Conifer Defense and Resistance to Bark Beetles

Paal Krokene , in Bark Beetles, 2015

2.1.5 The Vascular Cambium—a Defenseless Cell Factory

The vascular cambium is the main meristem in the stem, producing undifferentiated wood cells inwards and bark cells outwards. The thickness of the vascular cambium varies from around six cells during dormant periods to around 14 during the most active periods of growth ( Figure 5.4A–C). Being a meristem the cambium consists of flattened, undifferentiated cells. These undifferentiated cells possess no defense capabilities, although the cambium quickly can be reprogrammed to produce cells that are differentiated into PP cells or traumatic resin ducts. Since the cambium itself is defenseless, but crucial for maintaining stem growth and tree integrity, it must be protected by the different defense structures in the secondary phloem, cortex, and periderm.

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From Cambium to Early Cell Differentiation Within the Secondary Vascular System

Peter Barlow , in Vascular Transport in Plants, 2005

Publisher Summary

Vascular cambium of both roots and shoots contains two types of cells: long, spindle-shaped fusiform cells and smaller, cuboidal ray parenchyma cells. Ray initials are regularly interspersed with the fusiform initials on the cambial perimeter and the radially elongated files to which they give rise intrude, like the spokes of a bicycle wheel, into both secondary xylem and phloem. Irrespective of whether they are ray or fusiform cells, cambial initial cells are bidirectional in their cell production. Each initial produces alternating sequences of new cells from either its inward- or outward-facing surfaces that pass into the secondary xylem and phloem domains, respectively. Among the differentiated cells produced by the cambial fusiform cells are those which have become adapted for long-distance vertical transport of solutes (tracheids, xylem vessel elements, and phloem sieve cells) and for the assistance of these processes. Other cells (fibers, and also the tracheids) are adapted for the mechanical support of the plant. Ray cells also synthesize and transport radially secondary metabolites into the interior of the wood, as well as storing and transporting trophic materials to the cambium. From a mechanical point of view, rays physically bolt together the annual rings of xylem, thus preventing shearing of these groups of cells when the stem is bent. This chapter highlights the features of the cambial meristem, mainly in trees, that bear on the development of the vertical and radial transport systems of stems and roots and discusses some of the earliest stages of xylem vessel, phloem, and ray development.

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Introduction

Donald E. Fosket , in Plant Growth and Development, 1994

The Vascular Cambium and Secondary Growth

The vascular cambium and cork cambium are secondary meristems that are formed in stems and roots after the tissues of the primary plant body have differentiated. The vascular cambium is responsible for increasing the diameter of stems and roots and for forming woody tissue. The cork cambium produces some of the bark. In dicot stems, the vascular cambium initially differentiates from procambial cells within the vascular bundles (Fig. 1.8A). This fascicular cambium may contribute additional cells to both the xylem and the phloem of the bundle. At some point the cambium expands into the ground tissue between the vascular bundles, forming an interfascicular cambium, completing the ring of vascular cambium (Fig. 1.8B). Cell division by the cambium produces cells that become secondary xylem and phloem. As secondary phloem and xylem tissue accumulates, it both increases the girth of the stem and forms wood and bark. Because cambial activity is seasonal in temperate zone plants, the wood and bark are laid down in distinct annual rings (Fig. 1.8C). Monocots do not have a vascular cambium, even though some of them, such as palms and the Joshua tree, exhibit secondary growth. Instead, they have a thickening meristem that produces secondary ground tissue. This increases the girth of the stem and additional vascular bundles differentiate within the secondary ground tissue.

Figure 1.8. Secondary growth: the origin and structure of vascular cambium in the stem

The vascular cambium is formed in mature dicot stems after stem elongation stops. (A) Primary xylem and phloem differentiate from procambial tissue in the vascular bundles, and a fascicular cambium is formed from procambial tissue separating these tissues. (B) Later, an interfascicular cambium appears between the vascular bundles that is continuous with the fascicular cambium. (C) The further development of the cambium results in the formation of a cylinder of vascular tissue. (D) The vascular cambium is a layer of pluripotent dividing cells whose derivatives differentiate as either xylem elements (vessel members, tracheids, fibers, or xylem parenchyma) or phloem elements (sieve tube members, companion cells, fibers, or parenchyma). (E) The dividing cells of the vascular cambium consist of long, narrow fusiform initials, from which the tracheary elements are derived, and ray initials, from which ray parenchyma is formed.

