Phosphorylation of serine-15 of maize leaf sucrose synthase. Occurrence in vivo and possible regulatory significance

Structure and Expression Analysis of PtrSUS, PtrINV, PtrHXK, PtrPGM, and PtrUGP Gene Families in Populus trichocarpa Torr. and Gray

Exogenous nitrogen and carbon can affect plant cell walls, which are composed of structural carbon. Sucrose synthase (SUS), invertase (INV), hexokinase (HXK), phosphoglucomutase (PGM), and UDP-glucose pyrophosphorylase (UGP) are the key enzymes of sucrose metabolism involved in cell wall synthesis. To understand whether these genes are regulated by carbon and nitrogen to participate in structural carbon biosynthesis, we performed genome-wide identification, analyzed their expression patterns under different carbon and nitrogen treatments, and conducted preliminary functional verification. Different concentrations of nitrogen and carbon were applied to poplar (Populus trichocarpa Torr. and Gray), which caused changes in cellulose, lignin, and hemicellulose contents. In poplar, 6 SUSs, 20 INVs, 6 HXKs, 4 PGMs, and 2 UGPs were identified. Moreover, the physicochemical properties, collinearity, and tissue specificity were analyzed. The correlation analysis showed that the expression levels of PtrSUS3/5, PtrNINV1/2/3/5/12, PtrCWINV3, PtrVINV2, PtrHXK5/6, PtrPGM1/2, and PtrUGP1 were positively correlated with the cellulose content. Meanwhile, the knockout of PtrNINV12 significantly reduced the cellulose content. This study could lay the foundation for revealing the functions of SUSs, INVs, HXKs, PGMs, and UGPs, which affected structural carbon synthesis regulated by nitrogen and carbon, proving that PtrNINV12 is involved in cell wall synthesis.

Identification of sucrose synthase from Micractinium conductrix to favor biocatalytic glycosylation

Sucrose synthase (SuSy, EC 2.4.1.13) is a unique glycosyltransferase (GT) for developing cost-effective glycosylation processes. Up to now, some SuSys derived from plants and bacteria have been used to recycle uridine 5'-diphosphate glucose in the reactions catalyzed by Leloir GTs. In this study, after sequence mining and experimental verification, a SuSy from Micractinium conductrix (McSuSy), a single-cell green alga, was overexpressed in Escherichia coli, and its enzymatic properties were characterized. In the direction of sucrose cleavage, the specific activity of the recombinant McSuSy is 9.39 U/mg at 37°C and pH 7.0, and the optimum temperature and pH were 60°C and pH 7.0, respectively. Its nucleotide preference for uridine 5'-diphosphate (UDP) was similar to plant SuSys, and the enzyme activity remained relatively high when the DMSO concentration below 25%. The mutation of the predicted N-terminal phosphorylation site (S31D) significantly stimulated the activity of McSuSy. When the mutant S31D of McSuSy was applied by coupling the engineered Stevia glycosyltransferase UGT76G1 in a one-pot two-enzyme reaction at 10% DMSO, 50 g/L rebaudioside E was transformed into 51.06 g/L rebaudioside M in 57 h by means of batch feeding, with a yield of 76.48%. This work may reveal the lower eukaryotes as a promising resource for SuSys of industrial interest.

Elshikh M, Jamil A, Nasir JA, Dvořáčková H, Dvořáček J. Genome-wide analysis of sucrose synthase family in soybean and their expression in response to abiotic stress and seed development

The sucrose synthase (SS) is an important enzyme family which play a vital role in sugar metabolism to improve the fruit quality of the plants. In many plant species, the members of SS family have been investigated but the detailed information is not available in legumes particularly and Glycine max specifically. In the present study, we found thirteen SS members (GmSS1-GmSS13) in G. max genome. High conserved regions were present in the GmSS sequences that may due to the selection pressure during evolutionary events. The segmental duplication was the major factor to increase the number of GmSS family members. The identified thirteen GmSS genes were divided into Class I, Class II and Class III with variable numbers of genes in each class. The protein interaction of GmSS gave the co-expression of sucrose synthase with glucose-1-phosphate adenylyltransferase while SLAC and REL test found number of positive sites in the coding sequences of SS family members. All the GmSS family members except GmSS7 and few of class III members, were highly expressed in all the soybean tissues. The expression of the class I members decreased during seed development, whireas, the class II members expression increased during the seed developing, may involve in sugar metabolism during seed development. Solexa sequencing libraries of acidic condition (pH 4.2) stress samples showed that the expression of class I GmSS genes increased 1- to 2-folds in treated samples than control. The differential expression pattern was observed between the members of a paralogous. This study provides detailed genome-wide analysis of GmSS family in soybean that will provide new insights for future evolutionary and soybean breeding to improve the plant growth and development.

Novel QTL and Meta-QTL Mapping for Major Quality Traits in Soybean

Isoflavone, protein, and oil are the most important quality traits in soybean. Since these phenotypes are typically quantitative traits, quantitative trait locus (QTL) mapping has been an efficient way to clarify their complex and unclear genetic background. However, the low-density genetic map and the absence of QTL integration limited the accurate and efficient QTL mapping in previous researches. This paper adopted a recombinant inbred lines (RIL) population derived from 'Zhongdou27'and 'Hefeng25' and a high-density linkage map based on whole-genome resequencing to map novel QTL and used meta-analysis methods to integrate the stable and consentaneous QTL. The candidate genes were obtained from gene functional annotation and expression analysis based on the public database. A total of 41 QTL with a high logarithm of odd (LOD) scores were identified through composite interval mapping (CIM), including 38 novel QTL and 2 Stable QTL. A total of 660 candidate genes were predicted according to the results of the gene annotation and public transcriptome data. A total of 212 meta-QTL containing 122 stable and consentaneous QTL were mapped based on 1,034 QTL collected from previous studies. For the first time, 70 meta-QTL associated with isoflavones were mapped in this study. Meanwhile, 69 and 73 meta-QTL, respectively, related to oil and protein were obtained as well. The results promote the understanding of the biosynthesis and regulation of isoflavones, protein, and oil at molecular levels, and facilitate the construction of molecular modular for great quality traits in soybean.

Effect of ammonium and nitrate supplies on nitrogen and sucrose metabolism of Cabernet Sauvignon (Vitis vinifera cv.)

BACKGROUND

The quality of wine is highly dependent on the quality of berries. Development of berries is influenced by the type and ratio of different nitrogen supplies in the soil. To understand the impact of varying sources and levels of nitrate and ammonium on sucrose and nitrogen metabolism of Vitis vinifera cv. Cabernet Sauvignon, we tested nutrient solutions with four NO₃ ^- -N:NH₄ ⁺ -N ratios (100:0, 75:25, 50:50, 0:100) through the root system.

RESULTS

The form and quantity of nitrogen affected berries and leaves with source-sink relationships. Soluble sugar levels were significantly higher in plants treated with mixed nitrogen sources (75:25 and 50:50) compared to single nitrogen sources (100:0 and 0:100). In particular, treating plants with mixed nitrogen source at a 75:25 ratio resulted in 22% higher fructose levels in berries compared to the 50:50 treatment. In addition, mixed nitrogen treatments resulted in significantly higher amino acid levels and protein content. Mixed nitrogen substrates also increased the expression of enzymes involved in both nitrogen and sucrose metabolism.

CONCLUSION

Plants did not maximize the nitrogen supply when single form nitrogen was provided, and the mixed nitrogen substrates consistently increased the amount of available carbon and nitrogen in the berries and leaves. We found that NO₃ ^- -N:NH₄ ⁺ -N ratio of 75:25 was the optimum formula for improving nitrogen and sucrose metabolism, and reducing the competition between nitrogen and sucrose. By examining the nutrient utilization of plants cultivated with different nitrogen forms, the present study provides insights into improving cultivation and production practices. © 2020 Society of Chemical Industry.

