18O enrichment of leaf cellulose correlated with 18O enrichment of leaf sucrose but not bulk leaf water in a C3 grass across contrasts of atmospheric CO2 concentration and air humidity

· The 18O composition of plant cellulose is often used to reconstruct past climate and plant function. However, uncertainty remains regarding the estimation of the leaf sucrose 18O signal and its subsequent attenuation by 18O exchange with source water during cellulose synthesis.· We grew Lolium perenne at three CO2 concentrations (200, 400 or 800 mmol mol-1) and two relative humidity (RH) levels (50% or 75%), and determined 18O enrichment of leaf sucrose (Δ18OSucrose), bulk leaf water (Δ18OLW), leaf cellulose (Δ18OCellulose) and water at the site of cellulose synthesis (Δ18OCelSynW). · Δ18OCellulose correlated with Δ18OSucrose (R2=0.87) but not with Δ18OLW (R2=0.04), due to a variable 18O discrepancy (range 2.0-9.0‰) between sucrose synthesis water (Δ18OSucSynW, estimated from Δ18OSucrose) and bulk leaf water. The discrepancy resulted mainly from an RH effect. The proportion of oxygen in cellulose that exchanged with medium water during cellulose formation (pex), was near-constant when referenced to Δ18OSucSynW (pex-SucSynW = 0.52±0.02 SE), but varied when related to bulk leaf water (pex-LW = –0.01 to 0.46). · We conclude that previously reported RH-dependent variations of pex-LW in grasses are related to a discrepancy between Δ18OSucSynW and Δ18OLW that may result from spatial heterogeneity in 18O gradients of leaf water and photosynthetic sucrose synthesis.

Changes in sucrose metabolism in maize varieties with different cadmium sensitivities under cadmium stress

Sucrose metabolism contributes to the growth and development of plants and helps plants cope with abiotic stresses, including stress from Cd. Many of these processes are not well-defined, including the mechanism underlying the response of sucrose metabolism to Cd stress. In this study, we investigated how sucrose metabolism in maize varieties with low (FY9) and high (SY33) sensitivities to Cd changed in response to different levels of Cd (0 (control), 5, 10, and 20 mg L−1 Cd). The results showed that photosynthesis was impaired, and the biomass decreased, in both varieties of maize at different Cd concentrations. Cd inhibited the activities of sucrose phosphate synthase (SPS) and sucrose synthase (SS) (sucrose synthesis), and stimulated the activities of acid invertase (AI) and SS (sucrose hydrolysis). The total soluble sugar contents were higher in the Cd-treated seedlings than in the control. Also, Cd concentrations in the shoots were higher in SY33 than in FY9, and in the roots were lower in SY33 than in FY9. The decreases in the photosynthetic rate, synthesis of photosynthetic products, enzyme activity in sucrose synthesis direction, and increases in activity in hydrolysis direction were more obvious in SY33 (the sensitive variety) than in FY9 (the tolerant variety), and more photosynthetic products were converted into soluble sugar in SY33 than in FY9 as the Cd stress increased. The transcript levels of the sugar transporter genes also differed between the two varieties at different concentrations of Cd. These results suggest that sucrose metabolism may be a secondary response to Cd additions, and that the Cd-sensitive variety used more carbohydrates to defend against Cd stress rather than to support growth than the Cd-tolerant variety.

Effects of Reduced and Enhanced Glycogen Pools on Salt-Induced Sucrose Production in a Sucrose-Secreting Strain of Synechococcus elongatus PCC 7942

