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Background Sucrose phosphate synthase (SPS) genes play vital roles in sucrose production across various plant species. Modern sugarcane cultivar is derived from the hybridization between the high sugar content species Saccharum officinarum and the high stress tolerance species Saccharum spontaneum , generating one of the most complex genomes among all crops. The genomics of sugarcane SPS remains under-studied despite its profound impact on sugar yield. Results In the present study, 8 and 6 gene sequences for SPS were identified from the BAC libraries of S. officinarum and S. spontaneum , respectively. Phylogenetic analysis showed that SPSD was newly evolved in the lineage of Poaceae species with recently duplicated genes emerging from the SPSA clade. Molecular evolution analysis based on Ka/Ks ratios suggested that polyploidy reduced the selection pressure of SPS genes in Saccharum species. To explore the potential gene functions, the SPS expression patterns were analyzed based on RNA-seq and proteome dataset, and the sugar content was detected using metabolomics analysis. All the SPS members presented the trend of increasing expression in the sink-source transition along the developmental gradient of leaves, suggesting that the SPSs are involved in the photosynthesis in both Saccharum s pecies as their function in dicots. Moreover, SPSs showed the higher expression in S. spontaneum and presented expressional preference between stem ( SPSA ) and leaf ( SPSB ) tissue, speculating they might be involved in the differentia of carbohydrate metabolism in these two Saccharum species, which required further verification from experiments. Conclusions SPSA and SPSB genes presented relatively high expression and differential expression patterns between the two Saccharum species, indicating these two SPSs are important in the formation of regulatory networks and sucrose traits in the two Saccharum species. SPSB was suggested to be a major contributor to the sugar accumulation because it presented the highest expressional level and its expression positively correlated with sugar content. The recently duplicated SPSD2 presented divergent expression levels between the two Saccharum species and the relative protein content levels were highest in stem, supporting the neofunctionalization of the SPSD subfamily in Saccharum .

Sucrose is the primary form of mobile photoassimilates, and its level is regulated by sucrose-phosphate synthase (SPS) in plants. Increasing in the SPS activity was accompanied by an increase in sucrose accumulation. This study was designed to examine the effect of the overexpression sugarcane SoSPS1 gene on sucrose metabolizing enzymes, growth, and grain yield of indica rice. The SoSPS1 gene was constructed in a binary vector under the control of a rice ubiquitin promoter and transformed into indica rice using an Agrobacterium vector. Five lines of transgenic rice were selected to develop homozygous transgenic lines and used for analysis. The overexpression of the SoSPS1 gene significantly increased the transcript and protein levels, followed by increasing in SPS activity and sucrose content in the leaves of the transgenic rice lines. Moreover, the activity of soluble acid invertase (SAI) was elevated rather than sucrose synthase (SuSy) in the transgenic lines. The increase in the sucrose-degrading enzymes leads to an increase in plant growth and development. The plant height and number of tillers were significantly higher in the transgenic line compared to non-transgenic (NT) rice. In addition, the amylose content, the number of seeds per panicle, and the weight of 1000 grains of seed, including dry biomass weight, were increased in the transgenic lines. The results indicated that enhancement of SPS activity, as well as sucrose content, provides a higher carbon partitioning for higher growth and productivity of the transgenic rice lines.

In an attempt to increase source capacity, transgenic corn was generated by expressing a truncated maize sucrose phosphate synthase (ZmSPSΔ482) under two leaf mesophyll cellspecific promoters (CAB and PPDK). The endogenous and truncated SPS proteins from transgenic leaf extracts were distinguishable by protein immunoblot analysis. The expression of transgenic SPS protein across events varied from very low to very high and included several cosuppressed events. SPS activity showed a diurnal pattern in both transgenic and wild-type maize leaves. In greenhouse experiments, transgenic maize had higher leaf sucrose and lower leaf starch, suggesting a shift in carbon partitioning from starch to sucrose. Conversely, cosuppressed events had lower leaf sucrose and higher leaf starch. A field test was performed to compare sucrose and starch in positive and negative isolines of hybrid maize CAB and PPDK ZmSPSΔ482 events. In the field, many positive isolines had higher levels of both leaf sucrose and starch than the negative isolines. This suggests that in the field, with higher light intensity the shift in carbon partitioning from starch to sucrose, observed under greenhouse conditions did not occur. This in turn suggests that the environment affects the phenotype of the transgenics and that in the field, there was an overall increase in carbon assimilation. Six events from each construct were tested in a pilot multi-density yield trial but overall, no effect on yield was observed. Therefore, although the transgenic plants had more source capacity, this did not translate into higher seed yield.

