Water moves through the perforation plates to travel up the plant. Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers.
A series of sieve-tube cells also called sieve-tube elements are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells.
Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles. Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem.
The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith , while the layer of tissue between the vascular tissue and the epidermis is known as the cortex. Like animals, plants contain cells with organelles in which specific metabolic activities take place. Unlike animals, however, plants use energy from sunlight to form sugars during photosynthesis. In addition, plant cells have cell walls, plastids, and a large central vacuole: structures that are not found in animal cells.
Each of these cellular structures plays a specific role in plant structure and function. In plants, just as in animals, similar cells working together form a tissue. When different types of tissues work together to perform a unique function, they form an organ; organs working together form organ systems.
Vascular plants have two distinct organ systems: a shoot system, and a root system. The shoot system consists of two portions: the vegetative non-reproductive parts of the plant, such as the leaves and the stems, and the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis.
The root system , which supports the plants and absorbs water and minerals, is usually underground. Figure 4 shows the organ systems of a typical plant. Figure 4. The shoot system of a plant consists of leaves, stems, flowers, and fruits. The root system anchors the plant while absorbing water and minerals from the soil. Improve this page Learn More. Skip to main content. Module 8: Plant Structure and Function. Search for:. Plant Tissues and Organs Learning Outcomes Identify the different tissue types and organ systems in plants.
Try It. Many of the genes involved in the biosynthesis of each of the three main components have been identified, and the transcriptional network described in Fig. However, relatively few studies have demonstrated direct activation of specific genes through use of chromatin immunoprecipitation, electrophoretic mobility shift assays, or protoplast transfection systems Jin et al.
Histochemical studies have indicated that lignification of the secondary cell wall generally occurs after the initial deposition of the cellulosic components, and that it is initiated in a spatially distinct manner, beginning with the lignification of the middle lamella Saka and Thomas, ; Donaldson, One of the future challenges will be to understand how the temporal pattern of biosynthesis and deposition of the different components of the secondary cell wall is established.
Secondary cell wall formation must also be spatially regulated, even within one cell. Comparative studies of the patterns of secondary cell wall deposition in xylem tracheary elements in many different vascular plants have shown that the patterning of secondary cell wall deposition in these cells is a highly conserved process Esau, a , b ; Meylan and Butterfield, The fact that cellulose makes up the bulk of the secondary cell wall implies that regulation of spatial patterning must be intimately linked to regulation of the biosynthesis and deposition of cellulose microfibrils in the cell wall.
Much of what has been learned about cellulose biosynthesis has been gleaned from studies on cellulose deposition in primary cell walls, but there appear to be significant parallels between primary and secondary cell wall cellulose synthesis.
In both cases, cellulose synthesis complexes CSCs composed of 36 CesA cellulose synthesis A isoform subunits catalyse the linear polymerization of glucose molecules, a process that is also thought to drive the lateral movement of CSCs within the plasma membrane reviewed in Taylor, Distinct subgroups of CesA genes appear to be involved in cellulose biosynthesis in primary or in secondary cell walls. In Arabidopsis , the products of the CesA4 , CesA7 , and CesA8 genes appear to function non-redundantly to catalyse cellulose biosynthesis in secondary cell walls.
All three proteins were reported to be necessary to form the hexameric rosette-shaped secondary cell wall-associated CSCs whose form appears very similar to that of CSCs engaged in primary cell wall biosynthesis Haigler and Brown, ; Taylor, Nevertheless, secondary cell wall cellulose synthesis differs in some important respects from the primary cell wall-associated process.
It would be informative to determine whether the processivity of secondary cell wall CSC polymerization is significantly higher than that of primary CSCs, but limitations in the optical resolution of deeply buried xylem cell types have so far precluded the use of live cell imaging to assess the velocities of fluorescent protein-labelled secondary CSCs in plasma membranes accurately Wightman et al.
The spatial distribution of secondary cell wall CSCs appears to be correlated with sites of wall deposition, since high densities of CSCs have been localized to specific plasma membrane domains immediately below the developing secondary cell walls Wightman et al. The delivery and maintenance of CSCs in specific plasma membrane domains of developing tracheary elements is influenced by the subtending cortical microtubule array Wightman and Turner, , In Arabidopsis root protoxylem, microtubule bundles oriented along the borders of the developing secondary cell wall thickenings have been hypothesized to help organize functional CSCs at particular regions of the plasma membrane Wightman and Turner, In contrast to the linear cellulose polymer, hemicelluloses are branched cell wall matrix polysaccharides composed of backbones of glucose, mannose, or xylose, substituted to various degrees by other sugars.
