Chapter 17 Plant cell biology By Clive Lloyd

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.

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.

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.

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

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.

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.

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

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.

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 17.7 The cytokinetic apparatus builds a new wall in the plane anticipated by the preprophase band

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.

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.

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.

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

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 The large forces of turgor pressure are resisted by the strength of cellulose microfibrils in the cell wall

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.

17.13 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.

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.

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 Cortical microtubules are thought to organize components in the cell wall

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.

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

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.

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.

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

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 Differentiation of xylem cells requires extensive specialization

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.

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 Tip growth allows plant cells to extend processes

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.

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

17.21 Chloroplasts manufacture food from atmospheric CO 2 Photosynthesis occurs in specialized plastids called chloroplasts. Leaves maximize the amount of light for photosynthesis. Mesophyll cells are shaped for maximal gas exchange.