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PubMed2006

Cloning, characterization and localization of three novel class III peroxidases in lignifying xylem of Norway spruce (Picea abies).

Plant class III peroxidases (POXs) take part in the formation of lignin and maturation of plant cell walls. However, only a few examples of such peroxidases from gymnosperm tree species with highly lignified xylem tracheids have been implicated so far. We report here cDNA cloning of three xylem-expressed class III peroxidase encoding genes from Norway spruce (Picea abies). The translated proteins, PX1, PX2 and PX3, contain the conserved amino acids required for heme-binding and peroxidase catalysis. They all begin with putative secretion signal propeptide sequences but diverge substantially at phylogenetic level, grouping to two subclusters when aligned with other class III plant peroxidases. In situ hybridization analysis on expression of the three POXs in Norway spruce seedlings showed that mRNA coding for PX1 and PX2 accumulated in the cytoplasm of young, developing tracheids within the current growth ring where lignification is occurring. Function of the putative N-terminal secretion signal peptides for PX1, PX2 and PX3 was confirmed by constructing chimeric fusions with EGFP (enhanced green fluorescent protein) and expressing them in tobacco protoplasts. Full-length coding region of px1 was also heterologously expressed in Catharanthus roseus hairy root cultures. Thus, at least the spruce PX1 peroxidase is processed via the endoplasmic reticulum (ER) most likely for secretion to the cell wall. Thereby, PX1 displays correct spatiotemporal localization for participation in the maturation of the spruce tracheid secondary cell wall.

Amino Acid Sequence↗
PubMedDate not supplied

Immunolocalization of phenylalanine ammonia-lyase and cinnamate-4-hydroxylase in differentiating xylem of poplar.

Phenylalanine ammonia-lyase (PAL; EC 4.3.1.5) and cinnamate-4-hydroxylase (C4H; EC 1.14.13.11) are pivotal enzymes involved in lignification. We synthesized peptides as the epitopes according to the amino acid sequences of these enzymes, coupled them with hemocyanin, and injected them into mice. The antiserums against peptides of PAL and C4H specifically detected PAL and C4H in the crude enzymes extracted from differentiating xylem of poplar, respectively. PAL and C4H were localized in differentiating xylem of poplar. PAL labeling was mainly localized in the cytosol, and somewhat localized on the rough-endoplasmic reticulum (r-ER) and the Golgi apparatus. In contrast, C4H was mainly observed on r-ER and the Golgi apparatus. These findings suggest that conversion of phenylalanine to cinnamic acid occurs in the cytosol and the following reaction occurs near the membrane of r-ER and the Golgi apparatus. The possibility of coordinated localization of PAL and C4H is discussed.

Cytochrome P-450 Enzyme System↗
PubMed2006

Acoustic emission analysis and experiments with physical model systems reveal a peculiar nature of the xylem tension.

Advanced acoustic emission analysis, special microscopic examinations and experiments with physical model systems give reasons for the assumption that the tension in the water conducting system of vascular plants is caused by countless minute gas bubbles strongly adhering to the hydrophobic lignin domains of the xylem vessel walls. We ascertained these bubbles for several species of temperate deciduous trees and conifers. It is our hypothesis that the coherent bubble system of the xylem conduits operates as a force-transmitting medium that is capable of transporting water in traveling peristaltic waves. By virtue of the high elasticity of the gas bubbles, the hydro-pneumatic bubble system is capable of cyclic storing and releasing of energy. We consider the abrupt regrouping of the wall adherent bubble system to be the origin of acoustic emissions from plants. For Ulmus glabra, we recorded violent acoustic activity during both transpiration and re-hydration. The frequency spectrum and the waveforms of the detected acoustic emissions contradict traditional assumptions according to which acoustic emissions are caused by cavitation disruption of the stressed water column. We consider negative pressure in terms of the cohesion theory to be mimicked by the tension of the wall adherent bubble system.

Acoustics↗
PubMed2001

Cell-wall proteins from Sitka spruce xylem are selectively insolubilised during formation of dehydrogenation polymers of coniferyl alcohol.

