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39
Plant Responses to
Environmental Challenges
39 Plant Responses to Environmental Challenges
• 39.1 How Do Plants Deal with Pathogens?
• 39.2 How Do Plants Deal with Herbivores?
• 39.3 How Do Plants Deal with Climate
Extremes?
• 39.4 How Do Plants Deal with Salt and
Heavy Metals?
39.1 How Do Plants Deal with Pathogens?
Plants and pathogens have been
evolving together in an evolutionary
“arms race.”
Pathogens evolve mechanisms to attack
plants, plants evolve defenses against
pathogens.
Plants use both mechanical and chemical
defenses.
39.1 How Do Plants Deal with Pathogens?
The epidermis and cork tissues protect
the outer surfaces; they often have
cutin, suberin, or waxes.
If pathogens pass these barriers, other
nonspecific mechanisms are used.
39.1 How Do Plants Deal with Pathogens?
Plants generally do not repair tissue
damaged by pathogens, but instead
seal it off to prevent the rest of the plant
from being infected.
Because plants are modular, they can
grow new modules to replace damaged
ones.
39.1 How Do Plants Deal with Pathogens?
One of plant cell’s first lines of defense is
to deposit more polysaccharides on the
inside of the cell wall to reinforce this
barrier.
The polysaccharides block the
plasmodesmata, and serve as a base
for laying down lignin. Lignin is a
mechanical barrier, and the precursors
are toxic.
Figure 39.1 Signaling between Plants and Pathogens
39.1 How Do Plants Deal with Pathogens?
Plants produce many kinds of defensive
compounds.
Phytoalexins are toxic to many fungi
and bacteria. Most are phenolics or
terpenes, and protect against
herbivores as well.
Table 39.1 Secondary Plant Metabolites Used in Defense
39.1 How Do Plants Deal with Pathogens?
Enzymes from a pathogenic fungus
cause plant cell walls to release
signaling molecules, oligosaccharins,
which trigger phytoalexin production.
The action is nonspecific, so they can kill
many species in addition to the one that
triggered their production.
39.1 How Do Plants Deal with Pathogens?
Pathogenesis-related proteins (PR
proteins):
Some are enzymes that break down the
cell walls of pathogens. Chemicals
released from the walls can trigger other
defense mechanisms.
Other PR proteins serve as signals of
attack to other cells.
39.1 How Do Plants Deal with Pathogens?
Hypersensitive response: cells around
site of infection die, preventing nutrients
from reaching site. Some produce
phytoalexins before they die.
Dead tissue is called a necrotic lesion,
contains and isolates what is left of the
microbial invasion.
Figure 39.2 The Aftermath of a Hypersensitive Response
39.1 How Do Plants Deal with Pathogens?
One chemical produced during the
hypersensitive response is salicylic
acid, from which aspirin is derived.
People have used willow (Salix) since
ancient times for pain and fever.
Now it appears that all plants have some
salicylic acid. It often evokes a second
complex defensive response.
39.1 How Do Plants Deal with Pathogens?
Systemic acquired resistance: general
increase in resistance of whole plant to
a wide range of pathogens. Can be
long-lasting.
Accompanied by production of PR
proteins.
Salicylic acid treatment provides
protection against tobacco mosaic virus
and other viruses.
39.1 How Do Plants Deal with Pathogens?
Salicylic acid also acts as a hormone.
A microbial infection in one part of a plant
leads to export of salicylic acid to other
parts of the plant, and PR proteins are
produced to help stop the spread of
infection.
Infected plant parts also produce methyl
salicylate, which is volatile and may travel
in the air to nearby plants, where it
triggers PR protein production.
39.1 How Do Plants Deal with Pathogens?
Triggering of both responses is a very
specific mechanism: gene-for-gene
resistance.
Plants have many R genes (resistance
genes); pathogens have sets of Avr
genes (avirulence genes).
Dominant R alleles favor resistance;
dominant Avr alleles make pathogens
less effective.
39.1 How Do Plants Deal with Pathogens?
If a plant has a dominant allele of one R
gene and a pathogen has a dominant
allele for the corresponding Avr gene,
the plant will be resistant.
This is true even if none of the other R–
Avr pairs have corresponding dominant
alleles (epistasis).
Most gene-for-gene interactions trigger
the hypersensitive response.
Figure 39.3 Gene-for-Gene Resistance
39.1 How Do Plants Deal with Pathogens?
Plant response to RNA viruses:
• Plant uses its own enzymes to convert
virus RNA to double-stranded RNAs
(dsRNA), and to chop it into small
pieces—small interfering RNAs
(siRNA).
