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Simcha Lev-Yadun, University of Haifa-Oranim, Tivon, Israel Article Contents . Introduction . Structure and Development

. Regulation of Development

. Dilatation

Online posting date: 16th May 2011

Bark comprises all the tissues outside the vascular cam- Structure and Development bium of a vascular . The majority of the bark of woody develops from three : the vascular The bark includes primary and secondary , , that gives rise to the secondary phloem, the first periderm, sequent periderms (rhytidome) and tissues phellogen that gives rise to the cork and the dilatation formed by dilatation growth (Esau, 1965, 1969; Roth, 1981; that produces cells to prevent Fahn, 1990; Junikka, 1994). The structure of the bark in cracking when the axis increases in diameter. Bark tissues of a given species is usually somewhat less compli- have a critical role in defending plants from pathogens cated than in the shoot, and the description here refers to the shoot rather than to the . The wealth of known struc- and through their physical and chemical tures and functions of the bark is never found in a single properties. They also defend from environmental hazards species, and there are also many variations at different ages such as sun irradiation, desiccation, wind, flooding, hail, and with changing growth conditions in the same individual snow and even fire by forming a thick cork layer. The bark (Borger, 1973; Roth, 1981; Lev-Yadun and Aloni, 1990). has a critical role in storage and transport of organic Additional variation is expressed following wounding or molecules and in many plants the bark also contributes to pathogen attacks (Borger, 1973). See also: Phloem Struc- photosynthesis. Many of the various defensive and toxic ture and Function substances found in barks are used by humans as medi- In woody plants, two regions are distinguished within cines, and for various industries. Gene exploring in the bark: the inner bark, which is alive and where certain barks is expected to result in many beneficial molecules cells may redifferentiate and become meristematic or for agriculture, medicine, food and industry. change their fate (e.g., parenchyma cells that turn into ); and the outer, dead bark cut off from live tissues by dead isolating cork layers – the rhytidome. We distinguish between primary bark (originating from primary meristems – protoderm, ground meristem and procambium) and secondary bark (originating from sec- Introduction ondary meristems – the , the phellogen and dilatation meristem) (Esau, 1969; Fahn, 1990). Bark, which includes all tissues formed outside the vascular The contribution of the primary meristems to the pri- cambium, is structurally, physiologically and functionally a mary bark is as follows. The protoderm gives rise to the very complex part of the plant. The major functions of the , which may exist for many years or may be bark are translocation and storage of organic materials, replaced by cork (periderm). The ground meristem gives water storage and wound healing, protection from herbi- rise to the cortex, made of parenchyma, collenchyma, vores and pathogens, protection from environmental haz- fibres, sclereids, idioblasts of various types, ducts, ards, and in the shoot, photosynthesis. In many leafless or gum ducts or laticifers. The procambium gives rise to the almost leafless plants, all or most of a plant’s photosynthesis primary phloem, including the primary phloem fibres. In a is performed by the bark. An ecologically important func- number of plants, the border between the cortex and the tion of bark is to protect from fires. Thick barks, being primary phloem is marked by the starch sheath, a layer rich poor conductors of heat, isolate the sensitive live tissues of in starch grains, considered to be homologous with the many species from fires. See also: Epidermis: Outer Cell endodermis of the root. An endodermis, with its typical Layer of the Plant , is known from the shoot of only a small number of plants. In addition to these components, in ELS subject area: Plant Science shoots, traces composed of and phloem cross the cortex before they fuse with the central vascular cylinder How to cite: (Esau, 1965; Fahn, 1990). See also: Cork; Starch and Starch Lev-Yadun, Simcha (May 2011) Bark. In: Encyclopedia of Life Sciences Granules (ELS). John Wiley & Sons, Ltd: Chichester. The vascular cambium (a secondary lateral meristem) is DOI: 10.1002/9780470015902.a0002078.pub2 the origin of the secondary phloem. The distinction

ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net 1 Bark between the primary and secondary phloem is easy in most gymnosperms and dicotyledons since the secondary phloem has a radial component (the vascular rays) in addition to the axial component, whereas the primary phloem does not include rays. However, hundreds of spe- cies produce secondary phloem with no rays as a special adaptation (Larson, 1994; Lev-Yadun and Aloni, 1995). The secondary phloem of many species has many bands of fibres (bast fibres) (Esau, 1965, 1969). When cambial activity produces large amounts of xylem, the phloem is pushed outward, and the old, nonconducting and soft sieve cells collapse and flatten, but the bands of fibres, axial parenchyma, sclereides and rays remain intact (Esau, 1965, 1969; Fahn, 1990). See also: Lateral Meristems; Meristems In many conifers, resin ducts develop in both the primary and secondary bark. They produce resin with species or Figure 1 Cross-section of the stem of a small tree of Calotropis procera showing a microscopic view of the secondary xylem with the pores of the even genotype-specific chemical composition. In many water conducting vessels (red bottom part); live part of the bark with the angiosperms, laticifers or gum ducts develop and produce band of latex-forming ducts in the middle and the outer layers of cork with special and latexes. Wounding or pathogen attacks a typical in the centre (the green stained). result in the differentiation of additional (traumatic) resin ducts in conifers and gum ducts in many dicotyledons old phloem (Fahn, 1990). Although for each organ and (Fahn, 1979; Fink, 1999). See also: Conifers; Gymno- species there is a typical cell layer in which phellogen is sperms; Latex and Laticifers; Plant Gums initiated, there are many exceptions. Genetic, physiological At a certain stage of shoot development, a periderm and environmental factors largely influence the timing and (cork ) may appear. The periderm is a secondary location of phellogen initiation. In some plants, phellogen tissue formed from a secondary meristem – the phellogen, initiation occurs within a short time (only a few days) after also known as . The function of the periderm an organ is formed, but in others it can be delayed for is to isolate and thus protect the live tissues from both biotic decades. In some plants, only a first periderm is formed, and and abiotic damaging factors (Fahn, 1990; Sandved et al., this periderm may continue its activity for the rest of the 1993). In certain trees, the periderm may reach a thickness plant’s life. The best-known case of such periderm is the of dozens of centimetres, but usually it is only several cork of the Iberian peninsula, from which most of millimetres to several centimetres thick. A well-known case the global commercial cork originates (Fahn, 1990). is the giant sequoia of the western USA, in which bark In many woody plants, there is a second stage of cork thickness of mature trees ranges from 25 to 80 cm. The formation – the rhytidome or subsequent periderms. In periderm is composed mostly of cork (phellem) cells, which plants that form subsequent periderms, live parenchyma die after differentiation, and their secondary cell walls cells of the secondary phloem redifferentiate and form an are rich with . The periderm also includes a certain internal zone of phellogen. The activity of this phellogen amount of parenchyma (phelloderm) cells, and some- produces a layer of cork cells that isolate all tissues outward times, for instance in , layers of hard, heavily lignified to them from nutrients and so cause them to die. All tissues sclereids within the phellem zone (Fahn, 1990). As with (cork and other tissues) found outward from the innermost resin and gum ducts, wounding and pathogenesis induces subsequent periderm compose the rhytidome (Fahn, 1990). the formation of additional cork tissues (wound periderm) See also: Parenchyma (Borger, 1973). See also: Cork In several tropical tree species, especially trees of the In many plant species, there are many , which savannah, spines develop on the surface of the trunks (e.g., serve as gas exchange shafts through the almost imper- crepitans). Usually these spines are made of a special meable cork. Lenticels are made of many loosely arranged type of cork that develops from islands of phellogen (Roth, cells that form a continuity of intercellular spaces with the 1981). inner tissues and appear as dots and stripes on the bark In trees, bark tissues usually comprise a much smaller surface (Fahn, 1990; Figure 1). fraction of the volume than . Similarly, the Initials of the phellogen mostly divide outward to pro- effort to study bark development, anatomy and physiology duce phellem (cork) cells, but in many species a small is only a small fraction of that given to wood production. fraction of its cell divisions are inward, to form the par- Thus, we do not know much concerning bark biology. The enchymatic phelloderm. Derivatives of the phellogen are best model plant, Arabidopsis thaliana, has both primary usually arranged in radial files. Initiation of periderm starts and secondary bark, although it is a rather small annual at different distances from the shoot apex and from different and most of its secondary bark tissues are formed in the tissues. The site of phellogen initiation is in the in main root. Certain aspects of bark development, however, the roots, but in the shoot it can initiate in the epidermis, can be studied in this plant. The secondary phloem, which subepidermis or in much deeper layers, such as the cortex or composes most of the inner bark in trees, is formed by