Hemicelluloses are synthesized in the Golgi, and hemicellulose-containing vesicles may be trafficked to specific plasma membrane domains during secondary cell wall formation in a manner similar to that shown for CSCs. In Arabidopsis , several Golgi-localized glycosyltransferases have been identified that are involved in the biosynthesis of the xylose sugar backbone, and in the addition of the glucuronic acid side groups of glucuronoxylan.
Glucuronoxylan is the most abundant hemicellulose found in the secondary cell walls of dicotyledonous plants and is thought to function as the major cellulose cross-linking component in secondary cell walls. Lignification of the largely cellulosic secondary cell wall makes a major contribution to the functionality of mature tracheary elements and fibres. This phenolic polymer imparts both increased structural stability and water impermeability to the cell wall, and failure of lignin biosynthesis or polymerization often leads to IRX phenotypes similar to those observed in cellulose- or hemicellulose-deficient mutants Jones et al.
Because of its importance to the integrity of the vascular system, and the severe consequences of ectopic lignification in non-vascular tissues, the process of lignification is tightly controlled, both spatially and temporally. In protoxylem, for example, lignin polymerization is restricted to the narrow helical or annular zones of secondary cell wall deposition.
This provides a degree of wall rigidification while leaving intervening areas of the wall still capable of expansion Fig.
At the other extreme, total lignification of the massive secondary cell walls in fibre cells must be closely coordinated with the loss of metabolic capacity in these cells as they undergo PCD. The developmental progression of xylem cell types in the Arabidopsis inflorescence stem. A diagrammatic representation of the morphology of vessel elements VE , xylary fibres XF , and xylem parenchyma cells XP near the apex and at the base of the stem.
Monolignols are indicated by coloured spots in the cytosol, while solid colour within cell walls indicates the location of H-rich green , G-rich blue , and S-rich magenta lignin. Vessel element lignification progresses from initial H-lignin deposits in the middle lamella and cell corners to accumulation of G-lignin-rich secondary cell wall thickenings over the course of development, while fibres form a more massive S-lignin-rich secondary cell wall.
Both vessel elements and xylary fibres ultimately undergo programmed cell death, as indicated by the lack of cell contents of these cells near the base of the stem. Arrows from the xylem parenchyma cells to the vessel elements and xylary fibres indicate the putative metabolite contribution of neighbouring cells to the lignification process and demonstrate how lignification may proceed post-mortem.
Lignin formation has been intensively studied for several decades and, as a result, most of the enzymatic machinery involved, and the corresponding genes, have been well characterized.
Attention has recently been focused on the regulation of the pathway; particularly, how it is integrated into the larger secondary cell wall transcriptional network. The lignin biosynthesis pathway can be divided into two parts. The first is the general phenylpropanoid pathway, a multistep reaction sequence that generates precursors not only for synthesis of lignin monomers monolignol alcohols , but also for the synthesis of other phenylpropanoid compounds such as flavonoids, tannins, phenolic esters, and acids Hahlbrock and Scheel, The regulation of monolignol biosynthesis and subsequent lignin polymer formation is a complex process that is influenced by a variety of developmental, physiological, and environmental cues Zhao and Dixon, Therefore, different layers of regulatory mechanism s are likely to have evolved to respond to specific situations.
Transcriptional regulation appears to play a major role in controlling lignin biosynthesis, and several transcription factors are known to affect the transcription of the corresponding genes directly or indirectly reviewed in Zhong and Ye, MYB75 was previously found to function as an activator of flavonoid biosynthesis, based on the phenotype of the PAP1-d enhancer trap mutant that has elevated MYB75 expression and increased anthocyanin content Borevitz et al.
However, subsequent analysis of the myb75 loss-of-function mutant uncovered cell wall thickness phenotypes similar to those seen in the knat7 mutant, and both mutants were found to have increased expression of secondary cell wall biosynthesis genes cellulose, hemicellulose, and lignin Bhargava et al.
KNAT7 and MYB75 share partially overlapping tissue expression domains in developing xylem tissues and can physically interact with each other in yeast two-hybrid and bimolecular fluorescence complementation assays Zhong et al. MYB58 and MYB63 directly activate the expression of nearly all the genes involved in the lignin biosynthetic pathway, and both genes are thought to bind at AC regulatory elements also known as H-boxes or PAL boxes , conserved regulatory elements found upstream of most lignin biosynthetic genes Ohl et al.
The one lignin biosynthetic gene that is not activated by either MYB58 or MYB63 is the cytochrome P enzyme, ferulatehydroxylase F5H , which sits at a key branch point in the monolignol biosynthesis pathway. For instance, xylem vessels or tracheary elements have secondary cell walls primarily composed of G-lignin Fig.
