*photosynthetic, multicellular, terrestrial
TRANSCRIPT
© 2013 American Society of Plant Biologists
How to be a *plant
*photosynthetic, multicellular,
terrestrial
© 2013 American Society of Plant Biologists
Plants are multicellular, terrestrial
and photosynthetic
Multicellular: Different cells can
have various functions, but they
must integrate their activities
Terrestrial: Plant ancestors were
aquatic, but terrestrial plants
have to cope with very dry air
Photosynthetic: Plants and many
other organisms can convert solar
energy to chemical energy
Leaf cross section image from Bouton, J.H., et al., (1986). Photosynthesis, leaf anatomy, and morphology of progeny from hybrids between C3 and C3/C4 Panicum Species. Plant Physiol. 80: 487-492.
© 2013 American Society of Plant Biologists
Human physiology centers on
systems Respiratory
system Gas exchange
Digestive
system Water and
Nutrient uptake
Circulatory
system Nutrient
transport
Nervous
system
Perception,
control
Skeletal
system
Support
© 2013 American Society of Plant Biologists
Plant physiology: Same functions,
more broadly distributed Gas exchange
takes place
through
thousands of
stomata
Nutrients are
transported
from cell to cell
and through
vascular tissues
Water and nutrient uptake
take place across the root
(and sometimes shoot)
surfaces
Energy assimilation
takes place
throughout
photosynthetic
tissues
Signals to control and
coordinate plant functions
move from cell to cell and
through vascular tissues
Perception (of light,
pathogens etc.) occurs
through receptors and
other sensors found in
most cells
Support comes
from cell walls and
hydrostatic
pressure
© 2013 American Society of Plant Biologists
What are plants? Plants are
photosynthetic eukaryotes
ALL LIFE PHOTOSYNTHETIC
ORGANISMS
CYANOBACTERIA +
“DESCENDANTS”
EUKARYOTES WITH
CYANOBACTERIA-DERIVED
CHLOROPLASTS
Non-
photosynthetic
bacteria
Archaea
Fungi
Animals
You are here
Green
sulfur
bacteria
Purple
sulfur
bacteria
Other
bacteria
Cyanobacteria
Diatoms Red algae
Brown
algae
PLANTS
GREEN ALGAE AND
DESCEDANTS
(Circles not
drawn to
scale)
© 2013 American Society of Plant Biologists
Plants descended from a eukaryotic
ancestor + a cyanobacteria
ORIGIN OF LIFE
Bacteria Archaea Eukarya
Mitochondrial
endosymbiosis
Plastid
endo-
symbiosis
PL
AN
TS
AL
GA
E
FU
NG
I
AN
IMA
LS
>3.5 BYA
>1.5 BYA
>0.5 BYA
Photosynthesis evolved
in bacteria. All
photosynthetic
eukaryotes acquired this
ability through
endosymbiosis of
photosynthetic bacteria
Therefore, some “plant” genes
(those derived from the
ancestral bacteria) are more like
bacterial genes than the genes
of other eukaryotes
Adapted from Govindjee and Shevela, D. (2011). Adventures with cyanobacteria: a personal perspective. Frontiers in Plant Science. 2: 28.
© 2013 American Society of Plant Biologists
Two endosymbiotic organelles –
mitochondria and plastids
Cyanobacteria Proteobacteria
The host that acquired
mitochondria
Early diversification of eukarya
Ancient
proteobacterium
PLANTS
EUKARYOTES
Ancient cyanobacterium
Adapted from Timmis, J.N., Ayliffe, M.A., Huang, C.Y. and Martin, W. (2004). Endosymbiotic gene transfer: organelle genomes forge eukaryotic chromosomes. Nat Rev Genet. 5: 123-135.
© 2013 American Society of Plant Biologists
Family tree: Plants and green algae
Adapted from Hay, A. and Tsiantis, M. (2010). KNOX genes: versatile regulators of plant development and diversity. Development. 137: 3153-3165 and Prigge,
M.J. and Bezanilla, M. (2010). Evolutionary crossroads in developmental biology: Physcomitrella patens. Development. 137: 3535-3543.
1200?
