Evidence for plasma membrane-associated forms of sucrose synthase in maize

easyMF: A Web Platform for Matrix Factorization-Based Gene Discovery from Large-scale Transcriptome Data

With the development of high-throughput experimental technologies, large-scale RNA sequencing (RNA-Seq) data have been and continue to be produced, but have led to challenges in extracting relevant biological knowledge hidden in the produced high-dimensional gene expression matrices. Here, we develop easyMF ( https://github.com/cma2015/easyMF ), a web platform that can facilitate functional gene discovery from large-scale transcriptome data using matrix factorization (MF) algorithms. Compared with existing MF-based software packages, easyMF exhibits several promising features, such as greater functionality, flexibility and ease of use. The easyMF platform is equipped using the Big-Data-supported Galaxy system with user-friendly graphic user interfaces, allowing users with little programming experience to streamline transcriptome analysis from raw reads to gene expression, carry out multiple-scenario MF analysis, and perform multiple-way MF-based gene discovery. easyMF is also powered with the advanced packing technology to enhance ease of use under different operating systems and computational environments. We illustrated the application of easyMF for seed gene discovery from temporal, spatial, and integrated RNA-Seq datasets of maize (Zea mays L.), resulting in the identification of 3,167 seed stage-specific, 1,849 seed compartment-specific, and 774 seed-specific genes, respectively. The present results also indicated that easyMF can prioritize seed-related genes with superior prediction performance over the state-of-art network-based gene prioritization system MaizeNet. As a modular, containerized and open-source platform, easyMF can be further customized to satisfy users' specific demands of functional gene discovery and deployed as a web service for broad applications.

Switch from symplasmic to aspoplasmic phloem unloading in Xanthoceras sorbifolia fruit and sucrose influx XsSWEET10 as a key candidate for Sugar transport

The process of phloem unloading and post-unloading transport of photoassimilate is critical to crop output. Xanthoceras sorbifolia is a woody oil species with great biomass energy prospects in China; however, underproduction of seeds seriously restricts its development. Here, our cytological studies by ultrastructural observation revealed that the sieve element-companion cell complex in carpellary bundle was symplasmically interconnected with surrounding parenchyma cells at the early and late fruit developmental stages, whereas it was symplasmically isolated at middle stage. Consistently, real-time imaging showed that fluorescent tracer 6(5)carboxyfluorescein was confined to phloem strands at middle stage but released into surrounding parenchymal cells at early and late stages. Enzymatic assay showed that sucrose synthase act as the key enzyme catalyzing the progress of Suc degradation post-unloading pathway whether in pericarp or in seed, while vacuolar acid invertase and neutral invertase play compensation roles in sucrose decomposition. Sugar transporter XsSWEET10 had a high expression profile in fruit, especially at middle stage. XsSWEET10 is a plasma membrane-localized protein and heterologous expression in SUC2-deficient yeast strain SUSY7/ura3 confirmed its ability to uptake sucrose. These findings approved the transition from symplasmic to apoplasmic phloem unloading in Xanthoceras sorbifolia fruit and XsSWEET10 as a key candidate in sugar transport.

Purification and Biochemical Characterization of Sucrose synthase from the Stem of Nettle (Urtica dioica L.)

Sucrose synthase is a key enzyme in sucrose metabolism as it saves an important part of sucrose energy in the uridine-5'-diphosphate glucose (UDP-glucose) molecule. As such it is also involved in the synthesis of fundamental molecules such as callose and cellulose, the latter being present in all cell walls of plant cells and therefore also in the gelatinous cell walls of sclerenchyma cells such as bast fibers. Given the importance of these cells in plants of economic interest such as hemp, flax and nettle, in this work we have studied the occurrence of Sucrose synthase in nettle stems by analyzing its distribution between the cytosol, membranes and cell wall. We have therefore developed a purification protocol that can allow the analysis of various characteristics of the enzyme. In nettle, Sucrose synthase is encoded by different genes and each form of the enzyme could be subjected to different post-translational modifications. Therefore, by two-dimensional electrophoresis analysis, we have also traced the phosphorylation profile of Sucrose synthase isoforms in the various cell compartments. This information paves the way for further investigation of Sucrose synthase in plants such as nettle, which is both economically important, but also difficult to study.

Colocalization of sucrose synthase expression and sucrose storage in the sugarbeet taproot indicates a potential role for sucrose catabolism in sucrose accumulation

Sucrose metabolism is believed to have a central role in promoting sink strength and sucrose storage in the sugarbeet taproot. How sucrose accumulation is increased by sucrose-degrading enzymes, however, is a paradox. To elucidate roles for sucrose-degrading activities in sucrose accumulation, relationships between the intercellular location of sucrose-catabolizing enzymes and sites of sucrose accumulation were determined in the sugarbeet taproot. Sucrose storage was evident in parenchyma cells of the outer cortex, rays, and rings of parenchyma tissue, but was absent in phloem, the vascular cambium, cells surrounding these tissues, or cells surrounding xylem. Sucrose synthase, which was primarily responsible for sucrose catabolism throughout the taproot, was expressed in similar cell and tissue types to those accumulating sucrose. Colocalization of sucrose synthase with sucrose accumulation, as well as sucrose synthase localization near the tonoplast, suggests a role for the enzyme in generating metabolic energy to fuel sucrose sequestration in the vacuole. Localization near the plasma membrane also suggests a role for sucrose synthase in supplying substrates for cell wall biosynthesis. By utilizing sucrose for ATP or cell wall biosynthesis, sucrose synthase likely maintains the source-to-sink sucrose gradient that drives sucrose transport into the root, thereby promoting sugarbeet root sink strength.

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.

An Overview of Sucrose Synthases in Plants

Sucrose is the end product of photosynthesis and the primary sugar transported in the phloem of most plants. Sucrose synthase (SuSy) is a glycosyl transferase enzyme that plays a key role in sugar metabolism, primarily in sink tissues. SuSy catalyzes the reversible cleavage of sucrose into fructose and either uridine diphosphate glucose (UDP-G) or adenosine diphosphate glucose (ADP-G). The products of sucrose cleavage by SuSy are available for many metabolic pathways, such as energy production, primary-metabolite production, and the synthesis of complex carbohydrates. SuSy proteins are usually homotetramers with an average monomeric molecular weight of about 90 kD (about 800 amino acids long). Plant SuSy isozymes are mainly located in the cytosol or adjacent to plasma membrane, but some SuSy proteins are found in the cell wall, vacuoles, and mitochondria. Plant SUS gene families are usually small, containing between four to seven genes, with distinct exon-intron structures. Plant SUS genes are divided into three separate clades, which are present in both monocots and dicots. A comprehensive phylogenetic analysis indicates that a first SUS duplication event may have occurred before the divergence of the gymnosperms and angiosperms and a second duplication event probably occurred in a common angiosperm ancestor, leading to the existence of all three clades in both monocots and dicots. Plants with reduced SuSy activity have been shown to have reduced growth, reduced starch, cellulose or callose synthesis, reduced tolerance to anaerobic-stress conditions and altered shoot apical meristem function and leaf morphology. Plants overexpressing SUS have shown increased growth, increased xylem area and xylem cell-wall width, and increased cellulose and starch contents, making SUS high-potential candidate genes for the improvement of agricultural traits in crop plants. This review summarizes the current knowledge regarding plant SuSy, including newly discovered possible developmental roles for SuSy in meristem functioning that involve sugar and hormonal signaling.

