Georg Seifert

Irritable Walls: The Plant Extracellular Matrix and Signaling

The plant cell wall is a dynamic network of carbohydrates and proteins of enormous structural complexity that plays crucial roles in all aspects of plant life. Advances in molecular physiology and genetics have shed light on the relations between structure and function of various cell wall

Galactose Biosynthesis in Arabidopsis: Genetic Evidence for Substrate Channeling from UDP-D-Galactose into Cell Wall Polymers

The biosynthesis of plant cell wall polysaccharides requires the concerted action of nucleotide sugar interconversion enzymes, nucleotide sugar transporters, and glycosyl transferases. How cell wall synthesis in planta is regulated, however, remains unclear. The root epidermal bulger 1 (reb1) mutant in Arabidopsis thaliana is partially deficient in cell wall arabinogalactan-protein (AGP), indicating a role for REB1 in AGP biosynthesis. We show that REB1 is allelic to ROOT HAIR DEFICIENT 1 (RHD1), one of five ubiquitously expressed genes that encode isoforms of UDP-D-glucose 4-epimerase (UGE), an enzyme that acts in the formation of UDP-D-galactose (UDP-D-Gal). The RHD1 isoform is specifically required for the galactosylation of xyloglucan (XG) and type II arabinogalactan (AGII) but is not involved either in D-galactose detoxification or in galactolipid biosynthesis. Epidermal cell walls in the root expansion zone lack arabinosylated (1-->6)-beta-D-galactan and galactosylated XG. In cortical cells of rhd1, galactosylated XG is absent, but an arabinosylated (1-->6)-beta-D-galactan is present. We conclude that the flux of galactose from UDP-D-Gal into different downstream products is compartmentalized at the level of cytosolic UGE isoforms. This suggests that substrate channeling plays a role in the regulation of plant cell wall biosynthesis.

Post-transcriptional control of the Arabidopsis auxin efflux carrier EIR1 requires AXR1

The auxin efflux carrier EIR1 (also known as AGR and AtPIN2) is a key mediator of the response of Arabidopsis roots to gravity [1,2]. This response is thought to require the establishment of a transient auxin gradient in the root meristem, resulting in differential cell elongation [3]. Recent reports suggest that EIR1 is essential for the asymmetric distribution of auxin in the root meristem [4-7], but the regulatory aspects of this process are still not fully understood. Here, we studied the regulation of EIR1 in Arabidopsis using two reporters: one was a translational fusion that contained the entire EIR1 coding sequence, and the other a transcriptional fusion that had no EIR1 coding sequence. We found that EIR1 is controlled at the post-transcriptional level. The translational fusion was unstable in response to changes in auxin homeostasis, and was destabilized by cycloheximide. In contrast, the protein was stabilized in the axr1-3 mutant, which is auxin resistant and defective in auxin responses such as root gravitropism [8,9]. AXR1 is thought to participate in ubiquitin-mediated control of protein stability [10-12]. The dependence of EIR1 reporter expression on auxin concentrations and AXR1 suggests that auxin transport is regulated through a feedback regulatory loop that affects protein stability in response to auxin.

The Arabidopsis root hair cell wall formation mutant lrx1 is suppressed by mutations in the RHM1 gene encoding a UDP-L-rhamnose synthase.

Cell and cell wall growth are mutually dependent processes that must be tightly coordinated and controlled. LRR-extensin1 (LRX1) of Arabidopsis thaliana is a potential regulator of cell wall development, consisting of an N-terminal leucine-rich repeat domain and a C-terminal extensin-like domain typical for structural cell wall proteins. LRX1 is expressed in root hairs, and lrx1 mutant plants develop distorted root hairs that often swell, branch, or collapse. The aberrant cell wall structures found in lrx1 mutants point toward a function of LRX1 during the establishment of the extracellular matrix. To identify genes that are involved in an LRX1-dependent developmental pathway, a suppressor screen was performed on the lrx1 mutant, and two independent rol1 (for repressor of lrx1) alleles were isolated. ROL1 is allelic to Rhamnose Biosynthesis1, which codes for a protein involved in the biosynthesis of rhamnose, a major monosaccharide component of pectin. The rol1 mutations modify the pectic polysaccharide rhamnogalacturonan I and, for one allele, rhamnogalacturonan II. Furthermore, the rol1 mutations cause a change in the expression of a number of cell wall–related genes. Thus, the lrx1 mutant phenotype is likely to be suppressed by changes in pectic polysaccharides or other cell wall components.

