Sucrose and amino acids are translocated within the living cytoplasm of the sieve tubes. Companion cells - transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy.
A sieve tube is completely dependent on its companion cell s. Comparison of transport in the xylem and phloem Xylem Phloem Type of transport Physical process Requires energy Substances transported Water and minerals Products of photosynthesis; includes sucrose and amino acids dissolved in water Direction of transport Upwards from roots to leaves Upwards and downwards.
Type of transport. Physical process. Requires energy. Another group of cell wall structural proteins that play an important role in plant cell growth and developmental processes are expansins Cosgrove, , which, may be involved in cell wall modifications during xylogenesis in roots of P. AGPs are involved in many processes in plants including plant growth and development, root growth and development, xylem development, and PCD Motose et al.
The current study indicates that AGPs are mainly involved in the early stages of xylem development in roots and in the later stages of cell wall development in stems. Interestingly, AGPs in roots in primary growth were not localized in xylem cells; however, in roots in secondary growth, they were found in both primary and secondary lignified cell walls. In contrast, AGPs were observed in all tissues of stems in primary and secondary growth, including lignified TEs.
The current analysis confirmed that FLAs are associated with SCW development during both the primary and secondary xylem formation in pioneer roots, as well as in secondary xylem development in stems. Carbohydrates are the main components of the PCW, and among them, pectins are the most abundant.
In addition to pectins, there is also a high content of cellulose and hemicelluloses that are present in PCWs Plomion et al. In contrast, cellulose, with significantly fewer hemicelluloses and pectins, is the main component of SCWs Weng et al. In the current study, a coordinated expression of genes encoding cell wall components was observed; indicating that temporal changes in gene expression regulate primary and SCW formation in response to specific genetically defined programs of cell development.
Mellerowicz and Gorshkova studies reported that the primary and secondary walls of Populus xylem differ in their composition of carbohydrates.
On the other hand, the layers of SCWs are rich in cellulose with significantly lower levels of hemicelluloses glucuronoxylan and pectins Weng et al. The rearrangements that occur in the cell wall during the transition from primary to secondary growth indicate that the process is highly regulated by molecular mechanisms.
In our study, numerous genes encoding CesA were identified that are orthologues of A. However, the Ces7A-like gene, which is typically associated with SCW development, was down-regulated during secondary xylem development. On the other hand, the expression of PtiCesA , which is associated with PCW development, was up-regulated during secondary growth of roots.
These results suggest that some CesA genes that have been previously considered to be specific to either primary or SCW development may play a more general role in cell wall development. Similar observations were reported by Sundell et al. In addition to high levels of cellulose biosynthesis occurring during primary and SCW development, cellulose decomposition also occurs during the remodelling of the cell wall during the course of xylem maturation and lignification in roots.
Interestingly, more genes related to cellulose biosynthesis were found to be up-regulated in stems than in roots. These data suggest that a greater accumulation of cellulose occurs in the xylem cell wall of stems, which was also confirmed when cellulose content in cell walls was measured.
Hemicellulose synthesis-related genes were identified in both roots xylan and xyloglucan and stems xylan. Their expression pattern suggests that hemicelluloses are synthesized during both primary and SCW formation. IRX9 was up-regulated during both the primary and secondary xylem formation, while IRX14 was down-regulated in roots during secondary growth. Ratke et al. Our results suggest, however, that IRX14 is only involved in xylan biosynthesis during primary growth.
Xyloglucan was not observed in xylem TEs of roots with primary growth and was present to a lesser extent in roots with secondary growth, especially within TEs containing SCW thickenings. In contrast, xyloglucan was localized to primary xylem in stems exhibiting both primary and secondary growth. Compared to roots, a positive signal appeared in some walls of TEs that had thickened during differentiation. Similar to a previous observation by Bourquin et al. Hemicellulose degradation is also an integral part of cell wall remodelling Mellerowicz and Sundberg, ; Minic, In roots, our study indicates that hemicelluloses are broken down only during the primary xylem differentiation; whereas in stems, hemicelluloses are degraded during both primary and secondary development.