Plant Fructokinases: Evolutionary, Developmental, and Metabolic Aspects in Sink Tissues

Sucrose, a glucose-fructose disaccharide, is the main sugar transported in the phloem of most plants and is the origin of most of the organic matter. Upon arrival in sink tissues, the sucrose must be cleaved by invertase or sucrose synthase. Both sucrose-cleaving enzymes yield free fructose, which must be phosphorylated by either fructokinase (FRK) or hexokinase (HXK). The affinity of FRK to fructose is much higher than that of HXK, making FRKs central for fructose metabolism. An FRK gene family seems to exist in most, if not all plants and usually consists of several cytosolic FRKs and a single plastidic FRK. These genes are expressed mainly in sink tissues such as roots, stems, flowers, fruits, and seeds, with lower levels of expression often seen in leaves. Plant FRK enzymes vary in their biochemical properties such as affinity for fructose, inhibition by their substrate (i.e., fructose), and expression level in different tissues. This review describes recently revealed roles of plant FRKs in plant development, including the combined roles of the plastidic and cytosolic FRKs in vascular tissues and seed development.

Sucrose phloem unloading follows an apoplastic pathway with high sucrose synthase in Actinidia fruit

Phloem unloading plays a pivotal role in photoassimilate partitioning and the accumulation of sugars in sink organs, e.g. fruit. Here, we investigated the pathway of sucrose unloading in kiwifruit (Actinidia deliciasa cv. Qinmei) using a combination of electron microscopy, transport of the phloem-mobile symplastic tracer carboxyfluorescein and enzyme activity and gene expression assays. Our structural investigation revealed that the sieve element-companion cell complex of bundles feeding the fruit flesh was symplastically isolated from its surrounding parenchyma cells throughout fruit development, whereas numerous plasmodesmata were present between the phloem parenchyma cells. Consistent with this, carboxyfluorescein unloading showed that the dye remained confined in the phloem strands during fruit development. The activities and expression of cell wall acid invertase in fruit flesh were lower than those of other enzymes that catalyze sucrose dissociation. However, sucrose synthase showed higher enzyme activities and mRNA expression in fruit flesh compared with other detected enzymes. These results imply that, in kiwifruit flesh, phloem unloading of sucrose is predominantly an apoplastic pathway during fruit development, and that sucrose synthase is a key enzyme for sucrose post-unloading pathways.

Genome-Wide Association Study Identifies Candidate Genes for Starch Content Regulation in Maize Kernels

Kernel starch content is an important trait in maize (Zea mays L.) as it accounts for 65-75% of the dry kernel weight and positively correlates with seed yield. A number of starch synthesis-related genes have been identified in maize in recent years. However, many loci underlying variation in starch content among maize inbred lines still remain to be identified. The current study is a genome-wide association study that used a set of 263 maize inbred lines. In this panel, the average kernel starch content was 66.99%, ranging from 60.60 to 71.58% over the three study years. These inbred lines were genotyped with the SNP50 BeadChip maize array, which is comprised of 56,110 evenly spaced, random SNPs. Population structure was controlled by a mixed linear model (MLM) as implemented in the software package TASSEL. After the statistical analyses, four SNPs were identified as significantly associated with starch content (P ≤ 0.0001), among which one each are located on chromosomes 1 and 5 and two are on chromosome 2. Furthermore, 77 candidate genes associated with starch synthesis were found within the 100-kb intervals containing these four QTLs, and four highly associated genes were within 20-kb intervals of the associated SNPs. Among the four genes, Glucose-1-phosphate adenylyltransferase (APS1; Gene ID GRMZM2G163437) is known as an important regulator of kernel starch content. The identified SNPs, QTLs, and candidate genes may not only be readily used for germplasm improvement by marker-assisted selection in breeding, but can also elucidate the genetic basis of starch content. Further studies on these identified candidate genes may help determine the molecular mechanisms regulating kernel starch content in maize and other important cereal crops.

Functional and expression analyses of transcripts based on full-length cDNAs of Sorghum bicolor

Sorghum bicolor is one of the most important crops for food and bioethanol production. Its small diploid genome and resistance to environmental stress make sorghum an attractive model for studying the functional genomics of the Saccharinae and other C₄ grasses. We analyzed the domain-based functional annotation of the cDNAs using the gene ontology (GO) categories for molecular function to characterize all the genes cloned in the full-length cDNA library of sorghum. The sorghum cDNA library successfully captured a wide range of cDNA-encoded proteins with various functions. To characterize the protein function of newly identified cDNAs, a search of their deduced domains and comparative analyses in the Oryza sativa and Zea mays genomes were carried out. Furthermore, genes on the sense strand corresponding to antisense transcripts were classified based on the GO of molecular function. To add more information about these genes, we have analyzed the expression profiles using RNA-Seq of three tissues (spikelet, seed and stem) during the starch-filling phase. We performed functional analysis of tissue-specific genes and expression analysis of genes of starch biosynthesis enzymes. This functional analysis of sorghum full-length cDNAs and the transcriptome information will facilitate further analysis of the Saccharinae and grass families.

Transgenic switchgrass (Panicum virgatum L.) biomass is increased by overexpression of switchgrass sucrose synthase (PvSUS1)

Sucrose synthase (SUS) converts sucrose and uridine di-phosphate (UDP) into UDP-glucose and fructose. UDP-glucose is used by the cellulose synthase to produce cellulose for cell wall biosynthesis. For lignocellulosic feedstocks such as switchgrass, the manipulation of cell walls to decrease lignin content is needed to reduce recalcitrance of conversion of biomass into biofuels. Of perhaps equal importance for bioenergy feedstocks is increasing biomass. Four SUS genes were identified in switchgrass. Each gene contained 14 or 15 introns. PvSUS1 was expressed ubiquitously in the tissues tested. PvSUS2 and PvSUS6 were highly expressed in internodes and roots, respectively. PvSUS4 was expressed in low levels in the tissues tested. Transgenic switchgrass plants overexpressing PvSUS1 had increases in plant height by up to 37%, biomass by up to 13.6%, and tiller number by up to 79% compared to control plants. The lignin content was increased in all lines, while the sugar release efficiency was decreased in PvSUS1-overexpressing transgenic switchgrass plants. For switchgrass and other bioenergy feedstocks, the overexpression of SUS1 genes might be a feasible strategy to increase both plant biomass and cellulose content, and to stack with other genes to increase biofuel production per land area cultivated.

