Growth and metabolism in sugarcane are altered by the creation of a new hexose‐phosphate sink

Abstract

SummaryAn efficient in planta sugarcane-based production system may be realized by coupling the synthesis of alternative products to the metabolic intermediates of sucrose metabolism, thus taking advantage of the sucrose-producing capability of the plant. This was evaluated by synthesizing sorbitol in sugarcane (Saccharum hybrids) using the Malus domestica sorbitol-6-phosphate dehydrogenase gene (mds6pdh). Mature transgenic sugarcane plants were compared with untransformed sugarcane variety Q117 by evaluation of the growth, metabolite levels and extractable activity of relevant enzymes. The average amounts of sorbitol detected in the most productive line were 120 mg/g dry weight (equivalent to 61% of the soluble sugars) in the leaf lamina and 10 mg/g 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. The activity of oxidative reactions in the pentose phosphate pathway and the non-reversible glyceraldehyde-3-phosphate dehydrogenase reaction were elevated to replenish the reducing power consumed by sorbitol synthesis. 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 co-expressed 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 summary, sorbitol hyperaccumulation affected sugarcane growth and metabolism, but the outcome was not lethal for the plant. This work also demonstrated that impressive yields of alternative products can be generated from the intermediates of sucrose metabolism in Saccharum spp.

Molecular Breeding of Grasses by Transgenic Approaches for Biofuel Production

“…Transgenic sugarcane plants expressing the mds6pdh gene accumulated the sorbitol. 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, but the accumulation caused evident necrosis in expanding leaves and reduced growth (Chong et al, 2007). More recently, another group focused on production of the sucrose isomer trehalulose (Hamerli & Birch, 2011).…”

“…The same research group also examined sugar manipulation in sugarcane by engineering a new carbon sink for the six-carbon sugar alcohol sorbitol; sorbitol has intrinsic value as a non-caloric sweetener and is also used to manufacture ascorbic acid (Chong et al, 2007). Transgenic sugarcane plants expressing the mds6pdh gene accumulated the sorbitol.…”

Non-Invasive Monitoring of Sucrose Mobilization from Culm Storage Parenchyma by Magnetic Resonance Spectroscopy

“…1) Metabolic engineering of sugarcane has resulted in plants that accumulate alternative products (e.g., aromatics, polyols, and sugars), indicating a possible future role of the crop in the production of selected high-value products. [2][3][4][5][6][7] Sugarcane cultivation in the future may be characterized by a multiplication of products beyond sucrose as well as the use of the biomass as feedstock for cheap sugars to feed bio-refineries. 1,8) A highly desirable trait in an efficient sugarcane biofactory is the ability to mobilize stored sucrose to increase carbon precursor flux in the production of alternative products.…”

“…9) Accordingly, a sugarcane biofactory can accumulate high levels of alternative products as evidenced by high-level production of isomaltulose, sorbitol, p-hydroxybenzoic acid, and polyhydroxybutyrate (PHB) in transgenic varieties. [2][3][4][5][6]10) A theoretical high-yield biofactory can be idealized as containing culm tissue that functions as a secondary source tissue rather than a sink with regard to sucrose metabolism. Transforming culm into a source tissue to fuel heterotrophic growth might by analogy be used as a model to identify the mechanism of sucrose mobilization in culm tissue.…”

“…A biological polyester, polyhydroxybutyrate (PHB), has been produced in sugarcane by nuclear transformation of three chloroplast-targeted enzymes. 5,6) Other polyhydroxyalkanoates have been produced in sugarcane by targeting the enzymes to the peroxisomes, 20) and sorbitol was produced in the cytosol of sugarcane, 3) while isomaltulose was produced in the vacuole by targeting a sucrose isomerase to this compartment. 10) Since sucrose is found in various subcellular compartments, the yields of alternative products might be increased if sucrose is converted into sucrose derivatives as opposed to functioning as a carbon storage reserve.…”