The gene expression and enzyme activity of plant 3-deoxy-D-manno-2-octulosonic acid-8-phosphate synthase are preferentially associated with cell division in a cell cycle-dependent manner

Functional characterization of plant specific Indeterminate Domain (IDD) transcription factors in tomato (Solanum lycopersicum L.)

Plant-specific transcription factors (TFs) are responsible for regulating the genes involved in the development of plant-specific organs and response systems for adaptation to terrestrial environments. This includes the development of efficient water transport systems, efficient reproductive organs, and the ability to withstand the effects of terrestrial factors, such as UV radiation, temperature fluctuations, and soil-related stress factors, and evolutionary advantages over land predators. In rice and Arabidopsis, INDETERMINATE DOMAIN (IDD) TFs are plant-specific TFs with crucial functions, such as development, reproduction, and stress response. However, in tomatoes, IDD TFs remain uncharacterized. Here, we examined the presence, distribution, structure, characteristics, and expression patterns of SlIDDs. Database searches, multiple alignments, and motif alignments suggested that 24 TFs were related to Arabidopsis IDDs. 18 IDDs had two characteristic C2H2 domains and two C2HC domains in their coding regions. Expression analyses suggest that some IDDs exhibit multi-stress responsive properties and can respond to specific stress conditions, while others can respond to multiple stress conditions in shoots and roots, either in a tissue-specific or universal manner. Moreover, co-expression database analyses suggested potential interaction partners within IDD family and other proteins. This study functionally characterized SlIDDs, which can be studied using molecular and bioinformatics methods for crop improvement.

CsKDO is a candidate gene regulating seed germination lethality in cucumber

Seed germination plays an important role in the initial stage of plant growth. However, few related studies focused on lethality after seed germination in plants. In this study, we identified an Ethyl methanesulfonate (EMS) mutagenesis mutant Csleth with abnormal seed germination in cucumber (Cucumis sativus L.). The radicle of the Csleth mutant grew slowly and detached from the cotyledon until 14 d after sowing. Genetic analysis showed that the mutant phenotype of Csleth was controlled by a single recessive gene. MutMap⁺ and Kompetitive Allele Specific PCR (KASP) genotyping results demonstrated that Csa3G104930 encoding 3-deoxy-manno-octulosonate cytidylyltransferase (CsKDO) was the candidate gene of the Csleth mutant. The transition mutation of aspartate occurred in Csa3G104930 co-segregated with the phenotyping data. CsKDO was highly expressed in male flowers in wild type cucumbers. Subcellular localization results showed that CsKDO was located in the nucleus. Overall, these results suggest CsKDO regulates lethality during seed germination in cucumber.

Rhamnogalacturonan II: Chemical Synthesis of a Substructure Including α-2,3-Linked Kdo*

The synthesis of a fully deprotected Kdo-containing rhamnogalacturonan II pentasaccharide is described. The strategy relies on the preparation of a suitably protected homogalacturonan tetrasaccharide backbone, through a post-glycosylation oxidation approach, and its stereoselective glycosylation with a Kdo fluoride donor.

BnaA02.NIP6;1a encodes a boron transporter required for plant development under boron deficiency in Brassica napus