UDP-Glucose 4-Epimerase Isoforms UGE2 and UGE4 Cooperate in Providing UDP-Galactose for Cell Wall Biosynthesis and Growth of Arabidopsis thaliana

Five Arabidopsis thaliana genes that encode UDP-glucose 4-epimerase (UGE) and represent two ancient plant UGE clades might be involved in the regulation of cell wall carbohydrate biosynthesis. We tested this hypothesis in a genome-wide reverse genetic study. Despite significant contributions of each gene to total UGE activity, none was essential for normal growth on soil. uge2 uge4 displayed dramatic general growth defects, while other mutant combinations were partially aberrant. UGE2 together with UGE3 influenced pollen development. UGE2 and UGE4 synergistically influenced cell wall galactose content, which was correlated with shoot growth. UGE2 strongly and UGE1 and UGE5 lightly supported UGE4 in influencing root growth and cell wall galactose content by affecting galactan content. By contrast, only UGE4 influenced xyloglucan galactosylation in roots. Secondary hypocotyl thickening and arabinogalactan protein carbohydrate structure in xylem parenchyma depended on the combination of UGE2 and UGE4. As opposed to cell wall galactose content, tolerance to external galactose strictly paralleled total UGE activity. We suggest a gradual recruitment of individual UGE isoforms into specific roles. UGE2 and UGE4 influence growth and cell wall carbohydrate biosynthesis throughout the plant, UGE3 is specialized for pollen development, and UGE1 and UGE5 might act in stress situations.

Class I α-Mannosidases Are Required for N-Glycan Processing and Root Development in Arabidopsis thaliana

In eukaryotes, class I α-mannosidases are involved in early N-glycan processing reactions and in N-glycan–dependent quality control in the endoplasmic reticulum (ER). To investigate the role of these enzymes in plants, we identified the ER-type α-mannosidase I (MNS3) and the two Golgi-α-mannosidase I proteins (MNS1 and MNS2) from Arabidopsis thaliana. All three MNS proteins were found to localize in punctate mobile structures reminiscent of Golgi bodies. Recombinant forms of the MNS proteins were able to process oligomannosidic N-glycans. While MNS3 efficiently cleaved off one selected α1,2-mannose residue from Man9GlcNAc2, MNS1/2 readily removed three α1,2-mannose residues from Man8GlcNAc2. Mutation in the MNS genes resulted in the formation of aberrant N-glycans in the mns3 single mutant and Man8GlcNAc2 accumulation in the mns1 mns2 double mutant. N-glycan analysis in the mns triple mutant revealed the almost exclusive presence of Man9GlcNAc2, demonstrating that these three MNS proteins play a key role in N-glycan processing. The mns triple mutants displayed short, radially swollen roots and altered cell walls. Pharmacological inhibition of class I α-mannosidases in wild-type seedlings resulted in a similar root phenotype. These findings show that class I α-mannosidases are essential for early N-glycan processing and play a role in root development and cell wall biosynthesis in Arabidopsis.

PROPORZ1, a Putative Arabidopsis Transcriptional Adaptor Protein, Mediates Auxin and Cytokinin Signals in the Control of Cell Proliferation

Plants generate cells and organs throughout their life cycle. Plant cell proliferation relates to the activity of dividing meristematic cells, which subsequently differentiate in a position- and lineage-dependent manner. The events underlying the regulation of cell division and further differentiation processes are under tight control of both intrinsic and environmental parameters. Among the intrinsic factors, two groups of phytohormones, auxins and cytokinins, exhibit a combined regulatory impact on cell proliferation, as an important determinant for the totipotency of plant cells. Classical experiments demonstrated that application of both growth regulators in appropriate concentrations promotes callus formation. When the ratio between the phytohormones changes, callus cells acquire the competence to regenerate organs. Typically, a high auxin-to-cytokinin ratio promotes the formation of roots, whereas a low auxin-to-cytokinin ratio results in the regeneration of shoots. Conclusively, the concerted, proportional impact of both phytohormones functions as a determinant of plant cell proliferation; they act as cell cycle-promoting mitogens as well as morphogens that control plant organogenesis. Here, we describe PROPORZ1 (PRZ1), an Arabidopsis gene, essential for the developmental switch from cell proliferation to differentiation in response to variations in auxin and cytokinin concentrations. PRZ1 probably acts as a transcriptional adaptor protein that affects the expression of cell cycle regulators and might, thereby, mediate its effect on the control of cell proliferation.

