Sucrose synthase localizes to cellulose synthesis sites in tracheary elements

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

Wood transcriptome analysis of Pinus densiflora identifies genes critical for secondary cell wall formation and NAC transcription factors involved in tracheid formation

Although conifers have significant ecological and economic value, information on transcriptional regulation of wood formation in conifers is still limited. Here, to gain insight into secondary cell wall (SCW) biosynthesis and tracheid formation in conifers, we performed wood tissue-specific transcriptome analyses of Pinus densiflora (Korean red pine) using RNA sequencing. In addition, to obtain full-length transcriptome information, PacBio single molecule real-time iso-sequencing was carried out using RNAs from 28 tissues of P. densiflora. Subsequent comparative tissue-specific transcriptome analysis successfully pinpointed critical genes encoding key proteins involved in biosynthesis of the major secondary wall components (cellulose, galactoglucomannan, xylan and lignin). Furthermore, we predicted a total of 62 NAC (NAM, ATAF1/2 and CUC2) family transcription factor members and identified seven PdeNAC genes preferentially expressed in developing xylem tissues in P. densiflora. Protoplast-based transcriptional activation analysis found that four PdeNAC genes, homologous to VND, NST and SND/ANAC075, upregulated GUS activity driven by an SCW-specific cellulose synthase promoter. Consistently, transient overexpression of the four PdeNACs induced xylem vessel cell-like SCW deposition in both tobacco (Nicotiana benthamiana) and Arabidopsis leaves. Taken together, our data provide a foundation for further research to unravel transcriptional regulation of wood formation in conifers, especially SCW formation and tracheid differentiation.

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.

Xylan in the Middle: Understanding Xylan Biosynthesis and Its Metabolic Dependencies Toward Improving Wood Fiber for Industrial Processing

Lignocellulosic biomass, encompassing cellulose, lignin and hemicellulose in plant secondary cell walls (SCWs), is the most abundant source of renewable materials on earth. Currently, fast-growing woody dicots such as Eucalyptus and Populus trees are major lignocellulosic (wood fiber) feedstocks for bioproducts such as pulp, paper, cellulose, textiles, bioplastics and other biomaterials. Processing wood for these products entails separating the biomass into its three main components as efficiently as possible without compromising yield. Glucuronoxylan (xylan), the main hemicellulose present in the SCWs of hardwood trees carries chemical modifications that are associated with SCW composition and ultrastructure, and affect the recalcitrance of woody biomass to industrial processing. In this review we highlight the importance of xylan properties for industrial wood fiber processing and how gaining a greater understanding of xylan biosynthesis, specifically xylan modification, could yield novel biotechnology approaches to reduce recalcitrance or introduce novel processing traits. Altering xylan modification patterns has recently become a focus of plant SCW studies due to early findings that altered modification patterns can yield beneficial biomass processing traits. Additionally, it has been noted that plants with altered xylan composition display metabolic differences linked to changes in precursor usage. We explore the possibility of using systems biology and systems genetics approaches to gain insight into the coordination of SCW formation with other interdependent biological processes. Acetyl-CoA, s-adenosylmethionine and nucleotide sugars are precursors needed for xylan modification, however, the pathways which produce metabolic pools during different stages of fiber cell wall formation still have to be identified and their co-regulation during SCW formation elucidated. The crucial dependence on precursor metabolism provides an opportunity to alter xylan modification patterns through metabolic engineering of one or more of these interdependent pathways. The complexity of xylan biosynthesis and modification is currently a stumbling point, but it may provide new avenues for woody biomass engineering that are not possible for other biopolymers.

Isolation and functional characterization of SUCROSE SYNTHASE 1 and SUCROSE TRANSPORTER 2 promoters from ramie (Boehmeria nivea L. Gaudich)

