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PubMed2003

Gravitational and hormonal control in secondary xylem formation of Japanese flowering cherry.

It has been reported that Japanese flowering cherry promotes the formation of tension wood in the upper side of the inclined stem, which induces the negative gravitropism. In the other hand, the plant under simulated micro-gravity conditions reduces the width of the secondary xylem and increases the density of the vessels, which means decreased the density of the fiber cells. The plant under simulated micro-gravity conditions showed that the inhibition of xylem development and the decreased supporting mechanism in the stem. In this study we examined that the effects of auxin and gibberellin in the plant under simulated micro-gravity conditions and inclined stimulus. The gibberellin treatment promotes the secondary xylem development under simulated microgravity conditions and inclined stimulus. The upper side of the inclined stem contents much higher levels of IAA and gibberellin A1(GA1) than the lower side of that.

Gibberellins↗
PubMedDate not supplied

In situ Raman investigation of single lipid droplets in the water-conducting xylem of four woody plant species.

A micro-Raman spectroscopy approach was used for the direct in situ characterization of lipid bodies in the water-conducting branch xylem of an African resurrection plant and three deciduous European tree species. Because of average diameters of at least 1 microm, the lipid bodies of all investigated species proved to be easily accessible by this technique. All vesicle-forming xylem lipids were identified as fatty acid esters, which may correspond to phospholipids. Whereas in the resurrection plant saturated lipids were dominant, the lipid bodies of the European trees consisted of highly unsaturated fatty acids. A comparison of the spectra of lipid droplets of lime obtained in situ and from isolated xylem sap revealed slightly different signatures. This finding suggests that micro-Raman spectroscopy may be used to detect modifications of the chemical composition of biological substances as a result of the extraction mode.

Biophysical Phenomena↗
PubMed1995

Xylem-specific gene expression in loblolly pine.

Two genes preferentially expressed in differentiating xylem of loblolly pine (Pinus taeda L.) were cloned from cDNA and genomic libraries and designated PtX3H6 and PtX14A9. Transcripts of PtX3H6 and PtX14A9 are very abundant in differentiating xylem, less abundant in needles, and very low or non-detectable in embryos and megagametophytes. PtX3H6 contains a putative signal peptide, a threonine-rich region, a proline-rich region, and a hydrophobic tail. Repeats of Pro-Pro-Pro-Val-X-X are similar to repeats found in proline-rich cell wall proteins. The amino acid compositions of PtX3H6 and PtX14A9 are similar to those of arabinogalactan proteins (AGPs). PtX14A9 contains an 8 amino acid sequence similar to amino terminal sequences of ryegrass, carrot and rose AGPs. Upstream sequences have been determined from genomic clones encoding PtX3H6 and PtX14A9. A 7 bp sequence found in the 5' flanking regions of both genes has previously been shown to be involved in the vascular-specific expression of GRP 1.8, a glycine-rich protein found in bean. The sequence is also present upstream of another glycine-rich protein from bean, GRP 1.0, and may be partially responsible for the xylem-specific expression of pTx3H6 and PtX14A9.

Amino Acid Sequence↗
PubMed2005

Arsenic speciation in xylem sap of cucumber (Cucumis sativus L.).

Flow injection analysis (FIA) and high-performance liquid chromatography double-focusing sector field inductively coupled plasma mass spectrometry (HPLC-DF-ICP-MS) were used for total arsenic determination and arsenic speciation of xylem sap of cucumber plants (Cucumis sativus L.) grown in hydroponics containing 2 micromol dm(-3) arsenate or arsenite, respectively. Arsenite [As(III)], arsenate [As(V)] and dimethylarsinic acid (DMA) were identified in the sap of the plants. Arsenite was the predominant arsenic species in the xylem saps regardless of the type of arsenic treatment, and the following concentration order was determined: As(III) > As(V) > DMA. The amount of total As, calculated taking into consideration the mass of xylem sap collected, was almost equal for both treatments. Arsenite was taken up more easily by cucumber than arsenate. Partial oxidation of arsenite to arsenate (<10% in 48 h) was observed in the case of arsenite-containing nutrient solutions, which may explain the detection of arsenate in the saps of plants treated with arsenite.