These observations suggest that a continuous metabolism of hemicelluloses occurs, which may be due to more extensive modification of cell wall carbohydrates resulting from the greater level of lignification that occurs in stems relative to roots.
Data from the current study suggest that the early stage of pectin biosynthesis in roots is completed prior to secondary growth begins. In stem tissues, however, pectin biosynthesis continues even after the second stage of xylogenesis is initiated. HGs were observed in the cell walls within cortical parenchyma cells and all un-lignified tissues of roots in primary growth.
In roots in secondary growth, however, HGs were mainly present in secondary xylem cells; possibly in the PCW layers of these cells.
In stems in primary growth, HGs were localized in the cell walls of all tissues, whereas in stems with secondary growth, HGs were only located in the PCWs of cambial zone cells, phloem, and primary xylem. Galactan was localized in the cell walls of phloem cells in roots in primary and secondary growth, and slightly in the cell walls of secondary xylem tissue. In stems with primary growth, an accumulation of galactan was observed in cambial cells and a slight amount in primary xylem cells.
In stems with secondary growth, however, a strong galactan signal was observed in the secondary xylem, cambium, and phloem. Similar to galactan, arabinan was observed in all un-lignified cell walls in roots with primary growth, but not in already lignified primary xylem.
In roots in secondary growth, arabinan was localized to phellem cells, secondary phloem fibers, and secondary xylem; most likely in the PCW layers. In stems in primary growth, arabinan occurred mainly in phloem and to some extent xylem tissue.
In stems with secondary growth, however, a stronger signal was observed in cambial cells and phloem initials; as well as in primary xylem and a little in secondary xylem. Remodeling of cell walls in pioneer roots is associated with pectin degradation, pectin de-esterification, acetylation, and de-methylation; which occurs during both the primary and secondary xylem formation.
In contrast, pectins in stems are modified through de-methylation and degradation by pectinase. The level of glucose and arabinose decreased in roots and stems with secondary growth, which may be explained by the fact that they construct hemicelluloses and pectin RG-I arabinose that are typically found in PCWs Hoch, ; Mellerowicz and Gorshkova In contrast, the level of xylose increases; which is typical for the hemicelluloses xylan and pectins glucuronoxylan in SCWs Mellerowicz and Gorshkova, The majority of genes related to lignin biosynthesis were up-regulated during both the primary and secondary xylem formation, however, a greater level of up-regulation was observed in their expression during secondary growth in both roots and stems.
It is possible that enzymes associated with the early steps of the monolignol pathway are also involved in the biosynthesis of other phenylpropanoids Koutaniemi, ; Sundell, Moreover, the level of lignin increased with the development of secondary growth in pioneer roots and stems. Lignins were located mainly in the cell walls of xylem vessels, xylem fibers, and phloem fibers.
Therefore, the constant level of S-units in pioneer roots may be explained by rather stable over-all expression of genes encoding COMT. Since there is no need for additional support in the roots, there is no significant increase of sclerenchyma fibers containing G- and S-units. NAC domain and MYB transcription factors act as master switches regulating gene expression during secondary wall biosynthesis Zhong and Ye, In our study, genes encoding NAC domain proteins were mostly up-regulated in roots and stems during xylogenesis; suggesting that they may be involved in the regulation of genes involved in cell wall biosynthesis and cell wall modifications.
While MYB TFs in pioneer roots were both up- and down-regulated, they were mostly up-regulated in stems. Consequently, it is plausible that this up-regulation increases the expression of genes in pathways involved in SCW formation which in turn may be responsible for the greater development of secondary xylem in stems relative to roots.
Although many studies of xylogenesis have been conducted in stems, much less is known about xylem formation in roots. Our present study provides a detailed, comprehensive description of the expression of genes during cell wall developments and cell wall modifications occurring during xylogenesis of pioneer roots and stems in P.