Dynamic transcriptome landscape of maize embryo and endosperm development

Maize (Zea mays) is an excellent cereal model for research on seed development because of its relatively large size for both embryo and endosperm. Despite the importance of seed in agriculture, the genome-wide transcriptome pattern throughout seed development has not been well characterized. Using high-throughput RNA sequencing, we developed a spatiotemporal transcriptome atlas of B73 maize seed development based on 53 samples from fertilization to maturity for embryo, endosperm, and whole seed tissues. A total of 26,105 genes were found to be involved in programming seed development, including 1,614 transcription factors. Global comparisons of gene expression highlighted the fundamental transcriptomic reprogramming and the phases of development. Coexpression analysis provided further insight into the dynamic reprogramming of the transcriptome by revealing functional transitions during maturation. Combined with the published nonseed high-throughput RNA sequencing data, we identified 91 transcription factors and 1,167 other seed-specific genes, which should help elucidate key mechanisms and regulatory networks that underlie seed development. In addition, correlation of gene expression with the pattern of DNA methylation revealed that hypomethylation of the gene body region should be an important factor for the expressional activation of seed-specific genes, especially for extremely highly expressed genes such as zeins. This study provides a valuable resource for understanding the genetic control of seed development of monocotyledon plants.

Hexose kinases and their role in sugar-sensing and plant development

Hexose sugars, such as glucose and fructose produced in plants, are ubiquitous in most organisms and are the origin of most of the organic matter found in nature. To be utilized, hexose sugars must first be phosphorylated. The central role of hexose-phosphorylating enzymes has attracted the attention of many researchers, leading to novel discoveries. Only two families of enzymes capable of phosphorylating glucose and fructose have been identified in plants; hexokinases (HXKs), and fructokinases (FRKs). Intensive investigations of these two families in numerous plant species have yielded a wealth of knowledge regarding the genes number, enzymatic characterization, intracellular localization, and developmental and physiological roles of several HXKs and FRKs. The emerging picture indicates that HXK and FRK enzymes found at specific intracellular locations play distinct roles in plant metabolism and development. Individual HXKs were shown for the first time to be dual-function enzymes - sensing sugar levels independent of their catalytic activity and controlling gene expression and major developmental pathways, as well as hormonal interactions. FRK, on the other hand, seems to play a central metabolic role in vascular tissues, controlling the amounts of sugars allocated for vascular development. While a clearer picture of the roles of these two types of enzymes is emerging, many questions remain unsolved, such as the specific tissues and types of cells in which these enzymes function, the roles of individual HXK and FRK genes, and how these enzymes interact with hormones in the regulation of developmental processes. It is anticipated that ongoing efforts will broaden our knowledge of these important plant enzymes and their potential uses in the modification of plant traits.

A novel isoform of sucrose synthase is targeted to the cell wall during secondary cell wall synthesis in cotton fiber

Sucrose (Suc) synthase (Sus) is the major enzyme of Suc breakdown for cellulose biosynthesis in cotton (Gossypium hirsutum) fiber, an important source of fiber for the textile industry. This study examines the tissue-specific expression, relative abundance, and temporal expression of various Sus transcripts and proteins present in cotton. A novel isoform of Sus (SusC) is identified that is expressed at high levels during secondary cell wall synthesis in fiber and is present in the cell wall fraction. The phylogenetic relationships of the deduced amino acid sequences indicate two ancestral groups of Sus proteins predating the divergence of monocots and dicots and that SusC sequences form a distinct branch in the phylogeny within the dicot-specific clade. The subcellular location of the Sus isoforms is determined, and it is proposed that cell wall-localized SusC may provide UDP-glucose for cellulose and callose synthesis from extracellular sugars.

Structure, expression profile and subcellular localisation of four different sucrose synthase genes from barley

Genes encoding two new isoforms of sucrose synthase from barley, HvSs3 and HvSs4, have been characterised and their expression patterns compared with those previously described for HvSs1 and HvSs2, in different organs and during seed maturation and germination. Their response to several abiotic stimuli has also been investigated in leaves: HvSs1 is up-regulated by anoxia and HvSs3 by water deprivation while no response is observed to 150 mM NaCl treatment; HvSs1 and HvSs3 are also induced by cold temperatures. Using translational fusions and transient expression analyses, the four isozymes have been localised not only to the cytoplasm but also along several cytoplasmic tracks and at the inner side of the cell membrane; besides, HvSS1 is also associated with mitochondria, a localisation that has been predicted in silico with the TargetP and Predotar programmes. These data suggest distinct although partially overlapping roles, for the four barley sucrose synthase isoforms, in the channelling of carbon towards different metabolic pathways within the cell.

Knockdown of the symbiotic sucrose synthase MtSucS1 affects arbuscule maturation and maintenance in mycorrhizal roots of Medicago truncatula

The relevance of the symbiosis-induced Medicago truncatula sucrose synthase gene MtSucS1 for an efficient arbuscular mycorrhiza (AM) was studied using two independent antisense lines that displayed up to 10-fold reduced SucS1 levels in roots. Mycorrhizal MtSucS1-reduced lines exhibited an overall stunted aboveground growth under inorganic phosphorus limitation. Apart from a reduced plant height, shoot weight, and leaf development, a delayed flowering, resulting in a lower seed yield, was observed. In addition, the root-to-shoot and root weight ratios increased significantly. Gene expression studies demonstrated a major reversion of AM-associated transcription, exhibiting a significant repression of well-known plant AM marker and mycosymbiont genes, together indicating a diminished AM fungus colonization of MtSucS1-antisense lines. Concomitantly, gas chromatography-mass spectrometry-based metabolite profiling revealed that mycorrhizal MtSucS1-reduced lines were affected in important nodes of the carbon, nitrogen, and phosphorus metabolism, accentuating a physiological significance of MtSucS1 for AM. In fact, antisensing MtSucS1 provoked an impaired fungal colonization within the less abundant infected regions, evident from strongly reduced frequencies of internal hyphae, vesicles, and arbuscules. Moreover, arbuscules were early senescing, accompanied with a reduced development of mature arbuscules. This defective mycorrhiza status correlated with reduced phosphorus and nitrogen levels and was proportional to the extent of MtSucS1 knockdown. Together, our results point to an important role for MtSucS1 in the establishment and maintenance of arbuscules in the AM symbiosis.