Arsenates↗
PubMed2003

Nutrient translocation in the xylem of poplar--diurnal variations and spatial distribution along the shoot axis.

This investigation shows diurnal variations in the xylem sap composition of poplar (Populus tremula x P. alba). All major macronutrients reached a maximum concentration in the first half of the light period and decreased to the middle of the night. The relative abundance of the nutrients did not change during the day. The sap flow, which responded very fast to the environmental changes (2.2-fold increase within 10-20 min of illumination), reached a maximum value in the second half of the light period. Transpiration (and photosynthesis) was constant throughout the light phase. The calculated translocation rates displayed a maximum in the first half of the light period and therefore did not fit the time course of sap flow. During the night, translocation rates were 63-69% lower than the maximum. The regulation of nutrient translocation is discussed taking the active xylem loading into account. The axial distribution located the nitrate assimilation in younger leaves and storage of nitrate (and other macronutrients) in older leaves. Nitrate and potassium concentrations in the xylem sap did not change along the plant axis. However, the sap flow was greater in younger shoot sections than in older sections. We assume that the greater demand for nitrate in the younger shoot section was satisfied via an increased volume flow rather an increased nitrate concentration.

Biological Transport↗
PubMed2004

Native root xylem embolism and stomatal closure in stands of Douglas-fir and ponderosa pine: mitigation by hydraulic redistribution.

Hydraulic redistribution (HR), the passive movement of water via roots from moist to drier portions of the soil, occurs in many ecosystems, influencing both plant and ecosystem-water use. We examined the effects of HR on root hydraulic functioning during drought in young and old-growth Douglas-fir [ Pseudotsuga menziesii (Mirb.) Franco] and ponderosa pine ( Pinus ponderosa Dougl. Ex Laws) trees growing in four sites. During the 2002 growing season, in situ xylem embolism, water deficit and xylem vulnerability to embolism were measured on medium roots (2-4-mm diameter) collected at 20-30 cm depth. Soil water content and water potentials were monitored concurrently to determine the extent of HR. Additionally, the water potential and stomatal conductance ( g(s)) of upper canopy leaves were measured throughout the growing season. In the site with young Douglas-fir trees, root embolism increased from 20 to 55 percent loss of conductivity (PLC) as the dry season progressed. In young ponderosa pine, root embolism increased from 45 to 75 PLC. In contrast, roots of old-growth Douglas-fir and ponderosa pine trees never experienced more than 30 and 40 PLC, respectively. HR kept soil water potential at 20-30 cm depth above -0.5 MPa in the old-growth Douglas-fir site and -1.8 MPa in the old-growth ponderosa pine site, which significantly reduced loss of shallow root function. In the young ponderosa pine stand, where little HR occurred, the water potential in the upper soil layers fell to about -2.8 MPa, which severely impaired root functioning and limited recovery when the fall rains returned. In both species, daily maximum g(s) decreased linearly with increasing root PLC, suggesting that root xylem embolism acted in concert with stomata to limit water loss, thereby maintaining minimum leaf water potential above critical values. HR appears to be an important mechanism for maintaining shallow root function during drought and preventing total stomatal closure.

Disasters↗
PubMed2005

Nitrogen transport in the xylem sap of Quercus ilex: the role of ornithine.

