Sucrose Phosphate Synthase Expression Research Articles

A genome-wide investigation of the mechanism underlying the effect of exogenous boron application on sugar content and overall quality of “Benihoppe” strawberries

In recent years, the widespread application of growth regulators and nutrients to boost yield and quality of strawberry fruits has led to the rapid growth of strawberry industry globally. Although the effects of major nutrients on strawberry yield have been widely studied, investigations into the effect of trace elements such as boron remain limited. This study examined the effect of boron application on the yield and quality of “Benihoppe” strawberry fruits. Nutrient solutions with varying boron concentrations (0, 0.024, 0.048, 0.072, and 0.096 mM) were applied to the plants, and their effect on fruit quality was evaluated. The results indicated that boron application enhanced the yield per plant, nutrient composition (total amino acid and vitamin C content), antioxidant properties (total phenol) and volatile components (esters) in strawberry fruits. Specifically, treatment with 0.048 mM boron concentration significantly increased the accumulation of soluble sugars, such as sucrose, whose concentration was 154.29% higher than that of the control treated with 0 mM concentration. This enhancement is attributable to the regulated expression of sucrose phosphate synthase (maker-Fvb2-2-augustus-gene-229.38) and β-fructofuranosidase-1/2/3 (augustus-masked-Fvb5-4-processed-gene-2.0, maker-Fvb5-3-augustus-gene-272.30, and maker-Fvb5-1-augustus-gene-0.37) genes, which play crucial roles in sugar metabolism and enzyme activity. Overall, boron application enhanced the quality of “Benihoppe” strawberries. The findings of this study offer substantial theoretical and practical guidance for using boron fertilizers in strawberry farming.

Optimizing the leaf-to-bunch ratio in ‘Mazafati’ date trees by considering source-sink limitation and physiological-molecular responses

The balance between carbon production and consumption is regulated by source and sink, which can limit plant performance. When there is an imbalance between source and sink, the plant's performance is affected. Thus, source and sink are interdependent. We conducted a two-year study (220–221) to investigate how different leaf-to–bunch ratios (LBR) (7–12) affect source–sink balance and yield in ’Mazafati’ date trees. We also determined the optimal number of bunches per tree as the main carbon sinks competing with other organs. Trehalose is a non-reducing sugar that protects cellular structure and function under stress. We found that 9–10 LBR-trees had no biotic stress due to carbon deficiency or excess, as indicated by minimal expression of the trehalose-6-phosphate synthase (TPS) gene and synthesis of trehalose. This alleviated source-sink limitation and resulted in the highest yield, bunch weight, fruit weight and flavor, water use efficiency, and reduced abscised fruit. Hormones gibberellic acid (GA3) and abscisic acid (ABA), gene expression of sucrose phosphate synthase 1 (SPS1), enzyme activities of sucrose phosphate synthase (SPS) and sucrose synthase (SuSy) related to sucrose synthesis, reducing and non-reducing sugars, and starch increased with increasing LBR in date palm fruit. Based on our findings, we concluded that a LBR of 9–10 optimizes fruit quality in date palm.

Establishment of protocol to investigate the expression of Sucrose Phosphate Synthase 2 (SPS2) in vietnamese golden melon (cucumis melo L.)

The accumulation of soluble sugars, especially sucrose, in mature Vietnamese golden melon, has been reported to be the determining factor of the sweetness of fruits. The gene of SPS2 has been shown to be mainly involved in the metabolic signal pathway as those for the synthesis of sucrose. The purpose of the current study is to establish a procedure to investigate the expression of the gene of SPS2 in Cucumis melo L. The materials of melon’s exocarp and mesocarp tissues were applied to isolate the total of RNAs. The procedure of Real-time PCR within the primers of SPS-F and SPS-R was successfully established as seen through the results. As a result, the cycle threshold (Ct) of 27.19, and 28.12, respectively, were observed in the sample of exocarp and mesocarp tissues. For authenticity, the products were sequenced, as determined as the gene of SPS by BLAST (NCBI). In conclusion, the procedure of investigation of SPSs gene expression was successfully established in Cucumis melo L.

Drought stress-induced the formation of heteromorphic leaves of Populus euphratica Oliv: evidence from gene transcriptome.