Summary Bermudagrass (Cynodon dactylon) is one of the most widely cultivated warm‐season turfgrass species around the world. Cold stress has been a key environmental factor that adversely affects the growth, development, and geographical distribution of bermudagrass; however, the underlying mechanism of bermudagrass responsive to cold stress remains largely unexplored. Here, we identified a cold‐induced WRKY transcription factor CdWRKY2 from bermudagrass and demonstrated its function in cold stress response. Overexpression of CdWRKY2 enhanced cold tolerance in transgenic Arabidopsis and bermudagrass hairy roots, while knocking down CdWRKY2 expression via virus‐induced gene silencing increased cold susceptibility. RNA sequencing showed that overexpression of CdWRKY2 in Arabidopsis activated the expression of genes involved in sucrose synthesis and metabolism, including sucrose synthase 1 (AtSUS1) and sucrose phosphate synthase 2F (AtSPS2F). CdSPS1, the homology gene of AtSPS2F in bermudagrass, was subsequently proven to be the direct target of CdWRKY2 by yeast one‐hybrid, electrophoretic mobility shift assay, and transient expression analysis. As expected, overexpression of CdSPS1 conferred cold tolerance in transgenic Arabidopsis plants, whereas silencing CdSPS1 expression enhanced cold sensitivity in bermudagrass. Besides, CdCBF1 whose expression was dramatically up‐regulated in CdWRKY2‐overexpressing bermudagrass hairy roots but down‐regulated in CdWRKY2‐silencing bermudagrass both under normal and cold stress conditions was confirmed as another target of CdWRKY2. Collectively, this study reveals that CdWRKY2 is a positive regulator in cold stress by targeting CdSPS1 and CdCBF1 promoters and activating their expression to coordinately mediate sucrose biosynthesis and CBF‐signalling pathway, which provides valuable information for breeding cold‐resistant bermudagrass through gene manipulation.

High temperature (temperature over 35 °C) is an extremely important environmental factor that affects the maize grain quality in Southern China. The effects of heat stress after pollination on grain protein and starch deposition and activities of involved enzymes were studied in a pot trail in 2014 and 2015. Results showed that grain dry weight reductions at maturity were 19.8% and 19.1%, whereas starch contents (mg g −1 ) were reduced by 3.0% and 3.3%, and starch accumulation (mg grain −1 ) were reduced 22.2% and 21.8% in 2014 and 2015, respectively. Protein content was decreased by heat stress before 15 DAP and increased thereafter. At maturity, protein contents (mg g −1 ) were increased by 24.5% and 25.3% in 2014 and 2015, while protein accumulation (mg grain −1 ) were not affected by heat stress. In response to heat stress, glutamate synthase activity was enhanced by 29.1–82.9% in 2014 and 2.0–141.8% in 2015, whereas glutamine synthetase activity was reduced by 1.9–43.5% in 2014 and 0.1–27.4% in 2015 throughout the grain filling. The activities of sucrose phosphate synthase were decreased by heat stress at 10–25DAP (12.7–32.0%) in 2014 and 15–20 DAP (23.2–27.5%) in 2015, and activities of sucrose synthase were decreased by heat stress at 5–15 DAP (20.0–45.0%) in 2014 and 15 DAP (22.0%) in 2015, repectively. The activities of enyzmes that involved in starch synthessis were all suppressed by heat stress during grain filling, and the reduction of adenosine diphosphate-glucose pyrophosphorylase, soluble starch synthase, and starch branching enzyme were decreased by 21.3–43.1%, 19.1–29.2%, and 7.0–45.6% in 2014 and 1.8–78.5%, 21.4–51.2%, and 11.0–48.0% in 2015, respectively. Conclusively, grain weight and starch deposition were suppressed by heat stress due to the decreased activities of enzymes involved in starch synthesis, and the increased protein content was due to the enhanced activity of glutamate synthase.

Main conclusion The changes in the expression of key sugar metabolism enzymes (SPS and SUS), sucrose content and arrangement of chloroplast starch may play a significant role in the cold response in M. giganteus  and maize plants. To understand the mechanism of the chilling-response of two closely-related C 4 plants, we investigated the changes in the expression of sucrose phosphate synthase (SPS) and sucrose synthase (SUS) as well as changes in their potential products: sucrose, cellulose and starch in the leaves of Miscanthus  ×  giganteus and Zea mays . Low temperature (12–14 °C) increased SPS content in Miscanthus (MG) and chilling-sensitive maize line (Zm-S), but not in chilling-tolerant one (Zm-T). In Zm-S line, chilling also caused the higher intensity of labelling of SPS in the cytoplasm of mesophyll cells, as demonstrated by electron microscopy. SUS labelling was also increased by cold stress only in MG plants what was observed in the secondary wall between mesophyll and bundle sheath cells, as well as in the vacuoles of companion cells. Cold led to a marked increase in total starch grain area in the chloroplasts of Zm-S line. In turn, Fourier transform infrared spectroscopy (FTIR) showed a slight shift in the cellulose band position, which may indicate the formation of more compact cellulose arrangement in Zm-T maize line. In conclusion, this work presents new findings supporting diversified cold-response, not only between two C 4 plant species but also within one species of maize.