An Intrinsically Disordered Protein Interacts with the Cytoskeleton for Adaptive Root Growth under Stress

Intrinsically disordered proteins function as flexible stress modulators in vivo through largely unknown mechanisms. Here, we elucidated the mechanistic role of an intrinsically disordered protein, REPETITIVE PRO-RICH PROTEIN (RePRP), in regulating rice (Oryza sativa) root growth under water deficit. With nearly 40% Pro, RePRP is induced by water deficit and abscisic acid (ABA) in the root elongation zone. RePRP is sufficient and necessary for repression of root development by water deficit or ABA. We showed that RePRP interacts with the highly ordered cytoskeleton components actin and tubulin both in vivo and in vitro. Binding of RePRP reduces the abundance of actin filaments, thus diminishing noncellulosic polysaccharide transport to the cell wall and increasing the enzyme activity of Suc synthase. RePRP also reorients the microtubule network, which leads to disordered cellulose microfibril organization in the cell wall. The cell wall modification suppresses root cell elongation, thereby generating short roots, whereas increased Suc synthase activity triggers starch accumulation in "heavy" roots. Intrinsically disordered proteins control cell elongation and carbon reserves via an order-by-disorder mechanism, regulating the highly ordered cytoskeleton for development of "short-but-heavy" roots as an adaptive response to water deficit in rice.

Subcellular Targeting of Plant Sucrose Transporters Is Affected by Their Oligomeric State

Post-translational regulation of sucrose transporters represents one possibility to adapt transporter activity in a very short time frame. This can occur either via phosphorylation/dephosphorylation, oligomerization, protein-protein interactions, endocytosis/exocytosis, or degradation. It is also known that StSUT1 can change its compartmentalization at the plasma membrane and concentrate in membrane microdomains in response to changing redox conditions. A systematic screen for protein-protein-interactions of plant sucrose transporters revealed that the interactome of all three known sucrose transporters from the Solanaceous species Solanum tuberosum and Solanum lycopersicum represents a specific subset of interaction partners, suggesting different functions for the three different sucrose transporters. Here, we focus on factors that affect the subcellular distribution of the transporters. It was already known that sucrose transporters are able to form homo- as well as heterodimers. Here, we reveal the consequences of homo- and heterodimer formation and the fact that the responses of individual sucrose transporters will respond differently. Sucrose transporter SlSUT2 is mainly found in intracellular vesicles and several of its interaction partners are involved in vesicle traffic and subcellular targeting. The impact of interaction partners such as SNARE/VAMP proteins on the localization of SlSUT2 protein will be investigated, as well as the impact of inhibitors, excess of substrate, or divalent cations which are known to inhibit SUT1-mediated sucrose transport in yeast cells. Thereby we are able to identify factors regulating sucrose transporter activity via a change of their subcellular distribution.

Comparative phosphoproteomic analysis of developing maize seeds suggests a pivotal role for enolase in promoting starch synthesis

Maize (Zea mays) seeds are the major source of starch all over the world and the excellent model for researching starch synthesis. Seed starch content is a typical quantitative phenotype and many reports revealed that the glycolytic enzymes are involved in regulating starch synthesis, however the regulatory mechanism is still unclear. Here, we present a comparative phosphoproteomic study of three maize inbred lines with different seed starch content. It reveals that abundances of 62 proteins and 63 phosphoproteins were regulated during maize seed development. Dynamics of 17 enzymes related to glycolysis and starch synthesis were used to construct a phosphorylation regulatory network of starch synthesis. It shows that starch synthesis and glycolysis in maize seeds utilize the same hexose phosphates pool coming from sorbitol and sucrose as carbon source, and phosphorylation of ZmENO1 are suggested to contribute to increase starch content, because it is positively related to seed starch content in different developmental stages and different lines, and the phosphor-mimic mutant (ZmENO1^S43D) damaged its enzyme activity which is vital in glycolysis. Our results provide a new sight into regulatory process of seed starch synthesis and can be used in maize breeding for high starch content.

Overexpression of PsnSuSy1, 2 genes enhances secondary cell wall thickening, vegetative growth, and mechanical strength in transgenic tobacco

KEY MESSAGE

Two homologs PsnSuSy1 and PsnSuSy2 from poplar played largely similar but little distinct roles in modulating sink strength, accelerating vegetative growth and modifying secondary growth of plant. Co-overexpression of them together resulted in small but perceptible additive effects. Sucrose synthase (SuSy) acts as a crucial determinant of sink strength by controlling the conversion of sucrose into UDP-glucose, which is not only the sole precursor for cellulose biosynthesis but also an extracellular signaling molecule for plants growth. Therefore, modification of SuSy activity in plants is of utmost importance. We have isolated two SuSy genes from poplar, PsnSuSy1 and PsnSuSy2, which were preferentially expressed in secondary xylem/phloem. To investigate their functions, T2 tobacco transgenic lines of PsnSuSy1 and PsnSuSy2 were generated and then crossed to generate PsnSuSy1/PsnSuSy2 dual overexpression transgenic lines. SuSy activities in all lines were significantly increased though PsnSuSy1/PsnSuSy2 lines only exhibited slightly higher SuSy activities than either PsnSuSy1 or PsnSuSy2 lines. The significantly increased fructose and glucose, engendered by augmented SuSy activities, caused the alternations of many physiological, biochemical measures and phenotypic traits that include accelerated vegetative growth, thickened secondary cell wall, and increased stem breaking force, accompanied with altered expression levels of related pathway genes. The correlation relationships between SuSy activities and many of these traits were statistically significant. However, differences of almost all traits among three types of transgenic lines were insignificant. These findings clearly demonstrated that PsnSuSy1 and PsnSuSy2 had similar but little distinct functions and insubstantial additive effects on modulating sink strength and affecting allocation of carbon elements among secondary cell wall components.

Genome-wide identification and expression analysis of sucrose synthase genes in allotetraploid Brassica juncea

Sucrose plays pivotal role in energy metabolism and regulating gene expression of several physiological processes in higher plants. Here, fourteen sucrose synthase (SUS) genes have been identified in the allotetraploid genome of Indian mustard, Brassica juncea. The identified SUS genes in B. juncea (BjSUS) were derived from the two-progenitor species, B. rapa and B. nigra. Intron-exon analysis indicated loss or gain of 1-3 introns in diversification of SUS gene family. Phylogenetic analysis revealed discrete evolutionary paths for the BjSUS genes, originating from three ancestor groups, SUS I, SUS II and SUS III. Gene expression study revealed significant variability in expression of the BjSUS paralogs across the different tissues. BjSUS genes showed transcriptional activation in response to defense hormones and a late response to wounding. Tissue and temporal specificity of expression revealed importance of specific SUS paralogs at different developmental stages and under different stress conditions. The study highlighted differential involvement of SUS paralogs in sucrose metabolism across the tissues and stress-responses, in a major oilseed crop B. juncea.

Sucrose transport and carbon fluxes during wood formation

Wood biosynthesis defines the chemical and structural properties of wood. The metabolic pathways that produce the precursors of wood cell wall polymers have a central role in defining wood properties. To make rational design of wood properties feasible, we need not only to understand the cell wall biosynthetic machinery, but also how sucrose transport and metabolism in developing wood connect to cell wall biosynthesis and how they respond to genetic and environmental cues. Here, we review the current understanding of the sucrose transport and primary metabolism pathways leading to the precursors of cell wall biosynthesis in woody plant tissues. We present both old, persistent questions and new emerging themes with a focus on wood formation in trees and draw upon evidence from the xylem tissues of herbaceous plants when it is relevant.

Molecular regulation of sucrose catabolism and sugar transport for development, defence and phloem function

Sucrose (Suc) is the major end product of photosynthesis in mesophyll cells of most vascular plants. It is loaded into phloem of mature leaves for long-distance translocation to non-photosynthetic organs where it is unloaded for diverse uses. Clearly, Suc transport and metabolism is central to plant growth and development and the functionality of the entire vascular system. Despite vast information in the literature about the physiological roles of individual sugar metabolic enzymes and transporters, there is a lack of systematic evaluation about their molecular regulation from transcriptional to post-translational levels. Knowledge on this topic is essential for understanding and improving plant development, optimizing resource distribution and increasing crop productivity. We therefore focused our analyses on molecular control of key players in Suc metabolism and transport, including: (i) the identification of promoter elements responsive to sugars and hormones or targeted by transcription factors and microRNAs degrading transcripts of target genes; and (ii) modulation of enzyme and transporter activities through protein-protein interactions and other post-translational modifications. We have highlighted major remaining questions and discussed opportunities to exploit current understanding to gain new insights into molecular control of carbon partitioning for improving plant performance.

