Download Plant cell biology

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.
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.
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.
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 crosswall.
• The plane in which the cell plate grows
conforms:
– to the preprophase band
– not to the spindle midzone
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
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.
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.
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.
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.
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
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 waterconducting tubes.
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.
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.
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.
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.
17.20 Plants contain unique organelles called
plastids
• All plastids differentiate from
proplastids.
• Plastids arose during evolution by an
endosymbiotic event.
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.