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Plants in the Environment

Chapter 28 Plants in the Environment

Key Concepts

• 28.1 Plants Have Constitutive and Induced Responses to Pathogens

• 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

• 28.3 Plants Adapt to Environmental Stresses

Chapter 28 Opening Question

How can knowledge of plant and fungal biology be used to prevent the spread of wheat rust?

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Plant pathogens include fungi, bacteria, protists, and viruses.

Plants and pathogens have evolved together in a continuing “arms race.”

Plant responses can be either:

Constitutive—always present

Induced—produced in reaction to presence of a pathogen.

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Constitutive defenses

Leaves and stems have cutin, suberin, and waxes, which help prevent fungal spores and bacteria from entering.

Some plants make chemicals that inhibit pathogens.

Example: plantains produce iridoid glycosides, which inhibit growth of fungal pathogens.

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Induced responses

Gene-for-gene resistance:

Elicitors—molecules made by pathogens that trigger plant defenses.

Pathogen genes that code for elicitors are called avirulence (Avr) genes.

Plants have resistance (R) genes that encode receptors specific for one or a few elicitors.

Figure 28.1 Pathogens Induce Plant Resistance

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

If plant does not have a receptor to bind the elicitor, the pathogen is not recognized, and plant is susceptible to invasion.

A major goal of plant breeders has been to identify R and Avr genes and breed new R genes into crops to increase pathogen resistance.

Figure 28.2 Genes and the Response to a Pathogen

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Binding of elicitor and receptor starts a signal transduction pathway that results in local and then systemic defenses.

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Hypersensitive response (local); 3 components:

1. Phytoalexins are produced within hours; they are antibiotics—toxic to pathogens; example: camalexin

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Hypersensitive response

2. Pathogenesis-related (PR) proteins

Some break down pathogen cell walls; chitinase breaks down walls of fungal cells.

The breakdown products may act as elicitors that trigger further responses.

Other PR proteins signal plant cells that have not yet been attacked so they can initiate defenses.

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Hypersensitive response

3. Physical isolation—damaged plant tissue with pathogen is sealed off.

Cells around the infection die, cutting off nutrients to the pathogen. Some cells produce toxins before dying; others produce polysaccharides to seal off the plasmodesmata.

The dead tissue (a necrotic lesion) contains and isolates what is left of the infection.

Figure 28.3 Sealing Off the Pathogen and the Damage

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Systemic acquired resistance—general increase in resistance of the entire plant to a range of pathogens.

Initiated by salicylic acid, produced during the local hypersensitive response:

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Salicylic acid may be transported to other parts of the plant and trigger production of PR proteins.

Methyl salicylate is volatile and travels through the air. It may trigger production of PR proteins in neighboring plants.

Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens

Another type of systemic acquired resistance is specific against RNA viruses:

Plant uses its own enzymes to convert single- stranded virus RNA to double-stranded RNA and chop it into small interfering RNAs (siRNAs).

siRNAs help to degrade viral mRNAs, blocking replication (RNA interference (RNAi).

siRNAs spread via plasmodesmata, providing systemic resistance.

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Most herbivores (plant-eaters) are insects.

Plants have constitutive and induced mechanisms to protect themselves from herbivory.

Figure 28.4 Insect Herbivores (Part 1)

Figure 28.4 Insect Herbivores (Part 2)

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Constitutive defenses

Plants have many physical features to deter herbivores: trichomes (leaf hairs), thorns, spines, thick cell walls, tree bark.

Chemical defense: secondary metabolites— not used for basic cell metabolism.

More than 10,000 are known; there are diverse mechanisms of action.

Table 28.1 Secondary Metabolites Used in Plant Defense

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Canavanine—amino acid similar to arginine

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Canavanine is incorporated into the insect’s proteins where arginine should be, and tertiary structure is changed.

Leads to developmental abnormalities that kill the insect.

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Nicotine kills insects by inhibiting nervous system function.

In studies on tobacco plants, genes coding for an enzyme used in nicotine synthesis were turned off.

The resulting low-nicotine plants suffered much more damage from insect herbivores than normal plants.

Figure 28.5 Nicotine Is a Defense Against Herbivores (Part 1)

Figure 28.5 Nicotine Is a Defense Against Herbivores (Part 2)

Figure 28.5 Nicotine Is a Defense Against Herbivores (Part 3)

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Plants must first sense an herbivore attack; detection triggers signal transduction pathways that induce plant defenses.

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Membrane signaling:

When attacked by an herbivore, changes in the electric potential of the plasma membrane occur in the damaged area.

Continuity of the symplast ensures that the signal travels over much of the plant within 10 minutes.

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Chemical signaling:

Substances in insect’s saliva combine with plant fatty acids to form elicitors that trigger both local and systemic responses.

In corn, volicitin is an elicitor produced by a moth larva. It induces production of volatile signals that can travel to other plant parts and to neighboring corn plants.