Sucrose synthase and sucrose transporter are involved in sucrose metabolism and partitioning of photosynthetic products, respectively. In this study, we cloned SUCROSE SYNTHASE 1 and SUCROSE TRANSPORTER 2 genes from ramie. Real-time quantitative PCR revealed that BnSUS1 and BnSUT2 were widely expressed in the analyzed tissues. Subsequently, the two promoters of BnSUS1 and BnSUT2 were isolated and truncated. The two promoters and their truncated fragments were fused GUS to transform into Arabidopsis. GUS staining showed that BnSUS1_pro-1690 and BnSUS1_pro-1420 had vascular specificity in cotyledons and mature leaves while BnSUT2_pro-2239, BnSUT2_pro-1681, BnSUT2_pro-1199 and BnSUT2_pro-618 had a constitutive function in seedlings and mature organs. Notably, the activity of BnSUT2_pro-2239 and its fragments (except that of BnSUT2_pro-231) are strongly induced by mechanical wounding. Moreover, BnSUS1_pro-1051 and BnSUS1_pro-485 are sensitive to CuSO₄ treatment while BnSUT2_pro-2239 and BnSUT2_pro-1681 are sensitive to PEG and ABA treatments, respectively. Our findings will provide the foundation for deciphering the functions of BnSUS1 and BnSUT2, and also expand the promoter library to provide more options for plant genetic engineering.

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.

Analysis of a novel mutant allele of GSL8 reveals its key roles in cytokinesis and symplastic trafficking in Arabidopsis

BACKGROUND

Plant cell walls are mainly composed of polysaccharides such as cellulose and callose. Callose exists at a very low level in the cell wall; however, it plays critical roles at different stages of plant development as well as in defence against unfavorable conditions. Callose is accumulated at the cell plate, at plasmodesmata and in male and female gametophytes. Despite the important roles of callose in plants, the mechanisms of its synthesis and regulatory properties are not well understood.

RESULTS

CALLOSE SYNTHASE (CALS) genes, also known as GLUCAN SYNTHASE-LIKE (GSL), comprise a family of 12 members in Arabidopsis thaliana. Here, we describe a new allele of GSL8 (named essp8) that exhibits pleiotropic seedling defects. Reduction of callose deposition at the cell plates and plasmodesmata in essp8 leads to ectopic endomitosis and an increase in the size exclusion limit of plasmodesmata during early seedling development. Movement of two non-cell-autonomous factors, SHORT ROOT and microRNA165/6, both required for root radial patterning during embryonic root development, are dysregulated in the primary root of essp8. This observation provides evidence for a molecular mechanism explaining the gsl8 root phenotype. We demonstrated that GSL8 interacts with PLASMODESMATA-LOCALIZED PROTEIN 5, a β-1,3-glucanase, and GSL10. We propose that they all might be part of a putative callose synthase complex, allowing a concerted regulation of callose deposition at plasmodesmata.

CONCLUSION

Analysis of a novel mutant allele of GSL8 reveals that GSL8 is a key player in early seedling development in Arabidopsis. GSL8 is required for maintaining the basic ploidy level and regulating the symplastic trafficking. Callose deposition at plasmodesmata is highly regulated and occurs through interaction of different components, likely to be incorporated into a callose biosynthesis complex. We are providing new evidence supporting an earlier hypothesis that GSL8 might have regulatory roles apart from its enzymatic function in plasmodesmata regulation.

Cytosolic invertases contribute to cellulose biosynthesis and influence carbon partitioning in seedlings of Arabidopsis thaliana

In plants, UDP-glucose is the direct precursor for cellulose biosynthesis, and can be converted into other NDP-sugars required for the biosynthesis of wall matrix polysaccharides. UDP-glucose is generated from sucrose by two distinct metabolic pathways. The first pathway is the direct conversion of sucrose to UDP-glucose and fructose by sucrose synthase. The second pathway involves sucrose hydrolysis by cytosolic invertase (CINV), conversion of glucose to glucose-6-phosphate and glucose-1-phosphate, and UDP-glucose generation by UDP-glucose pyrophosphorylase (UGP). Previously, Barratt et al. (Proc. Natl Acad. Sci. USA, 106, 2009 and 13124) have found that an Arabidopsis double mutant lacking CINV1 and CINV2 displayed drastically reduced growth. Whether this reduced growth is due to deficient cell wall production caused by limited UDP-glucose supply, pleiotropic effects, or both, remained unresolved. Here, we present results indicating that the CINV/UGP pathway contributes to anisotropic growth and cellulose biosynthesis in Arabidopsis. Biochemical and imaging data demonstrate that cinv1 cinv2 seedlings are deficient in UDP-glucose production, exhibit abnormal cellulose biosynthesis and microtubule properties, and have altered cellulose organization without substantial changes to matrix polysaccharide composition, suggesting that the CINV/UGP pathway is a key metabolic route to UDP-glucose synthesis in Arabidopsis. Furthermore, differential responses of cinv1 cinv2 seedlings to exogenous sugar supplementation support a function of CINVs in influencing carbon partitioning in Arabidopsis. From these data and those of previous studies, we conclude that CINVs serve central roles in cellulose biosynthesis and carbon allocation in Arabidopsis.