Interestingly, the majority of DEGs in pioneer roots vs. Despite this major difference, however, many characteristics of xylogenesis are similar; such as increasing expression for HRGPs in primary call wall, decreasing expression for extesins, differentiated expression of genes encoding CesAs and increasing lignins synthesis with G-units being dominant over S-units in primary xylem. Also similar pattern of pectins biosynthesis and remodeling during primary xylogenesis was observed in both roots and stems.
Moreover, the composition of monosaccharides in both organs is also very similar. For other components, however, the timing of the up- or down-regulation is different due to diverse role of both organs and differences in environment under- and aboveground. For example, AGPs and most FLAs are only involved in primary xylogenesis in roots, hemicelluloses are only degraded in the PCW of roots; whereas, these features are expressed continuously throughout all stages of xylogenesis in stems due to intensive cell wall remodeling and secondary wood development.
Some processes appear to be unique to one organ, e. Others are more intensive in one organ, such as the level of pectin remodeling that occurs in roots. In roots, xylan helps to stabilized the structure of cell walls, and biosynthesis and remodeling of xyloglucan ensure stretch ability and stress resistance during cell growth.
While in stems, pectins biosynthesis and signaling molecules arising during pectins degradation lead to cell wall strengthening. Increasing biosynthesis of hemicellulose provides stable cell wall structure, while expanded level of crystalline cellulose ensures cell wall stiffness. Moreover, predominance of G-units over S-units in lignins in secondary xylem provides structural support for the growing stem Figure 7.
The present study provides the first comprehensive structural and molecular analysis, including an analysis of gene expression, of the differentiation of TEs vessels and supporting elements fibers within xylem in pioneer roots in comparison with stems of P.
The current and previously reported information clearly reveals the great complexity of molecular mechanisms underlying the cell wall formation and modifications that occur during xylogenesis.
Our research increases the knowledge and improves understanding of the cell wall development in under- and aboveground tree organs. Efforts to breed new tree varieties with higher yield and better wood quality will not be successful without recognizing and understanding the complicated transcriptional network underlying wood development.
All experiments were performed on seed-grown P. Gray ex Hook. After 3 months in April , plants were transferred into rhizotrons. Roots were grown in transparent-walled chambers filled with natural soil with shoots extending from the top into the air. The rhizotrons 50x30 cm were constructed of two transparent polycarbonate plates held 3 cm apart by thick-walled plastic tubing to provide adequate growing space.
Waterlogging was avoided by installing a drainage hole in the bottom of each rhizotron that permitted soil aeration and drainage of excess water.
Material was collected in July, in the middle of vegetative season. Pioneer roots in all of the experiments were divided into the following segments corresponding to their developmental stage: cm—root tips with apical meristem PR1 ; 4—6 cm—primary growth PR2 ; and 13—16 cm—secondary growth PR3.
Similarly, stems were also sampled based on developmental stages: 0—2 cm—apical meristem with primary growth PS1 ; 20—25 cm—secondary growth PS2 , and 40—45 cm—isolated secondary xylem PS3 Table 2. Root tips were treated as a negative control for the process of xylogenesis, since the process of xylogenesis is undetectable in root tips, while isolated secondary xylem served as a positive control for the xylogenesis process.
Table 2 Experimental design describing sampling of Populus trichocarpa pioneer roots and stems. The normalized data were statistically analyzed using GeneSpringGX7 The sectioned samples were subsequently treated for 2. Results of the immunolocalization assay were recorded with a Leica TCS SP5 II confocal microscope Leica Biosystems, Germany using lasers: diode emitting light at wavelengths of to observe autofluorescence of lignins and argon laser emitting light at wavelengths to observe fluorescence fluorochrome Alexa secondary antibodies using in IC reactions.
Lignin autofluorescence was also characterized at the same time the immunolocalization studies were conducted. At least five root and stem segments were harvested from each developmental category for the analysis of each tested antibody. Incubations without primary antibodies were used as a negative control. No detectable results were obtained with the negative controls.