Role of myo-inositol phosphate synthase and sucrose synthase genes in plant seed development

The aim of this review is to highlight the role of myo-inositol phosphate synthase (MIPS), which catalyses the first step in inositol biosynthesis and of sucrose synthase (Sus), an enzyme involved in UDP-glucose formation, the principal nucleoside diphosphate in the sucrose cleavage reaction and in trehalose biosynthesis. These two enzymes are involved in various physiological processes including seed growth and resistance to biotic and abiotic stresses. The study of mutated MIPS and Sus genes in some crops, such as soybean and cotton, has shown that these two proteins are directly involved in embryogenesis. They exhibit several isoforms that are essential for normal seed development. The possible role of both genes in seed development is discussed in this review.

Study of AtSUS2 localization in seeds reveals a strong association with plastids

Sucrose synthase (SUS) is a key enzyme in sucrose metabolism. This enzyme catalyzes the reversible conversion of sucrose and UDP to UDP-glucose and fructose. In the Arabidopsis SUS gene family (six members), SUS2 is strongly and specifically expressed in Arabidopsis seeds during the maturation phase. Using specific antibodies, we have shown that SUS2 is localized in the embryo, endosperm and seed coat with differential patterns. During the maturation phase, the SUS2 protein seems to be mainly co-localized with plastids in the embryo. This novel finding is discussed in relation to the role of this enzyme in storage organs.

Sucrose synthase is involved in the conversion of sucrose to polysaccharides in filamentous nitrogen-fixing cyanobacteria

Higher plants and cyanobacteria metabolize sucrose (Suc) by a similar set of enzymes. Suc synthase (SuS, UDP-glucose: D: -fructose 2-alpha-D: -glucosyl transferase, EC 2.4.1.13) catalyses the synthesis and cleavage of Suc, and in higher plants, it plays an important role in polysaccharides biosynthesis and carbon allocation. In this work, we have studied the functional relationship between SuS and the metabolism of polysaccharides in filamentous nitrogen-fixing cyanobacteria. We show that the nitrogen and carbon sources and light regulate the expression of the SuS encoding gene (susA), in a similar way that they regulate the accumulation of polysaccharides. Furthermore, glycogen content in an Anabaena sp. mutant strain with an insertion inactivation of susA was lower than in the wild type strain under diazotrophic conditions, while both glycogen and polysaccharides levels were higher in a mutant strain constitutively overexpressing susA. We also show that there are soluble and membrane-bound forms of SuS in Anabaena. Taken together, these results strongly suggest that SuS is involved in the Suc to polysaccharides conversion according to nutritional and environmental signals in filamentous nitrogen-fixing cyanobacteria.

Sucrose synthase is associated with the cell wall of tobacco pollen tubes

Sucrose synthase (Sus; EC 2.4.1.13) is a key enzyme of sucrose metabolism in plant cells, providing carbon for respiration and for the synthesis of cell wall polymers and starch. Since Sus is important for plant cell growth, insights into its structure, localization, and features are useful for defining the relationships between nutrients, growth, and cell morphogenesis. We used the pollen tube of tobacco (Nicotiana tabacum) as a cell model to characterize the main features of Sus with regard to cell growth and cell wall synthesis. Apart from its role during sexual reproduction, the pollen tube is a typical tip-growing cell, and the proper construction of its cell wall is essential for correct shaping and direction of growth. The outer cell wall layer of pollen tubes consists of pectins, but the inner layer is composed of cellulose and callose; both polymers require metabolic precursors in the form of UDP-glucose, which is synthesized by Sus. We identified an 88-kD polypeptide in the soluble, plasma membrane and Golgi fraction of pollen tubes. The protein was also found in association with the cell wall. After purification, the protein showed an enzyme activity similar to that of maize (Zea mays) Sus. Distribution of Sus was affected by brefeldin A and depended on the nutrition status of the pollen tube, because an absence of metabolic sugars in the growth medium caused Sus to distribute differently during tube elongation. Analysis by bidimensional electrophoresis indicated that Sus exists as two isoforms, one of which is phosphorylated and more abundant in the cytoplasm and cell wall and the other of which is not phosphorylated and is specific to the plasma membrane. Results indicate that the protein has a role in the construction of the extracellular matrix and thus in the morphogenesis of pollen tubes.

Expression of sucrose synthase in the developing endosperm is essential for early seed development in cotton

Successful seed development requires coordinated interaction of the endosperm and embryo. In most dicotyledonous seeds, the endosperm is crushed and absorbed by the expanding embryo in the later stages of seed development. Little is known about the metabolic interaction between the two filial tissues early in seed development. We examined the potential role of sucrose synthase (Sus) in the endosperm development of cotton. Sus was immunologically localised in the cellularising endosperm, but not in the heart-stage embryo at 10 days after anthesis. The activities of Sus and acid invertase were significantly higher in the endosperm than in the young embryos, which corresponded to a steep concentration difference in hexoses between the endosperm and the embryo. This observation indicates a role for the endosperm in generating hexoses for the development of the two filial tissues. Interestingly, Sus expression and starch deposition were spatially separated in the seeds. Silencing the expression of Sus in the endosperm using an RNAi approach led to the arrest of early seed development. Histochemical analyses revealed a significant reduction in cellulose and callose in the deformed endosperm cells of the Sus-suppressed seed. The data indicate a critical role of Sus in early seed development through regulation of endosperm formation.

Antisense repression of the Medicago truncatula nodule-enhanced sucrose synthase leads to a handicapped nitrogen fixation mirrored by specific alterations in the symbiotic transcriptome and metabolome

We analyzed the role of the sucrose (Suc) synthase MtSucS1 during nodulation of the model legume Medicago truncatula, integrating data for the developmental, transcriptional, and metabolic processes affected downstream of an impaired Suc cleavage in root nodules. To reduce carbohydrate supply to nodule tissues, transgenic plants expressing a p35S-driven MtSucS1-antisense fusion were constructed. These plants displayed an up to 90% reduction of MtSucS1 proteins in roots and nodules. Phenotypic studies of two independent MtSucS1-reduced lines demonstrated that only under conditions depending on nodulation, these plants appeared to be impaired in above-ground growth. Specifically plant height, shoot weight, leaf development, flowering, as well as seed maturation were reduced, and the efficiency of photosynthesis was affected. Concomitantly, a significantly enhanced root to shoot ratio with a marked increase in root tip numbers was observed. Root nodule formation was found retarded and the impaired nodulation was accompanied by a less efficient nitrogen (N) acquisition. The decreased total N content of MtSucS1-antisense lines and an enhanced carbon to N ratio in roots, nodules, and shoots correlated with the extent of MtSucS1 knockdown. On the level of transcription, effects of an MtSucS1 reduction were evident for genes representing important nodes of the nodule carbon and N metabolism, while metabolite profiling revealed significantly lower levels of amino acids and their derivatives particularly in strongly MtSucS1-reduced nodules. Our results support the model that nodule-enhanced Suc synthase 1 of the model legume M. truncatula is required for the establishment and maintenance of an efficient N-fixing symbiosis.

