a
subsequent transgenic approach, McQualter et al. (2005a) reported that transgenic
sugarcane plants harboring a chloroplast-targeted version of Escherichia coli
chorismate pyruvate-lyase (CPL) (Siebert et al. 1996) and a 4-hydroxycinnamoylCoA hydratase/lyase from Pseudomonas fluorescens (HCHL) (Gasson et al. 1998)
(both enzymes providing a one-enzyme pathway from a naturally occurring plant
intermediate), were able to synthetize p-hydroxybenzoic acid (pHBA, an aromatic
hydroxiacid which constitutes monomers of liquid crystal polymers used in the
electrical and optical industries),which was quantitatively converted to glucose
conjugates by endogenous uridine diphosphate (UDP)-glucosyl transferases and
apparently stored in the vacuole. The largest amounts detected in leaf and stem
tissue were 7.3 and 1.5 % dry weight, respectively, while there were no evident
phenotypic defects. However, as a result of diverting carbon away from the
phenylpropanoid pathway, there was a severe reduction in leaf chlorogenic acid,
subtle changes in lignin composition, and an apparent compensatory upregulation
of phenylalanine ammonia-lyase (McQualter et al. 2005b).
Brumbley et al. (2007) transformed sugarcane with three genes from the bacterium Ralstonia eutropha that encode the genetic pathway for the biosynthesis of
PHB. In the best transformed line, PHB accumulated at 2.5 % of leaf dry weight.
Furthermore, transgenic plants were evaluated as a production platform for pHBA
using two different bacterial genes, one from Escherichia coli and the other from
Pseudomonas fluorescens. Each of these genes modifies a different existing biochemical pathway in sugarcane. In the best line, a glycosylated form of pHBA
accumulated in the leaf and stem tissue at 7.3 and 1.5 % dry weight, respectively.
Purnell et al. (2007) demonstrated that several transgenic sugarcane lines
accumulating the bacterial PHB exhibited a vertical PHB concentration gradient,
while the polymer concentration showed the lowest level in the youngest leaves
and increased with leaf age. In addition, there was a horizontal gradient along the
134
F. C. Gómez-Merino et al.