Enhanced Sucrose Accumulation Research Articles

Transcription Factor CitERF16 Is Involved in Citrus Fruit Sucrose Accumulation by Activating CitSWEET11d.

Sugars are the primary products of photosynthesis and play an important role in plant growth and development. They contribute to sweetness and flavor of fleshy fruits and are pivotal to fruit quality, and their translocation and allocation are mainly dependent on sugar transporters. Genome-wide characterization of Satsuma mandarin identified eighteen SWEET family members that encode transporters which facilitate diffusion of sugar across cell membranes. Analysis of the expression profiles in tissues of mandarin fruit at different developmental stages showed that CitSWEET11d transcripts were significantly correlated with sucrose accumulation. Further studies indicated that overexpression of CitSWEET11d in citrus callus and tomato fruit showed a higher sucrose level compared to wild-type, suggesting that CitSWEET11d could enhance sucrose accumulation. In addition, we identified an ERF transcription factor CitERF16 by yeast one-hybrid screening assay which could directly bind to the DRE cis-element on the promoter of CitSWEET11d. Overexpression of CitERF16 in citrus callus significantly induced CitSWEET11d expression and elevated sucrose content, suggesting that CitERF16 acts as a positive regulator to promote sucrose accumulation via trans-activation of CitSWEET11d expression.

SnRK1 phosphorylation of SDH positively regulates sorbitol metabolism and promotes sugar accumulation in peach fruit.

Fruit quality depends largely on the type and amount of sugar accumulated in the fruit. In peach [Prunus persica (L.) Batsch], sorbitol is the main photosynthetic product and plays a crucial role in sugar metabolism. As a conserved energy sensor, SNF1-related kinase 1 (SnRK1) is involved in the regulation of carbon metabolism. In this study, SnRK1 was able to respond to induction by treatment with exogenous trehalose and sorbitol on ‘Ruipan 17’ peach fruit. After treatment with 100-mM trehalose for 3 h, the SnRK1 activity decreased by 18% and the activities of sorbitol dehydrogenase (SDH) and sucrose synthase (SS) also decreased significantly, but sucrose phosphate synthase (SPS) activity increased significantly; whereas sorbitol treatment under the same conditions resulted in a 12.6% increase in SnRK1 activity and the activities of SDH and SS synthase also increased significantly, compared with the control. The contents of glucose, fructose and sucrose in peach fruit increased significantly after 3 h of sorbitol treatment. In addition, the interactions between PpSnRK1α and enzymes PpSDH and PpSPS were confirmed by yeast two-hybrid method and the phosphorylation of PpSnRK1α and PpSDH was detected in vitro. Taken together, these results suggest that SnRK1 promotes sorbitol metabolism by activating SDH and it also regulates the activities of SS and SPS that enhance sucrose accumulation in peach fruit. SnRK1 protein kinase is involved in sugar metabolism and has the potential to be used for improving fruit quality.

Enhance sucrose accumulation in strawberry fruits by eliminating the translational repression of FabZIPs1.1

The sugar content and sweetness are two indices used to characterize fruit quality in horticultural production. However, the regulation of sugar synthesis and the functional involvement of sugar signaling are far from clear. Here we descripted a strawberry S1 group basic leucine zipper (bZIP) factor FabZIPs1.1, which was significantly repressed by both blue and red light at the transcriptional level. The gene was expressed extensively in both vegetative and reproductive organs. It had closest phylogenetic relationship to AtbZIP53, with three putative sucrose-controlled upstream open reading frames (sc-uORF) in the 5′-leader region. The transcriptional level of FabZIPs1.1 was not affected by sucrose under in vitro conditions. The sucrose-induced repression of translation (SIRT) effect of FabZIPs1.1 was confirmed using stable transgenic Arabidopsis lines with a 35S::FabZIPs1.1-5′-leader-GUS reporter construct. Glucose can also induce translational inhibition of FabZIPs1.1, and this effect was sensitive to light. Transient over-expression of FabZIPs1.1 lacking the leader sequence can enhance sucrose accumulation in strawberry fruits. Our results revealed the sugar sensing and regulation role of FabZIPs1.1 and highlighted the involvement of environmental factors such as light in this mediation process.

Inactivation of invertase enhances sucrose production in the cyanobacterium Synechocystis sp. PCC 6803.