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.

Rapid cell expansion and cellulose synthesis regulated by plasmodesmata and sugar: insights from the single-celled cotton fibre

Higher plants comprise mixtures of some 40 different cell types, and this often complicates the interpretation of data obtained at the tissue level. Studies for a given cell type may provide novel insights into the mechanisms underlying defined cellular and developmental processes. In this regard, the cotton fibre represents an excellent single-cell model to study the control of rapid cell elongation and cellulose synthesis. These single cells, initiated from the ovule epidermis at anthesis, typically elongate to ~3-5 cm in the tetraploid species before they switch to intensive secondary cell wall cellulose synthesis. By maturity, more than 94% of fibre weight is cellulose. To unravel the mechanisms of fibre elongation and cellulose synthesis, two hypotheses have been examined: (a) that sucrose degradation and utilisation mediated by sucrose synthase (Sus) may play roles in fibre development and (b) that symplastic isolation of the fibre cells may be required for their rapid elongation. Reverse genetic and biochemical analyses have revealed the critical role that Sus plays in fibre initiation and early elongation. Late in development, plasma-membrane and cell wall association of Sus protein seems to be involved in rapid cellulose synthesis. Cell biology and gene expression studies showed a temporary closure of fibre plasmodesmata (PD), probably due to the deposition of callose, at the rapid phase of elongation. The duration of the PD closure correlates positively with the final fibre length attained. These data support the view that PD closure may be required for fibres to achieve extended elongation. The branching of PD towards the secondary cell wall stage is postulated to function as a molecule sieve for tight control of macromolecule trafficking into fibres to sustain intensive cellulose synthesis.

Determination of structural requirements and probable regulatory effectors for membrane association of maize sucrose synthase 1

Sucrose (Suc) synthase (SUS) cleaves Suc to form UDP glucose and fructose, and exists in soluble and membrane-associated forms, with the latter proposed to channel UDP glucose to the cellulose-synthase complex on the plasma membrane of plant cells during synthesis of cellulose. However, the structural features responsible for membrane localization and the mechanisms regulating its dual intracellular localization are unknown. The maize (Zea mays) SUS1 isoform is likely to have the intrinsic ability to interact directly with membranes because we show: (1) partial membrane localization when expressed in Escherichia coli, and (2) binding to carbonate-stripped plant microsomes in vitro. We have undertaken mutational analyses (truncations and alanine substitutions) and in vitro microsome-binding assays with the SUS1 protein to define intrinsic membrane-binding regions and potential regulatory factors that could be provided by cellular microenvironment. The results suggest that two regions of SUS1 contribute to membrane affinity: (1) the amino-terminal noncatalytic domain, and (2) a region with sequence similarity to the C-terminal pleckstrin homology domain of human pleckstrin. Alanine substitutions within the pleckstrin homology-like domain of SUS1 reduced membrane association in E. coli and with plant microsomes in vitro without reducing enzymatic activity. Microsomal association of wild-type SUS1 displayed cooperativity with SUS1 protein concentration and was stimulated by both lowering the pH and adding Suc. These studies offer insight into the molecular level regulation of SUS1 localization and its participation in carbon partitioning in plants. Moreover, transgenics with active SUS mutants altered in membrane affinity may be of technological utility.

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.

An important pool of sucrose linked to starch biosynthesis is taken up by endocytosis in heterotrophic cells

We have recently shown the occurrence of endocytic sucrose uptake in heterotrophic cells. Whether this mechanism is involved in the sucrose-starch conversion process was investigated by comparing the rates of starch accumulation in sycamore cells cultured in the presence or absence of the endocytic inhibitors wortmannin and 2-(4-morpholynyl-)-8-phenyl-4H-1 benzopyran-4-1 (LY294002). These analyses revealed a two-phase process involving an initial 120 min wortmannin- and LY294002-insensitive starch accumulation period, followed by a prolonged phase that was arrested by the endocytic inhibitors. Both wortmannin and LY294002 led to a strong reduction of the intracellular levels of both sucrose and the starch precursor molecule, ADPglucose. No changes in maximum catalytic activities of enzymes closely linked to starch and sucrose metabolism occurred in cells cultured with endocytic inhibitors. In addition, starch accumulation was unaffected by endocytic inhibitors when cells were cultured with glucose. These results provide a first indication that an important pool of sucrose incorporated into the cell is taken up by endocytosis prior to its subsequent conversion into starch in heterotrophic cells. This conclusion was substantiated further by experiments showing that sucrose-starch conversion was strongly prevented by both wortmannin and LY294002 in both potato tuber discs and developing barley endosperms.

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.

Identification and characterization of plasma membrane proteins that bind to microtubules in pollen tubes and generative cells of tobacco

The organization and function of microtubules in plant cells are important in many developmental stages. Connections between microtubules and the endomembrane system of plant cells have been discovered by microscopy, but the molecular characteristics of these relationships are mostly unknown except for a few cases. Using two antibodies raised against microtubule-associated proteins (MAPs) from maize, we have identified two polypeptides that share properties of the MAP family in the pollen tube of Nicotiana tabacum. The two polypeptides (with an apparent Mr of 161 and 90 kDa) bind efficiently to animal and plant microtubules and are found in association with the cellular membranes of the pollen tube, from which they can be solubilized with a zwitterionic detergent. One of these proteins has been purified and shown to promote the assembly of tubulin and, to a lesser extent, the bundling of microtubules. Subcellular fractionation indicated that the two proteins are associated with the plasma membrane compartment. The two proteins are found to co-localize in situ with cortical microtubules in the vegetative cytoplasm of tobacco pollen tubes; co-localization is also evident in the generative cell. According to these data, both the 161 and 90 kDa polypeptides are likely to mediate the interactions between the plasma membrane and microtubules in pollen tubes. In addition, functional data indicate that these MAP-like proteins take part in the process of microtubule assembly and reorganization occurring during cell growth. The evidence that both proteins associate with different cellular compartments also suggests a broad-spectrum role in mediating the dynamic relationships between microtubules and plant cell membranes.