450
400
360
300
Ch
loro
ph
yte
s
Ch
aro
ph
yte
s
Bry
op
hyte
s
Lyco
ph
yte
s
Fern
s
Gym
no
sp
erm
s
An
gio
sp
erm
s
Green algae Plants
Terrestrialization
Stomata
Vascular tissues
Seeds
Flowers
© 2013 American Society of Plant Biologists
Green algae are photosynthetic
eukaryotes, but not all plants
Image credits: JGI; Spike Walker Wellcome Images; Jaspser Nance; Show_ryu
Chlamydomonas reinhardtii
10 μm
Ostreococcus
tauri
Volvox spp
Chlorophytes Spirogyra spp
Charophytes
Chara braunii
© 2013 American Society of Plant Biologists
Bryophytes are “non-vascular”
plants
Adapted from Chang, Y. and Graham, S.W. (2011). Inferring the higher-order phylogeny of mosses (Bryophyta) and relatives using a large, multigene plastid data set. Am. J. Bot. 98: 839-849 and Ligrone, R., Duckett, J.G. and
Renzaglia, K.S. (2012). Major transitions in the evolution of early land plants: a bryological perspective. Ann. Bot. 109: 851-871. Photo credits Tom Donald, Mary Williams and gjshepherd_br / Foter.com / CC BY-NC-SA
Green algae
Land plants
Liverworts (~7000
– 8500 species)
Mosses (~10,000 –
17,000 species)
Hornworts (~200 species)
Vascular
plants
© 2013 American Society of Plant Biologists
Tracheophytes are vascular plants
Bryophytes
Vascular tissue
Lycophytes
(~ 1200 species)
Ferns
(~ 13,000 species)
Gymnosperms
(~ 1000 species) Angiosperms
(~ 350,000 species)
Seeds
Photos by Tom Donald
© 2013 American Society of Plant Biologists
Introduction to bioenergetics
Photo credit: Tom Donald
© 2013 American Society of Plant Biologists
Energy is never gained or lost,
just transferred to other forms H
igher
energ
y
Chemical energy
transferred to
potential energy by
muscles
Potential energy
transferred to
kinetic energy
Kinetic
energy
transferred
to heat
© 2013 American Society of Plant Biologists
Energy exists in many forms and
can be interconverted between them
Chemical
energy
Kinetic
energy
Electromagnetic
(solar) energy
Po
ten
tial
Kin
eti
c
Chemical
energy
Image credit: NOAA
© 2013 American Society of Plant Biologists
Chemical energy can be stored in
bonds and as reducing power
Energy released
sugar sugar sugar sugar sugar
sugar sugar
sugar
sugar
sugar
Starch – a polymer made of sugar
A carbohydrate like
starch has energy
stored in its bonds
Energy is released
when the bonds
are broken
More energy is
released when the
carbon in the sugar
is oxidized
O2
CO2
Energy released
© 2013 American Society of Plant Biologists
Energy can be stored or released via
the redox state of an element
Higher
energy,
reduced
form
Carbohydrates
C6H12O6
CH2O
Ammonium
NH4+
Iron (II)
Lower
energy,
oxidized
form
Carbon Dioxide
CO2
Nitrate, nitrite
NO3-, NO2
-
Iron (III)
Oxidation involves
the removal of
electrons, and
requires an
electron acceptor
Reduction O
xid
atio
n
Energy
input
required
Energy
released
© 2013 American Society of Plant Biologists
Campfire: Oxidation of carbon with
energy released as heat and light High-
energy
reduced
carbon in
wood
C6H12O6 + 6 O2 6 CO2 + 6 H2O
Low energy
oxidized carbon
in carbon
dioxide
The non-carbon elements
in wood contribute to ash
Energy is released
as heat and light
Oxygen is required
for a fire to burn.
Oxygen is the
electron acceptor in
the oxidation
reaction
© 2013 American Society of Plant Biologists
Biological metabolism: Controlled
release and recapture of energy High-
energy
reduced
carbon in
food
C6H12O6 + 6 O2 6 CO2 + 6 H2O
Low energy
oxidized carbon
in carbon
dioxide
Energy is captured
for later use
Oxygen is required to
efficiently extract energy
from carbohydrates. (Oxygen is the electron
acceptor in the oxidation
reaction)
Metabolism
(Specifically,
oxidative
phosphorylation)
ATP (adenosine
triphosphate)
© 2013 American Society of Plant Biologists
Adapted from TXU Corp; Poco a poco
Analogy: Hydroelectricity is produced by the
controlled release and capture of kinetic
energy from falling water
Reserv
oir
Dam Power Plant
Generator
Turbine
Electricity Kinetic energy in the moving
water turns the turbine to
generate electricity
(electromagnetic energy)
The energy of a
natural waterfall is
transferred to the
rocks below it
© 2013 American Society of Plant Biologists
Cellular energy currencies, ATP and
NAD(P)H
H2O
-
-
ATP (Adenosine
triphosphate)
ADP (Adenosine
diphosphate)
H+
energy
NADP+ (nicotinamide
adenine dinucleotide
phosphate)
NADPH
H+ + 2e-
ATP stores energy in a
high-energy bond
NADPH stores reducing
power and electrons
© 2013 American Society of Plant Biologists
Introduction to photosynthesis
Daum, B., Nicastro, D., Austin, J., McIntosh, J.R. and Kühlbrandt, W. (2010). Arrangement of photosystem II and ATP synthase in chloroplast membranes of spinach and
pea. Plant Cell. 22: 1299-1312 Chuartzman, S.G., et al., (2008). Thylakoid membrane remodeling during state transitions in Arabidopsis. Plant Cell. 20: 1029-1039..