Overexpression of poplar xylem sucrose synthase in tobacco leads to a thickened cell wall and increased height

Sucrose synthase (SuSy) is considered the first key enzyme for secondary growth because it is a highly regulated cytosolic enzyme that catalyzes the reversible conversion of sucrose and UDP into UDP-glucose and fructose. Although SuSy enzymes preferentially functions in the direction of sucrose cleavage at most cellular condition, they also catalyze the synthetic reaction. We isolated a gene that encodes a SuSy from Populus simonii×Populus nigra and named it PsnSuSy2 because it shares high similarity to SuSy2 in Populus trichocarpa. RT-PCR revealed that PsnSuSy2 was highly expressed in xylem, but lowly expressed in young leaves. To characterize its functions in secondary growth, multiple tobacco overexpression transgenic lines of PnsSuSy2 were generated via Agrobacterium-mediated transformation. The PsnSuSy2 expression levels and altered wood properties in stem segments from the different transgenic lines were carefully characterized. The results demonstrated that the levels of PsnSuSy2 enzyme activity, chlorophyll content, total soluble sugars, fructose and glucose increased significantly, while the sucrose level decreased significantly. Consequently, the cellulose content and fiber length increased, whereas the lignin content decreased, suggesting that PsnSuSy2 plays a significant role in cleaving sucrose into UDP-glucose and fructose to facilitate cellulose biosynthesis and that promotion of cellulose biosynthesis suppresses lignin biosynthesis. Additionally, the noticeable increase in the lodging resistance in transgenic tobacco stem suggested that the cell wall characteristics were altered by PsnSuSy2 overexpression. Scanning electron microscopy was performed to study the cell wall morphology of stem, and surprisingly, we found that the secondary cell wall was significantly thicker in transgenic tobacco. However, the thickened secondary cell wall did not negatively affect the height of the plants because the PsnSuSy2- overexpressing lines grew taller than the wildtype plants. This systematic analysis demonstrated that PsnSuSy2 plays an important role in cleaving sucrose coupled with cellulose biosynthesis in wood tissue.

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.

Leaf traits in parental and hybrid species of Sorbus (Rosaceae)

PREMISE OF THE STUDY

Knowledge of functional leaf traits can provide important insights into the processes structuring plant communities. In the genus Sorbus, the generation of taxonomic novelty through reticulate evolution that gives rise to new microspecies is believed to be driven primarily by a series of interspecific hybridizations among closely related taxa. We tested hypotheses for dispersion of intermediacy across the leaf traits in Sorbus hybrids and for trait linkages with leaf area and specific leaf area.

METHODS

Here, we measured and compared the whole complex of growth, vascular, and ecophysiological leaf traits among parental (Sorbus aria, Sorbus aucuparia, Sorbus chamaemespilus) and natural hybrid (Sorbus montisalpae, Sorbus zuzanae) species growing under field conditions. A recently developed atomic force microscopy technique, PeakForce quantitative nanomechanical mapping, was used to characterize the topography of cell wall surfaces of tracheary elements and to map the reduced Young's modulus of elasticity.

KEY RESULTS

Intermediacy was associated predominantly with leaf growth traits, whereas vascular and ecophysiological traits were mainly parental-like and transgressive phenotypes. Larger-leaf species tended to have lower modulus of elasticity values for midrib tracheary element cell walls. Leaves with a biomass investment related to a higher specific leaf area had a lower density. Leaf area- and length-normalized theoretical hydraulic conductivity was related to leaf thickness.

CONCLUSIONS

For the whole complex of examined leaf traits, hybrid microspecies were mosaics of parental-like, intermediate, and transgressive phenotypes. The high proportion of transgressive character expressions found in Sorbus hybrids implies that generation of extreme traits through transgressive segregation played a key role in the speciation process.