Sucrose is naturally synthesized by many cyanobacteria under high salt conditions, which can be applied to produce this widely used feedstock. To improve sucrose production with the moderate halo-tolerant cyanobacterium Synechocystis sp. PCC 6803, we identified and biochemically characterized the sucrose-degrading invertase. Inactivating the invertase encoding gene sll0626 (inv) significantly increased cellular sucrose levels; interestingly sucrose over-accumulation was also observed under NaCl-free conditions. The subsequent inactivation of inv in the mutant ΔggpS, which cannot synthesize the major compatible solute glucosylglycerol, resulted in further enhanced sucrose accumulation in the presence of 1.5 % NaCl. Then, inv mutation was introduced into the previously obtained sucrose-producing strain WD25 (Du W, Liang F, Duan Y, Tan X, Lu X. Metab Eng 2013;19:17-25), which resulted in almost 40 % higher sucrose accumulation. These findings show that invertase is an interesting target in obtaining efficient sucrose production in cyanobacterial host cells.

Reduced abscisic acid content is responsible for enhanced sucrose accumulation by potassium nutrition in vegetable soybean seeds.

In order to understand the physiological mechanism of potassium (K) application in enhancing sugar content of vegetable soybean seeds, pot experiments were conducted in 2014 and 2015 with two vegetable soybean (Glycine max L. Merr.) cultivars (c.v. Zhongkemaodou 1 and c.v. 121) under normal rate of nitrogen and phosphorus application. Three potassium (K) fertilization treatments were imposed: No K application (K0), 120kg K2SO4 ha-1 at seeding (K1), and 120kg K2SO4 ha-1 at seedling + 1% K2SO4 foliar application at flowering (K2). Contents of indole-3-acetic acid (IAA), gibberellins (GA), cytokinins (ZR) and abscisic acid (ABA) in seeds were determined from 4 to 8 weeks after flowering. K fertilization increased the contents of IAA, GA, ZR, soluble sugar, sucrose and fresh pod yield, but reduced ABA content consistently. When the contents of soluble sugar and sucrose reached the highest level at 7 weeks after flowering for the 2 cultivars, the contents of IAA、GA、ZR all reached the lowest level in general. The content of ABA in seed was negatively correlated with the sucrose content (P < 0.01, r = -0.749**, -0.768** in 2014 and -0.535**, -0.791** in 2015 for c.v.121 and c.v. Zhongkemaodou 1 respectively). The changes in ratio of the ABA to (IAA + GA + ZR) from 4 to 8 weeks after flowering affected by K application were coincident to the changes of sucrose accumulation. The reduced ratio of ABA/(IAA + GA + ZR) affected by K nutrition particularly reduced abscisic acid content plays a critical role in enhancing sucrose content, which might be a partial mechanism involved in K nutrition to improve the quality of vegetable soybean.

Herbicides and plant hormesis

Herbicide hormesis is commonly observed at subtoxic doses of herbicides and other phytotoxins. The occurrence and magnitude of this phenomenon are influenced by plant growth stage and physiological status, environmental factors, the endpoint measured and the timing between treatment and endpoint measurement. The mechanism in some cases of herbicide hormesis appears to be related to the target site of the herbicide, whereas in other examples hormesis may be by overcompensation to moderate stress induced by the herbicides or a response to disturbed homeostasis. Theoretically, herbicide hormesis could be used in crop production, but this has been practical only in the case of the use of herbicides as sugar cane 'ripeners' to enhance sucrose accumulation. The many factors that can influence the occurrence, the magnitude and the dose range of hormetic increases in yield for most crops make it too unpredictable and risky as a production practice with the currently available knowledge. Herbicide hormesis can cause undesired effects in situations in which weeds are unintentionally exposed to hormetic doses (e.g. in adjacent fields, when shielded by crop vegetation). Some weeds that have evolved herbicide resistance may have hormetic responses to recommended herbicide application rates. Little is known about such effects under field conditions. A more complete understanding of herbicide hormesis is needed to exploit its potential benefits and to minimize its potential harmful effects in crop production.

A Medicago truncatula H+-pyrophosphatase gene, MtVP1, improves sucrose accumulation and anthocyanin biosynthesis in potato (Solanum tuberosum L.).

