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Plant Cell Differentiation Secondary article Article Contents Martin Hülskamp, University of Tübingen, Tübingen, Germany Hilmar Ilgenfritz, University of Tübingen, Tübingen, Germany . Differentiated Cell Types . Epidermis During plant cell development cells become specialized for a particular function. This process is called plant cell differentiation. . Trichomes . Root Hairs . Xylem . Phloem Differentiated Cell Types During plant development cells adopt a cell fate according to their developmental context. Subsequently, cells begin to differentiate and acquire distinct morphological, biochemical and physiological properties. Although many differentiated plant cells can dedifferentiate and may regenerate into whole plants under appropriate conditions, some cell types are less flexible or unable to change cell fate. These cells exit mitosis and start endoreduplication, which usually locks the cells in a nonmitotic state. Other cells undergo programmed cell death as part of their differentiation programme. To illustrate different aspects of plant cell differentiation, a few well-studied cell types are described with emphasis on the experimental approaches used. In addition, the role of the cytoskeleton and the cell wall in cell differentiation is discussed. Epidermis The plant epidermis is the single-celled outermost layer. It is established early and is maintained separate from underlying tissue types throughout development. It protects the plant against insects, the invasion of microorganisms and environmental conditions. The epidermis is important for the control of gas exchange (aerial part) and nutrient uptake (root). These different functions are accommodated for by the division of labour between cells. In aerial tissues three cell types are formed: stomata, trichomes and epidermal pavement cells. Stomata control the gas exchange through pores formed by guard cell pairs. Trichomes are hairs that are thought to serve to protect the plant against insects, intensive light and dehydration. Epidermal pavement cells represent the majority of the epidermal cells and are characterized by a thick apical wall that secretes an impermeable wax cuticle. In Arabidopsis it has been shown that the shape of pavement cells is regulated by several genes. During wild-type development of leaves in Arabidopsis, epidermal cells undergo extensive changes in cell form from initially regular round or elongated cells to puzzle-like pavement cells. In mutants affecting leaf shape, such as angustifolia (narrow leaves) or rotundifolia (wide leaves), the phenotype is expressed at the . Cell Shaping and the Cytoskeleton . Cell Walls and Differentiation cellular level such that the average length–width ratio of individual cells is altered. Trichomes Trichomes in higher plants show a wide range of morphological features. They may be unbranched or stellate, unicellular or multicellular, and often plant hairs differentiate into specialized secreting cells. In Arabidopsis, trichomes are single large cells with a DNA content that is 16-fold increased as compared to normal diploid cells. The first indication of trichome differentiation is an increase in nuclear DNA content and cell size. The incipient trichome cell extends out of the epidermal surface and initiates two successive branching events. While the morphological description does not allow us to draw conclusions about the underlying mechanisms, a genetic analysis has enabled the dissection of trichome development into five processes (Hülskamp et al., 1998): (1) regulation of endoreduplication, (2) local outgrowth, (3) branching, (4) extension growth and (5) wall maturation (Figure 1a). 1. The initial switch from mitosis to endoreduplication and the first three endoreduplication cycles are an immediate consequence of trichome initiation and are probably regulated by two genes involved in trichome patterning: GLABRA1 (GL1), which encodes a mybrelated transcription factor, and TRIPTYCHON (TRY). Further endoreduplication cycles require GLABRA3 (GL3) function. The total number of endoreduplication cycles is negatively regulated by TRY and three KAK genes. The KAK genes appear to represent components of a gibberellic acid-dependent pathway indicating that the number of endoreduplication cycles is controlled by plant hormones. 2. Local outgrowth requires the function of the GLABRA2 (GL2) gene, which encodes a homeodomain transcription factor. In gl2 mutants trichomes frequently do not extend out of the epidermal surface and remain flat. ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1 Plant Cell Differentiation (a) Trichomes (b) Root hairs Endoreduplication Local outgrowth GL1, TRY GL2 Primary branching STA, STI Endoreduplication Secondary branching Extension growth Cell wall maturation GL3 KAK, RFI, PYC, TRY AN, STI NOK Distorted genes UDT, TBR, CHA, CDO, RTS (c) Xylem Stage I Site selection RHD6 Local outgrowth RHD1 Tip growth RHD2 Growth direction TIP1, RHD3, RHD4 (d) Phloem Precursor cell Dedifferentiation Asymmetric cell division TED expression Stage II Accumulation of ER, microtubules, mitochondria, etc. PC=phloem cell CC=companion cell Callose deposition Branched plasmodesmata Rupture of tonoplast PC CC Mature sieve pores Loss of cell contents Organelle lysis Cell wall rigidification PC CC Figure 1 Schematic drawings of cell differentiation