Populus euphratica Oliv., a dominant species of arid desert community, grows heteromorphic leaves at different crown positions. Whether heteromorphic leaves are a strategy of plant adaptation to drought stress is rarely reported. This study sequenced the transcriptome of three typical heteromorphic leaves (lanceolate, ovate and broad-ovate leaves) of P. euphratica, and measured their drought stress. We wanted to reveal the molecular mechanisms underlying the formation of heteromorphic leaves. Drought stress was increased significantly from lanceolate to ovate to broad-ovate leaves. Gene ontology (GO) and KEGG enrichment analysis showed that the MADs-box gene regulated the expression of peroxidase (POD) in the phenylpropane biosynthetic pathway. The up-regulated expression of the chalcone synthase (CHS) gene in broad-ovate leaves significantly activated the flavonoid biosynthetic pathway. In the process of leaf shape change, the different expressions of homeodomain leucine zipper (HD-ZIP) among the three heteromorphic leaves had potential interactions on the AUX and ABA pathways. The expression of Sucrose phosphate synthase (SPS) and sucrose synthase (SUS) increased from lanceolate to broad-ovate leaves, resulting in a consistent change in starch and sucrose content. We concluded that these resistance-related pathways are expressed in parallel with leaf formation genes, thereby inducing the formation of heteromorphic leaves. Our work provided a new insights for desert plants to adapt to drought stress.

Organ-specific expression of genes associated with the UDP-glucose metabolism in sugarcane (Saccharum spp. hybrids)

BackgroundThe importance of uridine 5′-diphosphate glucose (UDP-G) synthesis and degradation on carbon (C) partitioning has been indicated in several studies of plant systems, whereby the kinetic properties and abundance of involved enzymes had a significant effect upon the volume of C moving into the hemicellulose, cellulose and sucrose pools. In this study, the expression of 136 genes belonging to 32 gene families related to UDP-G metabolism was studied in 3 major sugarcane organs (including leaf, internode and root) at 6 different developmental stages in 2 commercial genotypes.ResultsAnalysis of the genes associated with UDP-G metabolism in leaves indicated low expression of sucrose synthase, but relatively high expression of invertase genes, specifically cell-wall invertase 4 and neutral acid invertase 1–1 and 3 genes. Further, organs that are primarily responsible for sucrose synthesis or bioaccumulation, i.e., in source organs (mature leaves) and storage sink organs (mature internodes), had very low expression of sucrose, cellulose and hemicellulose synthesis genes, specifically sucrose synthase 1 and 2, UDP-G dehydrogenase 5 and several cellulose synthase subunit genes. Gene expression was mostly very low in both leaf and mature internode samples; however, leaves did have a comparatively heightened invertase and sucrose phosphate synthase expression. Major differences were observed in the transcription of several genes between immature sink organs (roots and immature internodes). Gene transcription favoured utilisation of UDP-G toward insoluble and respiratory pools in roots. Whereas, there was comparatively higher expression of sucrose synthetic genes, sucrose phosphate synthase 1 and 4, and comparatively lower expression of many genes associated with C flow to insoluble and respiratory pools including myo-Inositol oxygenase, UDP-G dehydrogenase 4, vacuolar invertase 1, and several cell-wall invertases in immature internodes.ConclusionThis study represents the first effort to quantify the expression of gene families associated with UDP-G metabolism in sugarcane. Transcriptional analysis displayed the likelihood that C partitioning in sugarcane is closely related to the transcription of genes associated with the UDP-G metabolism. The data presented may provide an accurate genetic reference for future efforts in altering UDP-G metabolism and in turn C partitioning in sugarcane.

Sucrose phosphate synthase (SPS), sucrose synthase (SUS) and their products in the leaves of Miscanthus\u2009\xd7\u2009giganteus and Zea mays at low temperature

Main conclusionThe 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 C4 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 C4 plant species but also within one species of maize.

Overexpression of GmSnRK1, a soybean sucrose non-fermenting-1 related protein kinase 1 gene, results in directional alteration of carbohydrate metabolism in transgenic Arabidopsis