Extracts from the lignifying xylem of Sitka spruce that were enriched in cell-wall-associated glycoproteins contained peroxidase and oxidase activity and readily formed lignin-like water-insoluble dehydrogenation polymers (DHPs) from coniferyl alcohol (CA) when supplied with H2O2. During the formation of DHPs, the abundance of a number of polypeptides in the extracts was diminished. However, these polypeptides were also diminished in control reactions that contained H2O2 but lacked CA. Polypeptides could be recovered from the DHPs by heating in SDS-PAGE sample buffer but no insolubilised polypeptides could be recovered from the + H2O2 reactions. Although most of the DHP-bound polypeptides were easily removed by pre-washing the DHPs, two polypeptides at 125 and 52 kDa remained tightly bound to the DHPs. The abundance of the two DHP-bound polypeptides mirrored the diminution of 120 and 46 kDa polypeptides in the extracts. The N-terminal protein sequences of the 125 and 52 kDa DHP-bound polypeptides were essentially identical to the sequences obtained from the 120 and 46 kDa polypeptides from the extracts, which confirmed that the DHP-bound polypeptides were derived from these soluble polypeptides. The 125-kDa DHP-bound polypeptide yielded an N-terminal protein sequence that was identical to a laccase-type oxidase previously identified in similar extracts from lignifying Sitka xylem. The N-terminal protein sequence of the 46-kDa polypeptide was homologous with a subset of plant peroxidases. The DHPs had tightly bound peroxidase and oxidase activity, which suggested that these polypeptides were active in their insolubilised state. The mechanism and selectivity of insolubilisation of these enzymes is discussed.

Amino Acid Sequence↗
PubMed1998

Identification and disruption of a plant shaker-like outward channel involved in K+ release into the xylem sap.

SKOR, a K+ channel identified in Arabidopsis, displays the typical hydrophobic core of the Shaker channel superfamily, a cyclic nucleotide-binding domain, and an ankyrin domain. Expression in Xenopus oocytes identified SKOR as the first member of the Shaker family in plants to be endowed with outwardly rectifying properties. SKOR expression is localized in root stelar tissues. A knockout mutant shows both lower shoot K+ content and lower xylem sap K+ concentration, indicating that SKOR is involved in K+ release into the xylem sap toward the shoots. SKOR expression is strongly inhibited by the stress phytohormone abscisic acid, supporting the hypothesis that control of K+ translocation toward the shoots is part of the plant response to water stress.

Amino Acid Sequence↗
PubMed1998

Cambium pre-activation in beech correlates with a strong temporary increase of calcium in cambium and phloem but not in xylem cells.

Using secondary ion mass spectrometry (SIMS), calcium was imaged in cambium cells and in the adjacent secondary phloem and xylem cells during the different phases of cambium functioning in beech (Fagus sylvatica L.). At the end of the period of quiescence, immediately before the resumption of cell divisions (i.e. at the cambium pre-activation phase), a strong temporary increase of calcium concentration was observed to take place in cambium and phloem but not in xylem cells.

Calcium↗
PubMed2000

Investigation of the transported heavy metal ions in xylem sap of cucumber plants by size exclusion chromatography and atomic absorption spectrometry.

An 'off-line' high performance liquid chromatography-graphite furnace atomic absorption spectrometry (HPLC-GF-AAS) method using a size exclusion chromatography (SEC) column was developed to investigate heavy metal ions in xylem sap samples of cucumber plants grown in hydroponics containing iron as Fe(III)-ethylenediaminetetraacetate (Fe(III) EDTA), Fe(III) citrate or FeCl3 and exposed to lead, nickel or vanadium contamination. The SEC chromatogram of the samples contained the peak of nitrate ions (in significant concentration approximately 1400 microg/ml) and some small, unidentified compounds with molecular weight lower than 700 Da. The results indicate that Cu and Mn--which were added to the hydroponics as nutrient elements--determined in the collected fractions during the chromatographic runs are transported in the xylem vessels together with small inorganic ions like nitrate ions. In case of nickel other low-molecular weight compounds eluting earlier than the nitrate ions may take part in its transport toward the shoots. Lead could not be detected in the above mentioned fractions. Determination of vanadium in the fractions was not expected since it could not be detected in the sap samples.

Chromatography, High Pressure Liquid↗
PubMed2002

A new cellulose synthase gene (PtrCesA2) from aspen xylem is orthologous to Arabidopsis AtCesA7 (irx3) gene associated with secondary cell wall synthesis.

