Rays Appear to Be Continuous With Xylem Rays
Introduction to Vascular Plant Morphology and Anatomy
Thomas N. Taylor , ... Michael Krings , in Paleobotany (Second Edition), 2009
Secondary Phloem
Secondary phloem, the tissue produced to the outside of the vascular cambium, is also a complex tissue that includes an axial and a ray system. Like the xylem, the axial system in secondary phloem includes conducting cells, either sieve cells in conifers or sieve tube members in the angiosperms, which conduct solutes from the sites of photosynthesis to other parts of the plant. Phloem parenchyma occurs in the axial system, as well as companion cells (angiosperms) and albuminous cells (conifers). Fibers are very common in the secondary phloem of both conifers and angiosperms ( FIG. 7.31), and the pattern of fiber production by the cambium can sometimes be used to identify secondary phloem and bark tissue taxonomically. Although some conifers can produce regular, repeating bands of sieve cells, fibers, and parenchyma, they do not seem to produce these on an annual cycle, so it is not possible to determine the age of bark as it is to date wood by counting the tree rings.
FIGURE 7.31. Cross section of Tilia sp. stem showing secondary xylem (X), phloem (P), and dilating vascular rays (V) (Extant). Bar=650 μm.
Usually only a narrow band of phloem close to the cambium is actively involved in conduction—the functional phloem or inner bark. As the older phloem becomes nonfunctional, there are many histological changes in the tissue, including the collapse of sieve elements, the development of sclereids from parenchyma cells, and/or the deposition of ergastic substances in parenchyma cells. These changes have also been identified in fossil phloem (Smoot, 1984c). It is in the nonfunctional phloem that subsequent cork cambia may arise in older axes.
Vascular rays in the secondary phloem are continuous from the secondary xylem into the secondary phloem and consist only of parenchymatous ray cells. In some plants, the secondary phloem increases tangentially as the stem increases in diameter. This increase can occur by a tangential elongation of either axial or ray parenchyma cells. Some parenchyma cells, especially ray cells, may become meristematic and divide radially to produce additional cells. This process is called dilatation growth and can substantially increase the width of phloem rays. Secondary phloem rays are also important in ethylene signaling during plant responses to wounding and pathogens (Hudgins and Franceschi, 2004).
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From Cambium to Early Cell Differentiation Within the Secondary Vascular System
Peter Barlow , in Vascular Transport in Plants, 2005
Secondary Phloem
Secondary phloem tissues of both gymnosperms and dicotyledonous angiosperms are concerned mainly with the basipetal transport of sugars. Just as the structure of secondary xylem tissue shows relative simplicity in conifers but is more complex in dicotyledons, so the same is true of the secondary phloem (Srivastava, 1963; Esau, 1969). The main cell types of the vertical phloem system of conifers are sieve cells, parenchyma cells, and fibers. In dicotyledons, the cell types are sieve tube members, companion cells, parenchyma cells, and fibers. The problem to be addressed now is how the cells of these two vertical phloem systems come to be distinctively distributed within the radial files of phloem. The cell divisions that accompany phloem cell differentiation seem to be quite distinctive (Esau and Cheadle, 1955), suggesting that this feature can be taken as a starting point in a discussion of phloem cell arrangements.
Cell differentiation patterns within radial files of phloem are proposed to reflect the spatiotemporal patterns of cell division within the phloem domain of the vascular cambium (Barlow and Lück, 2004). Here, the cell division system specifies the relative locations of cells within the radial files and the duration for which any location is occupied by a cell. Also important for phloem cell determination and development are radial gradients of morphogens such as auxin (Uggla et al., 1998) and sucrose (Warren Wilson, 1978).
Suggestive of a cell lineage-cell differentiation hypothesis is the regularity of the sequences of cell types within the radial files of secondary phloem of Cupressaceae (Abbe and Crafts, 1939; Bannan, 1955). Denoting the relevant cells as S, sieve cell; P, parenchyma; and F, fiber, the standard radial sequence of differentiated cells in the Cupressaceae is the quartet (F S P S), which is repeatedly produced from the cambium during the development of the secondary phloem. How can this sequence be derived in a way that the proposed solution also has physiological plausibility?
In brief, cellular fates can be approached using a theoretical system that generates files of cells with particular sequences of interdivisional durations (Barlow and Lück, 2004). Using this principle together with positional values specified by a morphogen gradient, the radial quartet (F S P S) described for Cupressaceae can be simulated. The derivation of this standard sequence is shown in Fig. 14.10. Commencing with cell I, the first division at timestep 0 produces a new I cell and a mother cell M. At timestep 1, cell M divides to produce a new M cell and another cell, which, at the conclusion of timestep 2, divides to produce inner and outer daughter cells, which later differentiate as a parenchyma cell P and a sieve cell S, respectively. The precursor of fiber cell F is produced at timestep 4. The meristem extends radially beyond the initial I for one or two cells.