ABSTRACTSucrose non-fermenting-1-related protein kinase 1 (SnRK1) plays an important role in plant carbohydrate metabolism and starch biosynthesis. The regulatory role of GmSnRK1 from soybean in regulating carbohydrate metabolism and starch accumulation has not been reported. In the present study, a gene encoding the SnRK1 protein, named GmSnRK1, was successfully isolated from soybean, and the functions of this gene were studied. Subcellular localisation analysis in onion epidermal cells indicated that GmSnRK1 protein was localised to the nucleus. The GmSnRK1 gene was introduced into Arabidopsis to obtain transgenic plants. Its overexpression significantly increased the starch content, as well as the sucrose, glucose and fructose content in the transgenic plants compared to the wild-type (WT). Real-time quantitative polymerase chain reaction (PCR) analysis showed that overexpression of GmSnRK1 up-regulated the genes involved in starch biosynthesis, including sucrose synthase (AtSUS), phosphoglucomutase (AtPGM), ADP-glucose pyrophosphorylase (AtAGPase), granule-bound starch synthases (AtGBSS I and AtGBSS II), soluble starch synthases (AtSSS I, AtSSS II, AtSSS III and AtSSS IV) and starch branching enzymes (AtSBE I and AtSBE II) genes. In contrast, the expression of sucrose phosphate synthase (SPS) gene was decreased in the transgenic plants. Meanwhile, the enzyme activity levels of the five starch biosynthetic enzymes (SUS, AGPase, GBSS, SSS and SBE) exhibited higher activities, while the SPS activity was decreased in the transgenic plants compared to WT. These results suggest that the manipulation of GmSnRK1 expression might be used for improving the starch content in engineered plants for biofuel production.

A sucrose non-fermenting-1-related protein kinase 1 gene from potato, StSnRK1, regulates carbohydrate metabolism in transgenic tobacco.

Sucrose non-fermenting-1-related protein kinase 1 (SnRK1) has been shown to play an essential role in regulating saccharide metabolism and starch biosynthesis of plant. The regulatory role of StSnRK1 from potato in regulating carbohydrate metabolism and starch accumulation has not been investigated. In this work, a cDNA encoding the SnRK1 protein, named StSnRK1, was isolated from potato. The open reading frame contained 1545 nucleotides encoding 514 amino acids. Subcellular localization analysis in onion epidermal cells indicated that StSnRK1 protein was localized to the nucleus. The coding region of StSnRK1 was cloned into a binary vector under the control of 35S promoter and then transformed into tobacco to obtain transgenic plants. Transgenic tobacco plants expressing StSnRK1 were shown to have a significant increased accumulation of starch content, as well as sucrose, glucose and fructose content. Real-time quantitative PCR analysis indicated that overexpression of StSnRK1 up-regulated the expression of sucrose synthase (NtSUS), ADP-glucose pyrophosphorylase (NtAGPase) and soluble starch synthase (NtSSS III) genes involved in starch biosynthesis in the transgenic plants. In contrast, the expression of sucrose phosphate synthase (NtSPS) gene was decreased in the transgenic plants. Meanwhile, enzymatic analyses indicated that the activities of major enzymes (SUS, AGPase and SSS) involved in the starch biosynthesis were enhanced, whereas SPS activity was decreased in the transgenic plants compared to the wild-type. These results suggest that the manipulation of StSnRK1 expression might be used for improving quality of plants in the future.

貯蔵温度がニホンナシ果実の品質および酸性インベルターゼおよびショ糖リン酸合成酵素の遺伝子発現に及ぼす影響

貯蔵温度がニホンナシ果実の品質および酸性インベルターゼおよびショ糖リン酸合成酵素の遺伝子発現に及ぼす影響

Transcriptomic analysis of Chinese bayberry (Myrica rubra) fruit development and ripening using RNA-Seq

BackgroundChinese bayberry (Myrica rubra Sieb. and Zucc.) is an important subtropical fruit crop and an ideal species for fruit quality research due to the rapid and substantial changes that occur during development and ripening, including changes in fruit color and taste. However, research at the molecular level is limited by a lack of sequence data. The present study was designed to obtain transcript sequence data and examine gene expression in bayberry developing fruit based on RNA-Seq and bioinformatic analysis, to provide a foundation for understanding the molecular mechanisms controlling fruit quality changes during ripening.ResultsRNA-Seq generated 1.92 G raw data, which was then de novo assembled into 41,239 UniGenes with a mean length of 531 bp. Approximately 80% of the UniGenes (32,805) were annotated against public protein databases, and coding sequences (CDS) of 31,665 UniGenes were determined. Over 3,600 UniGenes were differentially expressed during fruit ripening, with 826 up-regulated and 1,407 down-regulated. GO comparisons between the UniGenes of these two types and interactive pathways (Ipath) analysis found that energy-related metabolism was enhanced, and catalytic activity was increased. All genes involved in anthocyanin biosynthesis were up-regulated during the fruit ripening processes, concurrent with color change. Important changes in carbohydrate and acid metabolism in the ripening fruit are likely associated with expression of sucrose phosphate synthase (SPS) and glutamate decarboxylase (GAD).ConclusionsMass sequence data of Chinese bayberry was obtained and the expression profiles were examined during fruit ripening. The UniGenes were annotated, providing a platform for functional genomic research with this species. Using pathway mapping and expression profiles, the molecular mechanisms for changes in fruit color and taste during ripening were examined. This provides a reference for the study of complicated metabolism in non-model perennial species.