The storage and remobilization of nitrogen in deciduous and evergreen species is a major source of N, supporting the seasonal growth of trees. In evergreens, in addition to wood and roots, older leaves are important reservoirs of N used in the growth of new foliage. Just before bud burst, when transpiration is inactive or low, and when uptake of nitrogen by the roots may be restricted due to low temperatures, levels of organic N in the xylem are high. Amino acids usually comprise the bulk of this organic N. Changes in amino acid concentrations in early spring are thought to result mainly from hydrolysis of N reserves, and not from current N uptake. The seasonal profiles of amino acids in the xylem sap of Quercus ilex, an evergreen Mediterranean tree, were investigated. The first amino acid detected in the xylem sap before spring was ornithine, which may result from the breakdown of arginine present in storage proteins. Arginine is one of the main amino acids present in storage proteins because each arginine molecule has four nitrogen atoms. When protein degradation increases the free arginine pool, the arginase activity is enhanced and, consequently, the conversion of arginine to ornithine. It seems that ornithine has an important role in N transport early in the growth season of Q. ilex.

Biological Transport, Active↗
PubMed2005

Phloem and xylem specification: pieces of the puzzle emerge.

The plant vascular system is composed of two tissue types, xylem and phloem, which originate from the vascular meristem, the procambium. Recently, several regulatory mechanisms that control the specification of these two tissue types have been uncovered. These include the asymmetric patterning of xylem and phloem in the vascular bundle by the class III HD-ZIP and KANADI genes, the tissue-type-specific control of vascular cell proliferation by brassinosteroids and class III HD-ZIP genes, the regulation of vascular tissue identity by the MYB-like transcription factor APL, and inductive signalling during xylem differentiation by xylogen. These findings define an emerging developmental framework for the control of vascular tissue specification.

Body Patterning↗
PubMed2005

Metabolic distinction of Ulmus minor xylem tissues after inoculation with Ophiostoma novo-ulmi.

Dutch elm disease (DED) is the most devastating and widespread disease of elms. The pathogen, Ophiostoma novo-ulmi, spreads systemically causing xylem vessels blocking and cavitation, and ultimately resulting in the development of a wilt syndrome. Twig samples from susceptible and resistant Ulmus minor trees were harvested at 0, 5, 15, 30, 60, and 120 days post-inoculation (dpi) with O. novo-ulmi. Fourier transform-infrared (FT-IR) spectroscopy, in tandem with chemometrics, was used to monitor changes in wood chemistry as consequence of infection. Principal component analysis distinguished between spectra from inoculated and control elms, and from susceptible- and resistant-inoculated elms. By 30 dpi, infected xylem showed reduced relative levels of carbohydrates and enhanced relative levels of phenolic compounds, probably due to the degradation of cell wall polysaccharides by fungal enzymes and the synthesis of host defence compounds. On 15 dpi, samples from resistant-inoculated elms showed higher levels of starch than samples from susceptible-inoculated elms, suggesting that availability of starch reserves could affect the tree's capacity for defensive responses. The results showed the power of FT-IR spectroscopy for analysing changes in the major components of elm xylem as consequence of infection by DED, and its potential for detecting metabolic profiles related to host resistance.

Ascomycota↗
PubMed2002

Movement to bark and metabolism of xylem cytokinins in stems of Lupinus angustifolius.

Following uptake of [(3)H]zeatin riboside and [(3)H]dihydrozeatin riboside by girdled lupin (Lupinus angustifolius L.) stems via the transpiration stream, rapid lateral movement of the radioactivity from xylem to bark was observed. Short-term studies with intact stems, and other studies with excised stem tissues, revealed that the ribosides and/or the corresponding nucleotides were the cytokinin forms which actually moved into the bark tissues. Relative to cytokinin metabolism in xylem plus pith, metabolism in bark was both more rapid and more complex. Riboside cleavage and formation of the O-acetylzeatin and O-acetyldihydrozeatin ribosides and nucleotides were almost completely confined to bark tissues. Exogenous (3)H-labelled O-acetylzeatin riboside was converted to zeatin riboside in bark tissue, but the presence of the acetyl group suppressed degradation to adenine metabolites. The sequestration and modification of xylem cytokinins by stem tissues probably contributes significantly to the cytokinin status of the shoot. New cytokinins identified by mass spectrometry in lupin were: O-acetyldihydrozeatin 9-riboside, a metabolite of exogenous dihydrozeatin riboside in stem bark; O-methylzeatin nucleotide and O-methyldihydrozeatin 9-riboside, metabolites of endogenous cytokinins in stem bark; O-methylzeatin nucleotide and O-methylzeatin 9-riboside, metabolites of exogenous zeatin riboside in excised pod walls.