We recently cloned MtVP1, a type I vacuolar-type H(+)-translocating inorganic pyrophosphatase from Medicago truncatula. In the present study, we investigated the cellular location and the function of this H(+)-PPase in Arabidopsis and potato (Solanum tuberosum L.). An MtVP1::enhanced green fluorescent protein fusion was constructed, which localized to the plasma membrane of onion epidermal cells. Transgenic Arabidopsis thaliana overexpressing MtVP1 had more robust root systems and redder shoots than wild-type (WT) plants under conditions of cold stress. Furthermore, overexpression of MtVP1 in potato accelerated the formation and growth of vegetative organs. The tuber buds and stem base of transgenic potatoes became redder than those of WT plants, but flowering was delayed by approximately half a month. Interestingly, anthocyanin biosynthesis was promoted in transgenic Arabidopsis seedlings and potato tuber buds. The sucrose concentration of transgenic potato tubers and tuber buds was enhanced compared with that of WT plants. Furthermore, sucrose concentration in tubers was higher than that in tuber buds. Although there was no direct evidence to support Fuglsang's hypothetical model regarding the effects of H(+)-PPase on sucrose phloem loading, we speculated that sucrose concentration was increased in tuber buds owing to the increased concentration in tubers. Therefore, overexpressed MtVP1 enhanced sucrose accumulation of source organs, which might enhance sucrose transport to sink organs, thus affecting anthocyanin biosynthesis.

Herbicides as Ripeners for Sugarcane

Chemical ripening of sugarcane is an important component to profitable sugar production in the United States as well as other sugarcane industries throughout the world. Harvesting of sugarcane often begins before the sugarcane reaches the desirable maturity level. This is especially true in the Louisiana sugarcane industry where the window for harvesting is limited because of the risk of freezing temperatures encountered in a temperate climate. Research on the application of chemicals, mostly of herbicide origin, to enhance sucrose accumulation (ripening) or limit flowering to conserve stored sucrose has been conducted for more than 60 yr. The only sugarcane ripener currently registered for use in the United States is glyphosate applied before harvest. The herbicide fluazifop is used as the primary ripener of sugarcane in South Africa. The herbicides glyphosate, fluazifop, and sulfometuron-methyl and the growth regulators ethephon and trinexapac-ethyl are registered for use in Brazil. There is a continuing need to evaluate sugarcane ripeners to increase the utility of currently registered ripeners and to find additional ripeners for use by sugarcane industries. The need for alternatives to glyphosate is especially critical before a glyphosate-tolerant sugarcane can be utilized to improve control of problematic weeds.

Physiological basis for enhanced sucrose accumulation in an engineered sugarcane cell line

Transgenic sugarcane (Saccharum officinarum L. interspecific hybrids) line N3.2 engineered to express a vacuole-targeted sucrose isomerase was found to accumulate sucrose to twice the level of the background genotype Q117 in heterotrophic cell cultures, without adverse effects on cell growth. Isomaltulose levels declined over successive subcultures, but the enhanced sucrose accumulation was stable. Detailed physiological characterisation revealed multiple processes altered in line N3.2 in a direction consistent with enhanced sucrose accumulation. Striking differences from the Q117 control included reduced extracellular invertase activity, slower extracellular sucrose depletion, lower activities of symplastic sucrose-cleavage enzymes (particularly sucrose synthase breakage activity), and enhanced levels of symplastic hexose-6-phosphate and trehalose-6-phosphate (T6P) in advance of enhanced sucrose accumulation. Sucrose biosynthesis by sucrose phosphate synthase (SPS) and sucrose phosphate phosphatase (SPP) was substantially faster in assays conducted to reflect the elevation in key allosteric metabolite glucose-6-phosphate (G6P). Sucrose-non-fermenting-1-related protein kinase 1 (SnRK1, which typically activates sucrose synthase breakage activity while downregulating SPS in plants) was significantly lower in line N3.2 during the period of fastest sucrose accumulation. For the first time, T6P is also shown to be a negative regulator of SnRK1 activity from sugarcane sink cells, hinting at a control circuitry for parallel activation of key enzymes for enhanced sucrose accumulation in sugarcane.

Sucrose accumulation in sugarcane is influenced by temperature and genotype through the carbon source - sink balance