Expression of Sucrose-Phosphate Synthase (SPS) in Non-photosynthetic Tissues of Maize

Expression of Sucrose-Phosphate Synthase (SPS) in Non-photosynthetic Tissues of Maize

Gene expression studies on developing kernels of maize sucrose synthase (SuSy) mutants show evidence for a third SuSy gene.

Previous studies have identified two tissue- and cell-specific, yet functionally redundant, sucrose synthase (SuSy) genes, Sh1 and Sus1, which encode biochemically similar isozymes, SH1 and SUS1 (previously referred to as SS1 and SS2, respectively). Here we report evidence for a third SuSy gene in maize, Sus3, which is more similar to dicot than to monocot SuSys. RNA and/or protein blot analyses on developing kernels and other tissues show evidence of expression of Sus3, although at the lowest steady-state levels of the three SuSy gene products and without a unique pattern of tissue specificity. Immunoblots of sh1sus1-1 embryos that are either lacking or deficient for the embryo-specific SUS1 protein have shown a protein band which we attribute to the Sus3 gene, and may contribute to the residual enzyme activity seen in embryos of the double mutant. We also studied developing seeds of the double mutant sh1sus1-1, which is missing 99.5% of SuSy enzyme activity, for evidence of co-regulation of several genes of sugar metabolism. We found a significant reduction in the steady-state levels of Miniature-1 encoded cell wall invertase2, and Sucrose transporter (Sut) mRNAs in the double mutant, relative to the lineage-related sh1Sus1 and sh1Sus1 kernels. Down-regulation of the Mn1 gene was also reflected in significant reductions in cell wall invertase activity. Co-regulatory changes were not seen in the expression of Sucrose phosphate synthase, UDP-glucose pyrophosphorylase, and ADP-glucose pyrophosphorylase.

Potential for manipulating carbon metabolism in wheat

SummaryThe transformation of wheat opens up opportunities for manipulation of key steps in plant metabolic pathways. Attempts to alter the expression of sucrose phosphate synthase (SPS) and glycine decarboxylase (GDC) in wheat leaves have been made.Using a maize SPS gene under the control of either a constitutive promoter (in p35SMSPS) or a green tissue specific promoter (in pRGMSPS), transformation of wheat (Triticum aestivum L. cv. Cadenza) led to the production of 22 independent transformed lines that contained the SPS transgene. Expression of the transgene, analysed by RT‐PCR, revealed that the transcript was present in nine of the 12 p35SMSPS plants and six of the 10 pRGMSPS plants. The maize SPS protein could not, however, be detected in any of these lines.The expression of GDC, and in particular that of the P subunit, has been shown to be altered in plant species with the intermediate C3/C4 photosynthetic phenotype. Attempts have been made to make similar changes in the expression of GDC in wheat leaves by transforming them with full length cDNA sequences for the P subunit of GDC in the sense and antisense orientation. More than 70 transformed lines were produced. Of these, 11 contained the gene in the sense, and 31 in the antisense, orientation, as determined by PCR. Results with Tt generation plants suggested that the activity of GDC had been reduced in some lines transformed with antisense constructs. However, there was no evidence for the expression of the sense constructs. GDC mRNA could not be detected by RT‐PCR in any of the 10 lines produced in the sense transformation experiments.Taken together, these results demonstrate the effectiveness of the technology to deliver large numbers of transgenic wheat plants. However, clearly this alone is not sufficient to guarantee the correct expression of the transgenes at sufficient levels to establish the presence of the gene product.