Phosphorylation of rice sucrose synthase isoforms promotes the activity of sucrose degradation

Sucrose is utilized as an initial material for production of the storage substances. Sucrose synthase reversibly catalyzes reactions of the sucrose degradation and its synthesis between sucrose with UDP and UDP-glucose with fructose. They also had the activity of the reactions for sucrose degradation of sucrose with ADP, and sucrose synthesis from ADP-glucose and fructose. Rice has three representative isoforms of sucrose synthase, Rsus1, Rsus2, and Rsus3, in which Rsus1 and Rsus3 are highly expressed in developing seeds. These three isoforms were phosphorylated by SPK, a calcium-dependent protein kinase. By phosphorylation, they showed increase of their reactivity for sucrose degradation on both reactions using UDP and ADP. In contrast, the synthetic activity of these isoforms was not altered by phosphorylation in any cases of the reactions with UDP-glucose and ADP-glucose. These results indicated that phosphorylation of sucrose synthase isoforms selectively led to enhance the reactivity for sucrose degradation.

A Role for Barley Calcium-Dependent Protein Kinase CPK2a in the Response to Drought

Increasing the drought tolerance of crops is one of the most challenging goals in plant breeding. To improve crop productivity during periods of water deficit, it is essential to understand the complex regulatory pathways that adapt plant metabolism to environmental conditions. Among various plant hormones and second messengers, calcium ions are known to be involved in drought stress perception and signaling. Plants have developed specific calcium-dependent protein kinases that convert calcium signals into phosphorylation events. In this study we attempted to elucidate the role of a calcium-dependent protein kinase in the drought stress response of barley (Hordeum vulgare L.), one of the most economically important crops worldwide. The ongoing barley genome project has provided useful information about genes potentially involved in the drought stress response, but information on the role of calcium-dependent kinases is still limited. We found that the gene encoding the calcium-dependent protein kinase HvCPK2a was significantly upregulated in response to drought. To better understand the role of HvCPK2a in drought stress signaling, we generated transgenic Arabidopsis plants that overexpressed the corresponding coding sequence. Overexpressing lines displayed drought sensitivity, reduced nitrogen balance index (NBI), an increase in total chlorophyll content and decreased relative water content. In addition, in vitro kinase assay experiments combined with mass spectrometry allowed HvCPK2a autophosphorylation sites to be identified. Our results suggest that HvCPK2a is a dual-specificity calcium-dependent protein kinase that functions as a negative regulator of the drought stress response in barley.

Sucrose synthase: A unique glycosyltransferase for biocatalytic glycosylation process development

Sucrose synthase (SuSy, EC 2.4.1.13) is a glycosyltransferase (GT) long known from plants and more recently discovered in bacteria. The enzyme catalyzes the reversible transfer of a glucosyl moiety between fructose and a nucleoside diphosphate (NDP) (sucrose+NDP↔NDP-glucose+fructose). The equilibrium for sucrose conversion is pH dependent, and pH values between 5.5 and 7.5 promote NDP-glucose formation. The conversion of a bulk chemical to high-priced NDP-glucose in a one-step reaction provides the key aspect for industrial interest. NDP-sugars are important as such and as key intermediates for glycosylation reactions by highly selective Leloir GTs. SuSy has gained renewed interest as industrially attractive biocatalyst, due to substantial scientific progresses achieved in the last few years. These include biochemical characterization of bacterial SuSys, overproduction of recombinant SuSys, structural information useful for design of tailor-made catalysts, and development of one-pot SuSy-GT cascade reactions for production of several relevant glycosides. These advances could pave the way for the application of Leloir GTs to be used in cost-effective processes. This review provides a framework for application requirements, focusing on catalytic properties, heterologous enzyme production and reaction engineering. The potential of SuSy biocatalysis will be presented based on various biotechnological applications: NDP-sugar synthesis; sucrose analog synthesis; glycoside synthesis by SuSy-GT cascade reactions.

Analysis of the sucrose synthase gene family in tobacco: structure, phylogeny, and expression patterns

MAIN CONCLUSION

Provide an evolutionary and an empirical molecular genetic foundation of the Sus gene family in tobacco and will be beneficial for further investigations of Sus gene functions Sucrose synthase (Sus) has been well characterized as the key enzyme participating in sucrose metabolism, and the gene family encoding different Sus isozymes has been cloned and characterized in several plant species. However, scant information about this gene family is available to date in tobacco. Here, we identified 14, 6, and 7 Sus genes in the genomes of Nicotiana tabacum, N. sylvestris and N. tomentosiformis, respectively. These tobacco Sus family members shared high levels of similarity in their nucleotide and amino acid sequences. Phylogenetic analysis revealed distinct evolutionary paths for the tobacco Sus genes. Sus1-4, Sus5, and Sus6-7 originated from three Sus precursors, respectively, which were generated by duplication before the split of monocots and eudicots. There were two additional duplications, before and after the differentiation of the Solanaceae, which separately gave rise to Sus3/4 and Sus1/2. Gene exon/intron structure analysis showed that the tobacco Sus genes contain varying numbers of conserved introns, resulting from intron loss under different selection pressures during the course of evolution. The expression patterns of the NtSus genes differed from each other in various tobacco tissues. Transcripts of Ntab0259170 and Ntab0259180 were detected in leaves at all tested developmental stages, suggesting that these two genes play a predominant role in sucrose metabolism during leaf development. Expression of Ntab0288750 and Ntab0234340 were conspicuously induced by low temperature and virus treatment, indicating that these two isozymes are important in meeting the increased glycolytic demand that occurs during abiotic stress. Our results provide an evolutionary and an empirical molecular genetic foundation of the Sus gene family in tobacco, and will be beneficial for further investigations of Sus gene functions in the processes of tobacco leaf development and tobacco resistance to environmental stresses.

Phosphorylation is an on/off switch for 5-hydroxyconiferaldehyde O-methyltransferase activity in poplar monolignol biosynthesis

Although phosphorylation has long been known to be an important regulatory modification of proteins, no unequivocal evidence has been presented to show functional control by phosphorylation for the plant monolignol biosynthetic pathway. Here, we present the discovery of phosphorylation-mediated on/off regulation of enzyme activity for 5-hydroxyconiferaldehyde O-methyltransferase 2 (PtrAldOMT2), an enzyme central to monolignol biosynthesis for lignification in stem-differentiating xylem (SDX) of Populus trichocarpa. Phosphorylation turned off the PtrAldOMT2 activity, as demonstrated in vitro by using purified phosphorylated and unphosphorylated recombinant PtrAldOMT2. Protein extracts of P. trichocarpa SDX, which contains endogenous kinases, also phosphorylated recombinant PtrAldOMT2 and turned off the recombinant protein activity. Similarly, ATP/Mn(2+)-activated phosphorylation of SDX protein extracts reduced the endogenous SDX PtrAldOMT2 activity by ∼ 60%, and dephosphorylation fully restored the activity. Global shotgun proteomic analysis of phosphopeptide-enriched P. trichocarpa SDX protein fractions identified PtrAldOMT2 monophosphorylation at Ser(123) or Ser(125) in vivo. Phosphorylation-site mutagenesis verified the PtrAldOMT2 phosphorylation at Ser(123) or Ser(125) and confirmed the functional importance of these phosphorylation sites for O-methyltransferase activity. The PtrAldOMT2 Ser(123) phosphorylation site is conserved across 93% of AldOMTs from 46 diverse plant species, and 98% of the AldOMTs have either Ser(123) or Ser(125). PtrAldOMT2 is a homodimeric cytosolic enzyme expressed more abundantly in syringyl lignin-rich fiber cells than in guaiacyl lignin-rich vessel cells. The reversible phosphorylation of PtrAldOMT2 is likely to have an important role in regulating syringyl monolignol biosynthesis of P. trichocarpa.