Role of the sucrose synthase encoding PrSus1 gene in the development of the parasitic plant Phelipanche ramosa L. (Pomel)

Phelipanche ramosa L. (Pomel) is a major root-parasitic weed attacking many important crops. Success in controlling this parasite is rare and a better understanding of its unique biology is needed to develop new specific control strategies. In the present study, quantitative polymerase chain reaction experiments showed that sucrose synthase encoding PrSus1 transcripts accumulate at their highest level once the parasite is connected to the host (tomato) vascular system, mainly in the parasite tubercles, which bear numerous adventitious roots. In situ hybridization experiments revealed strong PrSus1 expression in both shoot and root apices, especially in shoot apical meristems and in the vascular tissues of scale leaves and stems, and in the apical meristems and developing xylem in roots. In addition, immunolocalization experiments showed that a sucrose synthase protein co-localized with cell-wall thickening in xylem elements. These findings highlight the role of PrSus1 in the utilization of host-derived sucrose in meristematic areas and in cellulose biosynthesis in differentiating vascular elements. We also demonstrate that PrSus1 is downregulated in response to 2,3,5-triiodobenzoic acid-induced inhibition of polar auxin transport in the host stem, suggesting that PrSus1 activity in xylem maturation is controlled by host-derived auxin.

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.

Cellulose synthases and synthesis in Arabidopsis

Plant cell walls are complex structures composed of high-molecular-weight polysaccharides, proteins, and lignins. Among the wall polysaccharides, cellulose, a hydrogen-bonded β-1,4-linked glucan microfibril, is the main load-bearing wall component and a key precursor for industrial applications. Cellulose is synthesized by large multi-meric cellulose synthase (CesA) complexes, tracking along cortical microtubules at the plasma membrane. The only known components of these complexes are the cellulose synthase proteins. Recent studies have identified tentative interaction partners for the CesAs and shown that the migratory patterns of the CesA complexes depend on phosphorylation status. These advances may become good platforms for expanding our knowledge about cellulose synthesis in the near future. In addition, our current understanding of cellulose chain polymerization in the context of the CesA complex is discussed.

Distribution of callose synthase, cellulose synthase, and sucrose synthase in tobacco pollen tube is controlled in dissimilar ways by actin filaments and microtubules

Callose and cellulose are fundamental components of the cell wall of pollen tubes and are probably synthesized by distinct enzymes, callose synthase and cellulose synthase, respectively. We examined the distribution of callose synthase and cellulose synthase in tobacco (Nicotiana tabacum) pollen tubes in relation to the dynamics of actin filaments, microtubules, and the endomembrane system using specific antibodies to highly conserved peptide sequences. The role of the cytoskeleton and membrane flow was investigated using specific inhibitors (latrunculin B, 2,3-butanedione monoxime, taxol, oryzalin, and brefeldin A). Both enzymes are associated with the plasma membrane, but cellulose synthase is present along the entire length of pollen tubes (with a higher concentration at the apex) while callose synthase is located in the apex and in distal regions. In longer pollen tubes, callose synthase accumulates consistently around callose plugs, indicating its involvement in plug synthesis. Actin filaments and endomembrane dynamics are critical for the distribution of callose synthase and cellulose synthase, showing that enzymes are transported through Golgi bodies and/or vesicles moving along actin filaments. Conversely, microtubules appear to be critical in the positioning of callose synthase in distal regions and around callose plugs. In contrast, cellulose synthases are only partially coaligned with cortical microtubules and unrelated to callose plugs. Callose synthase also comigrates with tubulin by Blue Native-polyacrylamide gel electrophoresis. Membrane sucrose synthase, which expectedly provides UDP-glucose to callose synthase and cellulose synthase, binds to actin filaments depending on sucrose concentration; its distribution is dependent on the actin cytoskeleton and the endomembrane system but not on microtubules.

Imaging cell wall architecture in single Zinnia elegans tracheary elements

The chemical and structural organization of the plant cell wall was examined in Zinnia elegans tracheary elements (TEs), which specialize by developing prominent secondary wall thickenings underlying the primary wall during xylogenesis in vitro. Three imaging platforms were used in conjunction with chemical extraction of wall components to investigate the composition and structure of single Zinnia TEs. Using fluorescence microscopy with a green fluorescent protein-tagged Clostridium thermocellum family 3 carbohydrate-binding module specific for crystalline cellulose, we found that cellulose accessibility and binding in TEs increased significantly following an acidified chlorite treatment. Examination of chemical composition by synchrotron radiation-based Fourier-transform infrared spectromicroscopy indicated a loss of lignin and a modest loss of other polysaccharides in treated TEs. Atomic force microscopy was used to extensively characterize the topography of cell wall surfaces in TEs, revealing an outer granular matrix covering the underlying meshwork of cellulose fibrils. The internal organization of TEs was determined using secondary wall fragments generated by sonication. Atomic force microscopy revealed that the resulting rings, spirals, and reticulate structures were composed of fibrils arranged in parallel. Based on these combined results, we generated an architectural model of Zinnia TEs composed of three layers: an outermost granular layer, a middle primary wall composed of a meshwork of cellulose fibrils, and inner secondary wall thickenings containing parallel cellulose fibrils. In addition to insights in plant biology, studies using Zinnia TEs could prove especially productive in assessing cell wall responses to enzymatic and microbial degradation, thus aiding current efforts in lignocellulosic biofuel production.