Boron (B) is an essential micronutrient for the plant normal growth. In Arabidopsis, NIP6;1 is a boric acid channel required for the proper distribution of boric acid, especially in the nodal regions of shoots. BnaA02.NIP6;1a, a homologous gene of AtNIP6;1 in Brassica napus, was reported to play a key role in B transport activity. However, little is known about the other functions of BnaA02.NIP6;1a in Brassica napus. In this study, we found that BnaA02.NIP6; 1a was localized in both plasma membrane and cytoplasm, which was different from that in Arabidopsis. The transgenic Arabidopsis plant containing a BnaA02.NIP6;1a promoter driven GUS reporter gene displayed strong GUS activity in roots, stems, leaves, especially in buds and open flowers, which are different from the expression pattern from its homologous gene in Arabidopsis. Silencing BnaA02.NIP6;1a repressed vegetative growth under B-deficient condition in Brassica napus. More importantly, knockdown of BnaA02.NIP6;1a in rapeseed resulted in the reduction of boron accumulation in the flower under boron deficiency and lead to severe sterility, which has not yet been reported before. Furthermore, nip6;1 mutant in Arabidopsis only showed the loss of apical dominance phenotype under boron deficiency at reproductive stage, whereas BnaA02.NIP6;1 RNAi lines exhibited large amounts of abnormal development of the inflorescence as compared with the wild type under boron limitation. Taken together, our results demonstrate that BnaA02.NIP6;1a encodes a boron transporter required for plant development under boron deficiency in Brassica napus, which shows its novel and diverse function in rapeseed compared with model plant Arabidopsis.

Identification of QTLs for Domestication-Related Traits in Zombi Pea [Vigna vexillata (L.) A. Rich], a Lost Crop of Africa

Zombi pea [Vigna vexillata (L.) A. Rich] is a legume crop found in Africa. Wild zombi pea is widely distributed throughout the tropical and subtropical regions, whereas domesticated zombi pea is rarely cultivated. Plant domestication is an evolutionary process in which the phenotypes of wild species, including seed dormancy, pod shattering, organ size, and architectural and phenological characteristics, undergo changes. The molecular mechanism underlying the domestication of zombi pea is relatively unknown. In this study, the genetic basis of the following 13 domestication-related traits was investigated in an F₂ population comprising 198 individuals derived from a cross between cultivated (var. macrosperma) and wild (var. vexillata) zombi pea accessions: seed dormancy, pod shattering, days-to-flowering, days-to-maturity, stem thickness, stem length, number of branches, leaf area, pod length, 100-seed weight, seed width, seed length, and seeds per pod. A genetic map containing 6,529 single nucleotide polymorphisms constructed for the F₂ population was used to identify quantitative trait loci (QTLs) for these traits. A total of 62 QTLs were identified for the 13 traits, with 1-11 QTLs per trait. The major QTLs for days-to-flowering, stem length, number of branches, pod length, 100-seed weight, seed length, and seeds per pod were clustered in linkage group 5. In contrast, the major QTLs for seed dormancy and pod shattering belonged to linkage groups 3 and 11, respectively. A comparative genomic analysis with the cowpea [Vigna unguiculata (L.) Walp.] genome used as the reference sequence (i.e., the genome of the legume species most closely related to zombi pea) enabled the identification of candidate genes for the major QTLs. Thus, we revealed the genomic regions associated with domestication-related traits and the candidate genes controlling these traits in zombi pea. The data presented herein may be useful for breeding new varieties of zombi pea and other Vigna species.

Suppression of Arabidopsis GGLT1 affects growth by reducing the L-galactose content and borate cross-linking of rhamnogalacturonan-II

Boron is a micronutrient that is required for the normal growth and development of vascular plants, but its precise functions remain a subject of debate. One established role for boron is in the cell wall where it forms a diester cross-link between two monomers of the low-abundance pectic polysaccharide rhamnogalacturonan-II (RG-II). The inability of RG-II to properly assemble into a dimer results in the formation of cell walls with abnormal biochemical and biomechanical properties and has a severe impact on plant productivity. Here we describe the effects on RG-II structure and cross-linking and on the growth of plants in which the expression of a GDP-sugar transporter (GONST3/GGLT1) has been reduced. In the GGLT1-silenced plants the amount of L-galactose in side-chain A of RG-II is reduced by up to 50%. This leads to a reduction in the extent of RG-II cross-linking in the cell walls as well as a reduction in the stability of the dimer in the presence of calcium chelators. The silenced plants have a dwarf phenotype, which is rescued by growth in the presence of increased amounts of boric acid. Similar to the mur1 mutant, which also disrupts RG-II cross-linking, GGLT1-silenced plants display a loss of cell wall integrity under salt stress. We conclude that GGLT1 is probably the primary Golgi GDP-L-galactose transporter, and provides GDP-L-galactose for RG-II biosynthesis. We propose that the L-galactose residue is critical for RG-II dimerization and for the stability of the borate cross-link.