Adenosine↗
PubMed2000

Sequences upstream and downstream of two xylem-specific pine genes influence their expression.

The identification of regulatory elements conferring high levels of expression in differentiating pine xylem will be valuable for genetic engineering of wood properties and will contribute to our understanding of gene regulation in this important group of forest trees. We examined the roles of both upstream and downstream elements in regulating the expression of two genes with preferential expression in developing xylem of loblolly pine. Gene constructs containing a PtX3H6, PtX14A9, or CaMV 35S promoter, the uidA gene encoding beta-glucuronidase, and a PtX3H6, PtX14A9, or NOS terminator were used to transform tobacco and hybrid poplar. When combined with the NOS terminator, neither pine promoter conferred xylem-specific expression in tobacco. When combined with the PtX3H6 promoter, an element at the 3' end of PtX3H6 reduced GUS expression resulting in preferential expression in vascular tissues. This silencing effect was not observed when the pine terminator was tested in conjunction with the CaMV 35S promoter. The PtX14A9 terminator did not increase tissue specificity. In leaves of transgenic poplar, both pine promoters conferred preferential GUS expression in veins when combined with the NOS terminator. The PtX3H6 terminator greatly decreased expression in leaves and stems when combined with the PtX3H6 promoter but only slightly altered expression when combined with the CaMV 35S promoter. An element at the 3' end of PtX14A9 increased GUS expression in veins when used in conjunction with either the PtX14A9 or CaMV35S promoter.

Journal Article↗
PubMed1999

Secondary xylem-specific expression of caffeoyl-coenzyme A 3-O-methyltransferase plays an important role in the methylation pathway associated with lignin biosynthesis in loblolly pine.

Two types of structurally distinct O-methyltransferases mediate the methylation of hydroxylated monomeric lignin precursors in angiosperms. Caffeate 3-O-methyltransferase (COMT; EC 2.1.1.68) methylates the free acids and caffeoyl CoA 3-O-methyltransferase (CCoAOMT; EC 2.1.1.104) methylates coenzyme A esters. Recently, we reported a novel hydroxycinnamic acid/hydroxycinnamoyl CoA ester O-methyltransferase (AEOMT) from loblolly pine differentiating xylem that was capable of methylating both acid and ester precursors with similar efficiency. In order to determine the possible existence and role of CCoAOMT in lignin biosynthesis in gymnosperms, a 1.3 kb CCoAOMT cDNA was isolated from loblolly pine that showed 79-82% amino acid sequence identity with many angiosperm CCoAOMTs. The recombinant CCoAOMT expressed in Escherichia coli exhibited a significant methylating activity with hydroxycinnamoyl CoA esters whereas activity with hydroxycinnamic acids was insignificant. Moreover, 3.2 times higher catalytic efficiency for methylating caffeoyl CoA over 5-hydroxyferuloyl CoA was observed which could serve as a driving force towards synthesis of guaiacyl lignin. The secondary xylem-specific expression of CCoAOMT was demonstrated using RNA blot analysis, western blot analysis, and O-methyltransferase enzyme assays. In addition, Southern blot analysis indicated that CCoAOMT may exist as a single-copy gene in loblolly pine genome. The transgenic tobacco plants carrying loblolly pine CCoAOMT promoter-GUS fusion localized the site of GUS activity at the secondary xylem tissues. These data suggest that CCoAOMT, in addition to AEOMT, plays an important role in the methylation pathway associated with lignin biosynthesis in loblolly pine.