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.

Evidence that sucrose loaded into the phloem of a poplar leaf is used directly by sucrose synthase associated with various beta-glucan synthases in the stem

Sucrose (Suc) synthase (SuSy) is believed to function in channeling UDP-Glc from Suc to various beta-glucan synthases. We produced transgenic poplars (Populus alba) overexpressing a mutant form (S11E) of mung bean (Vigna radiata) SuSy, which appeared in part in the microsomal membranes of the stems. Expression of SuSy in these membranes enhanced the incorporation of radioactive Suc into cellulose, together with the metabolic recycling of fructose (Fru), when dual-labeled Suc was fed directly into the phloem of the leaf. This overexpression also enhanced the direct incorporation of the glucosyl moiety of Suc into the glucan backbone of xyloglucan and increased recycling of Fru, although the Fru recycling system for cellulose synthesis at the plasma membrane might differ from that for xyloglucan synthesis in the Golgi network. These findings suggest that some of the Suc loaded into the phloem of a poplar leaf is used directly by SuSys associated with xyloglucan and cellulose synthases in the stem. This may be a key function of SuSy because the high-energy bond between the Glc and Fru moieties of Suc is conserved and used for polysaccharide syntheses in this sink tissue.

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.

Structure and expression profile of the sucrose synthase multigene family in Arabidopsis

The release of the complete genome sequence of Arabidopsis enabled the largest sucrose synthase family described to date, comprising six distinct members, for which expression profiles were not yet available, to be identified. Aimed at understanding the precise function of each AtSUS member among the family, a comparative study of protein structure was performed, together with an expression profiling of the whole gene family using the technique of real-time quantitative reverse transcriptase-polymerase chain reaction. Transcript levels were analysed in several plant organs, including both developing and germinating seeds. A series of treatments such as oxygen deprivation, dehydration, cold treatment, or various sugar feedings were then carried out to characterize the members of the family further. The AtSUS genes exhibit distinct but partially redundant expression profiles. Under anaerobic conditions, for instance, both AtSUS1 and AtSUS4 mRNA levels increase, but in a distinct manner. AtSUS2 is specifically and highly induced in seeds at 12 d after flowering and appears as a marker of seed maturation. AtSUS3 seems to be induced in various organs under dehydration conditions including leaves deprived of water or submitted to osmotic stress as well as late-maturing seeds. AtSUS5 and AtSUS6 are expressed in nearly all plant organs and do not exhibit any transcriptional response to stresses. These results add new insights on the expression of SUS genes and are discussed in relation to distinct functions for each member of the AtSUS family.

Sucrose synthase isozyme SUS1 in the maize root cap is preferentially localized in the endopolyploid outer cells

The structure of the maize (Zea mays L.) root cap was studied to quantitatively evaluate the relationship among the size of the cells, their endopolyploidy level, and the abundance of the sucrose synthase isozyme SUS1. Median longitudinal root cap sections were analysed using immunolocalization, quantitative DNA staining, and image cytometry. Both the immunolocalization signal for the SUS1 protein and the endopolyploidy level increased from calyptrogen towards the root cap periphery and were thus the highest in the outer cells. These cells had a nuclear DNA content of mostly 8C or higher and the largest volumes of all root cap cells. The high amount of SUS1 protein in the outer, endopolyploid cells suggests an association between endoreduplication and the abundance of this enzyme. The outer cells are involved in mucilage production; hence, there is a possibility that sucrose synthase provides monosaccharide precursors for mucilage synthesis.Key words: nuclear DNA amount, endoreduplication, immunolocalization, image cytometry, Zea mays L.

In vivo and in vitro phosphorylation of membrane and soluble forms of soybean nodule sucrose synthase

Sucrose synthase (SS) is a known phosphoserine (SerP)-containing enzyme in a variety of plant "sink" organs, including legume root nodules, where it is phosphorylated primarily at Ser-11. Using immunofluorescence confocal microscopy, we documented that part of the total SS (nodulin-100) pool in mature soybean (Glycine max) nodules is apparently associated with the plasma membrane in situ, and we report that this association is very "tight," as evidenced by a variety of chemical and enzymatic pretreatments of the isolated microsomal fraction. To investigate the in situ and in planta phosphorylation state of the membrane (m) and soluble (s) forms of nodule SS, three complementary approaches were used. First, excised nodules were radiolabeled in situ with [(32)P]Pi for subsequent analysis of phosphorylated m- and s-SS; second, immunopurified s- and m-SS were used as substrate in "on-bead" assays of phosphorylation by nodule Ca(2+)-dependent protein kinase; and third, SS-Ser-11(P) phosphopeptide-specific antibodies were developed and used. The collective results provide convincing evidence that microsomal nodulin-100 is phosphorylated in mature nodules, and that it is hypophosphorylated relative to s-SS (on an equivalent SS protein basis) in attached, unstressed nodules. Moreover, the immunological data and related phosphopeptide mapping analyses indicate that a homologous N-terminal seryl-phosphorylation domain and site reside in microsomal nodulin-100. We also observed that mild, short-term inorganic nitrogen and salt stresses have a significant negative impact on the content and N-terminal phosphorylation state of nodule m- and s-SS, with the former being the more sensitive of the two SS forms.

The regulation of metabolic flux to cellulose, a major sink for carbon in plants

Cellulose is an important component of the cell walls of higher plants and the world's most abundant organic compound. As a major sink for carbon on earth, it is of interest to examine possible means by which the quality or quantity of cellulose deposited in various plant parts might be manipulated by metabolic engineering techniques. This review outlines basic knowledge about the genes and proteins that are involved in cellulose biosynthesis and presents a model that summarizes our current thinking on the overall cellulose biosynthesis pathway. Strategies that might be used for altering the flux of carbon into this pathway are discussed.

Carbon partitioning to cellulose synthesis

This article discusses the importance and implications of regulating carbon partitioning to cellulose synthesis, the characteristics of cells that serve as major sinks for cellulose deposition, and enzymes that participate in the conversion of supplied carbon to cellulose. Cotton fibers, which deposit almost pure cellulose into their secondary cell walls, are referred to as a primary model system. For sucrose synthase, we discuss its proposed role in channeling UDP-Glc to cellulose synthase during secondary wall deposition, its gene family, its manipulation in transgenic plants, and mechanisms that may regulate its association with sites of polysaccharide synthesis. For cellulose synthase, we discuss the organization of the gene family and how protein diversity could relate to control of carbon partitioning to cellulose synthesis. Other enzymes emphasized include UDP-Glc pyrophosphorylase and sucrose phosphate synthase. New data are included on phosphorylation of cotton fiber sucrose synthase, possible regulation by Ca2+ of sucrose synthase localization, electron microscopic immunolocalization of sucrose synthase in cotton fibers, and phylogenetic relationships between cellulose synthase proteins, including three new ones identified in differentiating tracheary elements of Zinnia elegans. We develop a model for metabolism related to cellulose synthesis that implicates the changing intracellular localization of sucrose synthase as a molecular switch between survival metabolism and growth and/or differentiation processes involving cellulose synthesis.