The inside and outside: topological issues in plant cell wall biosynthesis and the roles of nucleotide sugar transporters

The cell wall is a complex extracellular matrix composed primarily of polysaccharides. Noncellulosic polysaccharides, glycoproteins and proteoglycans are synthesized in the Golgi apparatus by glycosyltransferases (GTs), which use nucleotide sugars as donors to glycosylate nascent glycan and glycoprotein acceptors that are subsequently exported to the extracellular space. Many nucleotide sugars are synthesized in the cytosol, leading to a topological issue because the active sites of most GTs are located in the Golgi lumen. Nucleotide sugar transporters (NSTs) overcome this problem by translocating nucleoside diphosphate sugars from the cytosol into the lumen of the organelle. The structures of the cell wall components synthesized in the Golgi are diverse and complex; therefore, transporter activities are necessary so that the nucleotide sugars can provide substrates for the GTs. In this review, we describe the topology of reactions involved in polysaccharide biosynthesis in the Golgi and focus on the roles of NSTs as well as their impacts on cell wall structure when they are altered.

Plant cell wall imaging by metabolic click-mediated labelling of rhamnogalacturonan II using azido 3-deoxy-D-manno-oct-2-ulosonic acid

In plants, 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) is a monosaccharide that is only found in the cell wall pectin, rhamnogalacturonan-II (RG-II). Incubation of 4-day-old light-grown Arabidopsis seedlings or tobacco BY-2 cells with 8-azido 8-deoxy Kdo (Kdo-N3 ) followed by coupling to an alkyne-containing fluorescent probe resulted in the specific in muro labelling of RG-II through a copper-catalysed azide-alkyne cycloaddition reaction. CMP-Kdo synthetase inhibition and competition assays showing that Kdo and D-Ara, a precursor of Kdo, but not L-Ara, inhibit incorporation of Kdo-N3 demonstrated that incorporation of Kdo-N3 occurs in RG-II through the endogenous biosynthetic machinery of the cell. Co-localisation of Kdo-N3 labelling with the cellulose-binding dye calcofluor white demonstrated that RG-II exists throughout the primary cell wall. Additionally, after incubating plants with Kdo-N3 and an alkynated derivative of L-fucose that incorporates into rhamnogalacturonan I, co-localised fluorescence was observed in the cell wall in the elongation zone of the root. Finally, pulse labelling experiments demonstrated that metabolic click-mediated labelling with Kdo-N3 provides an efficient method to study the synthesis and redistribution of RG-II during root growth.

Role of AINTEGUMENTA-like gene NtANTL in the regulation of tobacco organ growth

The Nicotiana tabacum AINTEGUMENTA-like gene (NtANTL), encoding one of AP2/ERF transcription factors, is a putative ortholog of the AtANT gene from Arabidopsis thaliana. In wild-type tobacco plants, the NtANTL gene was expressed in the actively dividing young flowers, shoot apices, and calluses, while the level of its mRNA increased considerably after treatment with exogenous 6-benzylaminopurine, indoleacetic acid and 24-epibrassinolide. We found a positive correlation among the expression levels of NtANTL, cyclin NtCYCD3;1 and cyclin-dependent kinase NtCDKB1-1 genes, suggesting possible molecular links between AINTEGUMENTA and cell cycle regulators in tobacco plants. However, no correlation was observed between NtANTL, NtCYCD3;1 and NtCDKB1-1 expression levels in response to NaCl and ABA. These observations indicate that the transcription factor NtANTL was not involved in the regulation of the cellular response to salinity nor did it affect the expression of NtCYCD3;1 and NtCDKB1-1 when tobacco plants were exposed to salt stress and ABA. In addition, we generated transgenic tobacco plants with both up-regulated and down-regulated expression of the NtANTL gene. Constitutive expression of the NtANTL gene contributed to an increase in the size of leaves and corolla of transgenic plants. Transgenic plants with reduced expression of the NtANTL gene had smaller leaves, flowers and stems, but showed a compensatory increase in the cell size of leaves and flowers. The results show the significance of the NtANTL gene for the control of organ growth by both cell division and expansion in tobacco plants.