ATP synthase (yellow) embedded in
thylakoid membranes (green)
PSII supercomplexes in thylakoid membranes
lumen
Ultrastructure of a chloroplast
showing thylakoid membrane stacks
© 2013 American Society of Plant Biologists
Energy enters the biosphere mainly
through photosynthesis by plants
TERRESTRIAL
BIOSPHERE
MARINE
BIOSPHERE
Some energy is
introduced by the
action of
chemotrophs COAL, OIL
AND GAS
RESERVES
Field, C.B., Behrenfeld, M.J., Randerson, J.T. and Falkowski, P. (1998). Primary production of the biosphere: Integrating terrestrial and oceanic components. Science. 281: 237-240.
© 2013 American Society of Plant Biologists
Burning buried sunshine*
*Title credit from Dukes, J. (2003). Burning buried sunshine: Human consumption of ancient solar energy. Climatic Change. 61: 31-44.
COAL, OIL
AND GAS
RESERVES
One gallon of gasoline comes
from ~89 metric tons (~180,000
pounds) of ancient lycophytes and
ferns that dominated the earth 300
million years ago, but mostly have
been supplanted by seed plants
Fossil fuel reserves are
estimated to be about
5,000 gigatons (Gt) of
carbon, accumulated
over millions of years
89 metric tons
of gold is worth
$4 billion
($4 x 109)
89 metric tons is
about 89 elephants
“I get 500
meters per ton”
© 2013 American Society of Plant Biologists
In plants, photosynthesis captures
light energy as reduced carbon
Low energy,
oxidized carbon
in carbon
dioxide
Energy input
from sunlight
High
energy,
reduced
carbon
Oxygen is
released as
a byproduct
6 CO2 + 6 H2O C6H12O6 + 6 O2
The first step is the capture of
light energy as ATP and
reducing power, NADPH
The second step is the transfer of
energy and reducing power from
ATP and NADPH to CO2, to produce
high-energy, reduced sugars
ATP NADPH
© 2013 American Society of Plant Biologists
Photosynthesis is two sets of
connected reactions
Chloroplast 2 H2O O2 + 2 H+
ADP H+
ATP
The LIGHT-HARVESTING reactions take
place in the chloroplast membranes
The CARBON-FIXING
reactions take place in
the chloroplast stroma
Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70–78.
© 2013 American Society of Plant Biologists
Light harvesting reactions produce
O2, ATP and NADPH
2 H2O O2 + 2 H+
ADP
H+
ATP
ATP synthase Photosystem II (PSII)
Photosystem I
(PSI)
Cytochrome
b6f complex The reactions
require several
large multi-protein
complexes: two
light harvesting
photosystems (PSI
and PSII), a
cytochromome b6f
complex, and an
ATP synthase
Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70–78.
© 2013 American Society of Plant Biologists
Chlorophyll captures light energy to
initiate the light harvesting reactions
Hydrophobic
tail anchors
chlorophyll in
membrane
Porphyrin ring
captures photons
Photon capture by
chlorophyll excites the
chlorophyll (Chl*). Chl*
loses an electron (e-)
and relaxes back to its
unexcited state
Chl
Chl* e-
Photon
Chl
2 H2O
e-
4 H+
O2
Oxidized Chl is
reduced by stripping
an electron from
water, releasing
oxygen and protons
Buchanan, B.B., Gruissem, W. and Jones, R.L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists.
© 2013 American Society of Plant Biologists
Plants have two photosystems that
work together
Adapted from Govindjee and Shevela, D. (2011). Adventures with cyanobacteria: a personal perspective. Frontiers in Plant Science. 2: 28.
2 H2O e-
4 H+
O2
H+
Thylakoid
Membrane
PSII PSI
e-
e-
e-
e-
2 NADP+
2 NADPH
2 H+ e-
In photosystem II,
water is split to
release oxygen
Electrons are passed from
PSII to PSI and eventually to
NADP+, producing NADPH
Cytochrome
b6f complex
Protons accumulate in
the thylakoid lumen
Thylakoid lumen
© 2013 American Society of Plant Biologists
[H+]
Thylakoid
Membrane
Thylakoid lumen
ADP + Pi
H+
ATP
H+
Proton gradient from
high (in) to low (out)
2 H2O e-
4 H+
O2
H+
ATP
Synthase
H+
H+
H+ H+
H+
H+
H+
H+
H+
A proton gradient drives ATP
synthesis
Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.
155: 70–78 and Hohmann-Marriott, M.F. and Blankenship, R.E. (2011). Evolution of photosynthesis. Annu. Rev. Plant Biol. 62: 515-548.
© 2013 American Society of Plant Biologists
Through the Calvin-Benson cycle,
ATP and NADPH are used to fix CO2
Adapted from: Buchanan, B.B., Gruissem, W. and Jones, R.L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists.