 Based on Wilson, C. L., and Loomis, W. E. (1967). Botany. Holt, New York. Copyright © 1967

The vascular cambium is composed of two kinds of cells, ray initials and fusiform initials. In cross section these look very similar. Both are small, flattened cells with thin walls. When viewed in tangential section, however, ray initials can be seen to be relatively short, small cells, whereas fusiform initials are very long and narrow (Fig. 1.8D). In gymnosperms the fusiform initials often are several millimeters in length. Dicot fusiform initials are much shorter, but some still are up to 0.5 mm in length. Cell division in the fusiform initials usually is tangential and the cell is partitioned down its long axis, forming two equally long, narrow cells. Some of the cells produced by the cambial initials continue to divide, whereas others differentiate. Tracheary elements or sieve elements differentiate from derivatives of the fusiform initials, and derivatives of the ray initials differentiate as ray parenchyma. The ray parenchyma permits transport of water from the xylem into the cambium and the tissues of phloem, as well as transport of photosynthate from the phloem into the cambium and the living cells of the xylem.

The cork cambium also is a secondary meristem, containing meristematic cells. The cork cambium forms a major portion of the bark of woody plants. The secondary phloem also is part of the bark, but of course phloem is produced by the vascular cambium. The cork cambium first arises within the cortex as a concentric layer forming a cylinder of dividing cells (Fig. 1.9). The derivatives of this meristematic cell layer differentiate as cork, or phellem, toward the outside of the stem, whereas derivatives produced toward the inner part of the stem differentiate as phelloderm. Suberin is deposited in the cell walls of the phellem and they are dead at maturity. They protect the stem from water loss and from mechanical damage. As the tree increases in girth, the outer layers of bark are sloughed off. Additional cork cambia arise within the secondary phloem as the plant develops.

Figure 1.9. Cross section through the stem of a woody dicot showing the development of a cork cambium

 (A) Based on Raven, P. H., and Curtis, H. (1970). Biology of Plants. Worth Publishing Company, New York. (B) Redrawn with permission from Wilson, C. L. and Loomis, W. E. (1967). Botany. Holt, New York. Copyright © 1970

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Introduction to Vascular Plant Morphology and Anatomy

Thomas N. Taylor , ... Michael Krings , in Paleobotany (Second Edition), 2009

Vascular Cambium

The vascular cambium arises between the primary xylem and phloem of a young stem or root. Parenchymatous cells become meristematic and begin to produce secondary xylem or wood toward the inside of the cambium and secondary phloem toward the outside of the cambium. The cambium itself remains meristematic, except in some unusual cases, for example, in the Carboniferous arborescent lycopsids ( Chapter 9) and may range from a single layer to several layers of meristematic cells (FIG. 7.26). If the primary xylem is a solid core, as in some fossils, the cambium begins development as a complete cylinder (a ring, as seen in cross section) between the primary xylem and phloem. If the primary vascular tissue occurs in bundles, as is the case in woody dicots and gymnosperms, the cambium begins development within the bundle—the fascicular cambium. Then, parenchyma cells between the bundles become meristematic—the interfascicular cambium—and connect the fascicular cambia together so that the cambium eventually forms a complete ring around the axis, between the primary xylem and phloem.

FIGURE 7.26. Cross section of Pinus sp. stem showing radial files of vascular cambium initials (C) (Extant). Bar=100   μm.

Cambial cells or initials divide primarily by periclinal divisions (parallel to the surface of the axis) on their inner and outer faces, producing files of cells along the radii of the axis. The presence of these orderly files is one way to distinguish secondary growth in fossil axes. Cambial initials must also divide anticlinally (perpendicular to the surface) to produce more cambial cells as the circumference of the axis continues to increase due to the production of secondary tissue. There are two types of initial cells in the vascular cambium. Fusiform initials are elongate cells that produce the conducting cells in both the secondary xylem and secondary phloem and the other cells in the axial system. Ray initials are shorter, generally rectangular cells, which give rise to cells in the ray system (see section "Secondary Xylem"). Generally, many more secondary xylem cells are produced than secondary phloem; indeed, in most living trees the bulk of the trunk represents secondary xylem or wood.

The vascular cambium in roots arises in the same place as in stems, that is, between the primary xylem and phloem, but since the primary xylem in many roots is lobed or furrowed, the cambium initially also has this shape. As the root continues to develop, however, more secondary xylem is produced in the furrows so that the cambium eventually has a cylindrical shape, just as it does in stems. See section "Secondary Xylem" and "Phloem" (later) for the cell types produced by the vascular cambium.