Heterologous expression, purification, crystallization and preliminary X-ray diffraction analysis of KDO8P synthase from Arabidopsis thaliana

3-Deoxy-d-manno-octulosonate 8-phosphate synthase (KDO8PS) [EC 4.1.2.16] is the first and rate-limiting enzyme in the 3-deoxy-d-manno-octulosonate (KDO) biosynthetic pathway. The enzyme is widely expressed in bacteria and plants. Their well conserved protein sequences imply a similar oligomeric arrangement. However, the reported size exclusion chromatrographic analysis suggested a species-dependent self-assembling. To clarify the discrepancy and explore the self-assembling property of KDO8PS, we expressed and purified the Arabidopsis enzyme in Escherichia coli system. The enzyme was highly purified using a two-step purification strategy including nickel affinity and size exclusion chromatography with an expected pH activity profile. The identity of the purified enzyme was confirmed by Western-blot and mass fingerprints. Further analysis by analytical ultracentrifugation indicated that both bacteria and Arabidopsis enzymes are homotetramer. Furthermore, the purified enzyme from the plant has been crystallized and a complete set of X-ray data to 2.1Å resolution has been collected.

Plant nucleotide sugar formation, interconversion, and salvage by sugar recycling

Nucleotide sugars are the universal sugar donors for the formation of polysaccharides, glycoproteins, proteoglycans, glycolipids, and glycosylated secondary metabolites. At least 100 genes encode proteins involved in the formation of nucleotide sugars. These nucleotide sugars are formed using the carbohydrate derived from photosynthesis, the sugar generated by hydrolyzing translocated sucrose, the sugars released from storage carbohydrates, the salvage of sugars from glycoproteins and glycolipids, the recycling of sugars released during primary and secondary cell wall restructuring, and the sugar generated during plant-microbe interactions. Here we emphasize the importance of the salvage of sugars released from glycans for the formation of nucleotide sugars. We also outline how recent studies combining biochemical, genetic, molecular and cellular approaches have led to an increased appreciation of the role nucleotide sugars in all aspects of plant growth and development. Nevertheless, our understanding of these pathways at the single cell level is far from complete.

Specificity and mutational analysis of the metal-dependent 3-deoxy-D-manno-octulosonate 8-phosphate synthase from Acidithiobacillus ferrooxidans

3-deoxy-D-manno-octulosonate 8-phosphate synthase (KDO8PS) catalyzes the reaction between phosphoenol pyruvate and D-arabinose 5-phosphate to generate KDO8P. This reaction is part of the biosynthetic pathway to 3-deoxy-D-manno-octulosonate, a component of the lipopolysaccharide of the Gram-negative bacterial cell wall. Two distinct groups of KDO8PSs exist, differing by the absolute requirement of a divalent metal ion. In this study Acidithiobacillus ferrooxidans KDO8PS has been expressed and purified and shown to require a divalent metal ion, with Mn2+, Co2+ and Cd2+ (in decreasing order) being able to restore activity to metal-free enzyme. Cd2+ significantly enhanced the stability of the enzyme, raising the Tm by 14 degrees C. D-glucose 6-phosphate and D-erythrose 4-phosphate were not substrates for A. ferrooxidans KDO8PS, whereas 2-deoxy-D-ribose 5-phosphate was a poor substrate and there was negligible activity with D-ribose 5-phosphate. The 243AspGlyPro245 motif is absolutely conserved in the metal-independent group of synthases, but the Gly and Pro sites are variable in the metal-dependent enzymes. Substitution of the putative metal-binding Asp243 to Ala in A. ferrooxidans KDO8PS gave inactive enzyme, whereas substitutions Asp243Glu or Pro245Ala produced active enzymes with altered metal-dependency profiles. Prior studies indicated that exchange of a metal-binding Cys for Asn converts metal-dependent KDO8P synthase into a metal-independent form. Unexpectedly, this mutation in A. ferrooxidans KDO8P synthase (Cys21Asn) gave inactive enzyme. This finding, together with modest activity towards 2-deoxy-D-ribose 5-phosphate suggests similarities between the A. ferrooxidans KDO8PS and the related metal-dependent 3-deoxy-D-arabino-heptulosonate phosphate synthase, and highlights the importance of the AspGlyPro loop in positioning the substrate for effective catalysis in all KDO8P synthases.