Diurnal changes in sucrose, nucleotides, starch synthesis and AGPS transcript in growing potato tubers that are suppressed by decreased expression of sucrose phosphate synthase

Sucrose export from leaves is high during the day and lower at night, when it depends on starch remobilisation. We have investigated the consequences of diurnal changes of photoassimilate supply for starch synthesis and other metabolic processes in growing potato tubers. Sucrose, the levels of the transcripts for SUS and AGPS, the levels of key metabolites, and the rate of synthesis of starch and other major end products, including protein and cell wall polysaccharides, increased twofold or more between the start and end of the light period. The stimulation of starch synthesis was accompanied by an increase of UDPglucose and ADPglucose, whereas glycolytic intermediates remained unaltered, revealing that sucrose synthase and ADP-glucose pyrophosphorylase are being co-ordinately regulated. Sucrose synthase is stimulated via an increase of its substrates, UDP and sucrose. UDP increases due to an increase of the overall uridine nucleotide pool, and a decrease of the UTP/UDP ratio that occurs in parallel with a decrease of the ATP/ADP ratio and adenylate energy charge when biosynthetic fluxes are high at the end of the day. Within the time frame of the diurnal changes, the changes in the SUS and AGPS transcript levels do not lead to significant changes in the encoded enzymes. Transformants with a progressive decrease of sucrose phosphate synthase expression, where diurnal changes in leaf sugar levels were damped, exhibited a progressive attenuation of the diurnal changes of sucrose, nucleotide sugars and nucleotides, and fluxes in their tubers. It is concluded that metabolic processes in tubers are tightly linked to the momentary supply of sucrose.

Decreased expression of two key enzymes in the sucrose biosynthesis pathway, cytosolic fructose-1,6-bisphosphatase and sucrose phosphate synthase, has remarkably different consequences for photosynthetic carbon metabolism in transgenic Arabidopsis thaliana.

Photosynthetic carbon metabolism was investigated in antisense Arabidopsis lines with decreased expression of sucrose phosphate synthase (SPS) and cytosolic fructose-1,6-bisphosphatase (cFBPase). In the light, triose phosphates are exported from the chloroplast and converted to sucrose via cFBPase and SPS. At night, starch is degraded to glucose, exported and converted to sucrose via SPS. cFBPase therefore lies upstream and SPS downstream of the point at which the pathways for sucrose synthesis in the day and night converge. Decreased cFBPase expression led to inhibition of sucrose synthesis; accumulation of phosphorylated intermediates; Pi-limitation of photosynthesis; and stimulation of starch synthesis. The starch was degraded to maintain higher levels of sugars and a higher rate of sucrose export during the night. This resembles the response in other species when expression of enzymes in the upper part of the sucrose biosynthesis pathway is reduced. Decreased expression of SPS inhibited sucrose synthesis, but phosphorylated intermediates did not accumulate and carbon partitioning was not redirected towards starch. Sugar levels and sucrose export was decreased during the night as well as during the day. Although ribulose-1,5-bisphosphate regeneration and photosynthesis were inhibited, the PGA/triose-P ratio remained low and the ATP/ADP ratio high, showing that photosynthesis was not limited by the rate at which Pi was recycled during end-product synthesis. Two novel responses counteracted the decrease in SPS expression and explain why phosphorylated intermediates did not accumulate, and why allocation was not altered in the antisense SPS lines. Firstly, a threefold decrease of PPi and a shift of the UDP-glucose/hexose phosphate ratio favoured sucrose synthesis and prevented the accumulation of phosphorylated intermediates. Secondly, there was no increase of AGPase activity relative to cFBPase activity, which would prevent a shift in carbon allocation towards starch synthesis. These responses are presumably triggered when sucrose synthesis is decreased in the night, as well as by day.

Alteration of starch- sucrose transition in germinating wheat seed under sodium chloride salinity

Alterations in starch-sucrose transition during germination were studied in wheat seeds under saline conditions. NaCI significantly reduced the speed of germination and resultant seedling growth, but delayed the degradation of seed storage components. The endogenous level of ABA increased while osmotic potential decreased. NaCI also inhibited the expression of α-amylase. Increasing the concentration of NaCI induced the expression of sucrose phosphate synthase, and sugars, including sucrose, were accumulated in the seedlings. This accumulation of sugar closely correlated with an increase in ABA. However, sugar accumulation was reversible when the salt stress was removed. Overall our results strongly suggest that the germinating wheat seeds alter the starch-to-sucrose conversion to adapt for salt stress. This is probably mediated by the increase in ABA.

Decreased expression of sucrose phosphate synthase strongly inhibits the water stress-induced synthesis of sucrose in growing potato tubers.