in selected cell types. (a) Trichome cell differentiation, (b) root hair cell differentiation, (c) xylem cell differentiation, (d) phloem cell differentiation. Abbreviations: KAK, KAKTUS; RFI, RASTAFARI; PYC, POLYCHOME; STI, STICHEL; NOK, NOECK; UDT, UNDER DEVELOPED TRICHOME; TBR, TRICHOME BIREFRINGENCE; CHA, CHABLIS; CRD, CHARDONNAY; RTS, RETSINA 3. Trichome branching involves two qualitatively different branching events that depend on STACHEL (STA) activity and ANGUSTIFOLIA (AN) functions, respectively. Additional genes are involved in the regulation of both branching events, for example the ZWICHEL (ZWI) gene is required as a general factor for branching and growth. ZWI encodes a member of the kinesin-like family of microtubule motor proteins that contain a calmodulin-binding site. This indicates that the directional transport of cellular components to spatially defined regions is important for branching. Branch number strongly correlates with the DNA 2 The five processes generally reflect the temporal sequence of morphological changes, implying that each step is a prerequisite for the next to be initiated. The genetic analysis of the various mutants reveals that this is the case for some processes, e.g. local outgrowth is necessary for all subsequent events. However, other processes take place independently of each other, e.g. extension growth is independent of branching. PC CC Secondary cell wall formation Stage III content such that trichomes with a reduced DNA content (as in gl3 mutants) produce fewer branches and trichomes exhibiting an increased DNA content (e.g in kak mutants) produce more branches. This suggests that branch initiation is regulated by cell size or cell growth. In addition, branching is regulated by genes that act independently of the DNA content. 4. After branching is initiated, the trichome cell undergoes extension growth. Directionality of the extension growth requires the activity of eight genes of the DISTORTED group. Mutations in these genes cause a very similar phenotype: mature trichomes display irregular and twisted growth. 5. During trichome maturation the outer cell wall thickens and is strengthened by encrustation. To date, five genes are known to be involved in this process. Root Hairs Root hairs are single elongated epidermal cells that produce a tubular projection perpendicular to the cell axis. In Arabidopsis, the analysis of mutants affecting root hair differentiation revealed distinct regulatory steps (Aeschbacher et al., 1994) (Figure 1b). Root hairs develop from trichoblasts that are shorter and less vacuolated than atrichoblasts. Initiation of root hair differentiation requires GL2. Further outgrowth involves two processes: site selection and elongation. Root hairs grow out at the basal end (relative to the root axis) of the root hair cell. In ROOT HAIR DEFECTIVE6 (RHD6) mutants the site of emergence is frequently shifted to apical positions. Site selection also involves the two plant hormones auxin and ethylene. After site selection the cell starts to grow out locally. In rhd1 mutants the initial growth is not locally restricted, although subsequent root hair elongation is normal. Root hairs elongate by tip growth. This is characterized by a tiphigh gradient of calcium and the polarized secretion of plasma membrane and cell wall material into the growing tip. Four genes are known to be required during root hair elongation: RHD2, TIP1, RHD3 and RHD4. RHD2 is required for tip growth. tip1 mutants display branched root hairs, suggesting that TIP1 is required to maintain stable tip growth. RHD3 and RHD4 are required for the directionality of tip growth based on the phenotypes of rhd3 and rhd4 mutants in which root hairs have an ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Plant Cell Differentiation abnormal, corkscrew-like appearance. In rhd3 mutants it has been shown that irregular tip growth is correlated with an asymmetric distribution of vesicles. RHD3 encodes a member of a new class of GTP-binding proteins and thus represents the first identified component of a molecular pathway for localized secretion during root tip growth. Xylem the companion cell become branched on the companion cell side. Plasmodesmata between neighbouring phloem cells differentiate to sieve pores. This process involves the deposition of callose (a complex polymer of glucose) around the plasmodesmata, which is thought to replace the cellulose. Hydrolysis of callose in later stages results in the formation of wide pores. Later differentiation stages of the phloem include the breakdown of the nucleus, the ER and organelles. The plasmalemma, however, remains intact and becomes covered by an elaborate system of membranes known as the sieve element reticulum. Xylem cells are highly specialized cells of the vascular system required for the regulated transport of water and nutrients. They develop from procambial or cambial cells (i.e cells of the secondary meristem surrounding the vascular tissue) or can be induced to form from parenchymal cells by wound stress. Transdifferentiation from parenchymal cells into xylem cells can also be induced in vitro in Zinnia cell cultures by the appropriate combination of plant hormones. This has enabled three differentiation stages to be defined (Fukuda, 1997) (Figure 1c). During the initial stage, parenchymal cells dedifferentiate without undergoing cell