Characterization of a putative 3-deoxy-D-manno-2-octulosonic acid (Kdo) transferase gene from Arabidopsis thaliana

The structures of the pectic polysaccharide rhamnogalacturonan II (RG-II) pectin constituent are remarkably evolutionary conserved in all plant species. At least 12 different glycosyl residues are present in RG-II. Among them is the seldom eight-carbon sugar 3-deoxy-d-manno-octulosonic acid (Kdo) whose biosynthetic pathway has been shown to be conserved between plants and Gram-negative bacteria. Kdo is formed in the cytosol by the condensation of phosphoenol pyruvate with d-arabinose-5-P and then activated by coupling to cytidine monophosphate (CMP) prior to its incorporation in the Golgi apparatus by a Kdo transferase (KDTA) into the nascent polysaccharide RG-II. To gain new insight into RG-II biosynthesis and function, we isolated and characterized null mutants for the unique putative KDTA (AtKDTA) encoded in the Arabidopsis genome. We provide evidence that, in contrast to mutants affecting the RG-II biosynthesis, the extinction of the AtKDTA gene expression does not result in any developmental phenotype in the AtkdtA plants. Furthermore, the structure of RG-II from the null mutants was not altered and contained wild-type amount of Rha-alpha(1-5)Kdo side chain. The cellular localization of AtKDTA was investigated by using laser scanning confocal imaging of the protein fused to green fluorescent protein. In agreement with its cellular prediction, the fusion protein was demonstrated to be targeted to the mitochondria. These data, together with data deduced from sequence analyses of higher plant genomes, suggest that AtKDTA encodes a putative KDTA involved in the synthesis of a mitochondrial not yet identified lipid A-like molecule rather than in the synthesis of the cell wall RG-II.

Structural characterisation of the pectic polysaccharide rhamnogalacturonan II using an acidic fingerprinting methodology

Rhamnogalacturonan II (RG-II) is a structurally complex cell wall pectic polysaccharide. Despite its complexity, both the structure of RG-II and its ability to dimerise via a borate diester are conserved in vascular plants suggesting that RG-II has a fundamental role in primary cell wall organisation and function. The selection and analysis of new mutants affected in RG-II formation represents a promising strategy to unravel these functions and to identify genes encoding enzymes involved in RG-II biosynthesis. In this paper, a novel fingerprinting strategy is described for the screening of RG-II mutants based on the mild acid hydrolysis of RG-II coupled to the analysis of the resulting fragments by mass spectrometry. This methodology was developed using RG-II fractions isolated from citrus pectins and then validated for RG-II isolated from the Arabidopsis mur1 mutant and irx10 irx10-like double mutant.

