Cloning and expression analysis of sucrose-phosphate synthase from sugar beet (Beta vulgaris L.)

Sucrose Phosphate Synthase Genes in Plants: Its Role and Practice for Crop Improvement

Plants convert solar energy and carbon dioxide into organic compounds through photosynthesis. Sucrose is the primary carbonate produced during photosynthesis. Sucrose phosphate synthase (SPS) is the key enzyme controlling sucrose biosynthesis in plants. There are at least three SPS gene families in higher plants, named A, B, and C. However, in monocotyledonous plants from Poaceae, there are at least five SPS gene families, named A, B, C, DIII, and DIV. Each family of SPS genes in different plants shows a divergent expression pattern. So different families of SPS genes participate in diverse biological functions, including sucrose accumulation, plant growth and production, and abiotic stress tolerance. SPS activity in plants is regulated by exogenous factors through gene expression and reversible protein phosphorylation. It is a practicable way to improve crop traits through SPS gene transformation. This work analyzes the cloning, phylogeny, and regulatory mechanism of the SPS gene in plants, reviews its biological function as well as its role in crop improvement, and discusses the challenges and future perspectives. This paper can serve as a reference for further study on plant SPS genes and eventually for crop improvement.

Arabidopsis thaliana sucrose phosphate synthase (sps) genes are expressed differentially in organs and tissues, and their transcription is regulated by osmotic stress

Sucrose is synthesized from UDP-Glc and Fru-6-phosphate via the activity of sucrose-phosphate synthase (SPS) enzymes, which produce Suc-6-phosphate. Suc-6-phosphate is rapidly dephosphorylated by phosphatases to produce Suc and inorganic phosphate. Arabidopsis has four sps genes encoding SPS enzymes. Of these enzymes, AtSPS1F and AtSPS2F have been grouped with other dicotyledonous SPS enzymes, while AtSPS3F and AtSPS4F are included in groups with both dicotyledonous and monocotyledonous SPS enzymes. In this work, we generated Arabidopsis thaliana transformants containing the promoter region of each sps gene fused to gfp::uidA reporter genes. A detailed characterization of expression conferred by the sps promoters in organs and tissues was performed. We observed expression of AtSPS1F, AtSPS2F and AtSPS3F in the columella roots of the plants that support sucrose synthesis. Hence, these findings support the idea that sucrose synthesis occurs in the columella cells, and suggests that sucrose has a role in this tissue. In addition, the expression of AtSPS4F was identified in embryos and suggests its participation in this developmental stage. Quantitative transcriptional analysis of A. thaliana plants grown in media with different osmotic potential showed that AtSPS2F and AtSPS4F respond to osmotic stress.

Transcriptomic analysis of nitrogen starvation- and cultivar-specific leaf senescence in winter oilseed rape (Brassica napus L.)

High nitrogen (N) efficiency, characterized by high grain yield under N limitation, is an important agricultural trait in Brassica napus L. cultivars related to delayed senescence of older leaves during reproductive growth (a syndrome called stay-green). The aim of this study was thus to identify genes whose expression is specifically altered during N starvation-induced leaf senescence and that can be used as markers to distinguish cultivars at early stages of senescence prior to chlorophyll loss. To this end, the transcriptomes of leaves of two B. napus cultivars differing in stay-green characteristics and N efficiency were analyzed 4 days after the induction of senescence by either N starvation, leaf shading or detaching. In addition to N metabolism genes, N starvation mostly (and specifically) repressed genes related to photosynthesis, photorespiration and cell-wall structure, while genes related to mitochondrial electron transport and flavonoid biosynthesis were predominately up-regulated. A kinetic study over a period of 12 days with four B. napus cultivars differing in their stay-green characteristics confirmed the cultivar-specific regulation of six genes in agreement with their senescence behavior: the senescence regulator ANAC029, the anthocyanin synthesis-related genes ANS and DFR-like1, the ammonium transporter AMT1;4, the ureide transporter UPS5, and SPS1 involved in sucrose biosynthesis. The identified genes represent markers for the detection of cultivar-specific differences in N starvation-induced leaf senescence and can thus be employed as valuable tools in B. napus breeding.