Altered sucrose metabolism impacts plant biomass production and flower development

Nicotiana tabacum (tobacco) was transformed with three genes involved in sucrose metabolism, UDP-glucose pyrophosphorylase (UGPase, EC 2.7.7.9), sucrose synthase (SuSy, EC 2.4.1.13) and sucrose phosphate synthase (SPS, EC 2.4.1.14). Plants harbouring the single transgenes were subsequently crossed to produce double and triple transgenic lines, including: 2 x 35S::UGPase x SPS, 4CL::UGPase x SPS, 2 x 35S::SuSy x SPS, 4CL::SuSy x SPS, 2 x 35S::UGPase x SuSy x SPS, and 4CL::UGPase x SuSy x SPS. The ultimate aim of the study was to examine whether it is possible to alter cellulose production through the manipulation of sucrose metabolism genes. While altering sucrose metabolism using UGPase, SuSy and SPS does not have an end effect on cellulose production, their simultaneous overexpression resulted in enhanced primary growth as seen in an increase in height growth, in some cases over 50%. Furthermore, the pyramiding strategy of simultaneously altering the expression of multiple genes in combination resulted in increased time to reproductive bud formation as well as altered flower morphology and foliar stipule formation in 4CL lines. Upregulation of these sucrose metabolism genes appears to directly impact primary growth and therefore biomass production in tobacco.

What do we really know about cellulose biosynthesis in higher plants?

Cellulose biosynthesis is one of the most important biochemical processes in plant biology. Despite the considerable progress made during the last decade, numerous fundamental questions related to this key process in plant development are outstanding. Numerous models have been proposed through the years to explain the detailed molecular events of cellulose biosynthesis. Almost all models integrate solid experimental data with hypotheses on several of the steps involved in the process. Speculative models are most useful to stimulate further research investigations and bring new exciting ideas to the field. However, it is important to keep their hypothetical nature in mind and be aware of the risk that some undemonstrated hypotheses may progressively become admitted. In this review, we discuss the different steps required for cellulose formation and crystallization, and highlight the most important specific aspects that are supported by solid experimental data.

Proteomics of plasma membranes from poplar trees reveals tissue distribution of transporters, receptors, and proteins in cell wall formation

By exploiting the abundant tissues available from Populus trees, 3-4 m high, we have been able to isolate plasma membranes of high purity from leaves, xylem, and cambium/phloem at a time (4 weeks after bud break) when photosynthesis in the leaves and wood formation in the xylem should have reached a steady state. More than 40% of the 956 proteins identified were found in the plasma membranes of all three tissues and may be classified as "housekeeping" proteins, a typical example being P-type H(+)-ATPases. Among the 213 proteins predicted to be integral membrane proteins, transporters constitute the largest class (41%) followed by receptors (14%) and proteins involved in cell wall and carbohydrate metabolism (8%) and membrane trafficking (8%). ATP-binding cassette transporters (all members of subfamilies B, C, and G) and receptor-like kinases (four subfamilies) were two of the largest protein families found, and the members of these two families showed pronounced tissue distribution. Leaf plasma membranes were characterized by a very high proportion of transporters, constituting almost half of the integral proteins. Proteins involved in cell wall synthesis (such as cellulose and sucrose synthases) and membrane trafficking were most abundant in xylem plasma membranes in agreement with the role of the xylem in wood formation. Twenty-five integral proteins and 83 soluble proteins were exclusively found in xylem plasma membranes, which identifies new candidates associated with cell wall synthesis and wood formation. Among the proteins uniquely found in xylem plasma membranes were most of the enzymes involved in lignin biosynthesis, which suggests that they may exist as a complex linked to the plasma membrane.