Overexpression of a grapevine R2R3-MYB factor in tomato affects vegetative development, flower morphology and flavonoid and terpenoid metabolism

Although the terpenoid pathway constitutes, with the phenylpropanoid metabolism, the major pathway of secondary metabolism in plants, little is known about its regulation. Overexpression of a Vitis vinifera R2R3-MYB transcription factor (VvMYB5b) in tomato induced pleiotropic changes including dwarfism, modified leaf structure, alterations of floral morphology, pigmented and glossy fruits at the "green-mature" stage and impaired seed germination. Two main branches of secondary metabolism, which profoundly influence the organoleptic properties of the fruit, were affected in the opposite way by VvMYB5b overexpression. Phenylpropanoid metabolism was down regulated whereas the amount of beta-carotene was up regulated. This is the first example of the independent regulation of phenylpropanoid and carotenoid metabolism. The strongest modification concerns a decrease in beta-amyrin, the precursor of the oleanolic acid, which is the major component of grape waxes. Scanning electron microscopy analysis of fruits and leaves confirms the alteration of wax metabolism and a modification of cell size and shape. This may potentially impact resistance/tolerance to biotic and abiotic stresses. The results are compared with a similar approach using heterologous expression of VvMYB5b in tobacco.

The synthesis of the rhamnogalacturonan II component 3-deoxy-D-manno-2-octulosonic acid (Kdo) is required for pollen tube growth and elongation

Despite a very complex structure, the sugar composition of the rhamnogalacturonan II (RG-II) pectic fraction is extremely conserved. Among its constituting monosaccharides is the seldom-observed eight-carbon sugar 3-deoxy-D-manno-octulosonic acid (Kdo), whose phosphorylated precursor is synthesized by Kdo-8-P synthase. As an attempt to alter specifically the RG-II structure in its sugar composition and assess the consequences on the function of RG-II in cell wall and its relationship with growth, Arabidopsis null mutants were sought in the genes encoding Kdo-8-P synthase. Here, the isolation and characterization of one null mutant for the isoform 1 (AtkdsA1-S) and two distinct null mutants for the isoform 2 of Arabidopsis Kdo-8-P synthase (AtkdsA2-V and AtkdsA2-S) are described. Evidence is provided that AtkdsA2 gene expression is preferentially associated with plantlet organs displaying a meristematic activity, and that it accounts for 75% of the mRNAs to be translated into Kdo-8-P synthase. Furthermore, this predominant expression of AtKDSA2 over AtKDSA1 was confirmed by quantification of the cytosolic Kdo content in the mutants, in a variety of ecotypes. The inability to identify a double knockout mutant originated from pollen abortions, due to the inability of haploid pollen of the AtkdsA1- AtkdsA2- genotype to form an elongated pollen tube properly and perform fertilization.

Engineering of a sialic acid synthesis pathway in transgenic plants by expression of bacterial Neu5Ac-synthesizing enzymes

Plants are a low-cost and contamination-free factory for the production of recombinant pharmaceutical proteins. However, plant-made pharmaceuticals differ from their mammalian homologues by the structure of their N-linked glycans. For instance, most mammalian glycoproteins harbour terminal sialic acids that control their half-life in the bloodstream. The absence of the whole sialylation machinery in plants is of major concern as non-sialylated plant-made pharmaceuticals may not perform at their full potential in humans, because of their removal from the circulation through the involvement of hepatic cell receptors. In this context, we have investigated the synthesis of N-acetylneuraminic acid (Neu5Ac) in the cytosol of plants by either the re-routing of the endogenous 3-deoxy-d-manno-2-octulosonic acid (Kdo) biosynthetic pathway or the expression of microbial Neu5Ac-synthesizing enzymes. In this paper, we demonstrate that the plant Kdo-8P synthase is not able to use N-acetyl d-mannosamine as a substrate, and thus re-routing of the Kdo pathway for the synthesis of Neu5Ac is not possible. Consequently, we expressed genes encoding Neu5Ac lyase from Escherichia coli and Neu5Ac synthase (neuB2) from Campylobacter jejuni in plants. These resulted in the production of functional enzymes in the cytosol, which in turn can catalyse the synthesis of Neu5Ac in vitro. Experiments were carried out on two models, Bright Yellow 2 (BY2) tobacco cells and Medicago sativa (alfalfa), the perennial legume crop.