Biochemical and molecular characterization of RcSUS1, a cytosolic sucrose synthase phosphorylated in vivo at serine 11 in developing castor oil seeds

Sucrose synthase (SUS) catalyzes the UDP-dependent cleavage of sucrose into UDP-glucose and fructose and has become an important target for improving seed crops via metabolic engineering. A UDP-specific SUS homotetramer composed of 93-kDa subunits was purified to homogeneity from the triacylglyceride-rich endosperm of developing castor oil seeds (COS) and identified as RcSUS1 by mass spectrometry. RcSUS1 transcripts peaked during early development, whereas levels of SUS activity and immunoreactive 93-kDa SUS polypeptides maximized during mid-development, becoming undetectable in fully mature COS. The cytosolic location of the enzyme was established following transient expression of RcSUS1-enhanced YFP in tobacco suspension cells and fluorescence microscopy. Immunological studies using anti-phosphosite-specific antibodies revealed dynamic and high stoichiometric in vivo phosphorylation of RcSUS1 at its conserved Ser-11 residue during COS development. Incorporation of (32)P(i) from [γ-(32)P]ATP into a RcSUS1 peptide substrate, alongside a phosphosite-specific ELISA assay, established the presence of calcium-dependent RcSUS1 (Ser-11) kinase activity. Approximately 10% of RcSUS1 was associated with COS microsomal membranes and was hypophosphorylated relative to the remainder of RcSUS1 that partitioned into the soluble, cytosolic fraction. Elimination of sucrose supply caused by excision of intact pods of developing COS abolished RcSUS1 transcription while triggering the progressive dephosphorylation of RcSUS1 in planta. This did not influence the proportion of RcSUS1 associated with microsomal membranes but instead correlated with a subsequent marked decline in SUS activity and immunoreactive RcSUS1 polypeptides. Phosphorylation at Ser-11 appears to protect RcSUS1 from proteolysis, rather than influence its kinetic properties or partitioning between the soluble cytosol and microsomal membranes.

Phosphoproteome and proteome analyses reveal low-phosphate mediated plasticity of root developmental and metabolic regulation in maize (Zea mays L.)

Phosphate (Pi) deficiency has become a significant challenge to worldwide agriculture due to the depletion of accessible rock phosphate that is the major source of cheap Pi fertilizers. Previous research has identified a number of diverse adaptive responses to Pi starvation in the roots of higher plants. In this study, we found that accelerated axile root elongation of Pi-deprived maize plants resulted from enhanced cell proliferation. Comparative phosphoproteome and proteome profiles of maize axile roots were conducted in four stages in response to Pi deficiency by multiplex staining of high-resolution two dimensional gel separated proteins. Pro-Q DPS stained gels revealed that 6% of phosphoprotein spots displayed changes in phosphorylation state following low-Pi treatment. These proteins were involved in a large number of metabolic and cellular pathways including carbon metabolism and signal transduction. Changes in protein abundance of a number of enzymes indicated that low-Pi induced a number of carbon flux modifications in metabolic processes including sucrose breakdown and other downstream sugar metabolic pathways. A few key metabolic enzymes, including sucrose synthase (EC 2.4.1.13) and malate dehydrogenase (EC 1.1.1.37), and several signaling components involved in protein kinase or phosphatase cascades, auxin signaling and 14-3-3 proteins displayed low-Pi responsive changes in phosphorylation state or protein abundance. A variety of key enzymes and signaling components identified as potential targets for phosphorylation provide novel clues for comprehensive understanding of Pi regulation in plants. Protein phosphorylation, coordinating with changes in protein abundance, is required for maize root metabolic regulation and developmental acclimation to Pi starvation.

Structure and expression profile of the sucrose synthase gene family in the rubber tree: indicative of roles in stress response and sucrose utilization in the laticifers

Sucrose synthase (Sus, EC 2.4.1.13) is widely recognized as a key enzyme in sucrose metabolism in plants. However, nothing is known about this gene family in Hevea brasiliensis (para rubber tree). Here, we identified six Sus genes in H. brasiliensis that comprise the entire Sus family in this species. Analysis of the gene structure and phylogeny of the Sus genes demonstrates evolutionary conservation in the Sus families across Hevea and other plant species. The expression of Sus genes was investigated via Solexa sequencing and quantitative PCR in various tissues, at various phases of leaf development, and under abiotic stresses and ethylene treatment. The Sus genes exhibited distinct but partially redundant expression profiles. Each tissue has one abundant Sus isoform, with HbSus3, 4 and 5 being the predominant isoforms in latex (cytoplasm of rubber-producing laticifers), bark and root, respectively. HbSus1 and 6 were barely expressed in any tissue examined. In mature leaves (source), all HbSus genes were expressed at low levels, but HbSus3 and 4 were abundantly expressed in immature leaves (sink). Low temperature and drought treatments conspicuously induced HbSus5 expression in root and leaf, suggesting a role in stress responses. HbSus2 and 3 transcripts were decreased by ethylene treatment, consistent with the reduced sucrose-synthesizing activity of Sus enzymes in the latex in response to ethylene stimulation. Our results are beneficial to further determination of functions for the Sus genes in Hevea trees, especially roles in regulating latex regeneration.

Lignification of cell walls of infected cells in Casuarina glauca nodules that depend on symplastic sugar supply is accompanied by reduction of plasmodesmata number and narrowing of plasmodesmata

The oxygen protection system for the bacterial nitrogen-fixing enzyme complex nitrogenase in actinorhizal nodules of Casuarina glauca resembles that of legume nodules: infected cells contain large amounts of the oxygen-binding protein hemoglobin and are surrounded by an oxygen diffusion barrier. However, while in legume nodules infected cells are located in the central tissue, actinorhizal nodules are composed of modified lateral roots with infected cells in the expanded cortex. Since an oxygen diffusion barrier around the entire cortex would also block oxygen access to the central vascular system where it is required to provide energy for transport processes, here each individual infected cell is surrounded with an oxygen diffusion barrier. In order to assess the effect of these oxygen diffusion barriers on oxygen supply for energy production for transport processes, apoplastic and symplastic sugar transport pathways in C. glauca nodules were examined. The results support the idea that sugar transport to and within the nodule cortex relies to a large extent on the less energy-demanding symplastic mechanism. This is in line with the assumption that oxygen access to the nodule vascular system is substantially restricted. In spite of this dependence on symplastic transport processes to supply sugars to infected cells, plasmodesmal connections between infected cells, and to a lesser degree with uninfected cells, were reduced during the differentiation of infected cells.

Research progresses on the key enzymes involved in sucrose metabolism in maize

Sucrose, as the major product of photosynthesis, is a vital metabolite and signaling molecule in higher plants. Three enzymes are responsible for the synthesis, transport, and degradation of sucrose. In this article, the gene structure, expression and regulation, and the physiological functions of the key enzymes involved in sucrose metabolism in maize are reviewed, moreover, the existing problems of the sucrose metabolism research were discussed in detail, and we present our ideas for future research.

Analyses of the sucrose synthase gene family in cotton: structure, phylogeny and expression patterns

BACKGROUND

In plants, sucrose synthase (Sus) is widely considered as a key enzyme involved in sucrose metabolism. Several paralogous genes encoding different isozymes of Sus have been identified and characterized in multiple plant genomes, while limited information of Sus genes is available to date for cotton.

RESULTS

Here, we report the molecular cloning, structural organization, phylogenetic evolution and expression profiles of seven Sus genes (GaSus1 to 7) identified from diploid fiber cotton (Gossypium arboreum). Comparisons between cDNA and genomic sequences revealed that the cotton GaSus genes were interrupted by multiple introns. Comparative screening of introns in homologous genes demonstrated that the number and position of Sus introns are highly conserved among Sus genes in cotton and other more distantly related plant species. Phylogenetic analysis showed that GaSus1, GaSus2, GaSus3, GaSus4 and GaSus5 could be clustered together into a dicot Sus group, while GaSus6 and GaSus7 were separated evenly into other two groups, with members from both dicot and monocot species. Expression profiles analyses of the seven Sus genes indicated that except GaSus2, of which the transcripts was undetectable in all tissues examined, and GaSus7, which was only expressed in stem and petal, the other five paralogues were differentially expressed in a wide ranges of tissues, and showed development-dependent expression profiles in cotton fiber cells.

CONCLUSIONS

This is a comprehensive study of the Sus gene family in cotton plant. The results presented in this work provide new insights into the evolutionary conservation and sub-functional divergence of the cotton Sus gene family in response to cotton fiber growth and development.