Water stress stimulates sucrose synthesis and inhibits starch synthesis in wild-type tubers. Antisense and co-suppression potato transformants with decreased expression of sucrose-phosphate synthase (SPS) have been used to analyse the importance of SPS for the regulation of this water-stress induced change in partitioning. (i) In the absence of water stress, a 70-80% decrease in SPS activity led to a 30-50% inhibition of sucrose synthesis and a slight (10-20%) increase of starch synthesis in tuber discs in short-term labelling experiments with low concentrations of labelled glucose. Similar changes were seen in short-term labelling experiments with intact tubers attached to well-watered plants. Provided plants were grown with ample light and water, transformant tubers had a slightly lower water and sucrose content and a similar or even marginally higher starch content than wild-type tubers. (ii) When wild-type tuber slices were incubated with labelled glucose in the presence of mannitol to generate a moderate water deficit (between -0.12 and -0.72 MPa), there was a marked stimulation of sucrose synthesis and inhibition of starch synthesis. A similar stimulation was seen in labelling experiments with wild-type tubers that were attached to water-stressed wild-type plants. These changes were almost completely suppressed in transformants with a 70-80% reduction of SPS activity. (iii) Decreased irrigation led to an increase in the fraction of the dry-matter allocated to tubers in wild-type plants. This shift in allocation was prevented in transformants with reduced expression of SPS. (iv) The results show that operation of SPS and the sucrose cycle in growing potato tubers may lead to a marginal decrease in starch accumulation in non-stressed plants. However, SPS becomes a crucial factor in water-stressed plants because it is required for adaptive changes in tuber metabolism and whole plant allocation.

Sucrose metabolism in cold‐stored potato tubers with decreased expression of sucrose phosphate synthase

Transfer of potato tubers to low temperature leads after 2–4 d to a stimulation of sucrose synthesis, a decline of hexose‐phosphates and a change in the kinetic properties, and the appearance of a new form of sucrose phosphate synthase (SPS). Antisense and co‐suppression transformants with a 70–80% reduction in SPS expression have been used to analyse the contribution of SPS to the control of cold sweetening. The rate of sucrose synthesis in cold‐stored tubers was investigated by measuring the accumulation of sugars, by injecting labelled glucose of high specific activity into intact tubers, and by providing 50 mol m–3 labelled glucose to fresh tuber slices from cold‐stored tubers. A 70–80% decrease of SPS expression resulted in a reproducible but non‐proportional (10–40%) decrease of soluble sugars in cold‐stored tubers, and a non‐proportional (about 25%) inhibition of label incorporation into sucrose, increased labelling of respiratory intermediates and carbon dioxide, and increased labelling of glucans. The maximum activity of SPS is 50‐fold higher than the net rate of sugar accumulation in wild‐type tubers, and decreased expression of SPS in the transformants was partly compensated for increased levels of hexose‐phosphates. It is concluded that SPS expression per se does not control sugar synthesis. Rather, a comparison of the in vitro properties of SPS with the estimated in vivo concentrations of effectors shows that SPS is strongly substrate limited in vivo. Alterations in the kinetic properties of SPS, such as occur in response to low temperature, will provide a more effective way to stimulate sucrose synthesis than changes of SPS expression.

Sucrose Phosphate Synthase Expression at the Cell and Tissue Level Is Coordinated with Sucrose Sink-to-Source Transitions in Maize Leaf.

Immunohistological analyses for sucrose phosphate synthase (SPS) show that the protein is localized in both bundle-sheath cells (BS) and mesophyll cells (M) in maize (Zea mays) leaves. In young leaves, SPS protein was predominantly in the BS, whereas mature leaves showed nearly equal levels of signal in both BS and M. A cell-type-specific response was also seen in light and dark treatments. Dark treatments led to reduced signal in M; however, little or no change was detected in BS. We suggest that SPS in BS is engaged in sucrose biosynthesis by both photoassimilatory and starch turnover reactions in maize leaves. In addition, we suggest that the enzyme in BS may play a major role in the early biosynthesis of sucrose in young leaves. These cell-specific changes in expression in situ were in agreement with the estimates of extractable enzyme activity from isolated BS and M of mature leaves (R. Ohsugi, S.C. Huber [1987] Plant Physiol 84: 1096-1101). In contrast, western blot analyses did not show any significant changes in the levels of SPS protein in either young or mature leaves subsequent to similar dark treatments. It is interesting that the northern blot analyses indicate that the steady-state levels of SPS transcripts were markedly reduced after dark treatments of > 12 h. Overall, our results indicate that Sps gene expression in maize leaf is modulated at multiple levels of controls by both developmental and environmental factors.