Molecular characterization of a WEE1 gene homologue in tomato (Lycopersicon esculentum Mill.)

Early fruit development in tomato (Lycopersicon esculentum Mill.) proceeds in two distinct phases of growth that comprise cell division and cell expansion, respectively. In pericarp and the jelly like locular tissue of tomato fruit, the transition between cell division to cell expansion is characterized by the arrest of mitotic activity, numerous rounds of nuclear DNA endoreduplication and the inhibition of Cyclin-Dependent Kinase A (CDKA) activity. To investigate whether the WEE1 kinase may play a role during the endoreduplication process, we isolated and characterized the tomato homologue for WEE1. The LeWEE1 gene consisted of 10 exons with a predicted 510 amino acid-long protein. The accumulation of the corresponding transcripts was associated with mitotically active organs: developing fruits, seeds and roots. Interestingly, LeWEE1was expressed in the jelly like locular tissue concomitant with endoreduplication during fruit development. Using tobacco BY-2 synchronized cells, we showed that the WEE1 gene expression is cell-cycle regulated with a maximum transcript accumulation at S phase. Our data indicate the putative dual contribution of LeWEE1 in the classical cell cycle and the endocycle.

Functional and biochemical characterization of a recombinant Arabidopsis thaliana 3-deoxy-D-manno-octulosonate 8-phosphate synthase

An open reading frame, encoding for KDOPS (3-deoxy-D-manno-octulosonate 8-phosphate synthase), from Arabidopsis thaliana was cloned into a T7-driven expression vector. The protein was overexpressed in Escherichia coli and purified to homogeneity. Recombinant A. thaliana KDOPS, in solution, displays an apparent molecular mass of 76 kDa and a subunit molecular mass of 31.519 kDa. Unlike previously studied bacterial KDOPSs, which are tetrameric, A. thaliana KDOPS appears to be a dimer in solution. The optimum temperature of the enzyme is 65 degrees C and the optimum pH is 7.5, with a broad peak between pH 6.5 and 9.5 showing 90% of maximum activity. The enzyme cannot be inactivated by EDTA or dipicolinic acid treatment, nor it can be activated by a series of bivalent metal ions, suggesting that it is a non-metallo-enzyme, as opposed to the initial prediction that it would be a metallo-enzyme. Kinetic studies showed that the enzyme follows a sequential mechanism with K(m)=3.6 microM for phosphoenolpyruvate and 3.8 microM for D-arabinose 5-phosphate and kcat=5.9 s(-1) at 37 degrees C. On the basis of the characterization of A. thaliana KDOPS and phylogenetic analysis, plant KDOPSs may represent a new, distinct class of KDOPSs.

Rhamnogalacturonan II: structure and function of a borate cross-linked cell wall pectic polysaccharide

Rhamnogalacturonan II (RG-II) is a structurally complex pectic polysaccharide that was first identified in 1978 as a quantitatively minor component of suspension-cultured sycamore cell walls. Subsequent studies have shown that RG-II is present in the primary walls of angiosperms, gymnosperms, lycophytes, and pteridophytes and that its glycosyl sequence is conserved in all vascular plants examined to date. This is remarkable because RG-II is composed of at least 12 different glycosyl residues linked together by more than 20 different glycosidic linkages. However, only a few of the genes and proteins required for RG-II biosynthesis have been identified. The demonstration that RG-II exists in primary walls as a dimer that is covalently cross-linked by a borate diester was a major advance in our understanding of the structure and function of this pectic polysaccharide. Dimer formation results in the cross-linking of the two homogalacturonan chains upon which the RG-II molecules are constructed and is required for the formation of a three-dimensional pectic network in muro. This network contributes to the mechanical properties of the primary wall and is required for normal plant growth and development. Indeed, changes in wall properties that result from decreased borate cross-linking of pectin may lead to many of the symptoms associated with boron deficiency in plants.