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The Vascular Cambium of Trees and its Involvement in Defining Xylem Anatomy

Uwe Schmitt , ... Risto Jalkanen , in Secondary Xylem Biology, 2016

Abstract

The vascular cambium of trees is a secondary meristem and is responsible for the formation of the xylem and phloem. The main focus of this chapter is on the xylem, specifically on the following three topics, demonstrating that the cambium is not only responsible for the quantitative side of xylem formation, but also for the expression of stable anatomical features essential for wood identification. In this complex process, we first describe the seasonal cambial activity and its environmental control. Second, we discuss the cambium's involvement in the restoration of tissues after injuries. Third, we examine the cambium-dependent shaping of taxa-specific wood anatomical characteristics. The results are mainly based on light microscopy; however, electron microscopy was also occasionally used to reveal structural features on the cellular level.

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Functional Significance of Cambial Development in Vertebraria Roots

Anne-Laure Decombeix , Nicholas P. Rowe , in Transformative Paleobotany, 2018

4.1 Implications of a Derived Cambial Development

The derived vascular cambium present in Vertebraria resulted in a complex geometrical organization that likely had a significant effect on the functional biology and life history of the whole Glossopteris plant. The ensemble of developmental motifs behind this structural organization in Vertebraria is a remarkable example of how simple changes in developmental timing can lead to (1) a strong departure from a typical anatomical structure, (2) a wide diversity of geometries and shapes between developmental stages, and (3) potentially major changes in mechanical and hydraulic functioning between young and old stages and from the distal to proximal parts of the root system. So just what are the functional implications of these changes? How can they be interpreted at the level of the whole plant? And to what extent can they represent adaptations for life in high-latitude wetlands in the Palaeozoic?

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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

3.1. IAA Is an Important Factor in Reactivation of Cambium in Spring

In temperate climates, vascular cambium becomes dormant in the fall and resumes meristematic activity in the spring. It is commonly assumed that IAA is involved in cambial reactivation, i.e., induction of cell division activity. It has also been assumed that cambial activity proceeds from the top of the trunk to the base, a view that may be derived from the fact that IAA is produced in flushing apical and lateral buds and young shoots and flows basipetally. There is some evidence for a basipetal progression of cambial activation in diffuse porous woods based on bioassays. However, studies on cambia of conifers as well as diffuse- and ring-porous dicot woods, while demonstrating that IAA is required for cell divisions in the cambial zone, do not support the assumption that cambial activation proceeds basipetally in the main trunk. Such basipetal progression is seen only in young parts of a tree, usually the first year's growth; the rest of the trunk is reactivated more or less simultaneously.

Measurements of endogenous IAA in tree trunks at different heights using modern methods of analysis and quantitation are very few. They are also difficult because sampling pieces of bark, cambium, and wood from tree trunks takes time and quick freezing of relatively large samples in liquid nitrogen or isopentane still does not stop the mobility of small molecules and ions instantaneously. Nonetheless, studies have been made and indicate that the situation is more complex than previously realized. A vertical gradient in IAA concentration is seen mostly in young stems and branches and in trees that are growing vigorously. The gradient is not so clear and may even be nonexistent in older stems or in slow-growing trees. Moreover, not all IAA moving down basipetally comes from the shoot apex. Feeding 13C-labeled IAA to a decapitated pine shoot showed isotopic dilution down the trunk, which suggested that at least some IAA in the trunk is synthesized locally at lower levels. Finally, dormant cambium also has significant amounts of IAA, which could be mobilized in spring.

The site of polar transport of IAA in tree trunks is thought to be the cambial zone. It has been mentioned before that it is possible to measure very small quantities of hormones in tissue sections or small samples (see Chapter 5). In several papers, IAA concentrations were monitored in individual tangential sections of a pine stem and data were integrated to give a profile of IAA concentrations in the cambial zone and differentiating and mature secondary xylem and phloem cells on either side (Fig. 14-39). Data show that the highest concentrations of IAA occur in the cambial zone and fall off in a gradient on either side in the differentiating secondary xylem and secondary phloem, with fully mature tissues showing very little IAA.