Figure 14.10. Cell genealogy interpreting the standard recurring quartet of cell types (F S P S) within developing radial files of secondary phloem in the Cupressaceae. All divisions shown are periclinal, though transverse divisions could occur when there is a timestep without a periclinal division. The various cell types are determined according to the positions occupied within a morphogenic gradient across the phloem. This gradient is indicated by shading; the denser the shading, the higher the level of morphogen. F = fiber; P = parenchyma; S = sieve cell; I = initial cell; M = phloem mother cell.
All the radial sequences of differentiated secondary phloem cells mentioned by Bannan (1955) for Thuja occidentalis (Cupressaceae) can be generated in this way using, as the criterion for cell determination, the summation of the positional values that occurs as the cells are displaced through the meristem and immediately postmitotic zone (Barlow and Lück, 2004). The standard Cupressaceae-type sequence (F S P S) predominates as long as steady conditions apply. Variant sequences such as (F S P S P S) occur as a result of alteration to the duration of the interdivisional period in the initial cell I with respect to its cell productions into the phloem domain. One perturbation may, for example, be that the initial cell was diverted towards xylem cell production and a new initial cell was derived from a mother cell. But other questions remain. How, for example, do the repeating quartets of cells keep in register across the neighboring files, thereby resulting in a tangentially banded appearance of phloem cells, as is evident in the Cupressaceae? Do groups of initial cells divide in synchrony, or does some additional positional information regulate the outcome of cambial divisions? Similar considerations apply to the secondary phloem of the Pinaceae, whose species display repeating sequences of cells composed of P and S cells only (Barlow and Lück, 2004).
With regard to angiosperms, the types of cells differentiated within the radial files of the secondary phloem are slightly more varied than those of conifers, notably by the inclusion of a companion cell C. Nevertheless, the way in which differentiation in the two groups of trees is regulated in time and space in accordance with morphogen-related positional values is probably no different in principle. For example, the recurrent standard radial cellular sequence (F S P S) characteristic of Cupressaceae is also found in Robinia pseudoacacia (Derr and Evert, 1967) but with multiple copies of each of these cell types. In the secondary phloem of Liriodendron tulipifera, Cheadle and Esau (1964) described a consensus recurrent sequence (F F F P S C S C P S C). It, too, can be derived as the consequence of a particular cell division system with a phloem meristem up to four cells wide. The division that produces companion cell C is an innovation, suggesting the presence of supplementary morphogenic information. For example, although C cells can occupy various positions within the phloem, when they are in the vicinity of a ray, they generally make contact with a ray cell (Esau, 1969).
Alternative cell fates might be associated with different cell division patterns in different locations around the cambial perimeter. For example, Zee (1968) deduced two principal sequences of periclinal and radial divisions, in secondary phloem of pea (Pisum sativum) epicotyl, as well as an occasional third pathway. Whether a given radial file of the phloem consistently divides according to one or other of the first two pathways, or whether the pathways alternate within a single radial file, is not known.
During the phylogeny of phloem, it seems that there has been a move away from a strict stereotypical division pattern as an accompaniment of histogenesis (viz. conifers) to one where variations of division pattern are permitted (viz. angiosperms). Moreover, transverse divisions in the phloem may also promote diversity of cell types. Whereas it has been proposed (Barlow and Lück, 2004) that phloem fibers in the Cupressaceae are formed as part of a predetermined sequence of divisions within each radial cell file, in angiosperms (as in hybrid aspen) phloem fibers adopt patterns that seem governed by properties inherent to the tissue unit, as well as by those that pertain to the smaller scale of the cell file. The same idea may also be relevant to the question of whether radial cambial cell files have distinct outputs, in terms of the cells differentiated, which in turn relates to the position of the initial cells on the cambial perimeter. Although in many species phloem production precedes that of xylem at the start of the growing season (e.g., the mentioned example of Robinia studied by Derr and Evert, 1967), and for which environmental (Wareing and Roberts, 1956; Barlow, 2004) and endogenous hormonal controls (Digby and Wareing, 1966) may play important roles, the question remains of how a preferential direction of cell production from a potentially bidirectional cambial initial could be regulated.
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Senescence in Secondary Xylem
Rachel Spicer , in Vascular Transport in Plants, 2005
Analogous Cessation of Assimilate Transport in Secondary Phloem
Secondary phloem maintains living parenchyma cells for a number of years after conductive elements have ceased to function, much like secondary xylem. Extensive callose deposition (sometimes termed definitive callose) in sieve elements marks the end of their functional lifespan. In woody plants this can range from one (e.g., in Pyrus; Evert, 1963a, 1963b) to several (e.g., Tilia and Vitis; Esau, 1948, 1950) years and is accompanied by death of companion cells (in angiosperms) and albuminous cells (in conifers), as well as the death of some parenchyma after the breakdown of starch. However, most parenchyma stay alive for several years and continue to store starch, proteins, and polyphenols (Schneider, 1955; Evert, 1963b); some parenchyma may live for 20 or more years (Grillos and Smith, 1959). These cells remain alive until the development of a new phellogen (Esau, 1965), and ultimately are sloughed off as outer bark.