Four independent samples of cell walls were extracted from each studied developmental stage of roots and stems sampled as described above see section Plant Material and Experimental Design. Plant tissue was frozen in liquid nitrogen and ground in a ball mill Retsch, Germany to a fine powder. The material was air-dried and stored until further processing Foster et al. The acetylation of the alditols to alditol acetates was performed using acetic anhydride and pyridine.
Myo-inositol was used as an internal standard. Cell wall monosaccharide composition was measured for four replicates per each studied object. Samples were diluted with water and glucose content of the supernatant was assayed using the colorimetric anthrone assay.
Glucose and hence crystalline cellulose content was calculated based on the absorbance at mm compared to the glucose standard curve established on the same plate Foster et al.
Crystaline cellulose content was measured for four replicates per each studied object. Lignin composition was measured for four replicates per each studied object. A DB-5 bonded-phase fused-silica capillary column 30 m length, 0. The total time of GC analysis was 36 min.
One microliter of each sample was injected in splitless mode. In-source fragmentation was performed with a 70 eV energy. The metabolites were automatically identified using a library search NIST library. Artefacts [alkanes, column bleed, plasticizers, N-methyl-N- trimethylsilyl trifluoroacetamide, and reagents] were identified analogously and excluded from further analyses.
Unique quantification masses for each component were specified and the samples were reprocessed to obtain accurate peak areas for the deconvoluted components.
The obtained profiles were normalized against the sum of the chromatographic peak area using the total ion chromatogram TIC approach. The datasets generated for this study can be found in the Gene Expression Omnibus database: accession number GSE AB-Z conceived the original concept and research plan, supervised the experiments, and provided funding.
All authors discussed the results, read, and approved the final version of the manuscript. This work was supported by the grant no. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Aspeborg, H. Carbohydrate-active enzymes involved in the secondary cell wall biogenesis in hybrid aspen. Plant Physiol. Bagniewska-Zadworna, A. Avoiding transport bottlenecks in an expanding root system: Xylem vessel development in fibrous and pioneer roots under field conditions. New insights into pioneer root xylem development: evidence obtained from Populus trichocarpa plants grown under field conditions. Barros, J. The cell biology of lignification in higher plants. Bischoff, V. Boerjan, W.
Lignin Biosynthesis. Plant Biol. Post mortem function of AtMC9 in xylem vessel elements. New Phytol. Bonke, M. APL regulates vascular tissue identity in Arabidopsis. Nature , — The metabolic reactions required for monolignol biosynthesis are believed to operate in the cytosol, or perhaps in the region of the cytosol directly associated with the endoplasmic reticulum Boerjan et al.
However, in order for monolignols to participate in polymerization to form the final lignin structure, they must move from their site of synthesis, across the plasma membrane to the cell wall.
How this export is accomplished remains unclear. The results of recent studies are inconsistent with vesicle-mediated trafficking of monolignols to the cell wall Kaneda et al. On the other hand, the small size of monolignols, and their demonstrated ability to partition into the membrane of synthetic lipid disks, supports the idea that monolignols could potentially exit the cell by passive diffusion Boija and Johansson, ; Boija et al. In this model, monomer export would be driven by the concentration gradient between the cytosol, where monolignols are being actively synthesized, and the cell wall matrix, where they are rapidly polymerized into lignin.
However, only low levels of monolignol diffusion across the membrane of plasma membrane vesicles have been reported Miao and Liu, Since the rate of simple diffusion of the monolignols across the plasma membrane would have to be very high to account for the rapid and extensive lignification occurring in the maturing secondary cell wall, alternative models postulating that monolignol export to the cell wall should occur via plasma membrane-localized transporters have also been put forward Kaneda et al.
If appropriately configured at the plasma membrane, such putative transporters could not only meet the metabolite flux demands but could potentially account for the spatial precision with which lignin is deposited in the cell wall of different cell types e. The identification of a monolignol transporter protein from among the hundreds of active transporters encoded in a plant genome would fill a significant gap in our understanding of the lignification process.