FIGURE 14-39. Schematic drawing of the specimen block and radial distribution of IAA in the cambial zone and secondary tissues of pine (Pinus sylvestris). (A) Tangential longitudinal sections (30 μm in thickness, using a cryomicrotome at −20°C) were obtained starting from the outer phloem and into the xylem tissue. IAA content was measured in each section (sample) using a modified GC-MS procedure. Transverse sections at ends were used for the determination of sample position. (B) Radial distribution of IAA in two representative trees; one sampled in late June at the height of cambial activity and the other sampled during dormancy in mid-January. Each column represents the 30-μm tangential section. Endogenous IAA content per cm2 section is indicated with black squares. NFP, nonfunctional phloem; FP, functional phloem; CZ, cambial zone; ET and DT, expanding and differentiating tracheids; MT, mature tracheids. The average number of radial file cells in each developmental zone is given on the right.

 With permission from Uggla et al. (1996), ©1996 National Academy of Sciences, USA.

It would be expected that the IAA concentration in the cambial zone at any one location in the trunk would be higher in spring/summer when cambium is actively producing xylem and phloem than in winter when it is dormant. However, the summer and winter samples did not show much seasonal fluctuation, although there was a broadening of the IAA gradient in spring/summer and a narrowing of the gradient in winter (Fig. 14-39B). The presence of IAA in the dormant cambium suggests, by inference, that the cessation of cambial activity in late summer-early fall is not controlled by IAA, a suggestion that is supported by feeding experiments where IAA supplied to shoots does not prevent the cambium from becoming dormant. Environmental factors, such as temperature and shortening daylength, seem to be involved in the induction of cambial dormancy. Although the concentration of IAA did not show much seasonal variation, the active cambium contained a greater amount of IAA than the dormant cambium, which indicates that higher amounts of IAA are produced and utilized, i.e., there is a higher flux of IAA in the cambial zone in the summer months. The observation that the IAA content in differentiating xylem and phloem tissues was about the same is difficult to explain because higher IAA concentrations are known to promote xylem differentiation (see below). It could be that other factors besides IAA, such as sugars and gibberellins, may also control the developmental fate of cambial derivatives.

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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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Plant Morphology

Michael G. Simpson , in Plant Systematics (Third Edition), 2019

Twigs, Trunks, and Buds (Figure 9.7)

Twigs are the woody, recent-growth branches of trees or shrubs. Buds are immature shoot systems that develop from meristematic regions. In deciduous woody plants the leaves fall off at the end of the growing season and the outermost leaves of the buds may develop into protective bracts (modified leaves) known as bud scales. The bud of a twig that contains the original apical meristem of the shoot (which by later growth may result in further extension of the shoot) is called the terminal or apical bud. Buds formed in the axils of leaves are called axillary [axial] or lateral buds.

Figure 9.7

Figure 9.7. Twigs parts and bud types (l.s. = longitudinal section).

A given bud may be vegetative, if it develops into a vegetative shoot bearing leaves; floral or inflorescence, if it develops into a flower or inflorescence; or mixed, if it develops into both flower(s) and leaves. In some species more than one axillary bud forms per node. Two or more axillary buds that are oriented sideways are called collateral buds; two or more axillary buds oriented vertically are called superposed buds. If the original terminal apical meristem of a shoot aborts (e.g., by ceasing growth or maturing into a flower), then an axillary bud near the shoot apex may continue extension growth; because this axillary bud assumes the function of a terminal bud, it is called a pseudoterminal bud.

Several scars may be identified on a woody, deciduous twig. These include the leaf scar, leaf vascular bundle scars, stipule scars (if present), and bud scale scars. Bud scale scars represent the point of attachment of the bud scales of the original terminal bud after resumption of growth during the new season. Thus, bud scale scars represent the point where the branch ceased elongation the previous growing season; the region between adjacent bud scale scars represents a single year's growth in temperate climates, but could be shorter or longer in tropical climates.

Bark technically comprises all the tissue outside the vascular cambium of a plant with true wood (see Chapter 10). The outer bark, or periderm, are the tissues derived from the cork cambium itself. Morphologically, bark may refer to the outermost protective tissues of the stems or roots of a plant with some sort of secondary growth, whether derived from a true cork cambium or not. Bark types are often good identifying characteristics of plant taxa, particularly of deciduous trees during the time that the leaves have fallen. Various bark types include:

1.

Exfoliating, a bark that cracks or splits into large sheets

2.

Fissured, a bark split or cracked into vertical or horizontal grooves

3.

Plated, a bark split or cracked, with flat plates between the fissures

4.

Shreddy, bark coarsely fibrous

5.

Smooth, a non-fibrous bark without fissures, fibers, plates, or exfoliating sheets.

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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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