Plant cell walls. The Earth s biomass is largely plant cell walls

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1 Plant cell walls The Earth s biomass is largely plant cell walls

2 Plants have a very distinctive mode of growth Energy from sunlight & nutrients from soil Constantly producing new organs & extending their surfaces in air and soil

3 Elaboration of surfaces also applies at the cellular level and each plant cell has a thick cell wall Depicted as in a box But this wall/box is.. dynamic and metabolically complex

4 Plant cell walls The presence of a wall at the surface of plant cells has great significance for plant growth & development No cell movement Specific mechanisms of cytokinesis, cell expansion and cell adhesion/separation Cell walls are a major feature of plant cell differentiation

5 First plant cell component to be imaged least understood today Robert Hooke's image of cork published in Micrographia in 1665

6 ABUNDANT Over 70% of assimilated carbon from photosynthesis ends up in cell walls Most abundant renewable resource on earth ~ 100 billion tonnes of cell walls produced annually

7 Wood very tough cell walls 0.1 mm

8 BIOFUELS Cell Wall Biomass (lignocellulose) is a renewable resource Burn efficiently Saccharification enzymology of cell wall degradation Fast growing perennial grasses e.g.miscanthus

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10 TS Arabidopsis flowering stem Plant organs are composites of two types of cell walls Primary all cells, tensile properties generate/resist internal turgor pressure, prevent cell bursting EXTENDABLE ~ cell expansion Secondary certain cells only, resist compressive forces prevent cell collapse

11 All plant cells have a primary cell wall In addition, certain cells have secondary walls (e.g. xylem vessels & sclerenchyma fibres) The images above show the same section of an Arabidopsis stem labelled with a fluorescent probe for cellulose (blue) and a monoclonal antibody (with fluorescent tag) to pectin (green). The thick secondary walls at the top of the image (sclerified parenchyma) do not contain pectin

12 Plant cell walls What is the molecular nature of cell walls? Polymers of sugar molecules - Polysaccharides including cellulose and pectins - used as industrial polymers, materials and food additives CARBON HYDROGEN OXYGEN No nitrogen Great complexity More ways of linking than amino acids in proteins

13 Eukaryotic cell walls are fibrous composites: fibres embedded in a matrix components are largely polysaccharides (glycans) Fibres provide tensile strength & rigidity Matrix components connect and spaces fibres and control cell wall porosity

14 Cellulose fibre 3 nm Polymerized glucose

15 Glucose Galactose Mannose Glucuronic acid Galacturonic acid Rhamnose Xylose Fucose Arabinose

16 cellulose heteroxylans xyloglucans galactoglucomannans 1,3-glucans 1,3-1,4-glucans galacturonans rhamnogalacturonans xylogalacturonans galactans Non-cellulosic, non-pectic matrix glycans pectins

17 Matrix glycans GENERIC CELL WALL STRUCTURES Somerville et al. (2004) Science 306, 2206

18 Primary cell walls Outside the plasma membrane Rigid cell shape Tensile strength turgor pressure Extendable plant growth Function in cell adhesion

19 Removal of cell wall with wall-degrading enzymes - releases protoplasts Plasma membrane Primary cell wall CELL PROTOPLAST

20 CELL EXPANSION Cell elongation is a major feature of plant growth Credits: John Innes Centre

21 During plant cell expansion: Cellulose-matrix glycan network is stretched and microfibrils slide apart Extensibility of cell wall determined by cross-links between microfibrils New cell wall material is added and the primary cell wall is maintained at a even thickness The direction of cell expansion is determined by the orientation of the cellulose microfibrils elongation at right angles to microfibrils

22 The orientation of the cellulose determines how a plant cell will expand Transversely oriented cellulose microfibrils leads to cells expanding as cylinders, rather than spheres Other CW factors control extent of cell expansion

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24 Two mechanisms of turgor-driven cell growth Diffuse growth (adhered cells, in multicellular organs) Tip growth (polarised growth of single, unadhered cells)

25 Differential cell expansion of cells in an organ is the basis of tropisms Greater cell expansion

26 Cytokinesis When a plant cell divides it does not contract in the middle during cytokinesis A new wall is constructed as a partition across a cell to produce two daughter cells This is the cell plate Primary cell wall material is deposited on both sides & it becomes the middle lamella The middle lamella is the junction between adjacent plant cells

27 Animal cell Plant cell

28 Microtubules of the plant cytoskeleton are involved in determining the: orientation of cellulose microfibrils (interphase cortical array) separation of chromosomes at mitosis (mitotic array) orientation of the cell plate before mitosis (pre-prophase band) construction of the cell plate (phragmoplast)

29 Formation of a phragmoplast Figure Molecular Biology of the Cell ( Garland Science 2008)

30 Cell plate formation Plasma membrane Primary cell-wall At cytokinesis the cell plate grows from the centre of the cell The cell plate partitions the parent cell producing two daughter cells As the cells grow primary wall is deposited on both sides of the cell plate, which becomes the middle lamella

31 Intercellular space formation Cell 1 Cell 2 New middle lamella Middle lamella of parent cell Cell 3 Region of primary cell wall that is dismantled to allow... old and new middle lamellae to link up to form intercellular space between three cells

32 Cell adhesion/separation In plants, cell adhesion is a consequence of the mechanism of cytokinesis Plant cells have fixed neighbours for life Cell separation is a more active & highly regulated process in all tissues intercellular space leaf and fruit abscission pod dehiscence to release seed

33 Plasmodesmata (plasmodesma) Plasma membrane-lined pores across cell walls connecting adjacent plant cells. Also contain ER membrane

34 Plasmodesmata: membrane-lined channels linking plant cells across cell walls

35 Plasmodesmata (plasmodesma) PD often grouped together in thinner regions of cell walls called pit fields Transport small molecules up to 1000 mw Viruses exploit PD to move form cell to cell Evidence also for specific movement through PD of transcription factors/nucleic acids that can regulate gene expression & influence meristem development

36 Plant tissue & organs can be viewed as consisting of two continuums - separated by the plasma membrane Apoplast: cell wall & intercellular space outside the protoplast Symplast: inside of protoplasts - cytoplasm, ER etc. linked by PD Phloem transport symplastic Xylem transport apoplastic

37 Secondary cell walls Only in certain cells: e.g. xylem, sclerenchyma fibres Inside primary cell walls Thick: three layers with differing orientation of microfibrils

38 Secondary cell walls Often lignified LIGNIN: a phenolic macromolecule built up from monomers such as coniferyl alcohol Hydrophobic Resist compression & cell collapse

39 Polyphenolic lignin structure - extensively cross-linked, tough, hydrophobic polymer of secondary cell walls/wood

40 Secondary Cell Walls cannot extend (deposited after growth completed) A sclereid from pear fruit - a stone cell Protoplast must contract during development Cells with SCWs often dead at maturity

41 Cotton fibres non-lignified SCW