Miao and Liu provided some insight into this question by testing the ability of plasma membrane vesicles derived from Arabidopsis seedlings to export monolignols. Transport of the coniferyl alcohol monolignol into these vesicles was shown to be primarily energy dependent, in keeping with the active transport hypothesis. Disruption of trans-membrane pH or potential gradients with pharmacological inhibitors did not affect the observed monolignol transport, but treatment with chemicals known to act as ABC ATP-binding cassette transporter inhibitors, such as vanadate or nifedipine, greatly reduced monolignol accumulation in these vesicles Miao and Liu, No specific transport protein has yet been identified, however, and the plasma membrane vesicles used in the study of Miao and Liu were isolated from Arabidopsis seedlings in which only a small proportion of tissues would be undergoing lignification.
Because several ABC transporters have been shown to facilitate the export of a wide range of low molecular weight, hydrophobic substrates, including auxin Yazaki, ; Verrier et al. The extent to which such functional promiscuity would be biologically relevant to lignification remains to be established. A set of candidate ABC transporters for monolignol export was previously identified based on their co-expression with phenylpropanoid biosynthesis genes in developing Arabidopsis inflorescence stems Ehlting et al.
Instead, several of the loss-of-function ABC transporter mutants examined did display polar auxin transport defects Kaneda et al. This observation is consistent with the transporter multifunctionality mentioned above, and because local auxin concentrations play a key role in determining vascular cell fate, the ability of these particular transporters to transport auxin might account for the observed correlation between expression of the corresponding genes and increasing inflorescence stem lignification Ehlting et al.
In light of such functional redundancy, and the size of the ABC transporter gene family members in Arabidopsis , it is uncertain whether higher order mutant analysis would be capable of uncovering phenotypes consistent with defective monolignol export. Once in the cell wall, monolignols are oxidatively polymerized through the process of radical combinatorial coupling reviewed in Boerjan et al.
A recent study showed that laccases LAC4 and LAC17 are necessary for normal lignification of Arabidopsis fibre cell walls and, to some extent, of tracheary elements Berthet et al. Expression of the LAC4 gene was also found to be up-regulated in response to overexpression of the MYB58 transcription factor, which suggests that MYB58 could be activating genes involved in both monolignol biosynthesis and polymerization Zhou et al.
Unfortunately, the catalytic promiscuity and large gene families of peroxidases and laccases make it difficult to establish functional relationships between the activity of specific gene products and the spatiotemporal pattern of lignin deposition during xylem development. Many other aspects of the lignin polymerization process also remain unclear, including the physical nature of the association between lignin and other cell wall polymers, control of the spatial patterning of lignin deposition in the wall, and the relationship between the metabolic supply of G- and S-type monolignols and the composition of the final polymer.
In Arabidopsis , tracheary elements have secondary cell walls composed primarily of G-lignin while the walls of the interfasicular fibres are S-lignin rich Fig. While we might, by analogy, expect the xylary fibre cell wall also to be S-lignin rich, recent data in poplar suggest that the walls of xylary fibres close to, or surrounded by, tracheary elements have a lignin composition that is intermediate between the lignin of G-rich tracheary elements and S-rich fibres Gorzsas et al.
Spatial variability in lignin deposition was also revealed by a series of elegant autoradiography studies in pine, poplar, Japanese cedar, and Japanese black pine, demonstrating that the first stage of lignification during xylem development involves the incorporation of a mixture of H-lignin dominated by 4-hydroxy ring structures and G-lignin in the middle lamella and cell corners Fujita and Harada, ; Takabe et al.
Since this pectin-rich area of the cell wall is hydrophilic, whereas lignin is hydrophobic, it has been hypothesized that the deposition of the lignin polymer in the middle lamella and cell corners may displace or modify pectin.