The structure of sucrose synthase-1 from Arabidopsis thaliana and its functional implications

Sucrose transport is the central system for the allocation of carbon resources in vascular plants. During growth and development, plants control carbon distribution by coordinating sites of sucrose synthesis and cleavage in different plant organs and different cellular locations. Sucrose synthase, which reversibly catalyzes sucrose synthesis and cleavage, provides a direct and reversible means to regulate sucrose flux. Depending on the metabolic environment, sucrose synthase alters its cellular location to participate in cellulose, callose, and starch biosynthesis through its interactions with membranes, organelles, and cytoskeletal actin. The x-ray crystal structure of sucrose synthase isoform 1 from Arabidopsis thaliana (AtSus1) has been determined as a complex with UDP-glucose and as a complex with UDP and fructose, at 2.8- and 2.85-Å resolutions, respectively. The AtSus1 structure provides insights into sucrose catalysis and cleavage, as well as the regulation of sucrose synthase and its interactions with cellular targets.

Plasmodesmata distribution and sugar partitioning in nitrogen-fixing root nodules of Datisca glomerata

To understand carbon partitioning in roots and nodules of Datisca glomerata, activities of sucrose-degrading enzymes and sugar transporter expression patterns were analyzed in both organs, and plasmodesmal connections between nodule cortical cells were examined by transmission electron microscopy. The results indicate that in nodules, the contribution of symplastic transport processes is increased in comparison to roots, specifically in infected cells which develop many secondary plasmodesmata. Invertase activities are dramatically reduced in nodules as compared to roots, indicating that here the main enzyme responsible for the cleavage of sucrose is sucrose synthase. A high-affinity, low-specificity monosaccharide transporter whose expression is induced in infected cells prior to the onset of bacterial nitrogen fixation, and which has an unusually low pH optimum and may be involved in turgor control or hexose retrieval during infection thread growth.

Sugarcane genes associated with sucrose content

BACKGROUND

Sucrose content is a highly desirable trait in sugarcane as the worldwide demand for cost-effective biofuels surges. Sugarcane cultivars differ in their capacity to accumulate sucrose and breeding programs routinely perform crosses to identify genotypes able to produce more sucrose. Sucrose content in the mature internodes reach around 20% of the culms dry weight. Genotypes in the populations reflect their genetic program and may display contrasting growth, development, and physiology, all of which affect carbohydrate metabolism. Few studies have profiled gene expression related to sugarcane's sugar content. The identification of signal transduction components and transcription factors that might regulate sugar accumulation is highly desirable if we are to improve this characteristic of sugarcane plants.

RESULTS

We have evaluated thirty genotypes that have different Brix (sugar) levels and identified genes differentially expressed in internodes using cDNA microarrays. These genes were compared to existing gene expression data for sugarcane plants subjected to diverse stress and hormone treatments. The comparisons revealed a strong overlap between the drought and sucrose-content datasets and a limited overlap with ABA signaling. Genes associated with sucrose content were extensively validated by qRT-PCR, which highlighted several protein kinases and transcription factors that are likely to be regulators of sucrose accumulation. The data also indicate that aquaporins, as well as lignin biosynthesis and cell wall metabolism genes, are strongly related to sucrose accumulation. Moreover, sucrose-associated genes were shown to be directly responsive to short term sucrose stimuli, confirming their role in sugar-related pathways.

CONCLUSION

Gene expression analysis of sugarcane populations contrasting for sucrose content indicated a possible overlap with drought and cell wall metabolism processes and suggested signaling and transcriptional regulators to be used as molecular markers in breeding programs. Transgenic research is necessary to further clarify the role of the genes and define targets useful for sugarcane improvement programs based on transgenic plants.

Absolute quantification of Medicago truncatula sucrose synthase isoforms and N-metabolism enzymes in symbiotic root nodules and the detection of novel nodule phosphoproteins by mass spectrometry

Mass spectrometry (MS) has become increasingly important for tissue specific protein quantification at the isoform level, as well as for the analysis of protein post-translational regulation mechanisms and turnover rates. Thanks to the development of high accuracy mass spectrometers, peptide sequencing without prior knowledge of the amino acid sequence--de novo sequencing--can be performed. In this work, absolute quantification of a set of key enzymes involved in carbon and nitrogen metabolism in Medicago truncatula 'Jemalong A17' root nodules is presented. Among them, sucrose synthase (SuSy; EC 2.4.1.13), one of the central enzymes in sucrose cleavage in root nodules, has been further characterized and the relative phosphorylation state of the three most abundant isoforms has been quantified. De novo sequencing provided sequence information of a so far unidentified peptide, most probably belonging to SuSy2, the second most abundant isoform in M. truncatula root nodules. TiO(2)-phosphopeptide enrichment led to the identification of not only a phosphorylation site at Ser11 in SuSy1, but also of several novel phosphorylation sites present in other root nodule proteins such as alkaline invertase (AI; EC 3.2.1.26) and an RNA-binding protein.

Sucrose synthase oligomerization and F-actin association are regulated by sucrose concentration and phosphorylation

Sucrose synthase (SUS) is a key enzyme in plant metabolism, as it serves to cleave the photosynthetic end-product sucrose into UDP-glucose and fructose. SUS is generally assumed to be a tetrameric protein, but results in the present study suggest that SUS can form dimers as well as tetramers and that sucrose may be a regulatory factor for the oligomerization status of SUS. The oligomerization of SUS may also affect the cellular localization of the protein. We show that sucrose concentration modulates the ability of SUS1 to associate with F-actin in vitro and that calcium-dependent protein kinase-mediated phosphorylation of recombinant SUS1 at the Ser15 site is a negative regulator of its association with actin. Although high sucrose concentrations and hyperphosphorylation have been shown to promote SUS association with the plasma membrane, we show that the opposite is true for the SUS-actin association. We also show that SUS1 has a unique 28 residue coiled-coil domain that does not appear to play a role in oligomerization, but may prove to be significant in the future for interactions of SUS with other proteins. Collectively, these results highlight the multifaceted nature of SUS association with cellular structures.

Light and metabolic signals control the selective degradation of sucrose synthase in maize leaves during deetiolation

The content and activity of Suc (Suc) synthase (SUS) protein is high in sink organs but low in source organs. In this report, we examined light and metabolic signals regulating SUS protein degradation in maize (Zea mays) leaves during deetiolation. We found that SUS protein accumulated in etiolated leaves of the dark-grown seedlings but was rapidly degraded upon exposure to white, blue, or red light. This occurred concurrent with the accumulation of photosynthetic enzymes, such as Rubisco and Rubisco activase, and enzymes of Suc biosynthesis such as Suc-phosphate synthase. Deetiolation-induced SUS degradation was not inhibited by the proteasome inhibitor MG132. Moreover, neither full-length nor truncated SUS phosphorylated at the serine-170 site was found in the crude 26S proteasome fraction (150,000g postmicrosomal pellet) isolated in the presence of MG132. However, SUS degradation was strongly inhibited by feeding cycloheximide or amino acids to detached leaves, while Suc feeding had no effect. Of the amino acids tested, exogenous glutamate had the greatest effect. Collectively, these results demonstrate that SUS protein degradation during deetiolation: (1) is selective; (2) can be triggered by either blue- or red light-mediated signaling pathways; (3) does not involve the 26S proteasome; and (4) is inhibited by free amino acids. These findings suggest that SUS degradation is important to supply residues for the synthesis of other proteins required for autotrophic metabolism.

Serine/threonine/tyrosine protein kinase from Arabidopsis thaliana is dependent on serine residues for its activity

Genome-wide analysis of Arabidopsis thaliana with tyrosine kinase motif from animals predicted that tyrosine phosphorylation could be brought about only by dual-specificity protein kinases in plants. However, their regulation is poorly understood. In the present study, we have investigated the role of serines required for the activity of Arabidopsis thaliana serine/threonine/tyrosine protein kinase (AtSTYPK). There are eight serines in the kinase catalytic domain. The role of each serine residue was studied individually by substituting them with alanine. Serines at positions 215, 259, 269 and 315 are required for the kinase activity both in terms of auto and substrate phosphorylations of myelin basic protein. The mutant S265A showed slight increase in auto and substrate phosphorylations. Other serines at positions 165, 181 and 360 did not show any change in the phosphorylation status as compared to wild-type. In conclusion, these results suggest the importance of serine residues required for dual-specificity protein kinase activity.

