Metabolic engineering of sugars and simple sugar derivatives in plants

Abstract

SummaryCarbon captured through photosynthesis is transported, and sometimes stored in plants, as sugar. All organic compounds in plants trace to carbon from sugars, so sugar metabolism is highly regulated and integrated with development. Sugars stored by plants are important to humans as foods and as renewable feedstocks for industrial conversion to biofuels and biomaterials. For some purposes, sugars have advantages over polymers including starches, cellulose or storage lipids. This review considers progress and prospects in plant metabolic engineering for increased yield of endogenous sugars and for direct production of higher-value sugars and simple sugar derivatives. Opportunities are examined for enhancing export of sugars from leaves. Focus then turns to manipulation of sugar metabolism in sugar-storing sink organs such as fruits, sugarcane culms and sugarbeet tubers. Results from manipulation of suspected 'limiting' enzymes indicate a need for clearer understanding of flux control mechanisms, to achieve enhanced levels of endogenous sugars in crops that are highly selected for this trait. Outcomes from in planta conversion to novel sugars and derivatives range from severe interference with plant development to field demonstration of crops accumulating higher-value sugars at high yields. The differences depend on underlying biological factors including the effects of the novel products on endogenous metabolism, and on biotechnological fine-tuning including developmental expression and compartmentation patterns. Ultimately, osmotic activity may limit the accumulation of sugars to yields below those achievable using polymers; but results indicate the potential for increases above current commercial sugar yields, through metabolic engineering underpinned by improved understanding of plant sugar metabolism.

Sucrose Transporter ZmSut1 Expression and Localization Uncover New Insights into Sucrose Phloem Loading

“…In the stem storage parenchyma cells, ZmSUT1 may function to take up Suc from the apoplasm during expansive growth and to retrieve Suc leaked from cells during accumulation and to maintain turgor. A similar function has been proposed during sugar accumulation in sugarcane and sweet sorghum (Sorghum bicolor) stems (Bihmidine et al, 2013Patrick et al, 2013). pZmSut1::RFPer also was expressed in the region of the release phloem, which was marked by symplasmic CF unloading into the developing leaves and roots.…”

Tonoplast Sugar Transporters (SbTSTs) putatively control sucrose accumulation in sweet sorghum stems

“…[1][2][3][4][5] Multiple crops have been independently bred to store high concentrations of sucrose in terminal storage organs, namely, the taproots of sugar beet (Beta vulgaris L.), and the stems of sweet sorghum (Sorghum bicolor L. Moench) and sugarcane (Saccharum officinarum L.). [5][6][7][8][9] However, the sucrose contents of these crops appear to be approaching maximal levels attainable from breeding efforts 10,11 ; therefore, new approaches are needed to increase sucrose accumulation in storage organs. Hence, characterizing the genes that function in sucrose transport and storage will reveal potential new targets for future manipulations to enhance crop yields.…”

Rooting affects the photosystem II activity: in vitro and ex vitro studies on energy hybrid sorrel

“…Sáez et al (2012) reported that in C. sativa, the chloroplasts of plantlets that had been grown in vitro were devoid of starch, probably because of a low photosynthetic activity or because exogenous sucrose created negative feedback on the enzymes in the plastids that are responsible for starch biosynthesis. However, it should be emphasized that starch accumulation also depends on its degradation rate in sink heterotrophic organs (Patrick et al 2013). In addition, low plastoglobulus content under in vitro conditions may result in a low capacity of in vitro plantlets to prevent oxidative damage in the membrane systems of chloroplasts.…”