division, differentiating into a xylem precursor cell. These events resemble wound-induced xylem differentiation and involve the expression of wound-induced genes. The second stage is defined by three genes (TED2, TED3, TED4) expressed prior to secondary wall thickening. This phase is characterized by a general increase of transcriptional and physiological activities including an increase of the levels of the endoplasmic reticulum (ER), vesicle formation, mitochondria and tubulin. During the third stage, xylem cells form secondary cell walls and enter programmed cell death. The spatially ordered deposition of secondary cell walls depends on the coordinated organization of actin and microtubules which are thought to guide the movement of cellulose-synthesizing complexes in the plasma membrane. Rigidification of secondary cell walls involves several proteins that are specifically expressed during stage III including extensinlike proteins, arabinogalactan proteins, glycoproteins and enzymes involved in lignin biosynthesis. In late stage III, programmed cell death of the xylem cell occurs: the nucleus degenerates, organelles disappear, and the Golgi apparatus and the ER ruptures, resulting in a mature tracheary element without any cell content. The cytoskeleton is important for the spatial organization of intracellular processes and hence plays a crucial role in the regulation of cell shape, which is best understood for two different growth modes: tip growth and cell expansion. The role of actin microfilaments in targeted transport during tip growth, for example in pollen tubes, is evident from the application of inhibitor drugs. Cytochalasin, an inhibitor of actin-based transport processes, abolishes pollen tube growth. By contrast, colchicine, an inhibitor of microtubule-based transport processes, does not inhibit pollen tube growth. Cell expansion appears to be spatially controlled by the arrangement of microtubules. This is supported by a strong correlation between the orientation of cortical microtubules and growth direction. This is particularly evident when the directionality of growth, longitudinal versus radial expansion, is experimentally manipulated by the application of the plant hormone ethylene. In pea stem segments treated with ethylene reorientation of microtubules from a transverse to longitudinal orientation precedes the transition from longitudinal to radial growth. In other cell types this link has been further established by demonstrating that cellulose microfibrils coalign with the cortical microtubules. This suggests that enzymes involved in the synthesis of extracellular microfibrils are targeted by a microtubule-based system. Genetic evidence comes from the analysis of ton1 and ton2 mutants in Arabidopsis. In these mutants, irregular cell expansion is correlated with disorganized interphase cortical microtubules. Phloem Cell Walls and Differentiation The phloem consists of a series of connected sieve cells that form a syncytium and functions to transport assimilate molecules in the plant. Phloem cell differentiation starts with an unequal division that gives rise to a cambial initial cell and a companion cell (Figure 1d). During early differentiation stages, plasmodesmata undergo marked changes. Plasmodesmata connecting the phloem cell with Cell morphogenesis involves the spatially controlled remodelling of the cell wall (Roberts, 1994). Cell expansion is thought to be regulated by a balance between turgor (providing the drive for expansion) and localized wall loosening, i.e. the loosening of existing cellulose/pectic polysaccharide network and synthesis and intercalation of new wall material. Two classes of proteins appear to be Cell Shaping and the Cytoskeleton ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 3 Plant Cell Differentiation involved in this process. Xyloglucan-endotransglycosylases cleave the glycan backbone and transfer the cut end to an acceptor molecule, thus enabling controlled rearrangements of the cellulose network. In contrast, expansins appear to act nonenzymatically by loosening hydrogen bonds between cellulose/hemicellulose fibres. The extracellular matrix is not only subject to regulation as part of the differentiation process, but also regulates cell differentiation itself. This is most evident from experiments with the brown alga Fucus. During embryo development the first division gives rise to a thallus and a rhizoid cell. If the rhizoid cell is destroyed, the fate of the dividing thallus cell can be redirected to a rhizoid cell fate by contact to the remnants of the former rhizoid cell wall. A second example is provided by a particular class of extracellular matrix 4 proteins, the arabinogalactan proteins (AGPs). It has been shown that secreted AGPs from embryogenic tissues of carrots trigger somatic embryogenesis in tissue culture. References Aeschbacher R, Schiefelbein JW and Benfey PN (1994) The genetic and molecular basis of root development. Annual Review of Plant Physiology and Plant Molecular Biology 45: 25–45. Fukuda H (1997) Tracheary element differentiation. Plant Cell 9: 1147– 1156. Hülskamp M, Folkers U and Grini P (1998) Cell morphogenesis in Arabidopsis. BioEssays 20: 20–29. Roberts K (1994) The plant extracellular matrix: in a new expansive mood. Current Opinion in Cell Biology 6: 688–694. ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net