Analysis of the sucrose synthase gene family in Arabidopsis

The properties and expression patterns of the six isoforms of sucrose synthase in Arabidopsis are described, and their functions are explored through analysis of T-DNA insertion mutants. The isoforms have generally similar kinetic properties. Although there is variation in sensitivity to substrate inhibition by fructose this is unlikely to be of major physiological significance. No two isoforms have the same spatial and temporal expression patterns. Some are highly expressed in specific locations, whereas others are more generally expressed. More than one isoform is expressed in all organs examined. Mutant plants lacking individual isoforms have no obvious growth phenotypes, and are not significantly different from wild-type plants in starch, sugar and cellulose content, seed weight or seed composition under the growth conditions employed. Double mutants lacking the pairs of similar isoforms sus2 and sus3, and sus5 and sus6, are also not significantly different in these respects from wild-type plants. These results are surprising in the light of the marked phenotypes observed when individual isoforms are eliminated in crop plants including pea, maize, potato and cotton. A sus1/sus4 double mutant grows normally in well-aerated conditions, but shows marked growth retardation and accumulation of sugars when roots are subjected to hypoxia. The sucrose synthase activity in roots of this mutant is 3% or less of wild-type activity. Thus under well-aerated conditions sucrose mobilization in the root can proceed almost entirely via invertases without obvious detriment to the plant, but under hypoxia there is a specific requirement for sucrose synthase activity.

Application of high performance anion exchange-pulsed amperometric detection to measure the activity of key sucrose metabolising enzymes in sugarcane

A novel method using an HPAE-PAD system, which is routinely applied to detect carbohydrates at low levels (ng per sample injection), has been applied to the measurement of key sucrose metabolising enzyme activities in partially purified extracts of sugarcane tissues. Extraction and assay procedures tailored for the HPAE-PAD system enabled the accurate measurement of enzyme activities in more mature internodes than had previously been possible using enzyme coupled assay methodology. A major advantage of the HPAE-PAD method is the capability to monitor a broad range of sugars in each assay and provides an overarching perspective of the mix of competing enzymes that may be operating simultaneously in crude extracts. The technique has been successfully applied to measuring the activity of key sucrose metabolising enzymes in sugarcane stem tissue that is generally low in protein and high in endogenous sugars, primarily sucrose.

Sugar signalling and gene expression in relation to carbohydrate metabolism under abiotic stresses in plants

Sucrose is required for plant growth and development. The sugar status of plant cells is sensed by sensor proteins. The signal generated by signal transduction cascades, which could involve mitogen-activated protein kinases, protein phosphatases, Ca 2+ and calmodulins, results in appropriate gene expression. A variety of genes are either induced or repressed depending upon the status of soluble sugars. Abiotic stresses to plants result in major alterations in sugar status and hence affect the expression of various genes by down- and up-regulating their expression. Hexokinase-dependent and hexokinase-independent pathways are involved in sugar sensing. Sucrose also acts as a signal molecule as it affects the activity of a proton-sucrose symporter. The sucrose trans-porter acts as a sucrose sensor and is involved in phloem loading. Fructokinase may represent an additional sensor that bypasses hexokinase phosphorylation especially when sucrose synthase is dominant. Mutants isolated on the basis of response of germination and seedling growth to sugars and reporter-based screening protocols are being used to study the response of altered sugar status on gene expression. Common cis-acting elements in sugar signalling pathways have been identified. Transgenic plants with elevated levels of sugars/sugar alcohols like fructans, raffinose series oligosaccharides, trehalose and mannitol are tolerant to different stresses but have usually impaired growth. Efforts need to be made to have transgenic plants in which abiotic stress responsive genes are expressed only at the time of adverse environmental conditions instead of being constitutively synthesized.

The three maize sucrose synthase isoforms differ in distribution, localization, and phosphorylation

Although sucrose synthase (SUS) is widely appreciated for its role in plant metabolism and growth, very little is known about the contribution of each of the SUS isoforms to these processes. Using isoform-specific antibodies, we evaluated the three known isoforms individually at the protein level. SUS1 and SUS-SH1 proteins have been studied previously; however, SUS2 (previously known as SUS3) has only been studied at the transcript level. Using SUS2 isoform-specific antibodies, we determined that this isoform is present in several maize tissues. The intracellular localization of all SUS isoforms was studied by cellular fractionation of leaves and developing kernels. Interestingly, SUS1 and SUS-SH1 were associated with membranes while SUS2 was not. The lack of membrane-associated SUS2 indicates that it might have a unique role in cytoplasmic sucrose metabolism. Using co-immunoprecipitation with kernel extracts, it was also established that SUS2 exists predominantly as a hetero-oligomer with SUS1, while SUS-SH1 forms only homo-oligomers. Using sequence-specific and phospho-specific antibodies, we have established for the first time that SUS-SH1 is phosphorylated in vivo at the Ser10 site in kernels, similar to the SUS1 Ser15 site. In midveins, additional evidence suggests that SUS can be phosphorylated at a novel C-terminal threonine site. Together, these results show that the isoforms of SUS are important in both cytosolic and membrane-associated sucrose degradation, but that their unique attributes most probably impart isoform-specific functional roles.

Protein phosphorylation and membrane association of sucrose synthase in developing tomato fruit

Calcium-dependent protein kinase (CDPK) activities were detected both in the soluble and the membrane fraction of various tomato (Lycopersicon esculentum Mill.) organs, using a synthetic peptide mimicking the serine 11 phosphorylation site of a tomato sucrose synthase (SS, EC 2.4.1.13) isoform as substrate. The levels of membrane and soluble Ser-CDPK activities were differentially regulated during fruit development. The membrane Ser-CDPK activity was maximal in young fruit but decreased as the fruit developed, suggesting a specific role during fruit growth. Using an in gel assay with purified tomato SS as substrate, we showed that partially purified soluble and membrane Ser-CDPK preparations both contained a SS-kinase polypeptide of 55 kDa. The membrane and soluble Ser-CDPK activities were largely inactivated in the absence of calcium or when MgCl(2) was replaced by MnCl(2). Both soluble and membrane Ser-CDPK activities were very sensitive to staurosporine. Using Fe(III)-immobilized metal chromatography to determine the apparent phosphorylation status of the enzyme in vivo, we showed that soluble SS was largely dephosphorylated in fruits fed EGTA or staurosporine, compared to fruits fed water or sucrose. Moreover, the level of SS increased by about two-fold in the membrane fraction of fruits fed the Ser-CDPK inhibitors, compared to the control. The level of SS protein in the membrane and soluble fractions of tomato fruit was developmentally regulated, the membrane form being specifically detected in actively growing fruits. Together, our results suggest that a mechanism involving protein phosphorylation/dephosphorylation and/or calcium would in part control the association of SS isoforms with membranes in developing tomato fruit.

AtCHIP functions as an E3 ubiquitin ligase of protein phosphatase 2A subunits and alters plant response to abscisic acid treatment

CHIP proteins are E3 ubiquitin ligases that promote degradation of Hsp70 and Hsp90 substrate proteins through the 26S proteasome in animal systems. A CHIP-like protein in Arabidopsis, AtCHIP, also has E3 ubiquitin ligase activity and has important roles to play under conditions of abiotic stress. In an effort to study the mode of action of AtCHIP in plant cells, proteins that physically interact with it were identified. Like its animal orthologs, AtCHIP interacts with a unique class of ubiquitin-conjugating enzymes (UBC or E2) that belongs to the stress-inducible UBC4/5 class in yeast. AtCHIP also interacts with other proteins, including an A subunit of protein phosphatase 2A (PP2A). This PP2A subunit appears to be a substrate of AtCHIP, because it can be ubiquitylated by AtCHIP in vitro and because the activity of PP2A is increased in AtCHIP-overexpressing plants in the dark or under low-temperature conditions. Unlike the rcn1 mutant, that has reduced PP2A activity due to a mutation in one of the A subunit genes of PP2A, AtCHIP-overexpressing plants are more sensitive to ABA treatment. Since PP2A was previously shown to be involved in low-temperature responses in plants, the low-temperature-sensitive phenotype observed in AtCHIP-overexpressing plants might be partly due to the change in PP2A activity. These data suggest that the E3 ubiquitin ligase AtCHIP may function upstream of PP2A in stress-responsive signal transduction pathways under conditions of low temperature or in the dark.