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Conifer Defense and Resistance to Bark Beetles
Paal Krokene , in Bark Beetles, 2015
2.1.4.3 Resin-producing Structures
The secondary phloem of all members of the pine family contains preformed resin structures, in the form of resin cells or radially oriented resin ducts. Resin ducts are long intercellular spaces lined with plastid-enriched epithelial cells that produce and secrete resin into the duct lumen, where it is stored under pressure ( Charon et al., 1987; Gershenzon and Croteau, 1990; Nagy et al., 2000). Resin ducts in the secondary phloem are always oriented radially, and are located within the multiseriate radial rays (Fahn et al., 1979). Resin ducts form schizogenously as the epithelial cells pull apart during resin duct formation (Nagy et al., 2000). The simpler resin cells accumulate resin internally under pressure and may expand into quite large structures (Figure 5.4E). The epithelial cells lining the resin ducts are usually thin-walled and long-lived, in contrast to the epithelial cells of resin cavities, which are short-lived and gradually become lignified during development (Bannan, 1936; Fahn, 1979).
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Apical Dominance and Some Other Phenomena Illustrating Correlative Effects of Hormones
Lalit M. Srivastava , in Plant Growth and Development: Hormones and Environment, 2002
3.2. IAA and Gibberellins May Regulate the Production of Secondary Xylem and Phloem
The activation of cambium and the differentiation of secondary xylem and phloem can also be studied in cut stem segments of annuals with secondary growth (e.g., tomato, bean) or trees, such as birch (Betula), poplar (Populus), and black locust (Robinia). If tree samples are taken in the fall, it is essential to give them a cold treatment at 2-5°C for several weeks to break their winter dormancy. These samples, given appropriate IAA and/or gibberellin, treatment, show that while IAA and GA both promote cambial reactivation, IAA favors xylem and GA favors phloem differentiation (Fig. 14-40).
FIGURE 14-40. Activation of cambium and differentiation of xylem and phloem in stem segments of Robinia pseudoacacia (black locust). (A) Experimental protocol. Branches were sampled in the fall and placed in a cold room for 4-6 weeks to break winter dormancy. They were cut in 10- to 12-cm lengths and placed, morphological side up, in small petri plates with some water, and different concentrations of GA3 and/or IAA in lanolin were applied to the top end. After 4 weeks, sections were obtained from the segments, using a sliding microtome, and stained for xylem and phloem. (B) Cross sections of stem segments: (a) 100 μM GA 3 , (b) lanolin only, and (c) 100 μM IAA. Note that application of both IAA and GA promotes cambial activation, but IAA promotes xylem differentiation and GA promotes phloem differentiation.
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Xylogenesis in Trees: From Cambial Cell Division to Cell Death
Ryo Funada , ... Satoshi Nakaba , in Secondary Xylem Biology, 2016
Formation of cell wall
As soon as cambial cells lose the ability to divide, they start to differentiate into secondary phloem or xylem cells. The stages in the development of secondary xylem cells can be categorized as follows: cambial cell division, cell expansion or elongation, cell wall thickening, cell wall sculpturing (formation of modified structure), lignification, and cell death (cell autolysis) ( Funada, 2000, 2008). Fusiform cambial cells differentiate into longitudinal tracheids, vessel elements, wood fibers, and axial parenchyma cells, while ray cambial cells differentiate into ray parenchyma cells and, in some conifers, such as Pinus and Larix, ray tracheids. Cells derived from fusiform cambial cells increase in length and in diameter as they approach their final shape during differentiation (Kitin et al., 1999, 2001). For example, vessel elements and earlywood tracheids increase only slightly in length but they increase considerably in radial diameter.
Since tracheids or ray parenchyma cells derived from fusiform cambial cell or ray cambial cells are aligned in a radial direction, successive aspects of xylogenesis can be observed in a radial file within a single specimen. Thus, cambial derivatives are a suitable system to follow the process of differentiation of secondary xylem cells in situ.
The pressure of the protoplast against the cell wall (turgor pressure) within cells originates from the vacuole. It provides the driving force for the enlargement of cells in plants. The increase in the volume of the vacuole is derived from a gradient in the water potential between the cytoplasm and vacuole and the apoplast. When the turgor pressure in the cell exceeds the yield point of the cell wall, the cell can expand or elongate. As the cell expands or elongates, the cell wall becomes stiffer and, consequently, its yield point increases. Finally, cell expansion or elongation ceases.
The very thin and plastic cell wall that is characteristic for the stage of cell enlargement is called the primary wall. Cellulose that is highly crystalline and has very high tensile strength is the major component of the cell wall. Thus, cellulose microfibrils form a framework in the cell wall. The primary wall consists of loose aggregates of cellulose microfibrils (Abe and Funada, 2005). This structure allows expansion of the xylem cells derived from the cambium.