Amino Acid Sequence↗
PubMed2002

Secondary xylem development in Arabidopsis: a model for wood formation.

Our understanding of the molecular controls regulating the identity of the vascular cambium and the development of secondary xylem and phloem have not yet benefited much from the use of Arabidopsis as a genetic system. Under appropriate growth conditions Arabidopsis undergoes extensive secondary growth in the hypocotyl, with the development of both a vascular and a cork cambium. The secondary xylem of the hypocotyl develops in two phases, an early phase in which only vessel elements mature and a later stage in which both vessel elements and fibres are found. During this second phase the secondary xylem of Arabidopsis closely resembles the anatomy of the wood of an angiosperm tree, and can be used to address basic questions about wood formation. The development of the vascular cambium and secondary growth in Arabidopsis hypocotyl is described and its utility as a model for wood formation in trees is considered.

Journal Article↗
PubMed1970

Separation and identification of soluble nucleotides in cambial and young xylem tissue of Larix decidua Mill.

The nucleotides present in the cambial tissue (primary wall tissue) and in the not yet fully differentiated secondary xylem (secondary wall tissue) of Larix decidua Mill. were extracted and characterized. The method of extraction best suited to the material was investigated and the problems involved in desalting of extracts and their effect on the final nucleotide pattern obtained are discussed. UDP-glucose was found to be the most important sugar nucleotide isolated from both cambial and young xylem tissue. UDP-galactose, UDP-arabinose, UDP-xylose, UDP-fructose, GDP-glucose, GDP-galactose, GDP-mannose, ADP-glucose, ADP-galactose, ADP-fructose, ADP-ribose and ADP-ribose phosphate (the last two compounds are the acid breakdown products of NADH and NADPH respectively) were also found in both extracts. UDP-galacturonic acid was identified only in the extract of the primary wall material. Several nucleotide oligosaccharides were obtained from both the extract of cambial tissue and that of the young xylem.

Adenine Nucleotides↗
PubMed1977

Changes in enzymic activities of nucleoside diphosphate sugar interconversions during differentiation of cambium to xylem in sycamore and poplar.

During the transition from primary wall formation to secondary thickening there is a marked shift in the synthesis of pectin, hemicellulose and cellulose. The activities of the enzymes [UDP-D-galactose 4-epimerase (EC 5.1.3.2)8 UDP-l-arabinose 4-epimerase (EC 5.1.3.5), UDP-D-glucose dehydrogenase (EC 1.1.1.22) and UDP-D--glucuronate decarboxylase (EC 4.1.1.35)] were measured in cambial cells, differentiating xylem cells and differentiated xylem cells isolated from sycamore and poplar trees, and phloem cells from poplar. At the final stage of the differentiation of cambium to xylem there was a decrease in activity of the enzymes directly involved in producing the soluble precursors of pectin (DUP-D-galactose 4-epimerase and UDP-L-arabinose 4-epimerase and an increase in those producing the precursors of hemicellulose (UDP-D-glucose dehydrogenase and UDP-D-glucuronate decarboxylase). These results strongly suggest ahat the changes were correlated with the differences observed in the chemical composition of the wall during development. The changes found in the catalytic activity of the enzymes of nucleoside diphosphate sugar interconversion exert a coarse control over the synthesis of pectin and hemicelluloses. The tissues at all stages of development contained the necessary enzyme activities to produce all the precursors of pectin and hemicellulose, even at the final stage of differentiation when no pectin was formed.

Arabinose↗
PubMed1977

Changes in enzymic activities of nucleoside diphosphate sugar interconversions during differentiation of cambium to xylem in pine and fir.