Starch biosynthetic genes and enzymes are expressed and active in the absence of starch accumulation in sugar beet tap-root

BACKGROUND

Starch is the predominant storage compound in underground plant tissues like roots and tubers. An exception is sugar beet tap-root (Beta vulgaris ssp altissima) which exclusively stores sucrose. The underlying mechanism behind this divergent storage accumulation in sugar beet is currently not fully known. From the general presence of starch in roots and tubers it could be speculated that the lack in sugar beet tap-roots would originate from deficiency in pathways leading to starch. Therefore with emphasis on starch accumulation, we studied tap-roots of sugar beet using parsnip (Pastinaca sativa) as a comparator.

RESULTS

Metabolic and structural analyses of sugar beet tap-root confirmed sucrose as the exclusive storage component. No starch granules could be detected in tap-roots of sugar beet or the wild ancestor sea beet (Beta vulgaris ssp. maritima). Analyses of parsnip showed that the main storage component was starch but tap-root tissue was also found to contain significant levels of sugars. Surprisingly, activities of four main starch biosynthetic enzymes, phosphoglucomutase, ADP-glucose pyrophosphorylase, starch synthase and starch branching enzyme, were similar in sugar beet and parsnip tap-roots. Transcriptional analysis confirmed expression of corresponding genes. Additionally, expression of genes involved in starch accumulation such as for plastidial hexose transportation and starch tuning functions could be determined in tap-roots of both plant species.

CONCLUSION

Considering underground storage organs, sugar beet tap-root upholds a unique property in exclusively storing sucrose. Lack of starch also in the ancestor sea beet indicates an evolved trait of biological importance.Our findings in this study show that gene expression and enzymatic activity of main starch biosynthetic functions are present in sugar beet tap-root during storage accumulation. In view of this, the complete lack of starch in sugar beet tap-roots is enigmatic.

Sucrose induces expression of the sorbitol-6-phosphate dehydrogenase gene in source leaves of loquat

Rosaceae fruit trees use sorbitol and sucrose as translocating sugars and the sorbitol-to-sucrose ratio in source leaves determines apple fruit quality. Here, we investigate the effects of sugars on the expression of genes encoding key photosynthetic enzymes, including sorbitol-6-phosphate dehydrogenase (S6PDH, EC 1.1.1.200), sucrose phosphate synthase (SPS, EC 2.4.1.14), and ADP-glucose pyrophosphorylase (ADPGPPase, EC 2.7.7.27) to understand the sugar-signaling mechanism in Rosaceae fruit trees. Mature leaf-petiole cuttings of loquat (Eriobotrya japonica Lindl. cv. Mogi) were supplied with a water, sorbitol or sucrose solution for 2 days at 20°C. The relative levels of the transcripts were analyzed by real-time polymerase chain reaction (PCR). S6PDH transcription was decreased by sorbitol but drastically increased by sucrose. SPS and ADPGPPase large subunit transcription were decreased by sucrose and sorbitol. The simultaneous application of sorbitol and sucrose revealed that S6PDH transcription increased in a dose-dependent manner with sucrose. These results show that both sorbitol and sucrose work as signaling molecules in source organs of Rosaceae fruit trees. These trees have mechanisms to positively keep sorbitol as the dominant translocating sugar, suggesting that sorbitol plays an important role in their survival strategy. Effects of various sugars on S6PDH expression were investigated. Palatinose, a sucrose analog, increased S6PDH transcription much more drastically than sucrose. Mannose and 3-O-methylglucose, glucose analogs, also increased S6PDH transcription; however, glucose did not. Models of sugar signaling in source organs of Rosaceae fruit trees are discussed.

Functional differentiation of the sugar beet root system as indicator of developmental phase change

Developmental phase transitions in the plant root system have not been well characterized. In this study we compared the dynamics of sucrose accumulation with changes in gene expression analyzed with cDNA-amplified fragment length polymorphism (AFLP) in the developing tap root of sugar beet (Beta vulgaris, L.) during the first 9 weeks after emergence (WAE). Although differences between lines were evident as soon as 9 WAE, sucrose showed a marked increase in the rate of accumulation between 4 and 6 WAE and a remarkable shift in gene expression was observed between 5 and 6 WAE. These changes were evident in two unrelated genetic backgrounds and suggest that physiological and gene expression changes represent a functional differentiation of the tap root. These changes were considered as indicators of a developmental change in the sugar beet root system. To identify genes and metabolic pathways involved in this developmental shift, a root cDNA library was hybridized with probes enriched for 3- and 7-WAE transcripts and differentially expressed transcripts were analyzed by cDNA microarray. Several genes involved in the regulation of tissue development were found to be differentially regulated. Genes involved in protein metabolism, disease-related and secretory system were upregulated before the functional differentiation transition, while genes under hormonal control were upregulated after the functional differentiation transition. This developmental phase change of the root system is important to understand plant developmental regulation at the whole-plant level and will likely be useful as early selection parameter in breeding programs.