The orientation of cellulose microfibrils of the radial walls in differentiating tracheids changes during cell expansion (Abe et al., 1995b). The cellulose microfibrils on the innermost surface of the primary wall are not well-ordered. Most of cellulose microfibrils in the tracheids at the early stage of cell expansion are predominantly oriented longitudinally. Longitudinally oriented cellulose microfibrils might act to restrain the longitudinal elongation due to turgor pressure. Therefore, longitudinally oriented cellulose microfibrils in the primary wall of the fusiform cambial cells serve first to facilitate lateral expansion. As the cell expands, the predominant orientation of cellulose microfibrils on the innermost surface changes from longitudinal to transverse. At the final stage of cell expansion, cellulose microfibrils are oriented transversely to the cell axis. These observations suggest that it is not necessary to adopt the multinet growth hypothesis to explain the difference in orientation of cellulose microfibrils between the outer and inner parts of the primary wall in tracheids.
When cell expansion in differentiating tracheids is almost complete, well-ordered cellulose microfibrils are deposited on the inner surface of the primary wall, establishing the deposition of secondary wall (Abe et al., 1997; Abe and Funada, 2005). Once the formation of the secondary wall has begun, no further radial expansion of tracheids occurs. The secondary xylem cells of woody plants, such as tracheids, wood fibers, and vessel elements, have cell walls with a highly organized structure. Continuous deposition of the secondary wall increases the thickness of the cell wall. The thickness of the cell wall varies depending on cell function, cambial age, and the season at which the cell is formed, such as earlywood or latewood (Fig. 2.3a). In general, cells that function to support the tree, such as tracheids and wood fibers, form thick secondary walls. Thus, the ultrastructure of tracheids and wood fibers is of great importance to define the mechanical properties of wood. The cell wall supports the heavy weight of the tree itself and functions in the transport of water from roots to leaves, which can sometimes reach more than 100 m in height. In addition, the cell wall prevents microbial and insect attack, thereby protecting the tree during its very long life that, in some cases, can exceed several thousand years. In addition, the thickness of the cell wall of wood fibers varies depending on species (Fig. 2.3b).
Figure 2.3. Scanning electron micrographs of transverse section showing earlywood–latewood tracheids of Chamaecyparis obtusa (a) and wood fibers of Ochroma lagopus (b).
Arrows indicate cell walls. Scale bars = 20 μm (a) and 10 μm (b).
Courtesy of Dr Y. Sano.During formation of the secondary wall in tracheids or wood fibers, the cellulose microfibrils change their orientation progressively from a flat helix (S1 layer) to a steep Z-helix (S2 layer) in a clockwise rotation when viewed from the lumen side of cells. They are oriented at about 5–30° with respect to the cell axis. No cellulose microfibrils with an S-helix are observed during formation of the S2 layer. This shift in the angles of cellulose microfibrils is considered to generate a semihelicoidal structure (Prodhan et al., 1995; Abe and Funada, 2005).
The cellulose microfibrils of the S2 layer are closely aligned with a high degree of parallelism. When the rotational change in the orientation of cellulose microfibrils is arrested, a thick cell wall is formed as a result of the continuous deposition of cellulose microfibrils. The thickness of the secondary wall is important in terms of the properties of wood because it is closely related to the specific gravity of wood. The duration of the arrest in the orientation of cellulose microfibrils determines the thickness of the S2 layer and, thus, the thickness of the secondary wall.
At the final stage of the formation of the secondary wall, the orientation of newly deposited cellulose microfibrils changes from a steep Z-helix to a flat helix with counterclockwise rotation when viewed from the lumen side of cells. This corresponds to a directional switch in the orientation of the cellulose microfibrils from clockwise to counterclockwise, when viewed from the lumen side, during formation of the secondary wall. The deposition of cellulose microfibrils in a flat helix results in the S3 layer. The cellulose microfibrils in the S3 layer are deposited in bundles. This texture differs from that of the S2 layer where the cellulose microfibrils have a high degree of parallelism. The shift in angles of cellulose microfibrils is more abrupt during the transition from the S3 to the S3 layer than that from the S1 to the S2 layer (Abe and Funada, 2005). The rate of change in the orientation of cellulose microfibrils determines the structure of the cell wall layer.
A schematic model of the orientation of newly deposited cellulose microfibrils in a tracheid is shown in Fig. 2.4. The direction of orientation of cellulose microfibrils changes progressively with changing speed of rotation during the formation of the secondary wall (Funada, 2008).
Figure 2.4. A schematic model of cell wall structure in a tracheid.
The orientation of newly deposited cellulose microfibrils in primary wall (P) and secondary wall (S1, S2, and S3 layers).