2 ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net Bark cambial activity that usually produces 5–15 times more secondary xylem than phloem. Some species, that is, the cork oak, form growth-rings in the cork. However, dating various events using cork rings is not a common practise. The structure of the periderm influences the morphology of the outer surface of woody plants, especially trees. The bark can be smooth or covered by scales of various shapes and sizes. It can peel in small, medium or very large sheets or blocks and can differ in colour (Borger, 1973; Sandved et al., 1993). The functionality of the specific types of bark structure, morphology, shape and size of scales, colour and chemistry are almost not studied and therefore unknown. See also: Arabidopsis thaliana as an Experi- mental Organism

Regulation of Cork Development

Several environmental and endogenous conditions induce cork formation: submergence in water, direct strong sun irradiation, mechanical and biotic wounding and ageing. All these factors are known to increase the production of gaseous phytohormone ethylene. It was therefore proposed that ethylene is the major activator for phellogen initiation and activity (Lev-Yadun and Aloni, 1990). Since the cork layer formed is almost impermeable to gases, as a cork layer is formed, the inner cell layers are exposed to increasing levels of ethylene, and more cork formation is induced. In many plant species, there is a gap in phellogen initiat- ion around buds in the nodal region, or a cork-free region in stems and branches beneath buds and major veins of . Therefore, it was proposed that the basipetal polar auxin transport inhibits cork formation in these regions (Lev-Yadun and Aloni, 1990). These cork-free regions later Figure 2 Longitudinal tangential section of the bark of a large tree of Ficus enable suppressed buds to develop quickly and form new sycomorus showing a microscopic view of the bark in a region of old branches after damage to the , and not to be dis- phloem where dilatation started. The axial fibres and parenchyma form a net of strands while the radial component, the rays (which are turbed by a heavy, hard layer of cork (Lev-Yadun spindle-shaped), start to dilate. and Aloni, 1993). See also: Plant Growth Factors and Receptors ray dilatation occurs (Esau, 1965, 1969; Roth, 1981; Fahn, 1990). Dilatation may be very irregular, resulting in the Dilatation formation of whirled tissues and various shapes of rays. Similarly, meristematic dilatation zones in various other When trees and or even thick annuals grow in orientations and from other origins occur within the bark diameter, the outer tissues expand to a certain but limited in the process of dilatation (Roth, 1981; Lev-Yadun and degree. With additional diameter growth, there is a danger Aloni, 1992). Dilatation activity results in the production that cracks will be formed, and the sensitive inner tissues of parenchyma, and later, many of these parenchyma cells will be exposed to biotic and abiotic agents that can may redifferentiate to sclereids (Roth, 1981). Dilatation endanger the plant. To avoid the formation of such cracks, activity is suppressed in the lower side of leaning trunks, a special tissue is formed in the outer bark – a dilatation probably by the higher-than-usual auxin levels. Appli- tissue (Esau, 1965, 1969; Roth, 1981; Fahn, 1990). Dila- cation of ethylene, wounding, or environmental conditions tation is the outcome of one or two processes that co-occur that induce ethylene production results in higher dilatation in many species. The first process is dilatation growth as the activity. See also: Phloem Structure and Function result of cell expansion. Certain parenchyma cells of the cortex, of the axial phloem parenchyma or the phloem ray The regulation of traumatic resin and gum parenchyma expand to keep the bark intact (Figure 2). In ducts, and wound cork formation other cases, groups of these cells enter a phase of cell div- isions and form a dilatation meristem. When dilatation At least two plant hormones (ethylene and jasmonic acid), occurs in the phloem rays, a distinct funnel-shaped phloem which are expressed at higher levels than normal following

ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net 3 Bark wounding and pathogen attacks, induce the formation of Fibres in the bark traumatic resin and gum ducts and wound cork. Ethylene, one of the first known plant hormones, was recognised first Two types of fibres are found in the bark (bast fibres). as a major inducer of traumatic resin and gum ducts (Fahn, Some plants, for example, flax, produce only primary 1988). The role of ethylene in inducing cork formation phloem fibres of procambial origin. Others, for example, (Lev-Yadun and Aloni, 1990) and dilatation activity (Lev- hemp, kenaf and many trees produce in addition secondary Yadun and Aloni, 1992) followed. Later, methyl jasmonate phloem fibres of cambial origin (Esau, 1965, 1969; Fahn, was found to be involved in traumatic resin and gum duct 1990). formation (Franceschi et al., 2002; Hudgins et al., 2004). The stress hormone ABA seems to have a role in inducing Uses of bark suberin synthesis (Ginzberg, 2008). The role of other phy- Products made of bark have been important since ancient tohormones and their regulatory networks in these pro- times. Several plants are famous for their bast fibres cesses, if any, is not clear. The general picture emerging is (primary and secondary phloem fibres): flax, hemp, jute, that the three stress/wounding/defence phytohormones ramie and kenaf. Their fibres are used for textiles, cordage, ethylene and ABA are deeply involved in nontraumatic and matting, fishing nets, sails, sacks, paper production, com- traumatic bark formation and methyl jasmonate in trau- posite materials, etc. Before the invention of plastics, these matic bark tissue formation. The role of methyl jasmonate and other plant fibres were of utmost importance. Cork in nontraumatic bark formation is not yet known. The from the cork oak (Quercus suber) and a long list of sec- precise hormonal regulation of bark formation at the ondary cork-based products are used all over the world. molecular level is still far from being understood. Sealing wine bottles is only a small fraction of the uses of cork. hides with bark to produce leather Gene expression in the bark has been practiced for millennia. Before the age of rubber and plastics, leather was extremely important, and its The study of gene expression and genes involved in regu- production commonly needed bark tannins (Hill, 1952). lation of bark formation has practically only begun. We Rubber is mainly produced from the latex exuded from the have only a preliminary understanding of the genetic and artificially wounded bark of H. brasiliensis. Many other molecular processes involved in bark biology (e.g., Roach species produce latex in the bark, but only a small number and Deyholos, 2007; Soler et al., 2007, 2008; Barel and of species are used for rubber production (Loadman, 2005). Ginzberg, 2008; Wildhagen et al., 2010; Duan et al., 2010). Several barks provide chemicals used for medicine. Quin- The chances of finding commercially important molecules ine, the cure for malaria, still a major killer in the tropics, in the bark and the renewed interest in bast fibres have is produced from the bark of several species of the resulted in a recent modest increase in such studies of the . Taxol, a drug used against several types of bark but much more should be done. Genomic studies of cancer, is extracted from the bark of the Pacific yew differentiating cork in the most important source for (Taxus brevifolia). Curare, probably the best-known mus- commercial cork, the cork oak resulted in the classification cle-relaxing poison and a source for medicines, is produced of about 50 genes belonging to the main pathways for cork by mixing barks of several tropical species. Cinnamon, biosynthesis (Soler et al., 2007). The seasonal expression of camphor and several other spices are also produced from some of these genes was also studied and found to show tree barks (Sandved et al., 1993). See also: Plant Secondary highest expression in June, a crucial month for cork Metabolism; Taxol; Vegetable Tannins development in cork oak (Soler et al., 2008). In potato tuber skin, another important model for periderm for- mation, many of the expressed proteins are not only known References plant defence components, but also various enzymes involved in cork formation (Barel and Ginzberg, 2008). Barel G and Ginzberg I (2008) Potato skin proteome is enriched In Hevea brasiliensis, the most important source of with plant defense components. Journal of Experimental Botany natural rubber, induced by bark wounding, the treatments 59: 3347–3357. of wounding, application of methyl jasmonate or ethylene Borger GA (1973) Development and shedding of bark. In: Kozlowski TT (ed.) Shedding of Plant Parts, pp. 205–236. specifically induced the expression of various genes with New York: Academic Press. two genes up-regulated by all three treatments (Duan et al., Duan C, Rio M, Leclercq J et al. (2010) Gene expression pattern in 2010). Autumnal storage of proteins in poplar bark response to wounding, methyl jasmonate and ethylene in the after the re-translocation of amino acids from senescing bark of Hevea brasiliensis. Tree Physiology 30: 1349–1359. leaves is correlated with day length and temperature. Esau K (1965) , 2nd edn. New York: Wiley. Some of these proteins are known to be storage proteins, Esau K (1969) The Phloem, 2nd edn. Berlin: Gebru¨der Borntraeger. some have various physiological functions and the funct- Fahn A (1979) Secretory Tissues in Plants. London: Academic ion of others is not known yet (Wildhagen et al., 2010). Press. A much broader and deeper understanding of the genes Fahn A (1988) Secretory tissues and factors influencing their involved in all aspects of bark biology is expected in the development. Phyton (Austria) 28: 13–26. coming years. Fahn A (1990) Plant Anatomy, 4th edn. Oxford: Pergamon Press.