ABSTRACT Sucrose and glycogen syntheses in cyanobacteria share the common precursor glucose-1-phosphate. It is generally assumed that lowering glycogen synthesis could drive more carbon toward sucrose synthesis that can be induced by salt stress among cyanobacteria. By using a theophylline-dependent riboswitch system, the expression of glgC, a key gene in glycogen synthesis, was downregulated in a quantitative manner in a sucrose-secreting strain of Synechococcus elongatus PCC 7942. We observed that the stepwise suppression of glycogen synthesis limited rather than stimulated sucrose production in the salt-stressed cells, suggesting that glycogen could serve as a carbon pool for the synthesis of sucrose. Accordingly, we generated glycogen-overproducing strains, but the increased glycogen pool alone did not stimulate sucrose production, indicating that alternative steps limit the carbon flux toward the synthesis of sucrose. Consistent with previous studies that showed that sucrose-phosphate synthase (SPS) catalyzes the rate-limiting step in sucrose synthesis, the combination of glycogen overproduction and sps overexpression resulted in increased sucrose production. Our results indicate that the glycogen and sucrose pools are closely linked in Synechococcus elongatus PCC 7942, and we propose that enhancing the glycogen pool could be a promising strategy for the improvement of sucrose production by cyanobacteria in the presence of a strong sucrose synthesis sink. IMPORTANCE Many cyanobacteria naturally synthesize and accumulate sucrose when stressed by NaCl, which provides novel possibilities for obtaining sugar feedstock by engineering of cyanobacteria. It has been assumed that glycogen synthesis competes with sucrose synthesis for the carbon flux. However, our results showed that the suppression of glycogen synthesis decreased rather than stimulated sucrose production in a sucrose-secreting strain of Synechococcus elongatus PCC 7942. This result suggests that glycogen could serve as a supportive rather than a competitive carbon pool for the synthesis of sucrose, providing new insights about the relation between glycogen synthesis and sucrose synthesis in cyanobacteria. This finding is also useful to guide metabolic engineering work to optimize the production of sucrose and possibly other products by cyanobacteria.

Changes in Soluble Sugar Accumulation and Activities of Sucrose-Metabolizing Enzymes during Fruit Ripening of Jackfruit

Jackfruit (Artocarpus heterophyllus Lam.) is an important food crop widely grown in the tropical region. However, little is known about sugar metabolism during fruit ripening of jackfruit. Here we examined sugar profiles (sucrose, glucose and fructose) and corresponding enzyme activities (SPS, E.C.2.4.1.14; SuSy, EC 2.4.1.13; IV, EC 3.2.1.26) of four soft type and four firm type varieties of jackfruit during four stages of fruit ripening. We found that during fruit ripening, there was a rapid increase in contents of total soluble sugar and sucrose, whereas increases in glucose and fructose contents were much slower. Ratios of glucose versus fructose varied among different varieties and ripening stages but within the range of 0.9 to 1.2 in the ripe fruits. Five of these varieties exhibited markedly high levels of SuSy activity for sucrose synthesis at early ripening stage, and then decreased towards fully ripe stage. All soft type varieties exhibited a conspicuous peak of AIV activity and had overall higher AIV activities than NIV during ripening. The changing patterns for other enzymes varied among varieties. Our studies support the notion that sucrose was the major sugar species contributing to the fruit sweetness, followed by fructose and glucose. We also demonstrated that AIV and NIV were probably the primary enzymes responsible for sucrose hydrolysis during ripening, while SPS and SuSy were responsible for sucrose synthesis. We propose that during fruit ripening of jackfruit, glucose is released from starch hydrolysis, followed by sucrose hydrolysis leading to increase in both glucose and fructose contents.

Physiological factors determine the accumulation of D-glycero-D-ido-octulose (D-g-D-i-oct) in the desiccation tolerant resurrection plant Craterostigma plantagineum

The relationship between the accumulation of D-glycero-D-ido-octulose (D-g-D-i-oct) and sucrose and desiccation tolerance was analysed in leaves of Craterostigma plantagineum Hochst. in various conditions. The D-g-D-i-oct level is strictly controlled in C. plantagienum. Light is an important factor enhancing D-g-D-i-oct synthesis when exogenous sucrose is supplied. Desiccation tolerance is lost during natural senescence and during sugar starvation that leads to senescence. The differences in expression patterns of senescence-related genes and the carbohydrate status between vigorous and senescent plants indicate that desiccation tolerance and accumulation of octulose in C. plantagineum is dependent on the developmental stage. Sucrose synthesis is affected more by dehydration than by senescence. D-g-D-i-oct has superior hydroxyl scavenging ability to other common sugars accumulating in C. plantagineum. In the presence of reactive oxygen species (ROS) D-g-D-i-oct levels decreased, probably as a defence reaction.