• Some of the viral RNA is transcribed,
but the siRNAs degrade the mRNA,
blocking viral replication.
39.1 How Do Plants Deal with Pathogens?
• This is an example of RNA interference
(RNAi), or posttranscriptional gene
silencing.
• Immunity conferred by RNAi spreads
quickly through the plant.
• Plant viruses fight back by evolving
mechanisms to confound RNAi.
39.2 How Do Plants Deal with Herbivores?
Herbivores are predators that prey on
plants. They can have positive and
negative effects on plants.
Grazing: herbivores eat part of plant
without killing it.
Plants and their predators have evolved
together; has favored increased
production in some grazed-upon plant
species.
39.2 How Do Plants Deal with Herbivores?
Removing some leaves from a plant can
increase photosynthesis in remaining
leaves.
Nitrogen doesn’t have to be divided
among so many leaves; remaining
leaves may increase production to keep
up with demand from roots;
Grazing can decrease shading of
younger, more active leaves.
39.2 How Do Plants Deal with Herbivores?
Grasses grow from the base of the shoot,
so growth is not cut short by grazing.
Scarlet gilia, grazed by elk and mule
deer, regrows four stems instead of one,
and produce three times as many fruits
as ungrazed plants.
Figure 39.4 Overcompensation for Being Eaten
39.2 How Do Plants Deal with Herbivores?
Plants may benefit from herbivory
because the animals may spread its
seeds, fruit, or pollen.
But resisting herbivory is often
advantageous for plants.
Lepidopteran larvae feeding on leaves
Raphide extrusion in Dieffenbachia
39.2 How Do Plants Deal with Herbivores?
Many plants have chemical defenses,
which are secondary metabolites.
Primary metabolites are proteins, nucleic
acids, lipids, and carbohydrates that are
produced and used by all living things.
Secondary metabolites are compounds
not used for basic metabolism. Plants
can have vastly different secondary
metabolites.
39.2 How Do Plants Deal with Herbivores?
There are more than 10,000 known
secondary plant metabolites. Some are
produced by a single species, others
characteristic of whole groups.
They have many different effects. Some
act on the nervous system of
herbivores; some mimic insect
hormones; some damage digestive
tracts.
39.2 How Do Plants Deal with Herbivores?
Humans use many secondary
metabolites as pesticides and
pharmaceuticals.
Nicotine was one of first insecticides to
be used. This was tested in tobacco
plants in which the enzyme for nicotine
synthesis had been silenced. These
plants suffered much more damage
than wild types.
Figure 39.5 Some Plants Use Nicotine to Reduce Insect Attacks (Part 1)
Figure 39.5 Some Plants Use Nicotine to Reduce Insect Attacks (Part 2)
39.2 How Do Plants Deal with Herbivores?
Canavanine is an amino acid similar to
arginine, but is not found in proteins.
39.2 How Do Plants Deal with Herbivores?
Canavanine is a nitrogen-storing
compound in seeds, and also an
insecticide.
When insects eat plant tissue with
canavanine, it is incorporated into
proteins where arginine should be. This
results in defective proteins and
developmental abnormalities and death
for the insect.
39.2 How Do Plants Deal with Herbivores?
Some insect larvae can eat canavaninecontaining plants. They have tRNA that
can discriminate between arginine and
canavanine, so proteins are made
correctly.
39.2 How Do Plants Deal with Herbivores?
When some plants are attacked by
herbivorous insects, they synthesize
chemical signals to attract other insects
to prey on the herbivores.
Most signals are produced in the leaves,
but roots attacked by beetle larvae were
found to send signals to attract
predaceous nematodes.
Figure 39.6 Roots May Recruit Nematodes as Defenders (Part 1)
Figure 39.6 Roots May Recruit Nematodes as Defenders (Part 2)
Figure 39.6 Roots May Recruit Nematodes as Defenders (Part 3)
39.2 How Do Plants Deal with Herbivores?
Many plant defenses involve complex
signaling.
Systemin is a polypeptide hormone
formed in response to insect herbivores.
It initiates production of jasmonates from
linolenic acid. These activate genes that
encode for a protease inhibitor.
The inhibitor interferes with protein
digestion in the insect and stunts growth.
Figure 39.7 A Signaling Pathway for Synthesis of a Defensive Secondary Metabolite
39.2 How Do Plants Deal with Herbivores?
Jasmonates are also formed when
signals are released from leaves by
chewing caterpillars.
Jasmonates trigger production of volatile
compounds that attract insect predators
of the caterpillars.
