Biofuel and energy crops: high-yield Saccharinae take center stage in the post-genomics era

Abstract

The Saccharinae, especially sugarcane, Miscanthus and sorghum, present remarkable characteristics for bioenergy production. Biotechnology of these plants will be important for a sustainable feedstock supply. Herein, we review knowledge useful for their improvement and synergies gained by their parallel study.

Figure 1

The Saccharinae plants. (a) Glaucia Souza's group collecting photosynthetic data from sugarcane plants in Brazil. (b) A sorghum field in Mali; all plants are over 3 m high.

Figure 2

Simplified schematic representation of the cell wall. The wall is shown as a transverse section. Grasses and non-grass angiosperms possess different types of cell wall. The text in red denotes the main differences. Surrounding the cellulose microfibrils, the inner and outer hemicellulose circles show tightly and loosely bound polysaccharides, respectively. Grasses have glucuronoarabinoxylans (GAX) as the main cross-linking hemicellulose and a primary wall matrix enriched in mixed-linkage glucans, with lower pectin content. The thin red boundary in the primary wall of the grasses denotes the phenolic compounds, mainly ferulic acid, linked to GAX molecules. In grasses, seven cellulose microfibrils can be structured in a cellulose macrofibril. Typically, grasses have more lignin than other angiosperms. Non-grasses possess xyloglucan as the major cross-linking hemicellulose, a pectin-based matrix and structural proteins. In the secondary wall, note that pectins and mixed-linkage glucans are minor components. Also, we can see lignin forming a structural barrier surrounding the carbohydrates. Adapted from [39] and [110] with permission.

Figure 3

Simplified C4 and C3 pathways. (a) C4 pathway. (b) C3 pathway. Red numbers indicate the enzymes involved in the reaction: 1, carbonic anhydrase; 2, phosphoenolpyruvate carboxylase; 3, NADP malate dehydrogenase; 4, NADP malic enzyme; 5, ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco); 6, pyruvate, orthophosphate dikinase. The C4 pathway increases the CO₂ concentration in bundle sheath cells, minimizing the competition with O₂ for the Rubisco catalytic site, thus avoiding photorespiration. Glycerate-3-P, glycerate 3-phosphate; PEP, phosphoenolpyruvate.