We report here the molecular cloning and characterization of a new full-length cellulose synthase (CesA) cDNA, PtrCesA2 from aspen (Populus tremuloides) trees. The predicted PtrCesA2 protein shows a high degree of identity/similarity (87%/91%) to the predicted gene product of Arabidopsis AtCesA7 gene that has been associated with secondary cell wall development. Previously, a mutation in AtCesA7 gene (irx3) was correlated with a significant decrease in the amount of cellulose synthesized (about 70%) and genetic complementation of irx3 mutant with a wild-type AtCesA7 gene restored the normal phenotype. This is the first report of a full-length AtCesA7 ortholog from any non-Arabidopsis species. Interestingly, PtrCesA2 shares only 64% identity with our earlier reported PtrCesA1 from aspen suggesting its structural distinctness from the only other known CesA member from the aspen genome. PtrCesA1 is a xylem-specific and tension stress responsive gene that is highly similar to another Arabidopsis gene, AtCesA8 which also has been associated with secondary wall development. Moreover, AtCesA7 and AtCesA8 are suggested to be part of the same cellulose synthase complex. Isolation of PtrCesA2 from a xylem library enriched in cells with active secondary wall synthesis, PtrCesA2 expression levels similar to PtrCesA1 and high similarity of PtrCesA1 and PtrCesA2 to AtCesA8 and AtCesA7, respectively, suggest that both these aspen genes might be involved in the secondary wall development in aspen woody tissues. Availability of two aspen CesA orthologs will now enable us to examine if PtrCesA1 and PtrCesA2 functionally interact during aspen wood development that has long-term implications on genetic improvement of forest trees.

Arabidopsis Proteins↗
PubMed1998

Substrate profiles and expression of caffeoyl coenzyme A and caffeic acid O-methyltransferases in secondary xylem of aspen during seasonal development.

Seasonal expression of caffeoyl-CoA O-methyltransferase (EC 2.1.1.104) was analyzed in aspen developing secondary xylem in parallel with caffeate O-methyltransferase (EC 2.1.1.68). Enzyme activity and mRNA levels for both enzymes peaked in the middle of the growing season. These results strongly suggest that both forms of O-methyltransferase were actively participating in lignin precursor biosynthesis during the growing season. To determine the role of each enzyme form, xylem extracts from two days in the growing season were assayed with four substrates: caffeoyl-CoA, 5-hydroxyferuloyl-CoA, caffeate acid and 5-hydroxyferulic acid. Recombinant forms of caffeoyl-CoA and caffeate O-methyltransferase were also assayed with these substrates. The recombinant enzymes have different substrate specificity with the caffeoyl-CoA O-methyltransferase being essentially specific for CoA ester substrates with a preference for caffeoyl-CoA, while caffeate O-methyltransferase utilized all four substrates with a preference for the free acid forms. We suggest that caffeoyl-CoA O-methyltransferase is likely to be responsible for biosynthesis of lignin precursors in the guaiacyl pathway and may represent a more primitive enzyme form leftover from very early land plant evolution. Caffeate O-methyltransferase is more likely to be responsible for lignin precursor biosynthesis in the syringyl pathway, especially since it can catalyze methylation of 5-hydroxyferuloyl-CoA quite effectively. This latter enzyme form then may be considered a more recently evolved component of the lignin biosynthetic pathways of the evolutionarily advanced plants such as angiosperms.

Base Sequence↗
PubMed2003

An arabinogalactan protein associated with secondary cell wall formation in differentiating xylem of loblolly pine.

Arabinogalactan proteins (AGPs) are abundant plant proteoglycans implicated in plant growth and development. Here, we report the genetic characterization, partial purification and immunolocalization of a classical AGP (PtaAGP6, accession number AF101785) in loblolly pine (Pinus taeda L.). A PtaAGP6 full-length cDNA clone was expressed in bacteria. PtaAGP6 resembles tomato LeAGP-1 and Arabidopsis AtAGP17-19 in that they all possess a subdomain composed of basic amino acids. The accessibility of this domain in the glycoprotein makes it possible to label the PtaAGP6 epitopes on the cell surface or in the cell wall with polyclonal antibodies raised against this subdomain. The antibodies recognize the peptide of the basic subdomain and bind to the intact protein molecule. A soluble protein-containing fraction was purified from the differentiating xylem of pine trees by using beta-glucosyl Yariv reagent (beta-glcY) and was recognized by antibodies against the basic subdomain. Immunolocalization studies showed that the PtaAGP6 epitopes are restricted to a file of cells that just precede secondary cell wall thickening, suggesting roles in xylem differentiation and wood formation. The location of apparent labeling of the PtaAGP6 epitopes is separated from the location of lignin deposition. Multiple single nucleotide polymorphisms (SNPs) were detected in EST variants. Denaturing HPLC analysis of PCR products suggests that PtaAGP6 is encoded by a single gene. Mobility variation in denaturing gel electrophoresis was used to map PtaAGP6 SNPs to a site on linkage group 5.