39.2 How Do Plants Deal with Herbivores?
In experiments with wild and cultivated
beans, researchers determined that the
seed protein arcelin in the wild plants
conferred resistance to two species of
bean weevils.
Work is now being done using
recombinant DNA to introduce genes for
arcelin into crop plants.
39.2 How Do Plants Deal with Herbivores?
Crop plants that produce their own
pesticides have already been
engineered.
Tomatoes, corn, and cotton have been
engineered to express the gene for the
toxin from a bacterium Bacillus
thuringiensis, which kills insects.
39.2 How Do Plants Deal with Herbivores?
Plants that produce toxic secondary
metabolites protect themselves from the
toxin by:
• Isolating the toxin in special
compartments.
• Producing toxic substances only after
the cells have been damaged.
• Using modified enzymes or receptors
that do not recognize the toxins (e.g.,
canavanine-producing plants).
39.2 How Do Plants Deal with Herbivores?
Plants store toxins in vacuoles if toxin is
water-soluble.
If toxin is hydrophobic, it can be stored in
lacticifers (tubes containing rubbery
latex) or dissolved in waxes on the
cuticle.
39.2 How Do Plants Deal with Herbivores?
Some plants store precursors of the toxin
in one place, and enzymes to convert it
to a toxic form in another place. Toxin is
produced only if a cell is damaged.
Sorghum and some legumes produce
cyanide (inhibits cellular respiration) in
this way.
39.2 How Do Plants Deal with Herbivores?
Milkweeds are latex-producing plants;
they are lacticiferous).
When damaged, a large amount of latex
is released, keeping some insects from
eating the leaves.
One beetle that feeds on milkweed first
cuts some leaf veins and causes latex
flow, then moves to an “upstream” part
of the leaf to feed.
Figure 39.8 Disarming a Plant’s Defenses
39.3 How Do Plants Deal with Climate Extremes?
Desert plants may have structural
adaptations for water conservation, or
alternative strategies.
Some desert plants carry out their entire
life cycle in a brief period of rainfall.
Plants adapted to dry environments are
called xerophytes (xero = Greek, “dry”)
Figure 39.9 Desert Annuals Evade Drought
39.3 How Do Plants Deal with Climate Extremes?
Structural adaptations include thick
cuticles or a dense covering of hairs to
retard water loss.
The stomata may be in sunken pits below
the surface, which may have hairs as
well, called stomatal crypts.
Figure 39.10 Stomatal Crypts
39.3 How Do Plants Deal with Climate Extremes?
Many have thick, water storing leaves—
succulence.
Some produce leaves only when water is
available, (e.g., ocotillo).
Cacti have spines instead of typical
leaves; photosynthesis occurs in the
stems.
Figure 39.11 Opportune Leaf Production
39.3 How Do Plants Deal with Climate Extremes?
Corn and other grasses have leaves that
roll up in dry weather, reducing leaf
surface area for water loss.
Eucalyptus trees have leaves that hang
vertically all the time, avoiding direct
exposure to the sun.
Xerophytic adaptations can also minimize
uptake of CO2—many grow slowly but
use water more efficiently.
39.3 How Do Plants Deal with Climate Extremes?
Roots may also have adaptations.
Mesquite trees have long taproots,
reaching water far underground.
Cacti have shallow root systems that
intercept water near the surface; may
die back during dry periods.
Figure 39.12 Mining Water with Deep Taproots
39.3 How Do Plants Deal with Climate Extremes?
Some xerophytes accumulate proline and
other solutes in central vacuoles.
Solute potential and water potential of
cells become more negative, they can
extract more water from the soil.
Plants in salty environments also do this.
39.3 How Do Plants Deal with Climate Extremes?
Some plants live in environments with too
much water in the soil.
Saturated soils limit diffusion of oxygen to
roots.
Adaptations include shallow, slowgrowing roots that can carry out
alcoholic fermentation.
39.3 How Do Plants Deal with Climate Extremes?
Pneumatophores are root extensions
that grow up out of the water; have
many lenticels and spongy tissue for
gas exchange, (e.g., cypresses and
mangroves).
Figure 39.13 Coming Up for Air
39.3 How Do Plants Deal with Climate Extremes?
Aquatic plants may have large air spaces
in the parenchyma of leaf petioles,
called aerenchyma.
Oxygen produced by photosynthesis is
stored there, and diffuses to other plant
parts for respiration.
Aerenchyma also provides buoyancy.
Figure 39.14 Aerenchyma Lets Oxygen Reach Submerged Tissues
39.3 How Do Plants Deal with Climate Extremes?
Temperature extremes can also stress
plants.
• High temperatures can denature
proteins and destabilize membranes.