Integrating genes and phenotype: a wheat-Arabidopsis-rice glycosyltransferase database for candidate gene analyses

Glycosyltransferases (GTs) constitute a very large multi-gene superfamily, containing several thousand members identified in sequenced organisms especially in plants. GTs are key enzymes involved in various biological processes such as cell wall formation, storage polysaccharides biosynthesis, and glycosylation of various metabolites. GTs have been identified in rice (Oryza sativa) and Arabidopsis thaliana, but their precise function has been demonstrated biochemically for only a few. In this work we have established a repertoire of virtually all the wheat (Triticum aestivum) GT sequences, using the large publicly available banks of expressed sequences. Based on sequence similarity with Arabidopsis and rice GTs compiled in the carbohydrate active enzyme database (CAZY), we have identified and classified these wheat sequences. The results were used to feed a searchable database available on the web ( http://wwwappli.nantes.inra.fr:8180/GTIDB ) that can be used for initiating an exhaustive candidate gene survey in wheat applied to a particular biological process. This is illustrated through the identification of GT families which are expressed during cell wall formation in wheat grain maturation.

Cysteine proteases XCP1 and XCP2 aid micro-autolysis within the intact central vacuole during xylogenesis in Arabidopsis roots

Establishing the mechanisms regulating the autolysis of xylem tracheary elements (TEs) is important for understanding this programmed cell death process. These data demonstrate that two paralogous Arabidopsis thaliana proteases, XYLEM CYSTEINE PROTEASE1 (XCP1) and XCP2, participated in micro-autolysis within the intact central vacuole before mega-autolysis was initiated by tonoplast implosion. The data acquisition was aided by the predictable pattern of seedling root xylogenesis, the availability of single and double total knock-out T-DNA lines, anti-sera that recognized XCP1 and XCP2, and the microwave-assisted processing of whole seedlings prior to immunolabeling and observation in the transmission electron microscope. During secondary wall thickening, XCP1 and XCP2 (in wild type), XCP1 (in xcp2 seedlings) or XCP2 (in xcp1 seedlings) were imported into the TE central vacuole. Both XCP1 and XCP2 heavily labeled dense aggregates of material within the vacuole. However, because of XCP1 deficiency in xcp1 and xcp1 xcp2 TEs, non-degraded cellular remnants first accumulated in the vacuole and then persisted in the TE lumen (longer than in the wild type) after the final mega-autolysis was otherwise complete. This delayed TE clearing phenotype in xcp1 was rescued by complementation with wild-type XCP1. Although TEs in the xcp2 single knock-out cleared comparably with wild type, the non-degraded remnants in xcp1 xcp2 TEs were more densely packed than in xcp1 TEs. Therefore, XCP2 has a minor but distinct role in micro-autolysis. After tonoplast implosion, XCP1 and XCP2 remained associated with disintegrating cellular material as mega-autolysis, aided by additional lytic enzymes, destroyed the bulk of the cellular contents.

The cytoplasmic domain of the cellulose-synthesizing complex in vascular plants

The cytoplasmic domain of the rosette terminal complex has been imaged in situ in patches of plasma membrane isolated from tobacco BY-2 protoplasts. By partially extracting the plasma membrane lipids, cellulose microfibrils were observed through the plasma membrane. Rosette terminal complexes were identified on the basis of their association with the ends of these cellulose microfibrils. The cytoplasmic domain of the rosette terminal complex has been shown to be hexagonal in shape and has been measured to be 45-50 nm in diameter and 30-35 nm tall. These findings demonstrate that the terminal complex does indeed have a substantial cytoplasmic component, and that the hexagonal array observed in the lipid bilayer by freeze fracture is actually only a small part of the overall complex. These findings will allow better modeling of the terminal complex and may facilitate predictions of how many proteins are associated with the rosette terminal complex in vivo.

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.

Nano scale proteomics revealed the presence of regulatory proteins including three FT-Like proteins in phloem and xylem saps from rice