In the next phase of wall lignification, the primary cell wall and outer layers of the secondary cell wall are lignified primarily with G-lignin Terashima and Fukushima, The last stage of lignin deposition is directed to the innermost layer of the secondary cell wall Fujita and Harada, ; Takabe et al. If these temporal and spatial patterns are consistent across higher plant taxa, it is clear that highly integrated intracellular mechanisms must exist that focus both the genetic and metabolic resources of developing xylem cells on formation of the final cell wall structures.
Secondary cell wall formation and patterned deposition is tightly regulated in specific xylem cell types, but much about the fine regulation of the transcriptional network remains unknown. During fibre development, the thick secondary cell wall is formed over an extended period, whereas tracheary element differentiation progresses quickly from secondary cell wall deposition to PCD, the final stage of differentiation.
The expression of several genes functionally associated with PCD is correlated with xylem development Zhao et al. In contrast, SND1 has not been shown to regulate the expression of genes mediating PCD, which is consistent with a more specific role for SND1 as a regulator of secondary cell wall formation in fibres Zhong et al.
The proposed role of the cysteine proteases in executing PCD was confirmed by the examination of xcp1 single mutants and xcp1xcp2 double mutants, which displayed incompletely degraded cellular contents within tracheary elements Avci et al. XCP1 to specific cell types such as the tracheary elements Zhong et al. Gene transcript profiling in the Zinnia tracheary element differentiation cell culture system has helped identify which genes are specifically up-regulated prior to the onset of PCD in these cells Groover et al.
These data, together with several whole plant studies, suggest that PCD during tracheary element differentiation is an orderly and actively regulated cell-autonomous process Fukuda and Komamine, ; Groover et al.
Unlike many other types of PCD in plants, the large central vacuole of the nascent tracheary element cell plays a critical role during this process Roberts and McCann, Modifications or disruptions of the tonoplast vacuolar membrane , and accompanying changes in the vacuolar contents, define the initial stage of PCD.
The subsequent rupture of the vacuole and release of digestive enzymes such as nucleases and proteases results in digestion of all the cell contents, leaving only the cell wall intact Fukuda, XCP1 and XCP2 proteins appear to be localized within tracheary elements prior to this vacuolar implosion, and can still be detected, post-implosion, in the space formerly occupied by the vacuole Avci et al. Transcript profiling in the Zinnia cell culture system has also identified other nucleases, proteases, and lytic enzymes putatively stored in the vacuole, which are likely to be involved in the vacuole-mediated tracheary element PCD Groover et al.
A study in poplar Courtois-Moreau et al. Tracheary element PCD occurs rapidly, with the vacuolar implosion requiring only a few minutes, and the clearance of the remainder of the cell contents is completed within a few hours Groover et al. The timing of fibre PCD has not been extensively studied, but a study performed in poplar Courtois-Moreau et al. While PCD appears to operate as a cell-autonomous process, it has been hypothesized that lignification of the cell walls in tracheary elements may be non-cell autonomous Pickett-Heaps, The non-cell-autonomous lignification model suggests that non-lignifying cells such as xylary parenchyma cells, positioned adjacent to lignifying cells such as tracheary elements, are capable of synthesizing monolignols and exporting them to the cell wall of the neighbouring lignifying cells.
Studies in the Zinnia cell culture system have demonstrated that it is possible for lignification of tracheary element-like cells to proceed even after PCD. Thus, when dead tracheary elements were moved from the tracheary element induction medium to another medium containing added monolignols, the tracheary elements were able to use monolignols from the extracellular solution to continue lignification post-mortem Hosokawa et al. Because Zinnia cell cultures, in which mesophyll cells are induced to transdifferentiate to form tracheary element-like cells, contain many cells that remain in a state similar to xylem parenchyma cells McCann et al.
Further support for such a model Fig. There is no evidence that a similar cooperative lignification process occurs in fibres Baghdady et al. In contrast, the rapidity with which tracheary elements undergo PCD may leave limited time for cell-autonomous lignification, in which case the xylem parenchyma neighbours might continue to export monolignols to the cell wall after tracheary element PCD, and thereby further strengthen the wall.
