Chapter 17

1. Chapter 17 Plant cell biology By Clive Lloyd

2. 17.1 Introduction • Plant and animal cells grow in fundamentally different ways. • The tough cell wall prevents: • cell movement • uptake of large molecules as food • Plant development depends upon how immobile cells manipulate the cell wall.

3. 17.2 How plants grow • Plants extend into the environment using apical growing points. • Plant development continues beyond the embryonic stage. • Plant growth is sensitive to the environment.

4. 17.3 The meristem provides new growth modules in a repetitive manner • Apical meristems divide to produce new cells at the growing points. • Growth occurs by repeated addition of new growth modules.

5. 17.3 The meristem provides new growth modules in a repetitive manner • Cells divide, expand, then differentiate. • Massive expansion of cells behind the tips drives the growing points onward.

6. 17.4 The plane in which a cell divides is important for tissue organization • In the absence of cell movement, orientation of the division plane helps determine shape. • Formative divisions generate new cell types: • proliferative divisions add more cells.

7. 17.5 Cytoplasmic structures predict the plane of cell division before mitosis begins • The plane of cell division is predicted before mitosis by a ring of microtubules and actin filaments around the cortex. • A sheet of cytoplasm also predicts the plane of division in vacuolated cells.

8. 17.6 Plant mitosis occurs without centrosomes • The poles of plant mitotic spindles: • do not contain centrioles • can be much more diffuse than the poles of animal spindles

9. 17.7 The cytokinetic apparatus builds a new wall in the plane anticipated by the preprophase band • The cytokinetic apparatus—the phragmoplast—is a ring of cytoskeletal filaments that expands outward.

10. 17.7 The cytokinetic apparatus builds a new wall in the plane anticipated by the preprophase band • Vesicles directed to the midline of this double ring fuse to form the new cross-wall. • The plane in which the cell plate grows conforms: • to the preprophase band • not to the spindle midzone

11. 17.8 Secretion during cytokinesis forms the cell plate • The Golgi apparatus continues to make secretory vesicles throughout cytokinesis. • These vesicles fuse to make a cell plate lined with new plasma membrane.

12. 17.9 Plasmodesmata are intercellular channels that connect plant cells • Primary plasmodesmata are pores in the cell wall formed at cytokinesis. • Plasmodesmata interconnect cells into multicellular units called symplasts, within which signaling occurs. • Plasmodesmata can open and close • Their pore size can be increased by viruses.

13. 17.10 Cell expansion is driven by swelling of the vacuole • Uptake of water into the vacuole provides a unique, pressure-driven mechanism of cell expansion. • There is more than one type of vacuole.

14. 17.11 The large forces of turgor pressure are resisted by the strength of cellulose microfibrils in the cell wall • The plant cell wall is based largely on carbohydrate. • unlike the protein-rich extracellular matrix of animal cells

15. 17.11 The large forces of turgor pressure are resisted by the strength of cellulose microfibrils in the cell wall • The nonrandom arrangement of stiff cellulose microfibrils controls the swelling force of turgor pressure. • Proteins loosen the cell wall to allow cell expansion. • The orientation of cellulose microfibrils can change from layer to layer.

16. 17.12 The cell wall must be loosened and reorganized to allow growth • Proteins loosen the cell wall to allow cell expansion. • The orientation of cellulose microfibrils can change from layer to layer.

17. 17.13 Cellulose is synthesized at the plasma membrane, not preassembled and secreted like other wall components • Cellulose is polymerized by complexes embedded in the plasma membrane. • The synthesizing complexes move along the face of the plasma membrane.

18. 17.14 Cortical microtubules are thought to organize components in the cell wall • During interphase the microtubules in plant cells are primarily located immediately beneath the plasma membrane.

19. 17.14 Cortical microtubules are thought to organize components in the cell wall • Cortical microtubules are often coaligned with the newest cellulose microfibrils. • Cortical microtubules may organize the cell wall by providing tracks for the synthesis and assembly of cellulose microfibrils.

20. 17.15 Cortical microtubules are highly dynamic and can change their orientation • Plant microtubules polymerize from multiple sites. • Microtubules can move along the cortex after they have been nucleated.

21. 17.15 Cortical microtubules are highly dynamic and can change their orientation • Microtubule-associated proteins organize microtubules into parallel groups. • The microtubule array can reorient in response to: • hormones • gravity • light

22. 17.16 A dispersed Golgi system delivers vesicles to the cell surface for growth • The plasma membrane and cell wall materials needed for growth are provided by the ER/Golgi system. • The Golgi apparatus is dispersed in plants. • The actin system propels the dynamic Golgi apparatus over the ER network.

23. 17.17 Actin filaments form a network for delivering materials around the cell • Organelles and vesicles move around the cell by cytoplasmic streaming, powered by actin-myosin interaction. • Plants have two unique classes of myosin.

24. 17.18 Differentiation of xylem cells requires extensive specialization • Files of xylem cells undergo programmed cell death to form water-conducting tubes.

25. 17.18 Differentiation of xylem cells requires extensive specialization • The tubes are prevented from inward collapse by transverse patterns of secondary wall thickening. • Cortical microtubules bunch-up to form patterns that anticipate the pattern of secondary thickening.

26. 17.19 Tip growth allows plant cells to extend processes • Highly localized secretion of cell wall materials allows plant cells to extend long processes. • In tip-growing cells, actin filaments and microtubules generally run parallel to the direction of outgrowth.

27. 17.19 Tip growth allows plant cells to extend processes • Bundles of actin filaments direct the movement of vesicles to the tip. • There, they fuse with the plasma membrane, driving extension. • Microtubules seem to control the number and location of cell tips. • Symbiotic bacteria turn tip growth in on itself to gain access into the plant.

28. 17.20 Plants contain unique organelles called plastids • Plastids are membrane-bounded organelles that are unique to plants. • Several types of plastid exist, each with a different function.

29. 17.20 Plants contain unique organelles called plastids • All plastids differentiate from proplastids. • Plastids arose during evolution by an endosymbiotic event.

30. 17.21 Chloroplasts manufacture food from atmospheric CO2 • Photosynthesis occurs in specialized plastids called chloroplasts. • Leaves maximize the amount of light for photosynthesis. • Mesophyll cells are shaped for maximal gas exchange.