Differential transcriptional regulation of banana sucrose phosphate synthase gene in response to ethylene, auxin, wounding, low temperature and different photoperiods during fruit ripening and functional analysis of banana SPS gene promoter

Sucrose phosphate synthase (SPS) (EC 2.3.1.14) is the key regulatory component in sucrose formation in banana (Musa acuminata subgroup Cavendish, cv Giant governor) fruit during ripening. This report illustrates differential transcriptional responses of banana SPS gene following ethylene, auxin, wounding, low temperature and different photoperiods during ripening in banana fruit. Whereas ethylene strongly stimulated SPS transcript accumulation, auxin and cold treatment only marginally increased the abundance of SPS mRNA level, while wounding negatively regulated SPS gene expression. Conversely, SPS transcript level was distinctly increased by constant exposure to white light. Protein level, enzymatic activity of SPS and sucrose synthesis were substantially increased by ethylene and increased exposure to white light conditions as compared to other treatments. To further study the transcriptional regulation of SPS in banana fruit, the promoter region of SPS gene was cloned and some cis-acting regulatory elements such as a reverse GCC-box ERE, two ARE motifs (TGTCTC), one LTRE (CCGAA), a GAGA-box (GAGA...) and a GATA-box LRE (GATAAG) were identified along with the TATA and CAAT-box. DNA-protein interaction studies using these cis-elements indicated a highly specific cis-trans interaction in the banana nuclear extract. Furthermore, we specifically studied the light responsive characteristics of GATA-box containing synthetic as well as native banana SPS promoter. Transient expression assays using banana SPS promoter have also indicated the functional importance of the SPS promoter in regulating gene expression. Together, these results provide insights into the transcriptional regulation of banana SPS gene in response to phytohormones and other environmental factors during fruit ripening.

Prospects for increasing starch and sucrose yields for bioethanol production

In the short term, the production of bioethanol as a liquid transport fuel is almost entirely dependent on starch and sugars from existing food crops. The sustainability of this industry would be enhanced by increases in the yield of starch/sugar per hectare without further inputs into the crops concerned. Efforts to achieve increased yields of starch over the last three decades, in particular via manipulation of the enzyme ADPglucose pyrophosphorylase, have met with limited success. Other approaches have included manipulation of carbon partitioning within storage organs in favour of starch synthesis, and attempts to manipulate source-sink relationships. Some of the most promising results so far have come from manipulations that increase the availability of ATP for starch synthesis. Future options for achieving increased starch contents could include manipulation of starch degradation in organs in which starch turnover is occurring, and introduction of starch synthesis into the cytosol. Sucrose accumulation is much less well understood than starch synthesis, but recent results from research on sugar cane suggest that total sugar content can be greatly increased by conversion of sucrose into a non-metabolizable isomer. A better understanding of carbohydrate storage and turnover in relation to carbon assimilation and plant growth is required, both for improvement of starch and sugar crops and for attempts to increase biomass production in second-generation biofuel crops.

Transcript profiles at different growth stages and tap-root zones identify correlated developmental and metabolic pathways of sugar beet

Field-grown sugar beets were analysed for morphological characters, sucrose content, and reproducible transcript profiles by macroarray analyses with 11,520 unique sugar-beet cDNA targets in two different years. Seasonal differences were partly compensated by expressing sampling dates as thermal time. During early beet development the number of cambial rings, root length, and sucrose concentration had already achieved >40% of their final values. Sucrose levels rose from 10% to 17% over the thermal time of 1300-1400 degrees Cd with only small changes later when lower concentrations were restricted to the exterior zone at the minimum of the spatial sucrose gradient through the beet. The number of leaves and root diameter followed the same temporal growth pattern, but mass increased until beet maturity at around 2000 degrees Cd. Cluster analysis identified 543 transcripts with reproducible preferential expression between 1300-1400 degrees Cd, and 170 showing the highest transcript levels later. In maturing beets, 373 transcripts were over-represented in the inner zone and 148 in the outer zone. During early development, genes involved in cytoskeletal reorganization and transport processes showed the highest transcript levels. Cell wall biogenesis-, defence-, stress-, and degradation-related transcripts were identified in all samples, and associated with pathogen attack during late development and in the outer zone. Candidates with potential roles in carbohydrate metabolism appeared to serve anaplerotic functions by converting excess intermediates to sucrose production. Transcripts preferentially occurring in sucrose-accumulating young beet cells and newly generated peripheral cells of mature beets are discussed as potential breeding targets to improve sink strength and growth.

