Plant Responses to
Environmental Challenges
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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.
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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.
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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
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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
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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.
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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
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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.