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Plant Responses to
Environmental Challenges
40
Plant–Pathogen Interactions
• Pathogens have mechanisms for attacking plants,
while plants have mechanical and chemical
defenses to protect themselves.
• Each set of mechanisms uses information from
the other.
• For example, pathogens may break down a
plant’s cell walls, and the breakdown products
signal the plant that it is under attack.
40
Plant–Pathogen Interactions
• Plant tissues such as epidermis are protected from
pathogens by cutin, suberin, or waxes.
• Animals tend to repair damaged tissues; plants seal
them off so the rest of the plant is not infected.
40
Plant–Pathogen Interactions
• Chemical defenses include phytoalexins and
pathogenesis-related proteins.
• Infected plants cells produce phytoalexins within
hours.
• Phytoalexins destroy many species of fungi and
bacteria nonspecifically.
• Pathogenesis-related, or PR, proteins are
enzymes that digest the cell walls of pathogens.
• Other PR proteins may serve as alarm signals to
cells that have not yet been attacked.
Figure 40.1 Signaling between Plants and Pathogens
40
Plant–Pathogen Interactions
• Many plants that are resistant to fungal, bacterial,
or viral diseases use a strategy known as the
hypersensitive response.
• Cells around the site of infection rapidly die,
preventing access to nutrients by the pathogen.
• Phytoalexins and other chemicals are produced by
the dying cells. The invading pathogen is contained
within the dead tissue, called a necrotic lesion.
• Salicylic acid is a defense molecule produced by
willow and is the active ingredient of aspirin.
Figure 40.2 The Aftermath of a Hypersensitive Response
40
Plants and Herbivores: Benefits and Losses
• Grazing involves a predator eating part of a plant
without killing it.
• Grazing actually increases photosynthetic
production in some plants.
• Experimentally removing leaves from a plant has
the same effect. The remaining leaves have
increased production.
40
Plants and Herbivores: Benefits and Losses
• Scarlet gilia is an example of a plant that benefits
from grazing.
• Grazing removes 95% of aboveground parts.
• Each plant quickly grows four replacement stems for
each one eaten.
• Grazed plants produce three times as many fruits as
ungrazed plants.
• Some grazed trees and shrubs continue to grow
until much later in the season than ungrazed plants.
Figure 40.4 Overcompensation for Being Eaten
40
Plants and Herbivores: Benefits and Losses
• Many plant defenses are activated by a series of
signals.
• Tomato leaf damage by a caterpillar’s chewing
triggers a chain of events.
• The signaling process involves two hormones.
• The final step is the production of a protease
inhibitor, which interferes with the insect’s ability
to digest.
• The hormones also act as attractants to insects
that prey on the caterpillars.
Figure 40.5 A Signaling Pathway for Synthesis of a Defensive Secondary Metabolite
40
Plants and Herbivores: Benefits and Losses
• Other plants produce a toxin to ward off
predators.
• Arcelin is a protein produced by wild bean seeds
that confers resistance to bean weevils.
• Plants are now being genetically engineered to
produce pesticides such as arcelin.
• Many crop plants have been genetically
engineered to produce the Bt toxin.
40
40
40
Water Extremes: Dry Soils and Saturated Soils
• Some plants evade drought by carrying out their entire
life cycle from seed to seed during a brief period of
rainfall.
40
Water Extremes: Dry Soils and Saturated Soils
• Other plants have structural
adaptations to minimize water
loss. These include
 heavy cuticles, a dense
covering of epidermal hairs,
and sunken stomata.
 the possession of fleshy,
water-storing leaves.
 producing leaves only when
water is available.
 having spines instead of
leaves, and fleshy stems, as
cacti do.
 leaves that hang vertically,
avoiding the midday sun, as
seen in Eucalyptus trees.
Figure 40.8 Stomatal Crypts
Figure 40.9 Opportune Leaf Production
Leaves develop rapidly after rain.
40
Water Extremes: Dry Soils and Saturated Soils
• These adaptations to dry environments minimize
water loss, but also minimize the uptake of CO2.
• In consequence, these plants usually grow more
slowly but utilize water more efficiently than other
plants do, fixing more grams of carbon by
photosynthesis per gram of water lost to
transpiration.
40
Water Extremes: Dry Soils and Saturated Soils
• Mesquite trees can grow in arid environments
using a taproot that grows to great depths.
• Other plants have roots that die back during dry
seasons but grow rapidly during rainy seasons.
40
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Water Extremes: Dry Soils and Saturated Soils
• Some plants live at the other extreme – water
saturated environments where diffusion of oxygen
to the roots is limited.
• Some species have shallow root systems that
carry on alcoholic fermentation. This results in
slow growth.
• Some roots have extensions that grow out of the
water and up into the air.
Figure 40.11 Coming Up for Air
Mongroves
Cypress Tree
40
Water Extremes: Dry Soils and Saturated Soils
• Aquatic plants often have special tissue with large air
spaces that stores oxygen produced by photosynthesis
and permits its diffusion to other plant parts.
• This also imparts buoyancy.
40
Habitats Laden with Heavy Metals
• Some plants can survive on
mine tailings, which have
metal contaminants at levels
toxic to most other plants.
• Rather than excluding heavy
metals, tolerant plants deal
with them after taking them
up.
• Such tolerant plants may be
useful for bioremediation, or
the decontamination of an
area by using living
organisms.
Wild pansy can
grow in metalcontaminated
soil.
40
Hot and Cold Environments
• Temperatures that are too high or too low stress
plants.
• High temperatures destabilize membranes and
denature proteins.
• Low temperature causes membranes to lose
fluidity and alters their permeability to solutions.
• Freezing temperatures cause ice crystals to form,
damaging cellular membranes.
40
Hot and Cold Environments
• Plants produce heat shock proteins in response to
high temperatures.
• Some of these proteins are chaperonins that help other
proteins maintain their structures and avoid
denaturation.
40
Hot and Cold Environments
• Low (just above freezing) temperatures can harm
many plants.
• Many plants can be modified to resist the effects of
cold spells by producing more unsaturated fatty
acids in the plant membranes.
• Unsaturated fatty acids solidify at lower
temperatures than saturated ones do, so the
membranes retain their fluidity and function
normally at cooler temperatures.
• Some freezing-tolerant plants have antifreeze
proteins that inhibit the growth of ice crystals.