Carboxylation
Reduction
Rubisco 3 x CO2 3 x Ribulose-1,5-
bisphosphate
6 x 3-phosphogylcerate
6 ATP
6 ADP + 6 Pi
6 NADP+ + 6 H+
6 NADPH
Energy input
Reducing
power input
6 x glyceraldehyde
3-phosphate (GAP)
Regeneration
For every 3 CO2 fixed, one
GAP is produced for
biosynthesis and energy
5 x GAP
1 x GAP
6 ATP
6 ADP + 6 Pi
Each CO2 fixed
requires 3 ATP
and 2 NADPH
© 2013 American Society of Plant Biologists
Some plants have additional,
CO2-concentrating steps
Because Rubisco has
an oxygenase as well
as a carboxylase
activity, some plants
concentrate CO2 to
promote the
carboxylation reaction
PEP
carboxylase
Rubisco
CCC CO2
CCCC
CCC
CCCCC Calvin-
Benson
cycle
C4 cycle Energy input
Regeneration CO2
Ribulose-1,5-
bisphosphate
Phosphoenolpyruvate
This cycle produces a
four-carbon compound,
so it is called the C4 cycle
The result of the C4 cycle
is to elevate the
concentration of CO2 at
the site of Rubisco action
© 2013 American Society of Plant Biologists
Fixed carbon has many fates
Some is used for
biosynthesis of other
compounds
CO2
CO2
CO2
Some of the fixed carbon is
oxidized in the
mitochondria to produce
ATP, and released as CO2
Some is
transported to
growing tissues,
or stored as
starch or oil in
seeds
Some is transported to the roots
and used for growth, stored, used
to support nutrient uptake, or
exuded into the soil
© 2013 American Society of Plant Biologists
Summary: Plants are photosynthetic
eukaryotes
• Plants convert light energy to
chemical energy
• Photosynthesis evolved in
bacteria, and takes place in the
descendants of endosymbiotic
photosynthetic bacteria
• Through photosynthesis, plants
and algae are responsible for the
transfer of most of the energy that
enters the biosphere
Low energy,
oxidized carbon
in carbon
dioxide
Energy input
from sunlight
High
energy,
reduced
carbon
Oxygen is
released as
a byproduct
6 CO2 + 6 H2O C6H12O6 + 6 O2
© 2013 American Society of Plant Biologists
Plants are assembled from cells
VACUOLE
NUCLEUS CHLOROPLASTS
MIT
OC
HO
ND
RIA
Vacuolar membrane
Plasma membrane
Cell wall
A “typical” plant cell
The plasma membrane
is a barrier that lets cells
maintain a different
internal environment
from their surroundings
Plasma membrane
© 2013 American Society of Plant Biologists
Systems move towards uniformity,
unless energy is expended
Energy
Dispersed,
equally
distributed
Nonuniform
distribution,
Equilibrium Disequilibrium –
stored energy
Energy
Cells expend energy to maintain an
internal environment that is distinct
from the external environment
© 2013 American Society of Plant Biologists
Cells are surrounded by a semi-
permeable plasma membrane
Na+
Na+
K+
K+
H2O OUT
IN
CO2
The membrane is
permeable to water and
gases, but impermeable to
ions and larger molecules
Photo credit: Wenche Eikrem and Jahn Throndsen, University of Oslo
© 2013 American Society of Plant Biologists
Proteins in the plasma and vacuolar
membranes move molecules
Adapted from Hedrich, R. (2012). Ion channels in plants. Physiol. Rev. 92: 1777-1811.
These transporters bring
in needed ions and other
compounds, and export
unwanted molecules.
The transporters are also
needed to regulate the
cell’s osmotic potential
VACUOLE
Vacuolar membrane
Plasma membrane
Cell wall
ADP
H+
ATP
H+ X
H+
ADP
ATP
© 2013 American Society of Plant Biologists
Membrane proteins form pores,
channels or pumps for ion transport
K+
K+
K+
K+
The complex protein
structures are usually draw
in simplified forms
Ion transport across a
membrane usually directly
or indirectly requires energy
© 2013 American Society of Plant Biologists
Plant cells move water by osmosis
and pressure
Buer, C.S., Weathers, P.J. and Swartzlander, G.A. (2000). Changes in Hechtian strands in cold-hardened cells measured by optical microsurgery. Plant Physiol. 122: 1365-1378.
Cell wall
WATER
WATER
WATER
WATER
Cell showing the cytoplasm pull
away from the cell wall as it
loses volume (plasmolysis)
© 2013 American Society of Plant Biologists
Water moves down osmotic
gradients
Water moves across a
semipermeable membrane
into a compartment
containing salt
The water flows inwards
until the force of the
pressure inside balances
the osmotic driving force
The osmotic potential of pure water is 0. The addition
of solutes lowers the water potential. Water moves
towards a region with a lower water potential.