Amino Acid Sequence↗
PubMed1999

Characterization and heterologous expression of laccase cDNAs from xylem tissues of yellow-poplar (Liriodendron tulipifera).

Four closely related cDNA clones encoding laccase isoenzymes from xylem tissues of yellow-poplar (Ltlacc2.1-4) were identified and sequenced. The inferred yellow-poplar laccase gene products were highly related to one another (79-91% at the amino acid level) and showed significant similarity to other blue copper oxidases, especially with respect to the copper-binding domains. The encoded proteins had N-terminal signal sequences and 17-19 potential N-linked glycosylation sites. The mature proteins were predicted to have molecular masses of ca. 61 kDa (unglycosylated) and high isoelectric points (pI 9.3-9.5). The canonical copper ligands were conserved, with the exception of a Leu residue associated with the axial position of the Type-1 cupric ion. The residue at this position has been proposed to influence the redox potential of Type-1 cupric ions. Northern blot analysis revealed that the yellow-poplar laccase genes are differentially expressed in xylem tissues. The genes were verified as encoding active laccases by heterologous expression in tobacco cells and demonstration of laccase activity in extracts from transformed tobacco cell lines.

Amino Acid Sequence↗
PubMed2002

Arabidopsis boron transporter for xylem loading.

Boron deficiency hampers the productivity of 132 crops in more than 80 countries. Boron is essential in higher plants primarily for maintaining the integrity of cell walls and is also beneficial and might be essential in animals and in yeast. Understanding the molecular mechanism(s) of boron transport is crucial for alleviating boron deficiency. Here we describe the molecular identification of boron transporters in biological systems. The Arabidopsis thaliana mutant bor1-1 is sensitive to boron deficiency. Uptake studies indicated that xylem loading is the key step for boron accumulation in shoots with a low external boron supply and that the bor1-1 mutant is defective in this process. Positional cloning identified BOR1 as a membrane protein with homology to bicarbonate transporters in animals. Moreover, a fusion protein of BOR1 and green fluorescent protein (GFP) localized to the plasma membrane in transformed cells. The promoter of BOR1 drove GFP expression in root pericycle cells. When expressed in yeast, BOR1 decreased boron concentrations in cells. We show here that BOR1 is an efflux-type boron transporter for xylem loading and is essential for protecting shoots from boron deficiency.

Amino Acid Sequence↗
PubMed2004

Evolution of xylem lignification and hydrogel transport regulation.

In vascular plants, the polysaccharide-based walls of water-conducting cells are strengthened by impregnation with the polyphenolic polymer lignin. The fine-scale patterning of lignin deposition in water-conducting cells is shown here to vary phylogenetically across vascular plants. The extent to which water transport in xylem cells can be modified in response to changes in the ionic content of xylem sap also is shown to vary in correlation with variation in lignification patterns, consistent with the proposed mechanism for hydraulic response through size change of middle-lamella pectins. This covariation suggests that the fine-scale distribution of hydrophilic polysaccharides and hydrophobic lignin can affect hydraulic as well as mechanical properties, and that the evolutionary diversification of vascular cells thus reflects biochemical as well as morphological innovations evolved to fulfill opposing cell functions of transport and structural support.

Biological Evolution↗
PubMed2002

Visualization by comprehensive microarray analysis of gene expression programs during transdifferentiation of mesophyll cells into xylem cells.

Plants have a unique transdifferentiation mechanism by which differentiated cells can initiate a new program of differentiation. We used a comprehensive analysis of gene expression in an in vitro zinnia (Zinnia elegans L.) culture model system to gather fundamental information about the gene regulation underlying the transdifferentiation of plant cells. In this model, photosynthetic mesophyll cells isolated from zinnia leaves transdifferentiate into xylem cells in a morphogenic process characterized by features such as secondary-wall formation and programmed cell death. More than 8,000 zinnia cDNA clones were isolated from an equalized cDNA library prepared from cultured cells transdifferentiating into xylem cells. Microarray analysis using these cDNAs revealed several types of unique gene regulation patterns, including: the transient expression of a set of genes during cell isolation, presumably induced by wounding; a rapid reduction in the expression of photosynthetic genes and the rapid induction of protein synthesis-associated genes during the first stage; the preferential induction of auxin-related genes during the subsequent stage; and the transient induction of genes closely associated with particular morphogenetic events, including cell-wall formation and degradation and programmed cell death during the final stage. This analysis also revealed a number of previously uncharacterized genes encoding proteins that function in signal transduction, such as protein kinases and transcription factors that are expressed in a stage-specific manner. These findings provide new clues to the molecular mechanisms of both plant transdifferentiation and wood formation.