It has also been suggested that, in addition to the degradative enzymes released by the vacuole during tracheary element vacuole collapse, monolignols stored in that compartment could be released, and that lignification is therefore primarily a post-mortem process Pesquet et al.
In this model, monolignols would be synthesized prior to cell death, and small amounts might be deposited and polymerized in the cell wall. However, the bulk of the monolignol pool would be stored in the vacuole Pesquet et al. Most lignin polymerization would therefore occur after PCD of the tracheary element.
Monolignol localization using microautoradiography suggests that tracheary elements in Arabidopsis are still living while the cell wall is being lignified Kaneda et al. Smith, unpublished data , but this observation does not preclude lignification continuing to proceed following PCD, either through vacuolar release of monolignols or their acquisition by donation from neighbouring, non-lignifying cells. Gradual shifts in sites of free-auxin production during leaf-primordium development and their role in vascular differentiation and leaf morphogenesis in Arabidopsis.
Google Scholar. The Plant Journal. Eucalyptus gunnii CCR and CAD2 promoters are active in lignifying cells during primary and secondary xylem formation in Arabidopsis thaliana.
Plant Physiology and Biochemistry. The expression of the Athb-8 homebox gene is restricted to provascular cells in Arabidopsis thaliana. The arabidopsis ATHB-8 HD-zip protein acts as a differentiation-promoting transcription factor of the vascular meristems. Plant Physiology. Integration of transport-based models for phyllotaxis and midvein formation.
Genes and Development. Local, efflux-dependent auxin gradients as a common module for plant organ formation. Berleth T Jurgens G. The role of the Monopteros gene in organizing the basal body regions of the Arabidopsis embryo. Vascular continuity and auxin signals. Trends in Plant Science. The Plant Cell. Tissue- and cell-specific activity of a phenylalanine ammonia-lyase promoter in transgenic plants. EMBO Journal. MYB75 functions in regulation of secondary cell wall formation in the Arabidopsis inflorescence stem.
Lignin biosynthesis. Annual Review of Plant Biology. Boija E Johansson G. Interactions between model membranes and lignin-related compounds studied by immobilized liposome chromatography. Biochimica et Biophysica Acta. Evaluation of bilayer disks as plant cell membrane models in partition studies. Analytical Biochemistry. Activation tagging identifies a conserved MYB regulator of phenylpropanoid biosynthesis. Identification of novel genes in Arabidopsis involved in secondary cell wall formation using expression profiling and reverse genetics.
Vascular differentiation and transition in the seedling of Arabidopsis thaliana Brassicaceae. International Journal of Plant Sciences. Regulatory mechanisms for specification and patterning of plant vascular tissues. Annual Review of Cell and Developmental Biology. Secondary xylem development in Arabidopsis: a model for wood formation. Physiologia Plantarum. An Arabidopsis mutant defective in the general phenylpropanoid pathway.
Cell-specific and conditional expression of caffeoyl-coenzyme AO-methyltransferase in poplar. A unique program for cell death in xylem fibers of Populus stem. Pausing of Golgi bodies on microtubules regulates secretion of cellulose synthase complexes in Arabidopsis. Wood formation in Angiosperms. Comptes Rendus Biologies. Demura T Fukuda H. Molecular cloning and characterization of cDNAs associated with tracheary element differentiation in cultured Zinnia cells.
Transcriptional regulation in wood formation. Visualization by comprehensive microarray analysis of gene expression programs during transdifferentiation of mesophyll cells into xylem cells. Hormone interactions during vascular development. Plant Molecular Biology. An update on xylan synthesis. Molecular Plant. Donaldson LA. Lignification and lignin topochemistry—an ultrastructural view. Regulation of preprocambial cell state acquisition by auxin signaling in Arabidopsis leaves.