Cellulose is synthesized by enzyme complexes (terminal complexes) in the plasma membrane (Kimura et al., 1999). Observations in a wide variety of plant cells have revealed that cortical microtubules, one of cytoskeletons, play an important role in the orientation of newly deposited cellulose microfibrils (Giddings and Staehelin, 1991; Nick, 2000; Baskin, 2001; Funada, 2000, 2002, 2008; Funada et al., 2000). It has been postulated that cortical microtubules that are closely associated with the plasma membrane, guide the movement of terminal complexes because coalignment of cortical microtubules and newly deposited cellulose microfibrils has been often observed in the cells of lower and higher plants (Giddings and Staehelin, 1991). In addition, microtubule-depolymerizing agents, such as colchicine, disrupt the orientation of cellulose microfibrils. Moreover, direct visualization of cellulose synthase (CesA) in living cells of transgenic plants of Arabidopsis thaliana revealed that the CesA complexes moved in plasma membranes (Paredez et al., 2006). In addition, the movement of CesA complexes in linear tracks was coincident with cortical microtubules. These observations support strongly the hypothesis that cortical microtubules control the movement of cellulose synthase complexes in plasma membrane.
During the formation of the secondary wall after the cessation of cell expansion, the abundant cortical microtubules are aligned in well-ordered arrays. Successive changes in the orientation of cortical microtubules are observed in differentiating tracheids or wood fibers during the formation of secondary walls (Fig. 2.5a and b) (Abe et al., 1994, 1995a; Prodhan et al., 1995; Furusawa et al., 1998; Chaffey et al., 1997a, 1999, 2002; Begum et al., 2012a). The orientation of cortical microtubules changes by clockwise rotation from a flat S-helix to a steep Z-helix when viewed from the lumen side. This shift in the direction of cortical microtubules is completed within three or four tracheids or wood fibers in a radial file. Then, the cortical microtubules are oriented in a steep Z-helix at almost the same angle over the next 10–15 tracheids or wood fibers of the radial file. After further differentiation, the orientation of cortical microtubules returns from the steep Z-helix to a flat S-helix in tracheids or wood fibers. This shift is completed within one or two tracheids or wood fibers in a radial file.
Figure 2.5. Immunofluorescence images obtained by confocal laser scanning microscopy showing the orientation (a and b) and localization (c) of cortical microtubules, viewed from the lumen side of cells.
Successive changes in the orientation of cortical microtubules (arrows) from a flat S-helix to a steep Z-helix (a) and from a steep Z-helix to a flat S-helix (b) during formation of the secondary wall in differentiating tracheids of A. sachalinensis. Bands of helically oriented cortical microtubules (arrow heads) are visible at the final stage of formation of the secondary wall in differentiating tracheids of T. cuspidata (c). Scale bars = 50 μm.
These observations provide strong evidence that the orientation of cortical microtubules changes progressively in a similar manner to the changes in the orientation of newly deposited cellulose microfibrils during the formation of the secondary wall. Thus, there is a very close relationship between cortical microtubules and newly deposited cellulose microfibrils. The cortical microtubules control the ordered orientation of cellulose microfibrils in the semihelicoidal cell walls of tracheids or wood fibers.
During the formation of the secondary wall in tracheids or wood fibers, the orientation of cortical microtubules changes abruptly from a steep Z-helix to a flat S-helix in contrast to the gradual change from a flat S-helix to a steep Z-helix. As shown in Fig. 2.4, the shift in angles of newly deposited cellulose microfibrils is more abrupt during the transition from a Z-helix to a flat S-helix (from the S2 to the S3 layer) than the transition from a flat S-helix to a steep Z-helix (from the S1 to the S2 layer). The velocity of reorientation of microtubule might be closely related to the reorientation of newly deposited cellulose microfibrils. Therefore, the rotational motion of cortical microtubules reflects the thickness of intermediate layers and the structure of the secondary wall.
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Genetic Engineering for Secondary Xylem Modification: Unraveling the Genetic Regulation of Wood Formation
Jae-Heung Ko , ... Kyung-Hwan Han , in Secondary Xylem Biology, 2016
Secondary growth and wood formation
During secondary growth, cell division in the vascular cambium and subsequent cell differentiation result in the production of secondary xylem and phloem elements. The vascular cambium normally consists of 5 to 15 cambium initial cells occurring as a continuous ring of cells between the xylem and the phloem throughout the length of fully expanded shoots and roots (the so-called cambial zone) (Larson, 1994; Mauseth, 1998) (Fig. 10.1). Two types of initials are present in the cambium: (1) the fusiform initials leading to the axial system and (2) the ray initials, which produce the cells that differentiate into the system of rays throughout the wood of the stem (Lev-Yadun and Aloni, 1995). These initials serve as a conduit for radial (across the cambium) and longitudinal (along the cambium) transfer of developmental signals and nutrients. Adjusting to the demands of water transport required by the leaf biomass and of the mechanical strength necessary to support the crown and to withstand wind forces (Zimmermann and Brown, 1971), cambial growth promotes an increase in stem enlargement by the production of functional vascular elements through radial (or anticlinal) and tangential (or periclinal) divisions (Catesson et al., 1994). Diameter growth is also coordinated with changes in crown architecture and plant height (Larson, 1963), indicating a signaling system that integrates these growth responses. The exact molecular mechanisms underlying the regulation of cambial growth have not been elucidated.