© 2013 American Society of Plant Biologists
Osmotic forces drive the movement
of water into and out of cells
Salt water
Fresh water Fresh water
Salt water
An animal cell might burst Plant cell walls prevent them
from bursting
Cells have a lower osmotic
potential than pure water,
(because of the salts and
proteins in them), so water
moves into them
Salt water has a lower
osmotic potential than
cells, so water flows
outwards
Osmotic potential is written as Ψπ and measured in MegaPascals (MPa)
For seawater, Ψπ is about -2.5 MPa, and for a typical cell, Ψπ is about -0.8 MPa
© 2013 American Society of Plant Biologists
Water can be moved by tension
(pulling) or pressure (pushing)
Pulling up the
plunger causes
the pressure
inside the barrel
to be lower than
atmospheric.
Water from the
beaker moves
towards the
region of lower
pressure, into the
barrel.
Pushing down the
plunger causes
the pressure
inside the barrel
to be higher than
atmospheric.
Water from the
barrel moves out,
towards the
region of lower
pressure.
© 2013 American Society of Plant Biologists
Pressure can be positive or negative
Mschel; Image 14869 CDC/ Nasheka Powell
Household
water
pressure
0.3 MPa
Car tire
pressure
0.25 MPa
Pressure
required to
blow up a
balloon
0.01 MPa
Vacuum cleaner
-0.02 MPa
(household)
-0.1 MPa
(commercial)
Laboratory
vacuum
-0.01 MPa
Human blood
pressure
< 0.02 MPa
Inside typical plant cell
0.5 – 1.5 MPa
Pressure
washer
15 MPa
Inside xylem:
From +1 MPa to
-3 MPa or lower
© 2013 American Society of Plant Biologists
The water potential equation
incorporates osmotic and pressure
potentials
Ψw = Ψp + Ψπ
The water potential (Ψw) is the sum of the pressure
potential (Ψp) and the osmotic potential (Ψw) The water
potential
equation
Water flows
from higher to
lower water
potential
© 2013 American Society of Plant Biologists
Water moves towards lower water
potential
Initial conditions: Final conditions:
Ψπ = 0
Ψp = 0
Ψw = 0
+
Ψπ = -2
Ψp = 0
Ψw = -2
+
Water moves
towards lower
water potential
Ψπ = 0
Ψp = 0
Ψw = 0
+
Ψπ = -2
Ψp = 2
Ψw = 0
+
Water is at
equilibrium
© 2013 American Society of Plant Biologists
Plant cell walls are flexible, strong,
and complex The primary cell
wall is thin and
somewhat
flexible
Cytoplasm
Vacuole
Plasma membrane
The cell wall is synthesized
outside of the plasma
membrane from materials
transported outward through
the plasma membrane
Secondary cell walls form
inside of the primary cell wall.
Secondary walls are usually
rigid and contain lignin, a
complex, insoluble polymer
Primary cell wall
Secondary cell wall
Vanholme, R., Demedts, B., Morreel, K., Ralph, J. and Boerjan, W. (2010). Lignin biosynthesis and structure. Plant Physiol. 153: 895-905.
© 2013 American Society of Plant Biologists
Primary cell wall: cellulose, proteins
and other components
Reprinted from McCann, M.C., Bush, M., Milioni, D., Sado, P., Stacey, N.J., Catchpole, G., Defernez, M., Carpita, N.C., Hofte, H., Ulvskov, P., Wilson, R.H. and Roberts, K. (2001). Approaches to
understanding the functional architecture of the plant cell wall. Phytochemistry. 57: 811-821, with permission from Elsevier; Drawing credit LadyofHats.
© 2013 American Society of Plant Biologists
Plants are multicellular
Advantages:
• Bigger means better
able to compete for light,
nutrients
• Cells can specialize
Disadvantages:
• Need to move resources
and information,
coordinate actions
Photo credit: Tom Donald
© 2013 American Society of Plant Biologists
Multicellular organisms need to
transmit materials and information
Metabolic products
have to be distributed
throughout the body
Information has to be
transmitted to
integrate activities
Raw materials
have to be
distributed –
diffusion is
inadequate for
larger organisms
© 2013 American Society of Plant Biologists
(Most) plant cells are connected by
plasmodesmata
Reprinted from Lee, J.-Y. and Lu, H. (2011). Plasmodesmata: the battleground against intruders. Trends Plant Sci. 16: 201-210 with permission from Elsevier.
Plasmodesmata are
plasma-membrane
lined, regulated
cytoplasmic bridges
between plant cells
Signals, nutrients,
ions and water
can move to
adjacent cells, or
longer distances
via the phloem
Pathogens can
spread through
the plant through
plasmodesmata
Guard cells are
isolated, without
functional
plasmodesmata
© 2013 American Society of Plant Biologists
Transport in bryophytes – some
specialized transport cells
Hydroids (h) in
cross section
Water moves along
external surfaces,
within cells, between
cells, and through
specialized water-
conducting cells or
hydroids (h)
Bryophytes take up water from the air and substrate
Some bryophytes
have food-
conducting cells
or leptoids (l)
The end walls
between food-
conducting cells can
have enlarged
plasmodesmata-
Micrographs from Ligrone, R., Duckett, J.G. and Renzaglia, K.S. (2000). Conducting tissues and phyletic relationships of bryophytes. Philosophical
Transactions of the Royal Society of London. Series B: Biological Sciences. 355: 795-813 by permission of the Royal Society.