Global transcript profiling of primary stems from Arabidopsis thaliana identifies candidate genes for missing links in lignin biosynthesis and transcriptional regulators of fiber differentiation. Current Biology. Esau K. New York : Wiley and Sons. Google Preview. Esau K b Vascular differentiation in plants. New York : Holt, Rinehart and Winston. Establishment of polarity in lateral organs of plants. The PXY—CLE41 receptor ligand pair defines a multifunctional pathway that controls the rate and orientation of vascular cell division.
Tissue- and cell-specific expression of a cinnamyl alcohol dehydrogenase promoter in transgenic poplar plants. Fisher K Turner S. PXY, a receptor-like kinase essential for maintaining polarity during plant vascular-tissue development.
These two types of xylem perform the same function and are categorized by the type of growth for their formation. The primary growth of plant formation of primary xylem occurs at the tips of stems, roots, and flower buds. Also, the primary xylem helps the plant to grow taller and makes the roots longer. Thus, it occurs first in the growing season, so this is called primary growth.
The purpose of primary and secondary xylem is to transport water and nutrients. With the secondary growth of the plant, secondary xylem is formed that helps the plant to get wider over time. An example of the secondary growth of plants is wide tree trunks. It happens each year after the growth. Plus, the secondary xylem gives dark rings that determine the age of trees.
Xylem consists of four types of elements: 1 xylem vessels, 2 tracheids, 3 xylem fiber, and 4 xylem parenchyma. The xylem vessels are present in the angiosperms. They have a long cylindrical structure and have a tube-like appearance.
Walls contain a large central cavity, and walls are lignified. They lose their protoplasm, and thus, are dead, at maturity. They contain many cells vessel members that are interconnected through a perforation in common walls.
They are involved in the conduction of water, minerals and give mechanical strength to the plant. These are dead and are tube-like cells with a tapering end. They are found in the gymnosperm and angiosperm. These cells have a thick lignified cell wall and lack protoplasm. The main function they perform is water and mineral transportation. These are dead cells containing central lumen and lignified walls; they provide mechanical support to the plant and are responsible for water transportation.
The cells of xylem called parenchyma cells store food material and are considered the living cells of xylem. Moreover, they assist in the reduced distance transportation of water. Also, they are involved in the storage of carbohydrates, fats, and water conduction. The xylem structure can be understood by the types or divisions of xylem cells, including fiber cells, parenchyma cells, and tracheary elements. Xylem transports water and dissolved minerals as well as provides mechanical support to the plant.
They also convey phytohormonal signals in the plant body. Cohesive forces between water molecules work as a connecting way for the conduction of water within the xylem vascular system. Below are the precise functions of the xylem. How does xylem transport water? Cohesion-Adhesion theory is the hypothesis that attempts to explain how water travels upwards across the plant against gravity.
Transpiration in plants is a major factor that drives water to move up to replace water that has been lost by evaporation. Xylem picks the water from the roots to transfer to other parts of the plants. Several cells are involved in the process of conduction or transportation of water. Read: Plant Water Regulation Lesson free tutorial.
Tracheary elements including vessels and tracheids are dead cells after reaching maturity. Therefore, they act passively for water transportation. The water reaches upwards from roots towards the stem and leaves on the basis of two factors: root pressure and transpirational pull.
Around million years ago, the xylem was developed in plants due to adaptation to environmental requirements. The production of food through photosynthesis is characterized by water uptake and carbon dioxide. When plants colonized the land, they developed a more advanced transport system that increases their chances of survival on the ground. Eventually, plants evolved advanced structures, such as the xylem vascular system. The water concentration n the plant reduced through the transpirational process that occurs through stomata taking carbon dioxide in and water out.
As explained in the previous section, this transpiration helped pull water in the plant body against gravity. The development of the xylem is characterized by the bifacial lateral meristem cells and the vascular cambium that produces secondary xylem as well as secondary phloem.
Moreover, the development of xylem changes from one form to another.
0コメント