Sucrose synthase localizes to cellulose synthesis sites in tracheary elements

The synthesis of crystalline cellulose microfibrils in plants is a highly coordinated process that occurs at the interface of the cortex, plasma membrane, and cell wall. There is evidence that cellulose biogenesis is facilitated by the interaction of several proteins, but the details are just beginning to be understood. In particular, sucrose synthase, microtubules, and actin have been proposed to possibly associate with cellulose synthases (microfibril terminal complexes) in the plasma membrane. Differentiating tracheary elements of Zinnia elegans L. were used as a model system to determine the localization of sucrose synthase and actin in relation to the plasma membrane and its underlying microtubules during the deposition of patterned, cellulose-rich secondary walls. Cortical actin occurs with similar density both between and under secondary wall thickenings. In contrast, sucrose synthase is highly enriched near the plasma membrane and the microtubules under the secondary wall thickenings. Both actin and sucrose synthase lie closer to the plasma membrane than the microtubules. These results show that the preferential localization of sucrose synthase at sites of high-rate cellulose synthesis can be generalized beyond cotton fibers, and they establish a spatial context for further work on a multi-protein complex that may facilitate secondary wall cellulose synthesis.

Regulation of sucrose metabolism in higher plants: localization and regulation of activity of key enzymes

Sucrose (Suc) plays a central role in plant growth and development. It is a major end product of photosynthesis and functions as a primary transport sugar and in some cases as a direct or indirect regulator of gene expression. Research during the last 2 decades has identified the pathways involved and which enzymes contribute to the control of flux. Availability of metabolites for Suc synthesis and 'demand' for products of sucrose degradation are important factors, but this review specifically focuses on the biosynthetic enzyme sucrose-phosphate synthase (SPS), and the degradative enzymes, sucrose synthase (SuSy), and the invertases. Recent progress has included the cloning of genes encoding these enzymes and the elucidation of posttranslational regulatory mechanisms. Protein phosphorylation is emerging as an important mechanism controlling SPS activity in response to various environmental and endogenous signals. In terms of Suc degradation, invertase-catalyzed hydrolysis generally has been associated with cell expansion, whereas SuSy-catalyzed metabolism has been linked with biosynthetic processes (e.g., cell wall or storage products). Recent results indicate that SuSy may be localized in multiple cellular compartments: (1) as a soluble enzyme in the cytosol (as traditionally assumed); (2) associated with the plasma membrane; and (3) associated with the actin cytoskeleton. Phosphorylation of SuSy has been shown to occur and may be one of the factors controlling localization of the enzyme. The purpose of this review is to summarize some of the recent developments relating to regulation of activity and localization of key enzymes involved in sucrose metabolism in plants.

Altered patterns of sucrose synthase phosphorylation and localization precede callose induction and root tip death in anoxic maize seedlings

Root extracts made from maize (Zea mays) seedlings submerged for 2 h showed an increased (32)P-labeling of a 90-kD polypeptide in a Ca(2+)-dependent manner. This protein was identified as sucrose synthase (SS) by immunoprecipitation and mutant analysis. Metabolic labeling with (32)P(i) indicated that the aerobic levels of SS phosphorylation were maintained up to 2 h of anoxia. In contrast, during prolonged anoxia the protein was under-phosphorylated, and by 48 h most of the protein existed in the unphosphorylated form. In seedlings submerged for 2 h or longer, a part of SS became associated with the microsomal fraction and this membrane localization of SS was confined only to the root tip. This redistribution of SS in the root tip preceded callose induction, an indicator of cell death. The sh1 mutants showed sustained SS phosphorylation and lacked the anoxia-induced relocation of SS, indicating that it was the SH1 form of the enzyme that was redistributed during anoxia. The sh1 mutants also showed less callose deposition and greater tolerance to prolonged anoxia than their non-mutant siblings. EGTA accentuated anoxic effects on membrane localization of SS and callose accumulation, whereas Ca(2+) addition reversed the EGTA effects. These results indicate that the membrane localization of SS is an important early event in the anoxic root tip, probably associated with the differential anoxic tolerance of the two SS mutants. We propose that beside the transcriptional control of genes encoding SS, the reversible phosphorylation of SS provides a potent regulatory mechanism of sugar metabolism in response to developmental and environmental signals.

Soybean nodule sucrose synthase (nodulin-100): further analysis of its phosphorylation using recombinant and authentic root-nodule enzymes

Sucrose synthase (SS) is a known phosphoserine-containing enzyme in legume root nodules and various other plant "sink" tissues. In order to begin to investigate the possible physiological significance of this posttranslational modification, we have cloned a full-length soybean nodule SS (nodulin-100) cDNA and overexpressed it in Escherichia coli. Authentic nodule SS and recombinant wild-type and mutant forms of the enzyme were purified and characterized. We document that a conserved serine near the N-terminus (Ser(11)) is the primary phosphorylation site for a nodule Ca(2+)-dependent protein kinase (CDPK) in vitro. Related tryptic digestion and mass spectral analyses indicated that this target residue was also phosphorylated in planta in authentic nodulin-100. In addition, a secondary phosphorylation site(s) in recombinant nodule SS was implicated given that all active mutant enzyme forms (S11A, S11D, S11C, and N-terminal truncation between Ala(2) and Arg(13)) were phosphorylated, albeit weakly, by the CDPK. This secondary site(s) likely resides between Glu(14) and Met(193) as evidenced by CNBr cleavage and phosphopeptide mapping. Phosphorylation of the recombinant and authentic nodule Ser(11) enzymes in vitro by the nodule CDPK had no major effect on the sucrose-cleavage activity and/or kinetic properties. However, phosphorylation decreased the apparent surface hydrophobicity of the recombinant wild-type enzyme, suggesting that this covalent modification could potentially play some role in the documented partitioning of nodulin-100 between the nodule symbiosome/plasma membranes and cytosol in planta.