The main physiological roles of phloem and xylem in higher plants involve the transport of water, nutrients and metabolites. They are also involved, however, in whole plant events including stress responses and long-distance signaling. Phloem and xylem saps therefore include a variety of proteins. In this study, we have performed a shotgun analysis of the proteome of phloem and xylem saps from rice, taking advantage of the complete and available genomic information for this plant. Xylem sap was prepared using the root pressure method, whereas phloem sap was prepared with a unique method with the assistance of planthoppers to ensure the robustness of the detected proteins. The technical difficulties caused by the very limited availability of rice samples were overcome by the use of nano-flow liquid chromatography linked to a mass spectrometer. We identified 118 different proteins and eight different peptides in xylem sap, and 107 different proteins and five different peptides in phloem sap. Signal transduction proteins, putative transcription factors and stress response factors as well as metabolic enzymes were identified in these saps. Interestingly, we found the presence of three TERMINAL FLOWER 1/FLOWERING LOCUS T (FT)-like proteins in phloem sap. The detected FT-like proteins were not rice Hd3a (OsFTL2) itself that acted as a non-cell-autonomous signal for flowering control, but they were members of distinct subfamilies of the FT family with differential expression patterns. These results imply that proteomics on a nano scale is a potent tool for investigation of biological processes in plants.

Over-expression of an arabidopsis family A sucrose phosphate synthase (SPS) gene alters plant growth and fibre development

The objective of this study was to manipulate the intracellular pools of sucrose by differentially expressing exogenous sucrose phosphate synthase (SPS) and investigating its role in regulating plant growth and fibre development. Tobacco (Nicotiana tabacum cv. Xanthi) plants were transformed with an arabidopsis SPS gene under the regulation of the ubiquitously expressed tandem repeat of the 35S cauliflower mosaic virus promoter, and subject to growth trials and fibre characterization. It was apparent that over-expression of SPS resulted in substantially elevated concentrations of sink sucrose pools compared to wild-type plants, while source tissue sucrose pools remained the same. All transformed plants had significantly increased stem height, which was ascribed to internode elongation, and greater stem diameters, longer fibers and increased total dry biomass relative to the control plants. Difference in the chemical composition of either the storage or structural carbohydrates of the wild-type and SPS transgenic lines were only minor. The correlation between increased stem sucrose content and plant phenotypes with elevated SPS gene expression confirm a role for sucrose availability in controlling plant growth and fibre elongation.

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.

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.

Cytoskeletal organization during xylem cell differentiation

The water and mineral conductive tube, the xylem vessel and tracheid, is a highly conspicuous tissue due to its elaborately patterned secondary-wall deposition. One constituent of the xylem vessel and tracheid, the tracheary element, is an empty dead cell that develops secondary walls in the elaborate patterns. The wall pattern is appropriately regulated according to the developmental stage of the plant. The cytoskeleton is an essential component of this regulation. In fact, the cortical microtubule is well known to participate in patterned secondary cell wall formation. The dynamic rearrangement of the microtubules and actin filaments have also been recognized in the cultured cells differentiating into tracheary elements in vitro. There has recently been considerable progress in our understanding of the dynamics and regulation of cortical microtubules, and several plant microtubule associated proteins have been identified and characterized. The microtubules have been observed during tracheary element differentiation in living Arabidopsis thaliana cells. Based on this recently acquired information on the plant cytoskeleton and tracheary element differentiation, this review discusses the role of the cytoskeleton in secondary cell wall formation.

Molecular characterization and expression of four cDNAs encoding sucrose synthase from green bamboo Bambusa oldhamii

Bamboo is distinguished by its rapid growth. To investigate sucrose metabolism in this plant, we cloned the cDNAs encoding sucrose synthase (SuS) from Bambusa oldhamii and investigated their expression in growing shoots and leaves. Four cDNA clones, BoSus1, BoSus2, BoSus3 and BoSus4, were isolated by screening a cDNA library from etiolated bamboo shoots. Recombinant BoSuS proteins were produced in Escherichia coli and purified by immobilized metal affinity chromatography and ultrafiltration. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) was used to determine the abundance of the transcript of each gene. BoSus1 and BoSus3 may be duplicate or homeologous genes, the sequences of which show high identity. Similarly, BoSus2 shows high identity with BoSus4. Kinetic analysis showed that the two BoSuS isoforms of each type had similar michaelis constant (Km) values for sucrose, but different values for UDP. The four genes were expressed in various bamboo organs but were differentially regulated. The increase in the abundance of their mRNA paralleled the growth rate of the bamboo. The results suggest that, in bamboo, SuS is encoded by at least four genes, each with a specific role in providing substrates for the polysaccharide biosynthesis and/or energy production necessary to support the rapid growth of this species.