Figure 10.1. Cross-section of a poplar stem showing the organization of the cambial region and wood formation progress.
The bars above the stem section describe approximate regions of indicated developmental tissues. Vascular cambial zone has meristematic cells (i.e., fusiform initials and ray initials), which produce phloem mother cells outside and xylem mother cell inside. Sequential wood formation stages are shown. PF, phloem fiber; XV, xylem vessel; XF, xylary fiber; R, ray cell. Poplar stem (hybrid aspen clone 717 INRA) cross-sections stained with Calcofluor, auramine O, and propidium iodide were observed using confocal laser microscopy. Scale bars represent 200 mm.
Wood is produced by the successive addition of secondary xylem, which differentiates from the vascular cambium (Plomion et al., 2001). For wood formation, the cells on the xylem side of the cambium pass through four sequential developmental stages: (1) division of the xylem mother cells, (2) expansion of the derivative cells to their final size, (3) lignification and secondary cell wall formation (i.e., cell maturation), and (4) programmed cell death (Uggla et al., 1996, 1998; Chaffey, 1999) (Fig. 10.1). The resulting mature secondary xylem includes xylem parenchyma, fibers, vessels, and tracheary elements. This development of secondary xylem (i.e., xylogenesis) appears to be regulated by positional information that controls the cambial growth rate by defining the width of the cambial zone and, therefore, the radial number of dividing cells. Growth regulators, such as auxin, may be the source of this positional information (Wolpert, 1996; Bhalerao and Fischer, 2014), given IAA's polar basipital transport and the reported correlation of the IAA concentration gradient with cambial growth rate (Uggla et al., 1998). Gibberellin and the activation of its signaling pathway have also been shown to directly stimulate xylogenesis in Arabidopsis (Ragni et al., 2011).
Simultaneous increases in the radial number of dividing cells and the rate of cambial cell division result in increased productivity. Cambial growth and the subsequent differentiation of its derivatives appear to be under strict spatial and temporal control (Larson, 1994). Therefore, the quantity and quality of the final wood product is determined by a patterned control of numbers, places, and planes of cambial cell division, and a subsequent coordinated differentiation of the cambial derivatives into xylem tissues (Mauseth, 1998). This patterned growth requires that every cell must express the appropriate genes in a tightly coordinated manner upon receipt of positional information. As this regulation is under strong genetic control (Zobel and Jett, 1995), it should then be possible to genetically manipulate the quality and quantity of wood that is produced. Environmental factors, such as temperature, early season drought, and photoperiod, also affect wood formation, cell enlargement, and secondary wall thickening (Antonova and Stasova, 1997; Arend and Fromm, 2007).
While several plant hormones have been implicated in the regulation of wood formation, auxin appears to serve as a positional signal for the production of xylem and phloem by the vascular cambium (Little and Sundberg, 1991; Uggla et al., 1996, 1998; Sachs, 2000; Leyser, 2006; Bhalerao and Fischer, 2014). While gibberellins (GAs) are required for longitudinal growth (Wang et al., 1995). Uggla et al. (1996) observed a steep radial gradient of auxin across the cambial region in Pinus sylvestris, indicating that auxin acts as a positional signal that informs cambial derivatives of their radial position and regulates cambial growth rate by determining the radial population of dividing cambial-zone cells. In the presence of cytokinin, auxin induces xylem tracheary element differentiation in suspension culture cells of Zinnia (Fukuda, 1997). Klee et al. (1987) observed that auxin-overproducing transgenic petunia plants doubled in the amount of xylem and phloem production. Locally applied auxin can induce the formation of new vascular strands from parenchymatic cells (Sachs, 1981). Downregulation of auxin efflux carriers reduced auxin polar flow and consequently vascular cambium activity in the basal portions of the inflorescence stems (Zhong and Ye, 2001). Several Arabidopsis mutants with auxin transport or signaling defects show apparent interference with various aspects of vascular development (Hardtke and Berleth, 1998; Berleth and Sachs, 2001; Ko et al., 2004). The notion of auxin serving as a positional signal for wood formation, given its basipital movement, is consistent with the observation that stem-diameter growth is often greatest within the young crown and decreases gradually down the stem in forest trees.
