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Plant Tracheary ElementsHiroo Fukuda, University of Tokyo, Tokyo, Japan

Formation of tracheary elements, which are constituents of vessels and tracheids, is a

model of cell differentiation and programmed cell death in plants.

Introduction

The xylem, which is a tissue specific to the vascular plants,is composed of tracheary elements (TEs), parenchyma cellsand fibres. TEs are characterized by the formation of secondary cell walls with annular, spiral, reticulate orpitted wall thickenings. At maturity, TEs lose their nucleiand cell contents, to leave a hollow tube that is part of avessel or tracheid. Differentiation into TEs from procam-bial initials in situ and fromvarious types of cells in vitro is amodel of cell differentiation and programmed cell death in

plants.

Primary Xylem and Tracheary Elements

The primary xylem develops from procambial initials insitu. Procambial initials originate in the early embryo. InArabidopsis, the procambial primordium, which consists of eight narrow cells, is formed by the late globular stage of the embryo, and it forms the primary xylem tissues in thetorpedo-shaped embryo (Bowman, 1993). During vegeta-tive growth, the apical meristem serves as a continuoussource of the procambial initials. The differentiation of TEs from the procambial initials is divided into fourontogenetic processes: cell origination; cell elongation;secondary wall deposition; and wall lysis and cell death.Xylem differentiation can also be induced by wound stressand/or a combination of phytohormones in vitro. Thecontinuity of the xylem along the plant axis may be a resultof the steady polar flow of auxin from young leaves to roots(Sachs, 1981). Cytokinin from roots may also be acontrolling factor in xylem differentiation.

Cell Differentiation Pathway (Genes,

Cell Death)

Pattern formation of secondary walls

Microtubules determine the wall pattern by defining theposition and orientation of secondary walls, probably byguiding the movement of cellulose-synthesizing rosettes inthe plasma membrane. Microtubule arrays change drama-tically from a longitudinal to a transverse orientation priorto secondary wall formation in Zinnia. Double staining of 

microtubules and actin filaments and treatment witagents that depolymerize cytoskeletons indicate that theris a coordinated mechanism in which actin filaments arinvolved in the reorganization of microtubules which, iturn, regulate the spatial disposition of secondary wall(Kobayashi et al ., 1988). The dynamic change in microtubule organization is accompanied by an increase in thnumber of microtubules due to new expression of some othe tubulin genes.

Deposition of wall materials

During the formation of secondary walls, levels of cellulosand hemicellulose increase and the deposition of pecticeases, and a little later lignin deposition starts. Lignin anpolysaccharides are deposited in association with eacother. At the time of secondary wall formation, rosettes arlocated in the plasma membrane over regions of secondarwall thickenings, showing active synthesis of cellulosthere. Xylan is a major component of the increasehemicellulose and its synthesis is due to an increase i

xylan synthase activity during secondary wall formationSeveral proteins, including an extensin-like proteinarabinogalactan proteins and glycine-rich protein 1.8 arspecifically associated with secondary walls of Te(Fukuda, 1997).

The biosynthesis of lignin involves three pathwayknown as the shikimate, the general phenylpropanoid anthe specific lignin pathways. The general phenylpropanoipathway involves phenylalanine ammonia-lyase (PALcinnamate hydroxylase, O-methyltransferase, and 4-coumarate:CoA ligase (4CL), all of which are expressed iassociation with TE differentiation. Promoters of geneencoding PAL and 4CL share conserved sequence

resulting in the coordinated expression of these genes ithe xylem (Hatton et al ., 1995). Hydroxycinnamoyl-CoAesters, which are final products of the general phenylpropanoid pathway, are converted to their correspondinalcohols, which are polymerized into lignin on secondarwalls. Genes encoding enzymes that catalyse these processes are also expressed coupled with TE differentiation

Article Contents

Secondary article

. Introduction

. Primary Xylem and Tracheary Elements

. Cell Differentiation Pathway (Genes, Cell Death)