Sugars modulate an unusual mode of control of the cell-wall invertase gene (Incw1) through its 3' untranslated region in a cell suspension culture of maize

We show here that a cell-wall invertase encoded by the Incw1 gene is regulated at both the transcriptional and posttranscriptional levels by sugars in a heterotrophic cell suspension culture of maize. The Incw1 gene encoded two transcripts: Incw1-S (small) and Incw1-L (large); the size variation was attributable to different lengths in the 3' untranslated region. Both metabolizable and nonmetabolizable sugars induced Incw1-L RNA apparently by default. However, only the metabolizable sugars, sucrose and D-glucose, were associated with the increased steady-state abundance of Incw1-S RNA, the concomitant increased levels of INCW1 protein and enzyme activity, and the downstream metabolic repression of the sucrose synthase gene, Sh1. Conversely, nonmetabolizable sugars, including the two glucose analogs 3-O-methylglucose and 2-deoxyglucose, induced greater steady-state levels of the Incw1-L RNA, but this increase did not lead to either an increase in the levels of the INCW1 protein/enzyme activity or the repression of the Sh1 gene. We conclude that sugar sensing and the induction of the Incw1 gene is independent of the hexokinase pathway. More importantly, our results also suggest that the 3' untranslated region of the Incw1 gene acts as a regulatory sensor of carbon starvation and may constitute a link between sink metabolism and cellular translation in plants.

Purification of tomato sucrose synthase phosphorylated isoforms by Fe(III)-immobilized metal affinity chromatography

The major phosphorylation site of maize sucrose synthase (SuSy) is well conserved among plant species but absent in the deduced peptide sequence of the tomato SuSy cDNA (TOMSSF). In this study, we report the in vitro phosphorylation of 25-day-old tomato fruits SuSy on seryl residue(s) by an endogenous Ca2+-dependent protein kinase activity. Two distinct 32P-labeled peptides detected in the tryptic peptide map of in vitro 32P-radiolabeled tomato fruit SuSy were purified. Amino acid sequencing and phosphoamino acid analysis of the major 32P-labeled peptide revealed the presence of a SuSy isozyme in young tomato fruit having the N-terminus phosphorylation site present in other plant species. By using Fe(III)-immobilized metal affinity chromatography [Fe(III)-IMAC] as a final purification step of tomato fruit SuSy, two 32P-labeled tomato SuSy isoforms were separated from a nonradiolabeled SuSy fraction by using a pH gradient. The major 32P-SuSy isoform was phosphorylated exclusively at the seryl residue related to the phosphorylation site of maize SuSy. The multiphosphorylated state of the second radiolabeled SuSy fraction was indicated by a higher retention during Fe(III)-IMAC and by tryptic peptide mapping analysis. Kinetic analyses of SuSy isoforms purified by Fe(III)-IMAC have revealed that phosphorylation of the major phosphorylation site of tomato fruit SuSy was not sufficient by itself to modulate tomato SuSy activity, whereas the affinity for UDP increased about threefold for the multiphosphorylated SuSy isoform.

A cytokinesis-defective mutant of Arabidopsis (cyt1) characterized by embryonic lethality, incomplete cell walls, and excessive callose accumulation

The genetic control of cell division in eukaryotes has been addressed in part through the analysis of cytokinesis-defective mutants. Two allelic mutants of Arabidopsis (cyt1-1 and cyt1-2) altered in cytokinesis and cell-wall architecture during embryogenesis are described in this report. Mutant embryos appear slightly abnormal at the heart stage and then expand to form a somewhat disorganized mass of enlarged cells with occasional incomplete walls. In contrast to the keule and knolle mutants of Arabidopsis and the cyd mutant of pea, which also exhibit defects in cytokinesis during embryogenesis, cyt1 embryos cannot be rescued in culture, are desiccation-intolerant at maturity, and produce cell walls with excessive callose as revealed through staining with the aniline blue fluorochrome, Sirofluor. Some cyt1 defects can be partially phenocopied by treatment with the herbicide dichlobenil, which is thought to interfere with cellulose biosynthesis. The distribution of unesterified pectins in cyt1 cell walls is also disrupted as revealed through immunocytochemical localization of JIM 5 antibodies. These features indicate that CYT1 plays an essential and unique role in plant growth and development and the establishment of normal cell-wall architecture.

Identification of sucrose synthase as an actin-binding protein

Several lines of evidence indicate that sucrose synthase (SuSy) binds both G- and F-actin: (i) presence of SuSy in the Triton X-100-insoluble fraction of microsomal membranes (i.e. crude cytoskeleton fraction); (ii) co-immunoprecipitation of actin with anti-SuSy monoclonal antibodies; (iii) association of SuSy with in situ phalloidin-stabilized F-actin filaments; and (iv) direct binding to F-actin, polymerized in vitro. Aldolase, well known to interact with F-actin, interfered with binding of SuSy, suggesting that a common or overlapping binding site may be involved. We postulate that some of the soluble SuSy in the cytosol may be associated with the actin cytoskeleton in vivo.

Membrane association of sucrose synthase: changes during the graviresponse and possible control by protein phosphorylation

Sucrose synthase (SuSy) plays an important role in sucrose degradation and occurs both as a soluble and as a membrane-associated enzyme in higher plants. We show that membrane association can vary in vivo in response to gravistimulation, apparently involving SuSy dephosphorylation, and is a reversible process in vitro. Phosphorylation of SuSy has little effect on its activity but decreases its surface hydrophobicity as reported with the fluorescent probe bis-ANS. We postulate that phosphorylation of SuSy (and perhaps other membrane proteins) is involved in the release of the membrane-bound enzyme in part as a result of decreased surface hydrophobicity.

Seryl-phosphorylation of soybean nodule sucrose synthase (nodulin-100) by a Ca2+-dependent protein kinase

Sucrose synthase (SS; EC 2.4.1.13) was radiolabeled in situ by incubating detached soybean nodules with 32Pi. Phosphoamino acid analysis indicated that SS was phosphorylated on a serine residue(s). In-vitro phosphorylation of purified nodule SS by desalted nodule extracts was Ca2+-dependent. This SS-kinase was partially purified (approximately 2200-fold) from nodules harvested from illuminated plants. The molecular mass of the SS-kinase was about 55,000 on a Superdex 75 size-exclusion column or in a denaturing autophosphorylation gel. With either purified nodule SS or Syntide 2 as substrate, exogenous calmodulin and phosphatidylserine showed little or no effect on the in-vitro activity of this partially purified protein kinase. However, its activity was inhibited by W-7. The purified nodule SS-kinase (or CDPK) phosphorylated nodule PEP carboxylase (PEPC; EC 4.1.1.31) in the presence of Ca2+. In contrast, a partially purified nodule PEPC-kinase preparation was incapable of phosphorylating nodule SS. Unlike nodule PEPC [Zhang et al. (1995) Plant Physiol. 108, 1561-1568], the phosphorylation state of SS is not likely modulated in planta by photosynthate supply from the shoots.