Site-specific phosphorylation of L-form starch phosphorylase by the protein kinase activity from sweet potato roots

A 78-amino acid insertion (L78) is found in the low-affinity type (L-form) of starch phosphorylase (L-SP, EC 2.4.1.1). This insertion blocks the starch-binding site on the L-SP molecule, and it decreases the binding affinity of L-SP toward starch. The computational analysis of the amino acid sequence on L78 predicts several phosphorylation sites at its Ser residues. Indeed, from the immunoblotting results using antibodies against phosphoamino acids, we observed that the purified L-SP from mature sweet potato (Ipomoea batatas) roots is phosphorylated. This observation led us to the detection of a protein kinase activity in the protein fraction of the crude extract from the sweet potato roots. The kinase was partially purified by liquid chromatography, and its native molecular mass was estimated as 338 kDa. An expressed peptide (L78P) containing the essential part of L78 was intensively phosphorylated by the kinase. However, H-SP (the high-affinity isomer of SP lacking the L78 insertion) and the proteolytic modified L-SP, which lost its L78 fragment, could not be phosphorylated. Furthermore, using L78P mutants by site-directed mutagenesis at Ser residues on L78, we demonstrate that only one Ser residue on L78 is phosphorylated by the kinase. These results imply that this kinase is specific to L-SP, or more precisely, to the L78 insertion. The in vitro phosphorylated L-SP shows higher sensitivity to proteolytic modification, but has no change in its kinetic parameters.

Expression of sucrose synthase genes involved in enhanced elongation of pondweed (Potamogeton distinctus) turions under anoxia

BACKGROUND AND AIMS

Overwintering buds (turions) of the monocot aquatic pondweed species (Potamogeton distinctus) are highly tolerant to anoxic stress. Sucrose metabolism accompanied by enhanced activity of sucrose synthase (SuSy) operates actively during anaerobic elongation of pondweed turions. The aim of this study is to isolate SuSy genes from the turions and to investigate their transcriptional changes in response to anoxia and other stimuli.

METHODS

SuSy genes were isolated from pondweed turions by PCR methods and transcript levels of SuSy genes were examined in response to anoxia, sugars and plant hormones. In addition, the effects of anoxia on SuSy activity were examined both in the soluble fraction and in the microsomal fraction.

KEY RESULTS

cDNAs of two SuSy genes (PdSUS1 and PdSUS2) were cloned from pondweed turions. The levels of PdSUS1 transcripts increased under anoxia but did not with sugar treatments. Anoxia-stimulated elongation of turions was further enhanced by 2,4-dichlorophenoxyacetic acid (2,4-D) and suppressed by treatments with sorbitol, 2-deoxyglucose (2-dGlc) and abscisic acid (ABA). The levels of PdSUS1 transcripts were increased by 2,4-D and decreased by sorbitol under anoxia. The levels of PdSUS2 transcripts were not significantly affected by anoxia and any other treatments. SuSy activity of turions under anoxia was enhanced in the soluble fraction, but not in the microsomal fraction.

CONCLUSIONS

Up-regulation of PdSUS1 transcription under anoxia may not be attributed to sugar starvation under anoxia. A positive correlation between stem elongation and the level of PdSUS1 transcripts was observed in turions treated with anoxic conditions, 2,4-D and sorbitol. The increase in SuSy activity in the cytosol may contribute to sugar metabolism and sustain stem elongation under anoxia.

Sucrose synthase controls both intracellular ADP glucose levels and transitory starch biosynthesis in source leaves

The prevailing model on transitory starch biosynthesis in source leaves assumes that the plastidial ADPglucose (ADPG) pyrophosphorylase (AGP) is the sole enzyme catalyzing the synthesis of the starch precursor molecule, ADPG. However, recent investigations have shown that ADPG linked to starch biosynthesis accumulates outside the chloroplast, presumably in the cytosol. This finding is consistent with the occurrence of an 'alternative' gluconeogenic pathway wherein sucrose synthase (SuSy) is involved in the production of ADPG in the cytosol, whereas both plastidial phosphoglucomutase (pPGM) and AGP play a prime role in the scavenging of starch breakdown products. To test this hypothesis, we have compared the ADPG content in both Arabidopsis and potato wild-type (WT) leaves with those of the starch-deficient mutants with reduced pPGM and AGP. These analyses provided evidence against the 'classical' model of starch biosynthesis, since ADPG levels in all the starch-deficient lines were normal compared with WT plants. Whether or not SuSy is involved in the synthesis of ADPG accumulating in leaves was tested by characterizing both SuSy-overexpressing and SuSy-antisensed transgenic leaves. Importantly, SuSy-overexpressing leaves exhibited a large increase of both ADPG and starch levels compared with WT leaves, whereas SuSy-antisensed leaves accumulated low amounts of both ADPG and starch. These findings show that (i) ADPG produced by SuSy is linked to starch biosynthesis; (ii) SuSy exerts a strong control on the starch biosynthetic process; and (iii) SuSy, but not AGP, controls the production of ADPG accumulating in source leaves.

Partial purification and characterisation of sucrose synthase in sugarcane

Three sucrose synthase (SuSy) (EC 2.4.1.13) forms were isolated from sugarcane leaf roll tissue. During anion exchange chromatography, one peak of activity (SuSyA) eluted during the wash step and the other peak (SuSyB) during the salt gradient phase at 180mM KCl concentration. A third form of activity (SuSyC), which also eluted at 180mM KCl, was also present in the leaf roll and replaced SuSyB depending on the season of the year. Substrate Km values, as well as sucrose breakdown/synthesis ratios, differed between these forms. For SuSyA, SuSyB, and SuSyC, respectively, Km values+/-SE (mM) were: 41.8+/-3.4, 109+/-23, and 35.9+/-2.3 for sucrose, 1.07+/-0.08, 0.214+/-0.039, and 0.00191+/-0.00019 for UDP, 6.62+/-1.55, 11.7+/-2.6, and 6.49+/-0.61 for fructose, and 3.59+/-0.37, 0.530+/-0.142, and 0.234+/-0.025 for UDP-glucose. Sucrose breakdown/synthesis ratios+/-SE were 0.0791+/-0.0199, 0.330+/-0.180, and 0.426+/-0.069 for SuSyA, SuSyB, and SuSyC, respectively. The ratio of the area of peak 1 (low breakdown/synthesis ratio) to the area of peak 2 (high breakdown/synthesis ratio) in sucrose accumulating tissue (internode 9) was 0.88, while in non-accumulating (leaf roll) tissue it was 14.5 at the same time of year. The molecular mass of the denatured subunits of all three forms was 94kDa by SDS-PAGE. A polyclonal antiserum raised against SuSyB cross-reacted with all three forms on an immunoblot, but only SuSyA and SuSyB were immunoinactivated by this serum.

Variations in the level of enzyme activity and immunolocalization of calcium-dependent protein kinases in the phloem of different cucumber organs

Calcium-dependent protein kinases (CDPKs) constitute a unique family of enzymes in plants that are characterized by a C-terminal calmodulin (CaM)-like domain. Through protein kinase assays, we have examined the levels of cucumber calcium-dependent kinase (CsCDPK) activity in various organs of cucumber seedlings and plants. The activity of CsCDPK was highest in cucumber plant leaves followed by seedling roots and hypocotyls; however, cucumber plant flowers, seedling cotyledons, and hooks had levels that were barely detectable. The CsCDPKs were immunolocalized using polyclonal antibodies that are highly specific against a part of the kinase domain of a calcium-dependent protein kinase (CsCDPKS) in the phloem sieve elements (SEs) in various organs of cucumber. In addition, this study indicates the presence of CsCDPKs in organelle-like bodies associated with the plasma membrane of sieve elements in mature stems and roots as well as in the storage bodies of immature seeds. These findings are discussed in terms of the likely roles played by CDPKs in the signal transduction pathways for Ca2+-regulated phloem transport of assimilates from leaves to various organs during growth and development of cucumber seedlings and plants.

