transformation of sugarcane plants with a vacuolar-targeted trehalulose synthase
gene modified from the gene in Pseudomonas mesoacididophila MX-45 and
obtained transgenic lines reaching about 600 mM of trehalulose in mature stem
juice. These plants retained vigor and trehalulose production over multiple generations under glasshouse and field conditions.
Sucrose is the translocated photosynthate and the largest soluble carbon store in
sugarcane. The capacity to carry stored sucrose into pathways that provide substrates to produce alternative products would be highly advantageous in an efficient biofactory. Hence, a high-yielding sugarcane biofactory system would
ideally contain culm tissues that function as a secondary source tissue rather than a
sink in terms of sucrose balance (O’Neill 2011). To that end, O’Neill et al. (2012a)
demonstrated that sucrose is mobilized from set storage parenchyma via phloem to
the growing shoot tissue. Overall, metabolism in storage parenchyma shifts from
futile cycling to a more quiescent state during sucrose mobilization. Subsequently,
trehalose metabolism in sugarcane was engineered in an attempt to create a significant carbon drain of stored sucrose to impart value-added properties and
enhance abiotic stress tolerance (O’Neill et al. 2012b). To that end, two transgenes
were introduced into the sugarcane genome: trehalose-6-phosphate synthasephosphatase (TPSP) to increase trehalose biosynthesis, and an RNAi transgene
specific for trehalase to abrogate trehalose catabolism. In RNAi-expressing lines,
trehalase expression was abrogated in many plants although no decrease in trehalase activity was observed. In TPSP lines trehalase activity was significantly
higher. No events of co-integration of TPSP and RNAi transgenes were observed,
suggesting that trehalase activity is essential to mitigate embryonic lethal effects of
trehalose metabolism (O’Neill et al. 2012b).
Moreover, transgenic sugarcane plants expressing a vacuole-targeted isomaltulose synthase in seven recipient genotypes (elite cultivars) were evaluated
over 3 years under commercial field conditions (Basnayake et al. 2012). Isomaltulose concentration typically increased with internode maturity and comprised
up to 217 mm (33 % of total sugars) in whole-cane juice. There was generally a
comparable decrease in sucrose concentration, with no overall decrease in total
sugars. After several cycles of field propagation, selections were obtained with
cane yields similar to the recipient genotypes. Sucrose isomerase activity was low
in these transgenic lines, and the results indicate strong potential to develop
sugarcane for commercial-scale production of isomaltulose if higher activity can
be engineered in appropriate developmental patterns.
Bauer et al. (2012) reported the effect of high molecular weight bacterial
fructan (levan) and glucan (reuteran) on growth and carbohydrate partitioning in
transgenic sugarcane plants. These polysaccharides are products of bacterial glycosyltransferases, enzymes that catalyze the polymerization of glucose or fructose
residues from sucrose. Heterologous expression resulted in reduced total carbohydrate assimilation rather than a simple diversion of biopolymers by competition
for substrate.
Lately, transgenic sugarcane plants with developmentally controlled expression
of a silencing-resistant gene encoding a vacuole-targeted isomaltulose synthase
138
F. C. Gómez-Merino et al.
gene modified from the gene in Pseudomonas mesoacididophila MX-45 and
obtained transgenic lines reaching about 600 mM of trehalulose in mature stem
juice. These plants retained vigor and trehalulose production over multiple generations under glasshouse and field conditions.
Sucrose is the translocated photosynthate and the largest soluble carbon store in
sugarcane. The capacity to carry stored sucrose into pathways that provide substrates to produce alternative products would be highly advantageous in an efficient biofactory. Hence, a high-yielding sugarcane biofactory system would
ideally contain culm tissues that function as a secondary source tissue rather than a
sink in terms of sucrose balance (O’Neill 2011). To that end, O’Neill et al. (2012a)
demonstrated that sucrose is mobilized from set storage parenchyma via phloem to
the growing shoot tissue. Overall, metabolism in storage parenchyma shifts from
futile cycling to a more quiescent state during sucrose mobilization. Subsequently,
trehalose metabolism in sugarcane was engineered in an attempt to create a significant carbon drain of stored sucrose to impart value-added properties and
enhance abiotic stress tolerance (O’Neill et al. 2012b). To that end, two transgenes
were introduced into the sugarcane genome: trehalose-6-phosphate synthasephosphatase (TPSP) to increase trehalose biosynthesis, and an RNAi transgene
specific for trehalase to abrogate trehalose catabolism. In RNAi-expressing lines,
trehalase expression was abrogated in many plants although no decrease in trehalase activity was observed. In TPSP lines trehalase activity was significantly
higher. No events of co-integration of TPSP and RNAi transgenes were observed,
suggesting that trehalase activity is essential to mitigate embryonic lethal effects of
trehalose metabolism (O’Neill et al. 2012b).
Moreover, transgenic sugarcane plants expressing a vacuole-targeted isomaltulose synthase in seven recipient genotypes (elite cultivars) were evaluated
over 3 years under commercial field conditions (Basnayake et al. 2012). Isomaltulose concentration typically increased with internode maturity and comprised
up to 217 mm (33 % of total sugars) in whole-cane juice. There was generally a
comparable decrease in sucrose concentration, with no overall decrease in total
sugars. After several cycles of field propagation, selections were obtained with
cane yields similar to the recipient genotypes. Sucrose isomerase activity was low
in these transgenic lines, and the results indicate strong potential to develop
sugarcane for commercial-scale production of isomaltulose if higher activity can
be engineered in appropriate developmental patterns.
Bauer et al. (2012) reported the effect of high molecular weight bacterial
fructan (levan) and glucan (reuteran) on growth and carbohydrate partitioning in
transgenic sugarcane plants. These polysaccharides are products of bacterial glycosyltransferases, enzymes that catalyze the polymerization of glucose or fructose
residues from sucrose. Heterologous expression resulted in reduced total carbohydrate assimilation rather than a simple diversion of biopolymers by competition
for substrate.
Lately, transgenic sugarcane plants with developmentally controlled expression
of a silencing-resistant gene encoding a vacuole-targeted isomaltulose synthase
138
F. C. Gómez-Merino et al.