© 2013 American Society of Plant Biologists
Vascular plants have long-distance
transport systems
XYLEM Water moves from
the soil to the
atmosphere
through the hollow
dead cells of the
xylem
PHLOEM Photosynthetically-
produced sugars (and
other molecules) move
from their source to
sinks (non-
photosynthetic tissues)
through the phloem
© 2013 American Society of Plant Biologists
Water moves from the soil, into the
outer layers of the root , then into the
vascular cylinder and xylem
Water is pulled through the
hollow xylem by tension
developed at evaporative
sites in the leaves
In the leaf, water evaporates out of
the xylem into the intracellular
spaces, and then through the
stomata into the atmosphere
Stomate
Outer root
layer
Endodermis
Vascular
cylinder
Water uptake and movement in
vascular plants
© 2013 American Society of Plant Biologists
Water movement in the xylem is
driven by evaporation
Ψw= -0.2 MPa
Water moves
from moist soil to
the dry
atmosphere. The
plant simply taps
into this force to
draw water
through its body
Ψw= -15 to
-100 MPa
© 2013 American Society of Plant Biologists
Water movement in the xylem is
driven by evaporation
Adapted from Lucas, W.J. et al. (2013). The plant vascular system: Evolution, development and functions. J. Integr. Plant Biol. 55: 294-388.
Ψw= -0.2 MPa
Simple model:
Hollow tube
that water
evaporates
through
Better model:
Hollow tube
with a
selectivity
filter in the
roots and a
flow regulator
at the top
Guard
cells
Casparian
strip of the
endodermis
Ψw= -15 to
-100 MPa
© 2013 American Society of Plant Biologists
Xylem cells are highly specialized
water-conducting cells
Oda, Y., Mimura, T. and Hasezawa, S. (2005). Regulation of secondary cell wall development by cortical microtubules during tracheary element differentiation in Arabidopsis cell
suspensions. Plant Physiol. 137: 1027-1036. Reprinted from Höfte, H. (2010). Plant cell biology: How to pattern a wall. Curr. Biol. 20: R450-R452 with permission from Elsevier.
Secondary
wall
deposition
Elongation
Programmed
cell death
The cells form
thickened rings
of secondary wall
tissue and then
die, leaving
behind a hollow,
reinforced vessel
© 2013 American Society of Plant Biologists
The structure of xylem cell walls
prevents embolisms from spreading
Reprinted from Li, H.-F., et al. (2011). Vessel elements present in the secondary xylem of Trochodendron
and Tetracentron (Trochodendraceae). Flora. 206: 595-600 with permission from Elsevier.
Because water
moves under
tension, air
bubbles or
embolisms can
form. The small
gaps in the xylem
walls permit
water but not air
bubbles to move
through them
Air bubble
© 2013 American Society of Plant Biologists
Bryophotes
produce
monolignols:
UV protection?
Defense?
Tracheophytes produce
lignin polymer:
Structure for xylem,
support and strength for
size
Lignin-strengthened
cell walls are one of
the major features
that let plants get big
The secondary walls are reinforced by
lignin, a water-impermeable polymer
Reprinted from Höfte, H. (2010). Plant cell biology: How to pattern a wall. Curr. Biol. 20: R450-R452 with permission from Elsevier.
Weng, J.-K. and Chapple, C. (2010). The origin and evolution of lignin biosynthesis. New Phytol. 187: 273-285.
© 2013 American Society of Plant Biologists
Lignified xylem provides structural
support for vascular plants
11
5 m
Seq
uo
ia s
em
perv
iren
s
Sydney Opera House 65 m Taj Mahal 65 m
Statue of Liberty 93 m
St. Paul’s Cathedral 111 m
The tallest living trees
tower over many
familiar monuments
© 2013 American Society of Plant Biologists
The root endodermis acts as a
selectivity filter Na+
Na+
Na+
Na+
Na+ Na+
Na+
The endodermis produces a
water-impermeable layer,
the Casparian strip, that
provides selectivity Photo credit Michael Clayton
© 2013 American Society of Plant Biologists
The Casparian strip forces water to
cross a plasma membrane
Photo credit Michael Clayton; Adapted from Grebe, M. (2011). Plant biology: Unveiling the Casparian strip. Nature. 473: 294-295.
Na+
Na+
OUT IN
Water can enter
the root through
the apoplast (cell
wall spaces) until
it reaches the
Casparian strip,
which blocks
apoplastic water
flow
Water forced to
cross a plasma
membrane is
effectively filtered
© 2013 American Society of Plant Biologists
Waxy cuticles prevent water loss;
regulated pores allow it OPEN
CLOSED
Most plant aerial surfaces are
covered by a waxy cuticle. Pores
called stomata, usually covered by
pairs of guard cells, permit
transpiration
Guard cells change
their volume to
open and close the
pore. Guard cells
are sensitive to the
atmospheric
conditions and the
plant’s needs for
gas exchange and
water conservation
© 2013 American Society of Plant Biologists
Functional unit of the phloem: sieve
element – companion cell complex
Oparka, K.J. and Turgeon, R. (1999). Sieve elements and companion cells—Traffic control centers of the phloem. Plant Cell. 11: 739-750;
Mullendore, D.L., Windt, C.W., Van As, H. and Knoblauch, M. (2010). Sieve tube geometry in relation to phloem flow. Plant Cell. 22: 579-593.
