Abstract
BackgroundCellulose is not only a common component in vascular plants, but also has great economic benefits for paper, wood, and industrial products. In addition, its biosynthesis is highly regulated by carbohydrate metabolism and allocation in plant. MdFRK2, which encodes a key fructokinase (FRK) in apple, showed especially high affinity to fructose and regulated carbohydrate metabolism.ResultsIt was observed that overexpression of MdFRK2 in apple decreased sucrose (Suc) and fructose (Fru) with augmented FRK activity in stems, and caused the alterations of many phenotypic traits that include increased cellulose content and an increase in thickness of the phloem region. To further investigate the involved mechanisms, we generated FRK2-OE poplar lines OE#1, OE#4 and OE#9 and discovered (1) that overexpression of MdFRK2 resulted in the huge increased cellulose level by shifting the fructose 6-phosphate or glucose 6-phsophate towards UDPG formation, (2) a direct metabolic pathway for the biosynthesis of cellulose is that increased cleavage of Suc into UDP-glucose (UDPG) for cellulose synthesis via the increased sucrose synthase (SUSY) activity and transcript levels of PtrSUSY1, (3) that the increased FRK activity increases the sink strength overall so there is more carbohydrate available to fuel increased cambial activity and that resulted in more secondary phloem. These results demonstrated that MdFRK2 overexpression would significantly changes the photosynthetic carbon flux from sucrose and hexose to UDPG for increased cellulose synthesis.ConclusionsThe present data indicated that MdFRK2 overexpression in apple and poplar changes the photosynthetic carbon flux from sucrose and hexose to UDPG for stem cellulose synthesis. A strategy is proposed to increase cellulose production by regulating sugar metabolism as a whole.
Highlights
Cellulose is a common component in vascular plants, and has great economic benefits for paper, wood, and industrial products
These results indicated that the links between MdFRK2 and cellulose synthesis might be related to sugar metabolism in the sink
The expression level of PtrCWINV2 was unchanged relative to control levels in transgenic lines. These findings further suggested that the decreased Suc and Fru concentrations in MdFRK2-transgenic Populus were due to increased cleavage of Suc and Fru phosphorylation into hexose phosphates via the increased sucrose synthase (SUSY) and FRK activities and transcript levels of PtrSUSY1 and MdFRK2, respectively
Summary
Cellulose is a common component in vascular plants, and has great economic benefits for paper, wood, and industrial products. Its biosynthesis is highly regulated by carbohydrate metabolism and allocation in plant. The cell wall structure is complex and primarily consists of various polysaccharides, such as cellulose, hemicellulose, and pectin [2]. Cellulose is a major component of the cell wall, where it exists in the form of microfibrils. CesA encodes a glycosyltransferase that belongs to the GT2 family and has been reported to have key roles in cellulose biosynthesis [5]. Indirect evidence suggested that CESA-4 and CESA-8 are associated with secondary wall cellulose synthesis in Arabidopsis [6]. Regulation of cellulose synthesis has a direct impact on plant growth and development. It is remarkable to note that reduced cellulose synthesis affects the softening of apples [8]
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