The developmental and organ specific expression of sucrose cleaving enzymes in sugar beet suggests a transition between apoplasmic and symplasmic phloem unloading in the tap roots

Sucrose utilisation in sink tissues depend on its cleavage and is mediated by two different classes of enzymes, invertase and sucrose synthase, which determine the mechanism of phloem unloading. Cloning of two extracellular (BIN35 and BIN46) and one vacuolar invertase (BIN44) provided the basis for a detailed molecular analysis of the relative contribution of the sucrose cleaving enzymes to the sink metabolism of sugar beets (Beta vulgaris) during development. The determination of the steady state levels of mRNAs has been complemented by the analysis of the corresponding enzyme activities. The present study demonstrates an inverse regulation of extracellular invertase and sucrose synthase during tap root development indicating a transition between functional unloading pathways. Extracellular cleavage by invertase is the dominating mechanism to supply hexoses via an apoplasmic pathway at early stages of storage root development. Only at later stages sucrose synthase takes over the function of the key sink enzyme to contribute to the sink strength of the tap root via symplasmic phloem unloading. Whereas mRNAs for both extracellular invertase BIN35 and sucrose synthase were shown to be induced by mechanical wounding of mature leaves of adult plants, only sucrose synthase mRNA was metabolically induced by glucose in this source organ supporting the metabolic flexibility of this species.

Circadian and developmental regulation of vacuolar invertase expression in petioles of sugar beet plants

The expression pattern of the genes coding for vacuolar and extracellular invertase activity was analyzed in sugar beet (Beta vulgaris) and compared with the expression of sucrose synthase in this important sucrose-storing crop. Northern blot analysis revealed that sucrose synthase is the predominant sucrose-cleaving enzyme in tap roots, whereas vacuolar invertase was specifically expressed in petioles. Extracellular invertase transcripts showed low abundance in all the sugar beet organs and were not detected in northern blots. Relative RT-PCR analysis revealed differential expression of the two extracellular invertase genes: BVInv-CW1 was almost exclusively expressed in tap roots and BVInv-CW2 was widely expressed in all the organs analyzed. A remarkable result of this analysis was the high expression of vacuolar invertase (BVInv-V3) in petioles. Two factors had a clear influence on vacuolar invertase gene expression in petioles: light and the developmental stage, so that expression was higher in petioles from juvenile plants. BVInv-V3 transcripts showed circadian oscillation in petioles, with maximal accumulation during the light period. A similar pattern of diurnal oscillation was also observed for the vacuolar invertase activity, showing a delay with respect to the level of transcripts. The analysis of sugars in petioles revealed oscillation of the hexoses, with a remarkably higher content of glucose than fructose. In contrast, the level of sucrose in petioles was very low. This pattern of expression suggests an important role of petiole vacuolar invertase in plant development and photoassimilate partitioning.

Decreased sucrose-6-phosphate phosphatase level in transgenic tobacco inhibits photosynthesis, alters carbohydrate partitioning, and reduces growth

The aim of this work was to examine the role of sucrose-6-phosphate phosphatase (SPP; EC 3.1.3.24) in photosynthetic carbon partitioning. SPP catalyzes the final step in the pathway of sucrose synthesis; however, until now the importance of this enzyme in plants has not been studied by reversed-genetics approaches. With the intention of conducting such a study, transgenic tobacco plants with reduced SPP levels were produced using an RNA interference (RNAi) strategy. Transformants with less than 10% of wild-type SPP activity displayed a range of phenotypes, including those that showed inhibition of photosynthesis, chlorosis, and reduced growth rates. These plants had strongly reduced levels of sucrose and hexoses but contained 3-5 times more starch than the control specimens. The leaves were unable to export transient starch during extended periods of darkness and as consequence showed a starch- and maltose-excess phenotype. This indicates that no alternative mechanism for carbon export was activated. Inhibition of SPP resulted in an approximately 1,000-fold higher accumulation of sucrose-6-phosphate (Suc6P) compared to wild-type leaves, whereas the content of hexose-phosphates was reduced. Although the massive accumulation of Suc6P in the cytosol of transgenic leaves was assumed to impair phosphate-recycling into the chloroplast, no obvious signs of phosphate-limitation of photosynthesis became apparent. 3-Phosphoglycerate (3-PGA) levels dropped slightly and the ATP/ADP ratio was not reduced in the transgenic lines under investigation. It is proposed that in SPP-deficient plants, long-term compensatory responses give rise to the observed acceleration of starch synthesis, increase in total cellular Pi content, decrease in protein content, and related reduction in photosynthetic activity.