Companion
cell The companion cell loads
the sieve element and
provides it with genetic
materials, through
extensive plasmodesmatal
connections
Functional sieve
elements lack nuclei
and central vacuoles,
facilitating flow
The ends of
adjoining sieve
elements are
connected by pores
derived from
enlarged
plasmodesmata
© 2013 American Society of Plant Biologists
Transport in the phloem
Sugars
1. Sugars are loaded into the
phloem by active transport
2. Water moves
in by osmosis
3. The fluid
moves
through the
sieve
elements
under
pressure, by
bulk flow (like
water in a
hose) 4. Sugars are released
into the sink tissues
5. Water
follows by
osmosis
From source
to sink
CC SE
Adapted from Lucas, W.J. et al. (2013). The plant vascular system: Evolution, development and functions. J. Integr. Plant Biol. 55: 294-388.
© 2013 American Society of Plant Biologists
Vascular tissues are essential
conduits for information flow
Reprinted from Schachtman, D.P. and Goodger, J.Q.D. (2008). Chemical root to shoot signaling under drought. Trends Plant Sci. 13: 281-287 with permission from
Elsevier; see also Christmann, A., Grill, E. and Huang, J. (2013). Hydraulic signals in long-distance signaling. Curr. Opin. Plant Biol. 16: 293-300.
In animals, signals
moving through the
nervous and
circulatory systems
convey information
Some signals
move in the
xylem. Signals
from drought-
stressed roots
cause guard
cells to close
Other xylem-borne
signals convey
information about
nutrient availability
and soil-microbes
© 2013 American Society of Plant Biologists
The phloem is also responsible for
long-distance signaling
Time to reproduce:
Under appropriate day-
length conditions (i.e.
seasons) the protein
FLOWERING LOCUS T
(FT) and its orthologs
move from leaves to the
shoot apex, to promote
the transition to
reproductive growth
Coordination of root and
shoot functions:
When leaves are
phosphate-limited, a
microRNA (miR399)
moves through the phloem
to the root and promotes
phosphate uptake
FT
miR399
See for example Chiou, T.-J., et al. (2006). Regulation of phosphate homeostasis by microRNA in Arabidopsis. Plant Cell. 18: 412-421; McGarry, R.C. and Kragler, F. (2013). Phloem-mobile signals affecting
flowers: applications for crop breeding. Trends Plant Sci. 18: 198-206; Lucas, W.J. et al. (2013). The plant vascular system: Evolution, development and functions. J. Integr. Plant Biol. 55: 294-388.
© 2013 American Society of Plant Biologists
Opportunities and challenges of the
terrestrial environment
>450 million years ago, plants moved onto dry land
However, plants had to
overcome significant challenges
to adapt to dry land
The aquatic environment
was competitive and filled
with herbivores
Initially, land offered
less competition and
fewer herbivores
© 2013 American Society of Plant Biologists
The terrestrial environment is
challenging
Aquatic environment:
Buoyancy
Abundant water
Moderate temperatures
Filtered light
Terresrial environment:
No buoyancy
Scarce water
Extreme temperatures
Excess light including UV
Too heavy, dry, hot and cold and bright
© 2013 American Society of Plant Biologists
As sessile organisms, plants have to
respond and adjust to stress
Like animals,
plants sense and
respond to the
physical conditions
of their
environment:
temperature,
humidity, light
intensity
© 2013 American Society of Plant Biologists
Obtaining and retaining water is a
challenge for terrestrial plants
Freshwater green
algae easily take up
water from their
aquatic environment
The water potential of air
and soil is usually lower
than that of the plant
cells. How do terrestrial
plants survive?