. Zinnia as a Model System

. Function in Seedling Growth

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Cell death

In differentiating TEs, the visible degeneration of allorganelles, including the nucleus, starts only after thecollapse of the tonoplast, which occurs several hours afterthe secondary wall thickenings become visible. Followingthe collapse of the tonoplast, organelles such as Golgi

bodies and the endoplasmic reticulum become swollen andthen rupture. The nucleus is also degraded. This degrada-tion is preceded by the tonoplast collapse but not bynuclear shrinkage and fragmentation, which are typicalfeatures of apoptosis. This implies that the disruption of the tonoplast is a critical step towards cell death indifferentiating TEs. Specific hydrolytic enzymes such as a30 kDa cysteine protease, an RNase, and a 43 kDa S1-typenuclease are induced prior to the tonoplast collapse andseem to be accumulated in the vacuole (Fukuda, 1997).Because pH optima of these enzymes are around 5.5, thesehydrolytic enzymes may function in the vacuole or in thecytoplasm after the collapse of the tonoplast. In addition,

proteasomes in the cytoplasm may also be involved in celldeath programme of TEs.

Zinnia as a Model System

Induction of TE differentiation has been achieved in callus,in suspension-cultured cells, and in excised tissues andcells. Among the in vitro differentiation systems, the mostefficient is the system in which single mesophyll cellsisolated from Zinnia leaves can differentiate directly intoTEs without intervening cell division (Fukuda, 1997). Inthe Zinnia system, initiation of differentiation occurs by

wounding and by a combination of auxin and cytokinin.Localized secondary wall thickenings begin synchronouslyat 60 h of culture, and cell death occurs a little later. Theprocess of  in vitro TE differentiation is divided into threestages, I, II and III. Stage I, which immediately follows theinduction of differentiation, corresponds to the functionaldedifferentiation process. This stage occurs only in the caseof in vitro differentiation. In stage II, dedifferentiated cellschange to procambial cells, and then to TE precursor cells.During this stage, some vascular cell-specific genes such asTED2-TED4 are preferentially expressed (Demura andFukuda, 1994). Final determination occurs in the TEprecursor cells and they then enter stage III, involving TE-

specific events such as secondary wall deposition and celldeath. Endogenous brassinosteroids may be involved inthe final determination.

Function in Seedling Growth

The xylem plays a role in the transport of water to all thparts of the seedlings from the roots. TEs are emptied bthe loss of all cell contents to form hollow tubes as pathway for fluids, which is totally different from the casof the blood vessel. Transpiration and root pressure ar

engines for water transport. Because water is sucked up bhigh negative pressure, TEs should have a reinforced cewall to maintain the shape under such high pressure. Thxylem is also important from applied and biotechnologicaperspectives, because biomaterials, such as cellulose anlignin in xylem, represent the predominant part of thterrestrial biomass and therefore play a significant role ithe carbon cycle.

References

Bowman J (1993) Arabidopsis. New York: Springer-Verlag.Demura T and Fukuda H (1994) Novelvascularcell-specificgenes who

expression is regulated temporally and spatially during vascul

system development. Plant Cell 6: 967–981.

Fukuda H (1997) Tracheary element differentiation.Plant Cell 9: 1147

1156.

Hatton D, Sablowski R, Yung M-H, Smith C, Schuch W and Bevan

(1995) Two classes of  cis sequences contribute to tissue-speci

expressionof a PAL2 promoter in transgenictobacco.PlantJournal 

859–876.

Kobayashi H, Fukuda H and Shibaoka H (1988) Interrelationsh

between the spatial disposition of actin filaments and microtubu

during the differentiation of tracheary elements in cultured Zinn

cells. Protoplasma 143: 29–37.

Sachs T (1981) The control of the patterned differentiation of vascul

tissues. Advances in Botanical Research 9: 151–262.

Further Reading

Aloni R (1987) Differentiation of vascular tissues. Annual Review

Plant Physiology 38: 179–204.

Barnett JR (ed.) (1981) Xylem Cell Development. Tunbridge Wells, UK

Castle House.

Fukuda H (1996) Xylogenesis: initiation, progression and cell deat

Annual Review of Plant Physiology and Plant Molecular Biology 4

299–325.

Roberts LW, Gahan PBand Aloni R (1988)Vascular Differentiation an

Plant Growth Regulators. Berlin: Springer-Verlag.Torrey JG, Fosket DE and Hepler PK (1971) Xylem formation:

paradigm of cytodifferentiation in higher plants. American Scienti

59: 338–352.

Plant Tracheary Elements

2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net