Asteraceae↗
PubMed2001

Bundle sheath cells of small veins in maize leaves are the location of uptake from the xylem.

Rb(+) as a tracer for K(+) was used to test the hypothesis that uptake of K(+) from xylem vessels of small veins into the symplast of maize leaves occurs at the xylem/bundle sheath cell interface. 22.5 min after immersing cut leaves into 20 mM RbCl+1 mM KCl, Rb(+) appeared in the cells of the leaves. Sections of these leaves were freeze-dried. In cryo-thin sections (5 microm), (85)Rb(+) and (41)K(+) content was determined by laser microprobe mass analysis with a large resolution of about 1 microm. Determining the ratio of (85)Rb(+) to (41)K(+) in the cell walls and cytosols of bundle sheath cells, mesophyll cells, and in the cells between the xylem elements resulted in the following picture: In small veins, Rb(+) entered the symplast directly at the xylem/bundle sheath cell interface.

Ion Transport↗
PubMed2003

Diurnal variation in uptake and xylem contents of inorganic and assimilated N under continuous and interrupted N supply to Phleum pratense and Festuca pratensis.

Compensation by dark-period uptake of NH(4)(+) and NO(3)(-) in the grasses Phleum pratense L. and Festuca pratensis Huds. following N deprivation during the preceding light period was investigated in flowing solution culture under an artificial 10/14 h light/dark cycle. N was supplied as either NO(3)(-), NH(4)(+) or NH(4)NO(3) at 20+/-5 mmol m(-3), available continuously or only during the dark period, for 5-10 d. Intermittent N supply did not affect total daily N uptake, growth rate or net partitioning of dry matter. Net uptake and influx of NO(3)(-) varied similarly throughout the diurnal cycle when NO(3)(-) was supplied continuously, with a marginal contribution by NO(3)(-) efflux. Influx was significantly higher and efflux slightly higher following interruption of NO(3)(-) supply during the light period. Nitrate accounted for 80% of N in xylem exudate except between hours 6-9 of the light period when the amino acid concentration increased 3-fold, primarily as glutamine. Diurnal variation in relative NO(3)(-) uptake exhibited five phases of constant acceleration/deceleration, described reasonably well assuming NO(3)(-) influx was subject to metabolic co-regulation by NO(3)(-) and amino acid levels in the cytoplasmic compartment of the roots. Accordingly, influx is determined by variation in root NO(3)(-) levels throughout the dark period and the first half of the light period, but is down-regulated by increased amino acid levels during the second half of the light period. The sharp light/dark transitions affect transpiration rate and hence xylem N flux which, in turn, affect NO(3)(-) levels in the cytoplasmic compartment of the roots and the rate of NO(3)(-) assimilation in the shoot.

Circadian Rhythm↗
PubMed2003

Branch junctions and the flow of water through xylem in Douglas-fir and ponderosa pine stems.

Water flowing through the xylem from the roots to the leaves of most plants must pass through junctions where branches have developed from the main stem. These junctions have been studied as both flow constrictions and components of a hydraulic segmentation mechanism to protect the main axes of the plant. The hydraulic nature of the branch junction also affects the degree to which branches interact and can respond to changes in flow to other branches. The junctions from shoots of two conifer species were studied, with particular emphasis on the coupling between the downstream branches. Flow was observed qualitatively by forcing stain through the junctions and the resulting patterns showed that flow into a branch was confined to just part of the subtending xylem until a considerable distance below the junction. Junctions were studied quantitatively by measuring flow rates in a branch before and after flow was stopped in an adjacent branch and by measuring the hydraulic resistance of the components of the junction. Following flow stoppage in the adjacent branch, flow into the remaining branch increased, but considerably less than predicted based on a simple resistance analogue for the branch junction that assumes the two branches are fully coupled. The branches downstream from a junction, therefore, appear to be limited in their interconnectedness and hence in their ability to interact.

Biological Transport↗