Up-regulation of sucrose synthase and UDP-glucose pyrophosphorylase impacts plant growth and metabolism

The effects of the overexpression of sucrose synthase (SuSy) and UDP-glucose pyrophosphorylase (UGPase) on plant growth and metabolism were evaluated in tobacco (Nicotiana tabacum cv. Xanthi). T(1) transgenic plants expressing either gene under the control of a tandem repeat cauliflower mosaic virus 35S promoter (2x35S) or a xylem-localized 4CL promoter (4-coumarate:CoA ligase; 4CL) were generated, and reciprocally crossed to generate plants expressing both genes. Transcript levels, enzyme activity, growth parameters, fibre properties and carbohydrate content of stem tissue were quantified. The expression profiles of both genes confirmed the expression pattern of the promoters: 2x35S expressed more strongly in leaves, while 4CL expression was highest in stem tissue. In-depth plant characterization revealed that the single-transgene lines showed significant increases in the height growth compared with corresponding control lines. The double-transgene plants demonstrated an additive effect, proving to be even taller than the single-transgene parents. Several of these lines had associated increases in soluble sugar content. Although partitioning of storage carbohydrates into starch or cellulose was not observed, the increased height growth and increases in soluble carbohydrates suggest a role for SuSy as a marker in sink strength and lend credit to the function of UGPase in a similar role. The up-regulation of these two genes, although not increasing the percentage cellulose content, was effective in increasing the total biomass, and thus the overall cellulose yield, from a given plant.

Toward a systems approach to understanding plant cell walls

One of the defining features of plants is a body plan based on the physical properties of cell walls. Structural analyses of the polysaccharide components, combined with high-resolution imaging, have provided the basis for much of the current understanding of cell walls. The application of genetic methods has begun to provide new insights into how walls are made, how they are controlled, and how they function. However, progress in integrating biophysical, developmental, and genetic information into a useful model will require a system-based approach.

Nucleotide sugar interconversions and cell wall biosynthesis: how to bring the inside to the outside

Plants possess a sophisticated sugar biosynthetic machinery comprising families of nucleotide sugar interconversion enzymes. Literature published in the past two years has made a major contribution to our knowledge of the enzymes and genes involved in the interconversion of nucleotide sugars that are required for cell wall biosynthesis, including UDP-L-rhamnose, UDP-D-galactose, UDP-D-glucuronic acid, UDP-D-xylose, UDP-D-apiose, UDP-L-arabinose, GDP-L-fucose and GDP-L-galactose. Indirect evidence suggests that enzyme activity is crudely regulated at the transcriptional level in a cell-type and differentiation-dependent manner. However, feedback inhibition and NAD(+)/NADH redox control, as well as the formation of complexes between differentially encoded isoforms and glycosyltransferases, might fine-tune cell wall matrix biosynthesis. I hypothesise that the control of nucleotide sugar interconversion enzymes regulates glycosylation patterns in response to developmental, metabolic and stress-related stimuli, thereby linking signalling with primary metabolism and the dynamics of the extracellular matrix.

The white clover enod40 gene family. Expression patterns of two types of genes indicate a role in vascular function

Enod40 is one of the genes associated with legume nodule development and has a putative role in general plant organogenesis. We have isolated a small enod40 gene family from white clover (Trifolium repens), with three genes designated Trenod40-1, Trenod40-2, and Trenod40-3, all containing the conserved enod40 regions I and II. Trenod40-1 and Trenod40-2 share over 90% homology in the transcribed regions and high levels of similarity in their upstream regulatory sequences. Trenod40-1 and Trenod40-2 are similar to the enod40 genes of legumes forming indeterminate nodules (group II) and are predominantly expressed in nodules. Trenod40-3 shares only 32.8% identity with Trenod40-1 and Trenod40-2 within the transcribed region. Trenod40-3 is similar to the enod40 genes of legumes with determinate nodules (group I) and is not predominantly expressed in nodules. To our knowledge, this is the first report of both group I- and group II-type enod40 genes being expressed in a single legume species. In situ hybridization studies revealed that Trenod40 genes were highly expressed in non-symbiotic tissues, particularly in stolon nodes during nodal root and lateral shoot development. High levels of Trenod40 transcripts were also present in the vascular bundles of mature plant organs, mainly at sites of intensive lateral transport, suggesting a role in vascular tissue function. The expression pattern of Trenod40 genes was analyzed further using Trenod40 promoter-gus fusions in transgenic white clover and tobacco (Nicotiana tabacum), indicating that white clover and tobacco share the regulatory mechanisms for Trenod40-1/2 promoters and some aspects of Trenod40-3 regulation.

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.