In agreement with this model, the SND1 master switch from Arabidopsis , which is expressed in S-lignin-rich interfascicular fibres, was shown to directly activate the expression of reporter genes driven by the Medicago truncatula F5H promoter in transient protoplast expression assays Zhao et al. Consistent with this result, the overall cis -element structure of the Arabidopsis F5H promoter was found to be distinctly different from that of the homologous Medicago F5H promoter.
At the same time, loss of function at the AtMYB locus was correlated with a marked suppression of F5H expression in myb plants. 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.
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The pits are lined with a pit membrane composed of cellulose and pectins. According to the researchers, this control of water movement may involve pectin hydrogels which serve to glue adjacent cell walls together.
One of the properties of polysaccharide hydrogels is to swell or shrink due to imbibition. But when pectins shrink, the pores can open wide, and water flushes across the xylem membrane toward thirsty leaves above. Magnified horizontal view x of an inner perianth segment of a Brodiaea species in San Marcos showing a primary vascular bundle composed of several strands of vessels.
The strands consist of vessels with spirally thickened walls that appear like minute coiled springs. Although this species has been called B. This species contains at least 3 strands of vessels per bundle, while B. T he water-conducting xylem tissue in plant stems is actually composed of dead cells.
In fact, wood is essentially dead xylem cells that have dried out. The dead tissue is hard and dense because of lignin in the thickened secondary cell walls. Lignin is a complex phenolic polymer that produces the hardness, density and brown color of wood. Cactus stems are composed of soft, water-storage parenchyma tissue that decomposes when the plant dies. The woody lignified vascular tissue provides support and is often visible in dead cactus stems.
Left: Giant saguaro Carnegiea gigantea in northern Sonora, Mexico. The weight of this large cactus is largely due to water storage tissue in the stems. Right: A dead saguaro showing the woody lignified vascular strands that provide support for the massive stems. It is composed of sieve tubes sieve tube elements and companion cells. The perforated end wall of a sieve tube is called a sieve plate.
Thick-walled fiber cells are also associated with phloem tissue. I n dicot roots, the xylem tissue appears like a 3-pronged or 4-pronged star. The tissue between the prongs of the star is phloem. The central xylem and phloem is surrounded by an endodermis, and the entire central structure is called a stele. Microscopic view of the root of a buttercup Ranunculus showing the central stele and 4-pronged xylem.
The large, water-conducting cells in the xylem are vessels. Phloem tissue is produced on the outside of the cambium. The phloem of some stems also contains thick-walled, elongate fiber cells which are called bast fibers. Bast fibers in stems of the flax plant Linum usitatissimum are the source of linen textile fibers. Gymnosperms generally do not have vessels, so the wood is composed essentially of tracheids.
The notable exception to this are members of the gymnosperm division Gnetophyta which do have vessels. See Article About Welwitschia P ine stems also contain bands of cells called rays and scattered resin ducts. Rays and resin ducts are also present in flowering plants. In fact, the insidious poison oak allergen called urushiol is produced inside resin ducts. Wood rays extend outwardly in a stem cross section like the spokes of a wheel. The rays are composed of thin-walled parenchyma cells which disintegrate after the wood dries.
This is why wood with prominent rays often splits along the rays. In pines, the spring tracheids are larger than the summer tracheids. Because the summer tracheids are smaller and more dense, they appear as dark bands in a cross section of a log. Each concentric band of spring and summer tracheids is called an annual ring. By counting the rings dark bands of summer xylem in pine wood , the age of a tree can be determined.
Other data, such as fire and climatic data, can be determined by the appearance and spacing of the rings. Some of the oldest bristlecone pines Pinus longaeva in the White Mountains of eastern California have more than 4, rings. Annual rings and rays produce the characteristic grain of the wood, depending on how the boards are cut at the saw mill. Microscopic view of a 3-year-old pine stem Pinus showing resin ducts, rays and three years of xylem growth annual rings.
In ring-porous wood, such as oak and basswood, the spring vessels are much larger and more porous than the smaller, summer tracheids. This difference in cell size and density produces the conspicuous, concentric annual rings in these woods.
Because of the density of the wood, angiosperms are considered hardwoods, while gymnosperms, such as pine and fir, are considered softwoods. See Article About Hardwoods See Specific Gravity Of Wood T he following illustrations and photos show American basswood Tilia americana , a typical ring-porous hardwood of the eastern United States: A cross section of the stem of basswood Tilia americana showing large pith, numerous rays, and three distinct annual rings.
The large spring xylem cells are vessels. In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth.
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