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Special Features of Plant Development
Lalit M. Srivastava , in Plant Growth and Development: Hormones and Environment, 2002
3.2.3. Secondary Growth and Vascular Cambium
In gymnosperms and woody dicots, a vascular cambium makes its appearance in that region of root or stem that has ceased elongating and produces secondary xylem and phloem. The addition of secondary vascular tissues, especially xylem, adds to the girth of these organs and provides the needed structural support to trees. Small amounts of secondary growth may also occur in some species in petioles and midveins of leaves and in axes that bear flowers, but because these organs have only a limited life span, it is never extensive. Many herbaceous dicots also develop a cambium, but it may not form a complete ring and its activity may be restricted to the vascular bundles.
The vascular cambium is a layer of meristematic cells (or initials) that arises between primary xylem and phloem. Although it is a single layer of cells, in actual practice it is difficult to distinguish that layer from its immediate derivatives on either side. Hence, the term cambial zone is used (Fig. 1-14A). With few exceptions, the cambium consists of two types of initials; the fusiform and ray initials (Fig. 1-14B-D). Fusiform initials are elongated cells that divide periclinally and give rise to axially elongated cells in the xylem and phloem, i.e., is, tracheary cells, sieve elements, fibres, and parenchyma cells or vertical files of parenchyma cells, called parenchyma strands. Ray initials are more or less isodiametric and occur in clusters that appear spindle shaped in tangential sections. Ray initials give rise to xylem and phloem rays, which extend radially into the xylem and phloem and provide for the radial transport of water, minerals, and photoassimlate.
FIGURE 1-14. (A) Cross section of a pine (Pinus sp.) stem showing the location of the vascular cambium, secondary xylem, and secondary phloem. Tangential longitudinal sections through cambia of three woody trees, pine (B), birch (Betula sp.) (C), and black locust (Robinia pseudo-acacia) (D), showing the arrangement and orientation of the fusiform and ray initials. Note that in pine and birch the fusiform initials have ends that overlap with each other, whereas in black locust they are in tiers one upon another. Cambia with the former type of arrangement of fusiform initials are referred to as nonstoried cambia, whereas those with latter type of arrangement are referred to as storied cambia. Also note the differences in the width and the height of rays in the three species.
Reproduced with permission from Arnoldia (1973).The vascular cambium originates in roots and stems in slightly different locations (for origin in stems, see Fig. 1-1), but eventually in woody plants it forms a complete ring—it extends up and down the stem or root like a cylindrical sheath. How this sheath of cells with two distinct types of initials and a specific spatial arrangement comes to originate in procambial strands has not been studied closely and the details of transition are unknown.
Procambial strands are composed of narrow elongated cells. In dicots and gymnosperms, some of these cells escape differentiation as primary xylem or phloem cells and are left in a potentially meristematic state. Most likely, some of these cells become committed as fusiform initials, which, likewise, are elongated cells, whereas others give rise to ray initials after divisions. The actual process is probably more complicated and occurs over some time, but eventually results in the conferment of a new polarity, which is unique to cambium. Cambial cells divide in a strict periclinal plane and give rise to derivatives whose destinies are predetermined as xylem or phloem cells.
Cambium is not, however, a static cell layer placidly cutting out derivatives on each side, which differentiate as xylem and phloem cells; rather it is a seat of constant and dynamic change in interrelationships among fusiform and ray initials. In addition to dividing periclinally, cambial initials also divide periodically in an anticlinal plane (at right angles to the periphery of the stem or root) to add to their numbers and thus cope with the increasing diameter of the wood cylinder, a result of their own activity. In cambia that have been studied in detail, fusiform initials divide anticlinally with much greater frequency than required—far more cells are produced than needed. Excess cells are converted to ray initials by further divisions or they cease dividing and are lost from the cambial ring by differentiating as xylem or phloem cells. As a result, interrelationships among cambial initials are constantly changing and confer upon the cambium an added measure of plasticity. Such plasticity is useful in accommodating pathogens, such as mistletoe, which draw nutrients from host xylem and/or phloem, or in producing more wood on one side to cope with gravity or other environmental stresses, such as snow drifts and leaning boulders.
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Molecular Breeding of Woody Plants
Eric P. Beers , Chengsong Zhao , in Progress in Biotechnology, 2001
Vascular proteases
Secondary growth is evident in the root-hypocotyl of 8-day-old Arabidopsis seedlings 11 . By 14 days, the procambium-derived vascular cambium is producing secondary xylem internally and secondary phloem externally and the pericycle-derived cambium is also active. By 6 to 8 weeks of age, the cambium in the root-hypocotyl of Arabidopsis, grown as described here and originally by Lev-Yadun 14 , is a continuous lateral meristem producing secondary xylem internally and secondary phloem externally 14, 15 .