Cloning of a sucrose-phosphate synthase gene highly expressed in flowers from the tropical epiphytic orchid Oncidium Goldiana

Sucrose-phosphate synthase (SPS) is one of the key regulatory enzymes in carbon assimilation and partitioning in plants. It plays a crucial role in the production of sucrose in photosynthetic cells. The cloning and expression analysis of a full-length cDNA encoding SPS from tropical epiphytic orchid hybrid Oncidium Goldiana are reported here. The cDNA designated as sps1 is 3820 bp in length with an open reading frame of 3183 bp encoding 1061 amino acids. The deduced amino acid sequence of O. Goldiana sps1 shows 56% and 69% homology with those of maize SPS and spinach SPS, respectively. The high level expression of O. Goldiana sps1 in the flower suggests that it might play an important role in flowering. Growth under higher irradiance and elevated CO2 leads to an accumulation of the sps1 transcript in the photosynthetic leaves. It appears that SPS gene expression in photosynthetic leaves is associated with the leaf photosynthetic rate.

Possible structure and active site residues of starch, glycogen, and sucrose synthases

A group of enzymes that include muscle glycogen phosphorylase and sugar transferases involved in, for example, the glucosylation of DNA and the synthesis of peptidoglycan are known to possess the same basic three-dimensional fold. Here the possibility is examined that other monosaccharide transferases, those that catalyze synthesis of starch, glycogen, and the disaccharide sucrose, resemble the phosphorylase-type enzymes in structure. In particular, a clear relationship is shown, for the first time, between mammalian glycogen synthases and the phosphorylase structural group of proteins. Domain architecture and secondary structure are discussed, and the possible role of several conserved amino acids at the active site is explored.

Tissue-specific and developmental pattern of expression of the rice sps1 gene

Sucrose-phosphate synthase (SPS) is one of the key regulatory enzymes in carbon assimilation and partitioning in plants. SPS plays a central role in the production of sucrose in photosynthetic cells and in the conversion of starch or fatty acids into sucrose in germinating seeds. To explore the mechanisms that regulate the tissue-specific and developmental distribution of SPS, the expression pattern of rice (Oryza sativa) sps1 (GenBank accession no. U33175) was examined by in situ reverse transcriptase-polymerase chain reaction and the expression directed by the sps1 promoter using the beta-glucuronidase reporter gene. It was found that the expression of the rice sps1 gene is limited to mesophyll cells in leaves, the scutellum of germinating seedlings, and pollen of immature inflorescences. During leaf development, the sps1 promoter directs a basipetal pattern of expression that coincides with the distribution of SPS activity during the leaf sink-to-source transition. It was also found that during the vegetative part of the growth cycle, SPS expression and enzymatic activity are highest in the youngest fully expanded leaf. Additionally, it was observed that the expression of the sps1 promoter is regulated by light and dependent on plastid development in photosynthetic tissues, whereas expression in scutellum is independent of both light and plastid development.

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.

Sucrose-phosphate synthase from Synechocystis sp. strain PCC 6803: identification of the spsA gene and characterization of the enzyme expressed in Escherichia coli

The first identification and characterization of a prokaryotic gene (spsA) encoding sucrose-phosphate synthase (SPS) is reported for Synechocystis sp. strain PCC 6803, a unicellular non-nitrogen-fixing cyanobacterium. Comparisons of the deduced amino acid sequence and some relevant biochemical properties of the enzyme with those of plant SPSs revealed important differences in the N-terminal and UDP-glucose binding site regions, substrate specificities, molecular masses, subunit compositions, and regulatory properties.