© 2013 American Society of Plant Biologists
Desiccation tolerance or avoidance,
drought evasion or tolerance Most bryophytes can tolerate
desiccation (drying out extensively)
Some desert plants evade
drought. They survive the
dry season as seeds,
sprouting and flowering in
a brief period of rain
Some desert plants
tolerate dry conditions
through adaptations such
as deep roots, C4
photosynthesis, and tiny or
absent leaves
Most
tracheophytes
cannot tolerate
desiccation –
they die
Photo credits: Mary Williams; Amrum; Scott Bauer; James Henderson, Golden Delight Honey, Bugwood.org
© 2013 American Society of Plant Biologists
Desiccation tolerance or desiccation
avoidance
Photo credits: Mel Oliver, IRRI, IRRI
Most bryophytes (and a few
angiosperms) can tolerate
desiccation
The vegetative tissues of
most angiosperms cannot
tolerate desiccation, but the
seeds can
Desiccated
and alive
Desiccated
but DEAD
The cells adjust with external water
availability, and can survive desiccation
Cuticle
The cells do not really adjust with external water availability,
and die if totally desiccated
Decreasing water availability
© 2013 American Society of Plant Biologists
Strategies for desiccation tolerance
include stabilization and repair
Reprinted from Hoekstra, F.A., Golovina, E.A. and Buitink, J. (2001). Mechanisms of plant desiccation tolerance. Trends Plant Sci. 6: 431-438 with permission from Elsevier; Vander
Willigen, C., Pammenter, N.W., Mundree, S.G. and Farrant, J.M. (2004). Mechanical stabilization of desiccated vegetative tissues of the resurrection grass Eragrostis nindensis: does a
TIP 3;1 and/or compartmentalization of subcellular components and metabolites play a role? Journal Exp. Bot. 55: 651-661 by permission of Oxford University Press.
Craterostigma
pumilumI, an
angiosperm
desiccation tolerant
“resurrection plant”
Upon desiccation, the
leaves curl in, purple
anthocyanin
photoprotective pigments
accumulate, and small
molecules that stabilize
cell integrity accumulate
Desiccation affects
membranes and walls,
which must be rapidly
repaired on rehydration
© 2013 American Society of Plant Biologists
By contrast, most tracheophytes
avoid desiccation
Vascular
tissue
Roots
Tracheophytes have:
• A waxy cuticle that covers their aerial
tissues
• Regulated pores (stomata) for water
and gas exchange
• Lignified vascular tissues that
conduct water
• Roots specialized for water and
nutrient uptake
Cuticle Stomata
Image sources:B.W. Wells lantern slides, (UA023.039) NCSU Libraries, Yvan Lindekens; Girard, A.-L., et al., (2012). Tomato GDSL1 is required for cutin deposition in the fruit cuticle. Plant Cell. 24: 3119-3134;
Reprinted from Li, H.-F., et al. (2011). Vessel elements present in the secondary xylem of Trochodendron and Tetracentron (Trochodendraceae). Flora. 206: 595-600 with permission from Elsevier.
© 2013 American Society of Plant Biologists
Plants can survive across most of
the earth Mountain Arctic
Desert Antarctic
Photo credits: Hannes Grobe, AWI; Gnomefilliere; Liam Quinn; Florence Devouard
© 2013 American Society of Plant Biologists
Plants can adapt and acclimate to
temperature extremes
Adapted from Berry, J., and Björkman, O. (1980). Photosynthetic response and adaptation to temperature in higher plants. Annu. Rev. Plant Physiol. 31: 491 – 543. Photo credit Sue in Az
Leaf temperature (°C)
Photo
synth
etic r
ate
10 20 30 40 50
Desert
species
Coastal
species
Leaf temperature (°C)
Photo
synth
etic r
ate
10 20 30 40 50
Desert species acclimated
to high temperatures
Acclimated to
cool
temperatures
To a certain extent, most
plants can acclimate to
higher or lower
temperatures, by sensing
and responding to a
change in temperature
Some plants have
evolved to tolerate high
temperatures
Larrea tridentata, an
extremely thermotolerant
desert plant
© 2013 American Society of Plant Biologists
How long can bryophytes tolerate
extreme cold? 400 years!
Behind a retreating glacier,
scientists found viable
bryophytes that laid dormant
under the ice for ~400 years
La Farge, C., Williams, K.H. and England, J.H. (2013). Regeneration of Little Ice Age bryophytes emerging from a polar glacier
with implications of totipotency in extreme environments. Proc. Natl. Acad. Sci. 110: 9839-9844.
© 2013 American Society of Plant Biologists
Light is good, but too much light is
damaging
Image credits: Dennis Haugen, Bugwood.org; Tomas Sobek; Reprinted by permission from Macmillan Publishers Ltd from Kasahara, M., Kagawa, T.,
Oikawa, K., Suetsugu, N., Miyao, M. and Wada, M. (2002). Chloroplast avoidance movement reduces photodamage in plants. Nature. 420: 829-832.
When light is too
bright, chloroplasts
can move to reduce
light incidence Photoprotective
pigments can
protect delicate
light-harvesting
machinery
Vertical orientation
minimizes light
absorption at midday
© 2013 American Society of Plant Biologists
Image credit: Forest & Kim Starr, Starr Environmental, Bugwood.org
SUMMARY
Like animals, plants need
energy, water, and the ability to
tolerate environmental
challenges
Plants have endosymbiotic
photosynthetic organelles that
let them produce chemical
energy from light
The two groups of plants,
bryophytes and tracheophytes,
differ in size, how they move
materials, and how they deal
with desiccation
© 2013 American Society of Plant Biologists
Plants are green, but they aren’t
aliens. (They were here first!)
Photo credits: Tom Donald