From cDNA libraries constructed from xylem and bark isolated from the root-hypocotyl of 8-week-old plants we cloned two full-length cDNAs predicted to code for two closely related papain-like cysteine endopeptidases (XCP1 and XCP2) and one full-length cDNA predicted to code for a subtilisin-like serine endopeptidase (XSP1) l5. An additional papain-like enzyme possessing a granulin-like C-terminal extension, XBCP3, was also cloned. Using XSP1 as a marker for TE differentiation, competitive RT-PCR was conducted using RNA from 2-, 4-, 6- and 8-week-old Arabidopsis roots. The results shown in Figure 1 indicate that the highest level of gene expression associated with TE differentiation occurs in 4-week-old roots and is nearly 11 -fold greater than that observed for 8-week-old roots. These results indicate that 4-week-old roots may be better subjects for evaluation of TE-associated gene expression than the 8-week-old organs used to construct xylem and bark cDNA libraries 15
Figure 1. Quantitative RT-PCR for XSPI expression in roots from 2-, 4-, 6- and 8-week-old Arabidopsis. Levels of cDNA, relative to that for week-8 set at one unit, obtained from RNA isolated at the weeks indicated are shown. Quantitative RT-PCR was performed as described in Zhao et al. 15 .
Quantitative RT-PCR for various tissues and organs indicates that the expression levels for XCP2 are 10 to 20-fold higher than those observed for XCP1 15 . This is consistent with the observation that XCP2 promoter-GUS plants show GUS activity that is predictive (i.e., detectable prior to visible thickening of secondary cell walls of TEs) of tertiary vein positioning, while XCP1 promoter-GUS plants show activity only in late stage TEs (Table 1). In addition to TEs, both XCP1 and XCP2 promoter-GUS plants show GUS activity at the base of trichomes on young expanding leaves. Immunofluorescence confocal microscopy indicates that XCP1 localizes to TEs (E. Beers, unpublished observation), consistent with the localization of GUS activity for XCP1 promoter-GUS plants.
Table 1. Summary of GUS activity specified by putative promoters for the indicated peptidases isolated from Arabidopsis xylem and bark cDNA libraries. H, hydathodes; T, trichomes; PTE, protoxylem tracheary elements; MTE, metaxylem tracheary elements, STE, secondary xylem tracheary elements; C/P, cambium/phloem; XP, xylem parenchyma.
| Promoter- GUS fusion | Cell or tissue type | ||||||
|---|---|---|---|---|---|---|---|
| H | T | PTE | MTE | STE | C/P | XP | |
| XCP1 | - | + | + | + | + | - | - |
| XCP2 | - | + | + | + | + | - | - |
| XSP1 | - | - | + | + | + | - | - |
| XBCP3 | + | - | - | - | - | + | + |
The papain-like cysteine peptidases described here (XCP1 and XCP2) are typical three-domain zymogens (recently reviewed by Beers et al. 16 ), that exhibit 70% identity at the amino acid level. XCP1 is currently the only papain-like enzyme from among the 28 predicted papain-like enzymes encoded by the Arabidopsis genome for which there is experimental evidence for proteolytic activity. Under acid (pH 5.5) conditions, inactive polyhistidine-tagged proXCPl is apparently autocatalytically processed to yield the active mature form of XCP1 15 . When expressed ectopically in transgenic Arabidopsis, XCP1 is detectable by immunoblot as a 29 kD polypeptide that comigrates with proteolytic activity not detected in control plants (E. Beers, unpublished observation). Independent 35S::XCPI transformants exhibit phenotypes ranging from severely stunted plants to those without obvious abnormalities. Some stunted plants produce curled leaves or leaves that senesce prematurely. High XCP1 levels correlate with phenotype severity. XCP1 has been localized to isolated vacuoles purified from protoplasts prepared from 35S::XCP1 Arabidopsis (E. Beers, unpublished observation).
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Vegetative Growth
Dr. Stephen G. Pallardy , in Physiology of Woody Plants (Third Edition), 2008
Formation and Development of Rays
Some rays form near the pith from interfascicular parenchyma, connecting the pith with the cortex. Other rays originate from the cambium (Lev-Yadun and Aloni, 1995). The rays in the secondary xylem and phloem are produced by periclinal divisions of ray cell initials of the cambium. Ray development involves periodic changes in their number, height, and width as the tree grows.
Most new rays form in very young trees when peripheral expansion of the cambium is maximal. Thereafter the number of rays more or less stabilizes (Larson, 1994). In general ray height increases with tree age as a result of transverse divisions of ray cell initials, fusion of adjacent rays, or addition of segments from fusiform initials. When environmental stresses reduce the rate of cambial growth, the height of xylem rays may be reduced. The reduction occurs when rays are split by intrusion of fusiform initials into rays or as ray initials revert to fusiform initials. Some rays, especially small ones, simply disappear (Larson, 1994).
Ray widths in temperate-zone angiosperms often increase by anticlinal division of initial cells within rays or by merging of rays (Larson, 1994). The widths of multiseriate rays, especially, increase with increasing tree age in very young trees and stabilize thereafter. In tropical trees ray widths tend to increase progressively as trees age (Iqbal and Ghouse, 1985a). Ray widths in tropical trees also vary seasonally, increasing in the quiescent season and decreasing during the season of active growth when rays split (Iqbal and Ghouse 1985b; Larson, 1994).
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