• Low temperatures cause membranes to
loose fluidity and alter permeability.
• Freezing can cause ice crystals to form,
damaging membranes.
39.3 How Do Plants Deal with Climate Extremes?
Transpiration can cool a plant, but
increases its need for water.
Plants in hot environments have many
adaptations similar to xerophytes.
Spines and hairs help radiate heat.
CAM metabolism allows some metabolic
processes to occur at night.
39.3 How Do Plants Deal with Climate Extremes?
Plants can respond to high temperatures
by producing heat shock proteins.
Some are chaperonins, which help
prevent other proteins from denaturing.
Chilling and freezing can also produce
these proteins.
39.3 How Do Plants Deal with Climate Extremes?
Low temperatures (above freezing) can
harm some plants, (e.g., corn, cotton,
tropical plants)—chilling injury.
Many plants can be cold-hardened to
resist chilling injury—repeated exposure
to cool temperatures.
The relative amount of unsaturated fatty
acids in cell membranes increases—they
solidify at lower temperatures,
membranes retain fluidity.
39.3 How Do Plants Deal with Climate Extremes?
Low temperatures can induce formation
of heat shock proteins that protect
against chilling injury.
Sometimes heat shock proteins can
provide “cross protection,” protecting
against both heat and low temperatures.
39.3 How Do Plants Deal with Climate Extremes?
Ice crystals inside cells can puncture
organelles and membranes.
If ice crystals grow outside the cell, they
draw water from the cell and can
dehydrate them.
Freeze-tolerant plants have many
adaptations to cope, including
production of antifreeze proteins that
slow formation of ice crystals.
39.4 How Do Plants Deal with Salt and Heavy Metals?
Saline (salty) habitats support few plant
species. They can be hot and dry, or
cool and moist.
Salinization of agricultural land is an
increasing global problem, which can
occur in irrigated lands, and from salt
water intrusion.
39.4 How Do Plants Deal with Salt and Heavy Metals?
Saline environments pose osmotic
problems for plants, because of the
negative water potential of the
environment.
Plants must have an even more negative
water potential to obtain water from that
environment.
Salt ions are also toxic in large quantities.
39.4 How Do Plants Deal with Salt and Heavy Metals?
Halophytes are plants adapted to saline
conditions.
Most accumulate sodium and chloride
ions in vacuoles. The increased salt
concentration makes the water potential
of cells more negative.
39.4 How Do Plants Deal with Salt and Heavy Metals?
Some halophytes have salt glands in
their leaves that excrete salt (e.g., some
desert plants such as tamarisk, and
some mangroves).
One desert shrub has glands that secret
salt into small bladders on leaves which
increases gradient in water potential,
helping leaves get more water from
roots. Also reduces transpirational loss
of water.
Figure 39.15 Excreting Salt
39.4 How Do Plants Deal with Salt and Heavy Metals?
Halophytes and xerophytes have similar
adaptations.
Some accumulate proline in vacuoles,
making water potential of cells more
negative. It is not toxic, unlike sodium.
Succulents occur in saline environments
where uptake of water is difficult.
39.4 How Do Plants Deal with Salt and Heavy Metals?
Many succulents, both halophytes and
xerophytes, have CAM—crassulacean
acid metabolism.
CO2 can be taken up at night and stored
as carboxyl groups, the CO2 is released
for photosynthesis during the day.
Allows stomata to be closed during the
day, minimizing water losses.
Figure 39.16 Stomatal Cycles
39.4 How Do Plants Deal with Salt and Heavy Metals?
Heavy metals are also toxic. Some sites
naturally have high concentrations of
chromium, mercury, lead, and others.
Other sites include acid rain-affected
sites (leads to release of aluminum from
soils), and mine sites.
Mine tailings have high concentrations of
heavy metals, but some plants can
survive.
Figure 39.17 Life after Strip Mining
39.4 How Do Plants Deal with Salt and Heavy Metals?
Tolerant plants take up the heavy metals,
in concentrations that would kill other
plants. This is being used in
bioremediation—decontaminating sites
by using living organisms.
The mechanism for tolerance is known
for one plant—a buckwheat exposed to
aluminum secretes oxalic acid which
combines with the Al and does not
inhibit root growth.
39.4 How Do Plants Deal with Salt and Heavy Metals?
Tests of bent grass, which grows near
mines showed that plants tolerated the
metals that had been most abundant in
their environment, but were sensitive to
others.
Tolerant populations can colonize new
environments rapidly (e.g., bent grass
around one copper mine is tolerant of
copper, but the copper mining was only
100 years ago).