Complementary and alternative treatment methods in children with cancer: A population-based retrospective survey on the prevalence of use in Germany

INTRODUCTION Few studies have been conducted to date on the prevalence of use of complementary and alternative treatment methods (CAMs) in paediatric oncology, and those that have been conducted are often not representative. We therefore decided to study a representative sample of children with cancer in the German population. PATIENTS AND METHODS The study took the form of a retrospective survey amongst all parents whose children were first diagnosed with a disease covered by the German Childhood Cancer Registry in 2001. The primary objectives of the survey were to establish the prevalence of use of CAM and the factors related to its use. RESULTS Of the 1595 questionnaires sent out, 1063 (67%) could be evaluated. 35% of the responders had used CAM. The most frequently used methods were homeopathy, dietary supplements and anthroposophic medicine including mistletoe therapy. Factors which increased the probability of using CAM were the previous use of CAM, higher social status and poor prognosis of the childs disease. The most frequently named reasons for use of CAM were physical stabilisation, strengthening the immune system and improving the chance of cure. Whilst the sources of information about CAM were in most cases not doctors, 71% of users had nevertheless spoken to a doctor about using CAM. The effects of the CAM perceived by the parents were for the most part positive. 89% of the users reported that they would recommend CAM to other parents. CONCLUSIONS CAMs are administered alongside standard therapy to 35% of children with cancer in Germany, usually by the parents. Prospective studies on the effects and side-effects of the most frequently used methods are urgently needed, and paediatric oncologists should have sufficient knowledge of CAM to enable them to advise parents professionally and competently about these treatments, too.

Molecular mechanisms of mistletoe plant extract-induced apoptosis in acute lymphoblastic leukemia in vivo and in vitro

Viscum album (Mistletoe) is one of the most widely used alternative cancer therapies. Aqueous mistletoe extracts (MT) contain the three mistletoe lectins I, II and III as one predominant group of biologically active agents. Although MT is widely used, there is a lack of scientifically sound preclinical and clinical data. In this paper, we describe for the first time the in vivo efficacy and mechanism of action of MT in lymphoblastic leukemia. For this purpose, we first investigated both the cytotoxic effect and the mechanism of action of two standardized aqueous MTs (MT obtained from fir trees (MT-A); MT obtained from pine trees (MT-P)) in a human acute lymphoblastic leukemia (ALL) cell line (NALM-6). MT-A, MT-P and ML-I inhibited cell proliferation as determined by Casy Count analysis at very low concentrations with MT-P being the most cytotoxic extract. DNA-fragmentation assays indicated that dose-dependent induction of apoptosis was the main mechanism of cell death. Finally, we evaluated the efficacy of MT-A and MT-P in an in vivo SCID-model of pre-B ALL (NALM-6). Both MTs significantly improved survival (up to 55.4 days) at all tested concentrations in contrast to controls (34.6 days) without side effects.

Distinct Properties of the Five UDP-d-glucose/UDP-d-galactose 4-Epimerase Isoforms of Arabidopsis thaliana

Plant genomes contain genetically encoded isoforms of most nucleotide sugar interconversion enzymes. Here we show that Arabidopsis thaliana has five genes encoding functional UDP-d-glucose/UDP-d-galactose 4-epimerase (named UGE1 to UGE5). All A. thaliana UDP-d-glucose 4-epimerase isoforms are dimeric in solution, maximally active in vitro at 30-40 °C, and show good activity between pH 7 and pH 9. In vitro, UGE1, -3, and -5 act independently of externally added NAD+, whereas cofactor addition stimulates the activity of UGE2 and is particularly important for UGE4 activity. UGE1 and UGE3 are most efficiently inhibited by UDP. The five isoforms display \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(k_{\mathrm{cat}}^{\mathrm{UDP-Gal}}\) \end{document} values between 23 and 128 s- and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(K_{m}^{\mathrm{UDP-Gal}}\) \end{document} values between 0.1 and 0.3 mm. This results in enzymatic efficiencies ranging between 97 and 890 mm-1 s-1 for UGE4 = UGE1 < UGE3 < UGE5 < UGE2. The \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(K_{m}^{\mathrm{UDP-Glc}}\) \end{document} values, derived from the Haldane relationship, were 0.76 mm for UGE1, 0.56 mm for UGE4, and between 0.13 and 0.23 mm for UGE2, -3, and -5. The expression of UGE isoforms is ubiquitous and displays developmental and cell type-dependent variations. UGE1 and -3 expression patterns globally resemble enzymes involved in carbohydrate catabolism, and UGE2, -4, and -5 expression is more related to carbohydrate biosynthesis. UGE1, -2, and -4 are present in the cytoplasm, whereasUGE4 is additionally enriched close to Golgi stacks. All UGE genes tested complement the UGE4rhd1 phenotype, confer increased galactose tolerance in planta, and complement the galactose metabolization deficiency in the Saccharomyces cerevisiae gal10 mutant. We suggest that plant UGE isoforms function in different metabolic situations and that enzymatic properties, gene expression pattern, and subcellular localization contribute to the differentiation of isoform function.