Main conclusionZmSUS1 improved drought tolerance of maize by regulating sucrose metabolism and increasing soluble sugar content, and endowing transgenic maize with higher relative water content and photosynthesis levels.Sucrose synthase (SUS), a key enzyme of sugar metabolism, plays an important role in the regulation of carbon partitioning in plant, and affects important agronomic traits and abiotic responses to adversity. However, the function of ZmSUS1 in plant drought tolerance is still unknown. In this study, the expression patterns of ZmSUS1 in different tissues and under drought stress were analyzed in maize (Zea mays L.). It was found that ZmSUS1 was highly expressed during kernel development but also in leaves and roots of maize, and ZmSUS1 was induced by drought stress. Homozygous transgenic maize lines overexpressing ZmSUS1 increased the content and activity of SUS under drought stress and exhibited higher relative water content, proline and abscisic acid content in leaves. Specifically, the net photosynthetic rate and the soluble sugar contents including sucrose, glucose, fructose and SUS decomposition products including UDP-glucose (UDP-G) and ADP-glucose (ADP-G) in transgenic plants were significantly improved after drought stress. RNA-seq analysis showed that overexpressing of ZmSUS1 mainly affected the expression level of carbon metabolism-related genes. Especially the expression level of sucrose metabolism-related genes including sucrose phosphatase gene (SPP), sucrose phosphate synthase gene (SPS) and invertase gene (INV) were significantly up-regulated in transgenic maize. Overall, these results suggested that ZmSUS1 improved drought tolerance by regulating sucrose metabolism and increasing the soluble sugar content, and endowing transgenic maize with higher relative water content and photosynthesis levels, which can serve as a new gene candidate for cultivating drought-resistant maize varieties.

Many fruits are harvested at the early ripening stage and left to attain an edible stage during the post-ripening process or by ethylene treatment. However, the post-ripening quality of fruit is generally inferior to the quality of fruit that ripened naturally. Sugar metabolism plays critical roles in regulating fruit flavor and stress responses. Methyl Jasmonate (MeJA) treatment modulates sugar accumulations and postharvest fruit quality. However, the regulation mechanism of MeJA on sugar metabolism and its relationship with fruit post-ripening quality are unclear. The results indicated that MeJA-treated fruit possessed higher total antioxidant, lycopene and soluble solids content and lower electrical conductivity and malondialdehyde content. Besides, the post-ripening quality of fruit was strongly related to the starch and sucrose content. Compared to the control and ethephon treatments, MeJA treatment increased the activities and transcript levels of amylase, sucrose phosphate synthase (SPS) and sucrose synthase (SUS), but decreased the activities and transcript levels of acid invertase and neutral invertase. Further analysis suggested that the sucrose content had a strongly positive correlation with the activities and transcript levels of SUS and SPS, except SlSPS2. Thus, MeJA treatment improved fruit post-ripening quality by regulating sugar metabolism.

Sucrose synthase (SUS) is a common sugar-base transfer enzyme in plants, and sucrose phosphate synthase (SPS) is one of the major enzymes in higher plants that regulates sucrose synthesis. However, information of the SPS and SUS gene families in Actinidia , as well as their evolutionary and functional properties, is limited. According to the SPS and SUS proteins conserved domain of Arabidopsis thaliana , we found 6 SPS genes and 6 SUS genes from A. chinensis (cultivar: ‘Hongyang’), and 3 SPS genes and 6 SUS genes from A. eriantha (cultivar: ‘White’). The novel CDC50 conserved domains were discovered on AcSUS2, and all members of the gene family contain similar distinctive conserved domains. The majority of SUS and SPS proteins were hydrophilic, lipid-soluble enzymes that were expected to be found in the cytoplasm. The tertiary structure of SPS and SUS protein indicated that there were many tertiary structures in SPS, and there were windmill-type and spider-type tertiary structures in SUS. The phylogenetic tree was created using the neighbor-joining method, and members of the SPS and SUS gene families are grouped into three subgroups. Genes with comparable intron counts, conserved motifs, and phosphorylation sites were clustered together first. SPS and SUS were formed through replication among their own family members. AcSPS1 , AcSPS2 , AcSPS4 , AcSPS5 , AcSUS5 , AcSUS6 , AeSPS3 , AeSUS3 and AeSUS4 were the important genes in regulating the synthesis and accumulation of sucrose for Actinidia during the fruit growth stages.

Melatonin (MT) modulates a wide range of physiological functions with pleiotropic effects in plants. In this study, we studied the effect of exogenous applications of MT to pear trees on indicators of pear fruit quality. The results showed that MT increased the size of pear fruit by improving the net photosynthetic rate and maximal quantum efficiency of photosystem II photochemistry during the late stage of pear fruit development. Overall, MT increased fruit weight by 47.85% compared to the control. During the period of fruit maturation, MT increased the content of soluble sugars, particularly sucrose and sorbitol, likely as a result of improved starch accumulation. Expression levels of the invertase gene Pbinvertase 1/2 were lower in MT-treated fruit that resulted in lower enzyme activity, whereas activity of sucrose phosphate synthase was greater due to enhanced levels of expression of PbSPS1/2/3. Thus, application of melatonin improved sucrose content. This study shows that the application of MT to pear trees may be utilized for the production of larger and sweeter fruit of higher economic value.