A protein fraction [precipitate obtained between 40 and 65% (NH4)2SO4 satn.] prepared from cambial cells, differentiating xylem cells and differentiated xylem cells of pine and fir trees contained all the enzymes required for the nucleoside diphosphate sugar interconversions. By using UDP-D-[U-14C]glucose or UDP-D-[U-14C]galactose, UDP-D-[U-14C-]glucuronic acid and UDP-D-[U-14C]xylose as substrates, the activities of UDP-D-galactose 4-epimerase (DC 5.1.3.2), UDP-D-xylose 4-epimerase(EC 5.1.3.5), UDP-D-glucose dehydrogenase (EC 1.1.1.22) and UDP-D-glucuronate 4-epimerase (EC5.1.3.6), UDP-d-glucuronate decarboxylase (EC 4.1.1.35) were measured at different stages of cell-wall development. The specific activities and the activities per cell of these enzymes varied during differentiation of cambium to xylem according to the type polysaccharide synthesized. Variations were also found between the two species investigated. These data, compared with those obtained in out previous work on angiosperms [see the preceding paper, Dalessandro & Northcote (1977) Biochem. J. 162, 267-279], suggest that some control of polysaccharide synthesis operates at the level of the formation of the precursors of pectin and hemicellulose syntheses.

Carbohydrate Epimerases↗
PubMed1998

Molecular characterisation of a novel plant homeobox gene expressed in the maturing xylem zone of Populus tremula x tremuloides.

In trees, wood is produced from the vascular cambium. However, little is known about how differentiation of the cambial derivatives is controlled at the molecular level. Hybrid aspen, Populus tremula x tremuloides, is a fast-growing, wood-forming plant from which an efficient transformation and regeneration protocol has been developed. We use this species as a model to study xylem cell differentiation. In a cross-section of the stem, cells in the cambial region at different stages of development can be seen in a radial distribution. We dissected out the cambial region and constructed a cDNA-library from this tissue. The library was screened with a degenerated primer mix complementary to the third helix region of the homeobox. A number of cDNAs hybridising to the probe were isolated, purified, cloned and sequenced. Characterisation of the expression pattern of two such homeobox cDNAs revealed that one of the cDNAs, denoted PttHB1, was specifically expressed in the xylem maturation zone, indicating a role in the regulation of secondary wall formation, while the other, denoted PttHB2 was expressed in differentiating xylem and phloem cells. Structurally, the two hybrid aspen homeobox genes are homologous to each other, but no close relationship could be identified between these genes and any other homeobox sequence described thus far, in plants or other eukaryotes. We therefore conclude that the PttHB1 and PttHB2 genes belong to a novel hitherto undescribed sequence class of homeobox genes. We designate this as PALE.

Amino Acid Sequence↗
PubMed2000

A xylem-specific cellulose synthase gene from aspen (Populus tremuloides) is responsive to mechanical stress.

Angiosperm trees accumulate an elevated amount of highly crystalline cellulose with a concomitant decrease in lignin in the cell walls of tension-stressed tissues. To investigate the molecular basis of this tree stress response, we cloned a full-length cellulose synthase (PtCesA) cDNA from developing xylem of aspen (Populus tremuloides). About 90% sequence similarity was found between the predicted PtCesA and cotton GhCesA proteins. Northern blot and in situ hybridization analyses of PtCesA gene transcripts in various aspen tissues, and PtCesA gene promoter-beta-glucuronidase (GUS) fusion analysis in transgenic tobacco, demonstrated conclusively that PtCesA expression is confined to developing xylem cells during normal plant growth. During mechanical stress induced by stem bending, GUS expression remained in xylem and was induced in developing phloem fibers undergoing tension stress, but was turned off in tissues undergoing compression on the opposite side of the bend. Our results suggest a unique role for PtCesA in cellulose biosynthesis in both tension-stressed and normal tissues in aspen, and that the on/off control of PtCesA expression may be a part of a signaling mechanism triggering a stress-related compensatory deposition of cellulose and lignin that is crucial to growth and development in trees.

Arabidopsis Proteins↗