While substantial effort has been expended on molecular techniques in an attempt to break through the apparent ceiling for sucrose content (SC) in sugarcane stalks, molecular processes and genetics limiting sucrose accumulation remain unclear. Our own studies indicate that limiting expansive growth with water stress will enhance sucrose accumulation in both low- and high-sucrose clones. Sucrose accumulation was largely explained (72%) by an equation with terms for photosynthesis, plant extension rate (PER), and plant number. New research was conducted to determine if this simple model stands when using temperature rather than water stress to perturb the source–sink balance. We also applied a thinning treatment to test the proposal implicit in this equation that SC will increase if competition between plants for photo-assimilate is reduced. Four clones from a segregating population representing extremes in SC were planted in pots and subjected to warm and cool temperature regimes in a glasshouse facility. A thinning treatment was imposed on half the pots by removing all but 6 shoots per pot. Temperature as a means of reducing sink strength seemed initially to be more successful than water regime because PER was 43% lower in the cool than in the hot regime while photosynthesis was only 14% less. PER was a good indicator of dry matter allocation to expansive growth, limited by water stress but not by temperature, because stalks tended to thicken in low temperature. Thinning had little effect on any of the attributes measured. Nevertheless the clonal variation in plant numbers and the response of PER to temperature helped to explain at least 69% of the variation in sucrose accumulation observed in this experiment. Thus the earlier model for sucrose accumulation appeared to be valid for the effect on sucrose accumulation of both temperature and water stress on the source–sink balance. The next step is to include internodes in models of assimilate partitioning to help understand the limiting steps in sucrose accumulation from the basics of source–sink dynamics.

Fast changes in soluble carbohydrates and proline contents in tomato seedlings in response to ionic and non-ionic iso-osmotic stresses

Summary Qualitative and quantitative changes in soluble carbohydrates and proline contents were studied in young plants of two tomato cultivars ( Lycopersicon esculentum L. Mill.) differing in their salt-tolerance (cvs. Pera and Volgogradskij). Seedlings were treated for 24 h with ionic (300 mmol L −1 ) and non-ionic iso-osmotic (380 g kg −1 polyethylene glycol MW 6000) stresses. Salinity provoked a decrease in the hexose contents and it also dramatically enhanced sucrose accumulation in all organs of the plants, and to a greater extent in the salt-sensitive cv. Volgogradskij. The PEG iso-osmotic stress, compared with saline conditions, only affected the hexose contents in a similar way to salinity in the cv. Volgogradskij, while it increased the sucrose content 2.5 and 4 times in the roots of Pera and Volgogradskij, respectively. Proline accumulation was more important in the salt-sensitive cultivar under both conditions. The contribution of these solutes to the osmotic potential was up to −0.3 MPa under non-ionic stress, but they had little influence under salinity compared with control plants. The fast changes observed in soluble carbohydrates and proline composition are discussed in relation to the osmotic adjustment under saline and non-permeant iso-osmotic stresses.

Photoperiodically Induced Changes in Seed Growth Rate of Soybean as Related to Endogenous Concentrations of ABA and Sucrose in Seed Tissues

Short-day photoperiods can increase the partitioning of assimilates to filling seeds of soybean (Glycine max L. Merr.), resulting in higher seed growth rates. The plant growth substance ABA has been implicated in the regulation of assimilate transfer within filling soybean seeds. Thus, we hypothesized that an increased concentration of endogenous ABA in seeds may enhance sucrose accumulation and seed growth rate of soybeans exposed to short-day photoperiods. Plants of cv. Hood 75 were grown in a greenhouse under an 8-h short-day photoperiod (SD) until 11 d after anthesis (DAA) of the first flower, when half of the plants were transferred to a night-interruption (NI) treatment (3 h of low-intensity light inserted into the middle of the dark period). Plants remaining in SD throughout seed development had seed growth rates 43 % higher than that of plants shifted to NI (7-6 mg seed1 d1 vs. 5-3 mg seed1 d1). On a tissue-water basis, the concentration of ABA in SD seeds increased rapidly from 7-6 /tmol I1 at 11 DAA to 65-2 /tmol T1 at 18 DAA, but then declined to 6-6 /jmol I1 by 39 DAA. In contrast, the concentration of ABA increased more slowly in NI seeds, reaching only 47-4 /;mol I1 by 18 DAA, peaking at 57-0 fimo I1 on 25 DAA, and declining to 10-2/tmoll1 by 39 DAA. The concentration of sucrose in SD embryos peaked at 73-5 mmol I1 on 25 DAA and remained relatively constant for the remainder of the seed-filling period. In NI, the concentration of sucrose reached only 38-3 mmol I1 by 25 DAA, and peaked at 61-5 /jmol I1 on 32 DAA. Thus in both SD and NI, sucrose accumulated in embryos only after the peak in ABA concentration, suggesting that ABA may have stimulated sucrose movement to the seeds. The earlier accumulation of ABA and sucrose in SD suggests that ABA may have increased assimilate availability during the critical cell-division period, thus regulating cotyledon cell number and subsequent seed growth rate for the remainder of the seed-filling period.