4 ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net Bark

Fink S (1999) Pathological and Regenerative Plant Anatomy. Roth I (1981) Structural Patterns of Tropical Barks. Berlin: Berlin: Gebru¨der Borntraeger. Gebru¨der Borntraeger. Franceschi VR, Krekling T and Christianses E (2002) Application Sandved KB, Prance GT and Prance AE (1993) Bark. The For- of Methyl Jasmonate on Picea abies (Pinaceae) stems induces mation, Characteristics, and Uses of Bark Around the World. defense-related responses in phloem and xylem. American Portland: Timber Press. Journal of Botany 89: 578–586. Soler M, Serra O, Molinas M et al. (2007) A genomic approach to Ginzberg I (2008) Wound-periderm formation. In: Schaller A suberin biosynthesis and cork differentiation. Plant Physiology (ed.) Induced Plant Resistance to Herbivory, pp. 131–146. 144: 419–431. Berlin: Springer-Verlag. Soler M, Serra O, Molinas M et al. (2008) Seasonal variation Hill AF (1952) Economic Botany, 2nd edn. New York: McGraw- in transcript abundance in cork tissue analyzed by real time Hill Book Company Inc. RT-PCR. Tree Physiology 28: 743–751. Hudgins JW, Christianses E and Franceschi VR (2004) Induction Wildhagen H, Du¨rr J, Ehlting B and Rennenberg H (2010) Sea- of anatomically based defense responses in stems of diverese sonal nitrogen cycling in the bark of field-grown grey poplar is conifers by methyl jasmonate: a phylogenetic perspective. Tree correlated with meteorological factors and gene expression of Physiology 24: 251–264. bark storage proteins. Tree Physiology 30: 1096–1110. Junikka L (1994) Survey of English macroscopic bark terminol- ogy. IAWA Journal 15: 3–45. Larson PR (1994) The Vascular Cambium. Development and Structure. Berlin: Springer-Verlag. Lev-Yadun S and Aloni R (1990) Polar patterns of periderm Further Reading ontogeny, their relationship to leaves and buds, and the control of cork formation. IAWA Bulletin n.s. 11: 289–300. Dusotoit-Coucaud A, Kongsawadworakul P, Maurousset L et al. Lev-Yadun S and Aloni R (1992) Experimental induction of (2010) Ethylene stimulation of latex yield depends on the dilatation meristems in Melia azedarach L. Annals of Botany 70: expression of a sucrose transporter (HbSUT1B) in rubber tree 379–386. (Hevea brasiliensis). Tree Physiology 30: 1586–1598. Lev-Yadun S and Aloni R (1993) Bark structure and mode da Ponte-e-Sousa JCA, de A and Neto-Vaz AM (2011) Cork and of canopy regeneration in trees of Melia azedarach L. Trees 7: metals: a review. Wood Science and Technology 45: 183–202. 144–147. Saveyn A, Steppe K, Ubierna N and Dawson TE (2010) Woody Lev-Yadun S and Aloni R (1995) Differentiation of the ray system tissue photosynthesis and its contribution to trunk growth and in woody plants. Botanical Review 61: 45–88. bud development in young plants. Plant, Cell and Environment Loadman J (2005) Tears of the Tree. The Story of Rubber – A 33: 1949–1958. Modern Marvel. Oxford: Oxford University Press. Schreiber L (2010) Transport barriers made of cutin, suberin and Roach MJ and Deyholos MK (2007) Microarray analysis of flax associated waxes. Trends in Plant Science 15: 546–553. (Linum usitatissimum L.) stems identifies transcripts enriched in Serra O, Figueras M, Franke R, Prat S and Molinas M (2010) fibre-bearing phloem tissues. Molecular Genetics and Genomics Unraveling ferulate role in suberin and periderm biology by 278: 149–165. reverse genetics. Plant Signaling & Behavior 5: 953–958.

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