Cloning and molecular analysis of cDNAs encoding three sucrose phosphate synthase isoforms from a citrus fruit (Citrus unshiu Marc.)

Three partial cDNA clones (pSPS1, pSPS2 and pSPS3) encoding sucrose phosphate synthases (SPS) were isolated by Reverse Transcription (RT)-PCR using first-strand cDNA prepared from the leaf or fruit of citrus (Citrus unshiu Marc.). The nucleotide and deduced amino acid sequences of the three clones showed significant similarities to SPS previously isolated in other plants. A full-length, cDNA clone, CitSPS1, was isolated from a fruit (juice sacs and pulp segment) cDNA library using one (pSPS1) of the three partial clones as a probe. The 3539-bp CitSPS1 clone coded for a 1057-amino acid polypeptide with a predicted molecular mass of 117.8 kDa. The amino acid sequence deduced from the CitSPS1 clone showed homology with SPS from maize (55.8% identity) and spinach (74.0% identity). Genomic Southern blot analysis suggested that CitSPS1 clone represents a lowcopy-number gene. RNA blot analysis of leaf, flower and fruit showed that CitSPS1 and pSPS2 were expressed in all organs. However, the levels of expression of CitSPS1 in young leaves, flowers and immature fruits were low, but high in mature leaves, and fruit. pSPS2 transcripts were barely detectable in young leaves and immature fruits, low in mature leaves, and high in flowers and mature fruits. pSPS3 transcripts were only detected in young and in mature leaves.

ROLE AND REGULATION OF SUCROSE-PHOSPHATE SYNTHASE IN HIGHER PLANTS

Sucrose-phosphate synthase (SPS; E.C. 2.4.1.14) is the plant enzyme thought to play a major role in sucrose biosynthesis. In photosynthetic and nonphotosynthetic tissues, SPS is regulated by metabolites and by reversible protein phosphorylation. In leaves, phosphorylation modulates SPS activity in response to light/dark signals and end-product accumulation. SPS is phosphorylated on multiple seryl residues in vivo, and the major regulatory phosphorylation site involved is Ser158 in spinach leaves and Ser162 in maize leaves. Regulation of the enzymatic activity of SPS appears to involve calcium, metabolites, and novel "coarse" control of the protein phosphatase that activates SPS. Activation of SPS also occurs during osmotic stress of leaf tissue in darkness, which may function to facilitate sucrose formation for osmoregulation. Manipulation of SPS expression in vivo confirms the role of this enzyme in the control of sucrose biosynthesis.

CARBOHYDRATE-MODULATED GENE EXPRESSION IN PLANTS

Plant gene responses to changing carbohydrate status can vary markedly. Some genes are induced, some are repressed, and others are minimally affected. As in microorganisms, sugar-sensitive plant genes are part of an ancient system of cellular adjustment to critical nutrient availability. However, in multicellular plants, sugar-regulated expression also provides a mechanism for control of resource distribution among tissues and organs. Carbohydrate depletion upregulates genes for photosynthesis, remobilization, and export, while decreasing mRNAs for storage and utilization. Abundant sugar levels exert opposite effects through a combination of gene repression and induction. Long-term changes in metabolic activity, resource partitioning, and plant form result. Sensitivity of carbohydrate-responsive gene expression to environmental and developmental signals further enhances its potential to aid acclimation. The review addresses the above from molecular to whole-plant levels and considers emerging models for sensing and transducing carbohydrate signals to responsive genes.

Expression analysis of a sucrose synthase gene from sugar beet (Beta vulgaris L.)

To investigate the expression pattern of sucrose synthase, a cDNA from tap roots of sugar beet (Beta vulgaris L.) was isolated using a heterologous sucrose synthase cDNA from potato. The 2762 bp long cDNA clone designated SBSS 1 encodes for a 822 amino acid polypeptide of a predicted molecular mass of 93.7 kDa. The deduced amino acid sequence of sugar beet sucrose synthase has homologies of 65-70% when compared to predicted amino acid sequences of sucrose synthases from other species. RNA blot analysis shows that SBSS1 is expressed most predominant in tap root under normal growth conditions. Cold treatment and anaerobiosis lead to an increase in the steady-state levels of SBSS 1 mRNA in leaf and root tissue. In tap root slices, sugars in various concentrations had no influence on the SBSS 1 transcript level. On the other hand, wounding resulted in a decreased transcript level.