Ravaud et al. 2007). In turn, trehalose is implicated in anhydrobiosis as a result of
its high water retention capabilities, and is used in the food and cosmetic industries. The saccharide may form a gel phase as cells dehydrate, which prevents
disruption of internal organelles and may function as an antioxidant as well
(Reina-Bueno et al. 2012). Availability of these saccharides is currently limited by
the cost of fermentative conversion from sucrose (Mudge et al. 2013).
Wu and Birch (2007) engineered an efficient sucrose invertase isolated from the
bacterium Pantoea dispersa, with a monocot promoter and a vacuolar targeting
sequence (Gnanasambandam and Birch 2004) and transformed sugarcane explants
with this construct to produce isomaltulose. Isomaltulose accumulated in sugarcane stem storage tissues of transformed plants without any decrease in the stored
sucrose concentration, resulting in nearly doubled total sugar concentrations in
harvested juice. Transgenic plants also showed higher photosynthetic activity,
sucrose transport, and sink strength, which indicates a possible feedback signal for
sucrose biosynthesis, translocation, and storage (Wu and Birch 2007).
In order to develop an efficient in planta sugarcane-based production system by
coupling the synthesis of alternative products to the metabolic intermediates of
sucrose metabolism, Chong et al. (2007) evaluated the biosynthesis of sorbitol (a
polyalcohol used as sugar substitute) in sugarcane using the Malus domestica
sorbitol-6-phosphate dehydrogenase gene (mds6pdh). The average amounts of
sorbitol detected in the most productive line were 120 mg g
-1 dry weight
(equivalent to 61 % of the soluble sugars) in the leaf lamina and 10 mg g
-1 dry
weight in the stalk pith. The levels of enzymes involved in sucrose synthesis and
cleavage were elevated in the leaves of plants accumulating sorbitol, but this did
not affect sucrose accumulation in the culm. Sorbitol-producing sugarcane generated 30–40 % less aerial biomass and was 10–30 % shorter than control lines.
Leaves developed necrosis in a pattern characteristic of early senescence, and the
severity was related to the relative quantity of sorbitol accumulated. When the
Zymomonas mobilis glucokinase (zmglk) gene was coexpressed with mds6pdh to
increase the production of glucose-6-phosphate, the plants were again smaller,
indicating that glucose-6-phosphate deficiency was not responsible for the reduced
growth. In conclusion, sorbitol hyperaccumulation affected sugarcane growth and
metabolism, but the outcome was not deleterious for the plant.
Interested in the unusual development of the leaves in some transgenic sorbitolproducing sugarcane plants, Chong et al. (2010) compared the polar metabolite
profiles in the leaves of those plants against a group of control sugarcane plants.
Principal component analysis of the metabolites indicated that sorbitol, gentiobiose (a disaccharide), and gentiobiitol (a sugar alcohol) were strongly associated
with sorbitol-producing canes. Gentiobiose and gentiobiitol were positively correlated with sorbitol accumulation.
Trehalulose is also a structural isomer of sucrose that has a sweet taste with
similar physical and organoleptic properties to sucrose. Additionally, trehalulose is
acariogenic and can be applied in diabetic and sports foods and drinks as its
absorption reduces the rate at which monosaccharides and insulin are released into
the bloodstream (Ravaud et al. 2007). Hamerli and Birch (2011) reported the
5 Sugarcane as a Novel Biofactory: Potentialities and Challenges
137
its high water retention capabilities, and is used in the food and cosmetic industries. The saccharide may form a gel phase as cells dehydrate, which prevents
disruption of internal organelles and may function as an antioxidant as well
(Reina-Bueno et al. 2012). Availability of these saccharides is currently limited by
the cost of fermentative conversion from sucrose (Mudge et al. 2013).
Wu and Birch (2007) engineered an efficient sucrose invertase isolated from the
bacterium Pantoea dispersa, with a monocot promoter and a vacuolar targeting
sequence (Gnanasambandam and Birch 2004) and transformed sugarcane explants
with this construct to produce isomaltulose. Isomaltulose accumulated in sugarcane stem storage tissues of transformed plants without any decrease in the stored
sucrose concentration, resulting in nearly doubled total sugar concentrations in
harvested juice. Transgenic plants also showed higher photosynthetic activity,
sucrose transport, and sink strength, which indicates a possible feedback signal for
sucrose biosynthesis, translocation, and storage (Wu and Birch 2007).
In order to develop an efficient in planta sugarcane-based production system by
coupling the synthesis of alternative products to the metabolic intermediates of
sucrose metabolism, Chong et al. (2007) evaluated the biosynthesis of sorbitol (a
polyalcohol used as sugar substitute) in sugarcane using the Malus domestica
sorbitol-6-phosphate dehydrogenase gene (mds6pdh). The average amounts of
sorbitol detected in the most productive line were 120 mg g
-1 dry weight
(equivalent to 61 % of the soluble sugars) in the leaf lamina and 10 mg g
-1 dry
weight in the stalk pith. The levels of enzymes involved in sucrose synthesis and
cleavage were elevated in the leaves of plants accumulating sorbitol, but this did
not affect sucrose accumulation in the culm. Sorbitol-producing sugarcane generated 30–40 % less aerial biomass and was 10–30 % shorter than control lines.
Leaves developed necrosis in a pattern characteristic of early senescence, and the
severity was related to the relative quantity of sorbitol accumulated. When the
Zymomonas mobilis glucokinase (zmglk) gene was coexpressed with mds6pdh to
increase the production of glucose-6-phosphate, the plants were again smaller,
indicating that glucose-6-phosphate deficiency was not responsible for the reduced
growth. In conclusion, sorbitol hyperaccumulation affected sugarcane growth and
metabolism, but the outcome was not deleterious for the plant.
Interested in the unusual development of the leaves in some transgenic sorbitolproducing sugarcane plants, Chong et al. (2010) compared the polar metabolite
profiles in the leaves of those plants against a group of control sugarcane plants.
Principal component analysis of the metabolites indicated that sorbitol, gentiobiose (a disaccharide), and gentiobiitol (a sugar alcohol) were strongly associated
with sorbitol-producing canes. Gentiobiose and gentiobiitol were positively correlated with sorbitol accumulation.
Trehalulose is also a structural isomer of sucrose that has a sweet taste with
similar physical and organoleptic properties to sucrose. Additionally, trehalulose is
acariogenic and can be applied in diabetic and sports foods and drinks as its
absorption reduces the rate at which monosaccharides and insulin are released into
the bloodstream (Ravaud et al. 2007). Hamerli and Birch (2011) reported the
5 Sugarcane as a Novel Biofactory: Potentialities and Challenges
137