ARC3, a chloroplast division factor, is a chimera of prokaryotic FtsZ and part of eukaryotic phosphatidylinositol-4-phosphate 5-kinase

The arc3 (accumulation and replication of chloroplast) mutant of Arabidopsis thaliana has a small number of abnormally large chloroplasts in the cell, suggesting that chloroplast division is arrested in the mutant and ARC3 has an important role in the initiation of chloroplast division. To elucidate the role of ARC3, first we identified the ARC3 gene, and determined the location of ARC3 protein during chloroplast division because the localization and spatial orientation of such division factors are vital for correct chloroplast division. Sequencing analysis showed that ARC3 was a fusion of the prokaryotic FtsZ and part of the eukaryotic phosphatidylinositol-4-phosphate 5-kinase (PIP5K) genes. The PIP5K-homologous region of ARC3 had no catalytic domain but a membrane-occupation-and-recognition-nexus (MORN) repeat motif. Immunofluorescence microscopy, Western blotting analysis and in vitro chloroplast import and protease protection assays revealed that ARC3 protein was soluble, and located on the outer surface of the chloroplast in a ring-like structure at the early stage of chloroplast division. Prokaryotes have one FtsZ as a gene for division but have no ARC3 counterparts, the chimera of FtsZ and PIP5K, suggesting that the ARC3 gene might have been generated from FtsZ as another division factor during the evolution of chloroplast by endosymbiosis.

Sucrose metabolism: regulatory mechanisms and pivotal roles in sugar sensing and plant development

Sucrose cleavage is vital to multicellular plants, not only for the allocation of crucial carbon resources but also for the initiation of hexose-based sugar signals in importing structures. Only the invertase and reversible sucrose synthase reactions catalyze known paths of sucrose breakdown in vivo. The regulation of these reactions and its consequences has therefore become a central issue in plant carbon metabolism. Primary mechanisms for this regulation involve the capacity of invertases to alter sugar signals by producing glucose rather than UDPglucose, and thus also two-fold more hexoses than are produced by sucrose synthase. In addition, vacuolar sites of cleavage by invertases could allow temporal control via compartmentalization. In addition, members of the gene families encoding either invertases or sucrose synthases respond at transcriptional and posttranscriptional levels to diverse environmental signals, including endogenous changes that reflect their own action (e.g. hexoses and hexose-responsive hormone systems such as abscisic acid [ABA] signaling). At the enzyme level, sucrose synthases can be regulated by rapid changes in sub-cellular localization, phosphorylation, and carefully modulated protein turnover. In addition to transcriptional control, invertase action can also be regulated at the enzyme level by highly localized inhibitor proteins and by a system that has the potential to initiate and terminate invertase activity in vacuoles. The extent, path, and site of sucrose metabolism are thus highly responsive to both internal and external environmental signals and can, in turn, dramatically alter development and stress acclimation.

Phosphorylation of the amino terminus of maize sucrose synthase in relation to membrane association and enzyme activity

Sucrose synthase (SUS) is phosphorylated on a major, amino-terminal site located at Ser-15 (S15) in the maize (Zea mays) SUS1 protein. Site- and phospho-specific antibodies against a phosphorylated S15 (pS15) peptide allowed direct analysis of S15 phosphorylation in relation to membrane association. Immunoblots of the maize leaf elongation zone, divided into 4-cm segments, demonstrated that the abundance of soluble (s-SUS) and membrane (m-SUS) SUS protein showed distinct positional profiles. The content of m-SUS was maximal in the 4- to 8-cm segment where it represented 9% of total SUS and occurred as a peripheral membrane protein. In contrast, s-SUS was highest in the 12- to 16-cm segment. Relative to s-SUS, m-SUS was hypophosphorylated at S15 in the basal 4 cm but hyperphosphorylated in apical segments. Differing capabilities of the anti-pS15 and anti-S15 peptide antibodies to immunoprecipitate SUS suggested that phosphorylation of S15, or exposure of unphosphorylated SUS to slightly acidic pH, altered the structure of the amino terminus. These structural changes were generally coincident with the increased sucrose cleavage activity that occurs at pH values below 7.5. In vitro S15 phosphorylation of the S170A SUS protein by a maize calcium-dependent protein kinase (CDPK) significantly increased sucrose cleavage activity at low pH. Collectively, the results suggest that (1) SUS membrane binding is controlled in vivo; (2) relative pS15 content of m-SUS depends on the developmental state of the organ; and (3) phosphorylation of S15 affects amino-terminal conformation in a way that may stimulate the catalytic activity of SUS and influence membrane association.

Proteasome activity and the post-translational control of sucrose synthase stability in maize leaves

The serine-170 (S170) calcium-dependent protein kinase phosphorylation site of maize (Zea mays L.) sucrose synthase (SUS) (EC 2.4.1.13) has been implicated in the post-translational regulation of SUS protein stability. To clarify the proteolytic process and the role of phosphorylation, SUS degradation and proteasome activities were studied in the maize leaf elongation zone. Size-exclusion chromatography resolved two peaks of proteasome-like proteolytic activity. The large molecular mass ( approximately 1350 kDa) peak required Mg(2+) and ATP for maximal activity and was inhibited by the proteasome inhibitors MG132 and NLVS. Anion-exchange chromatography resolved a similar proteolytic activity that was activated by ATP, characteristics that are consistent with those of a 26S-proteasome. Appropriately, immunoblotting revealed the presence of a 26S-proteasome subunit and highly ubiquitinated proteins within the active fractions eluted from both columns. The smaller molecular mass ( approximately 600 kDa) peak represented only 40% of the total proteasome-like activity and is likely a maize 20S-proteasome as it was activated in vitro by low levels of sodium dodecyl sulfate (SDS). S170 phosphorylated SUS (pS170-SUS) was detected as both high molecular mass (HMM) forms and proteolytic fragments that co-eluted with 26S-proteasome activities on both size-exclusion and anion-exchange columns. Conditions that maintained maximal 26S-proteasome activity reduced the amounts of pS170-SUS recovered. In vitro, the 26S-proteasome degraded SUS and proteasome-specific inhibitors reduced SUS proteolysis. HMM-SUS conjugates were produced in vitro and immunoprecipitations suggested that some SUS might be ubiquitinated in vivo. The results suggest that S170 phosphorylation promotes the formation of HMM, ubiquitin-SUS conjugates that can be targeted for 26S-proteasome-dependent degradation.

Expression, purification and characterization of recombinant sucrose synthase 1 from Solanum tuberosum L. for carbohydrate engineering

The gene sus1 from Solanum tuberosum L. encoding for sucrose synthase 1 was cloned into the plasmid pDR195 under the control of the PMA1 promotor. After transformation of Saccharomyces cerevisiae strain 22574d sus1 was constitutively expressed giving a specific activity of 0.3Umg(-1) protein in the crude extract. A one-step purification by Q-Sepharose resulted in an 14-fold purified enzyme preparation in 74% yield. SuSy1 was subsequently purified by immobilized metal ion affinity chromatography and characterized for its utilization in synthesizing different nucleotide sugars and sucrose analogues. The kinetic constants for the cleavage and synthesis reaction were determined: K(m) (UDP) 4microM; K(iS) (UDP) 0.11mM; K(m) (sucrose) 91.6mM; K(m) (UDP-Glc) 0.5mM; K(iS) (UDP-Glc) 2.3mM; K(m) (D-fructose) 2.1mM; K(iS) (D-fructose) 35.9mM. Different nucleoside diphosphates as well as different donor substrate were accepted as follows: UDP>dTDP>ADP>CDP>GDP in the cleavage reaction and UDP-Glc>dTDP-Glc>ADP-Glc>CDP-Glc in the synthesis reaction. SuSy1 shows also a broad acceptance of D- and L-ketoses and D- and L-aldoses. The acceptance of aldoses was deduced from the binding of the inhibitor 5-deoxy-D-fructose (K(i) 0.3mM), an analogue of the natural substrate D-fructopyranoside. The broad substrate spectrum renders SuSy1 from potato a versatile biocatalyst for carbohydrate engineering.