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Signal transduction pathway:

Involves the plant hormone jasmonic acid (jasmonate)

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Jasmonate triggers many defenses, including synthesis of a protease inhibitor.

The inhibitor interferes with digestion of proteins in the insect’s gut, and thus stunts growth.

Also triggers production of volatile compounds to attract insects that prey on the herbivores.

Figure 28.6 A Signaling Pathway for Induced Defenses against Herbivory

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Why don’t the defensive chemicals harm the plants themselves?

• Toxic chemicals may be isolated in vacuoles, dissolved in latex in specialized compartments, or dissolved in waxes on the epidermal surface.

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

• The toxin precursors may be stored in one place, and enzymes to convert them stored in another place—they come in contact when cell is ruptured by insect chewing.

Sorghum and some legumes, which produce cyanide, use this method.

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

• The plant’s proteins are modified and don’t react to the toxin.

In plants that make canavanine, the plant enzyme that charges the arginine tRNA discriminates correctly between arginine and canavanine, so canavanine is not incorporated into the plant’s proteins.

Concept 28.2 Plants Have Mechanical and Chemical Defenses against Herbivores

Plants do not always win: Milkweeds store defensive chemicals in latex in specialized tubes called laticifers.

One type of beetle cuts a few leaf veins, causing leakage of the latex, then feeds on the leaf “downstream” of the leak.

In-Text Art, Ch. 28, p. 580

Concept 28.3 Plants Adapt to Environmental Stresses

Plants face many potential stresses in changing environments:

• Drought• Submersion• Heat• Cold• Salt • Heavy metals in the soil

Concept 28.3 Plants Adapt to Environmental Stresses

Adaptations to dry conditions—avoid or reduce water loss; tolerate high heat and light intensity.

• Drought avoiders—desert annuals carry out the entire life cycle during brief periods after a rain.

• Deciduous perennials in Africa and South America shed their leaves to conserve water during drought.

Figure 28.7 Desert Annuals Avoid Drought

Concept 28.3 Plants Adapt to Environmental Stresses

• Xerophytes—plants adapted to dry environments; have many adaptations in leaves and roots:

• A thick cuticle and many trichomes retard water loss

• Trichomes that diffract and diffuse sunlight

• Stomata may be located in sunken cavities (stomatal crypts) where they are sheltered from drying air currents.

Figure 28.8 Stomatal Crypts

Concept 28.3 Plants Adapt to Environmental Stresses

• Cacti have spines instead of leaves. The spines reflect solar radiation, dissipate heat, and deter herbivores.

• Succulence—having fleshy, water-storing leaves or stems. Plants take up water when it is available. Usually have fewer stomata.

• Shallow fibrous root systems intercept water at soil surface.

• The tamarugo tree has very deep taproots to reach groundwater.

Figure 28.9 Succulence

Figure 28.10 Mining Water with Deep Taproots

Concept 28.3 Plants Adapt to Environmental Stresses

Some xerophytes accumulate solutes—lowers water potential below that of soil and promotes water uptake by osmosis.

Concept 28.3 Plants Adapt to Environmental Stresses

Too much water:

In wet environments, diffusion of oxygen to roots is limited.

Shallow root systems—oxygen levels likely to be highest near surface.

Cypresses and mangroves have pneumatophores, root extensions that grow out of the water and up into the air; oxygen enters through lenticels.

Figure 28.11 Plant Adaptations to Saturated Habitats (Part 1)

Concept 28.3 Plants Adapt to Environmental Stresses

Submerged aquatic plants have large air spaces in the leaf and stem parenchyma and petioles, called aerenchyma.

Aerenchyma• Stores O2 produced by photosynthesis• Imparts buoyancy• Because it has fewer cells than other

tissues, metabolism is lower, requiring less oxygen.

Figure 28.11 Plant Adaptations to Saturated Habitats (Part 2)

Concept 28.3 Plants Adapt to Environmental Stresses

Drought stress:

Water deficits in plant cells reduce membrane integrity—forces that orient the lipid bilayer are reduced.

The 3-D structure of proteins can be changed.

Plant growth is reduced.

Concept 28.3 Plants Adapt to Environmental Stresses

Induced responses to drought stress:

When roots sense a water deficit, abscisic acid is produced, which starts a signaling pathway.

Abscisic acid travels to the shoot and causes closure of stomata and initiates gene transcription for other responses.

Late embryogenesis abundant (LEA) proteins bind to other proteins to stabilize them and prevent aggregation.

LEA proteins are also in maturing seeds.

Figure 28.12 A Signaling Pathway in Response to Drought Stress

Concept 28.3 Plants Adapt to Environmental Stresses

Temperature extremes:

High temperatures destabilize membranes and denature proteins, especially enzymes of photosynthesis.

Low temperatures alter permeabilities and cause membranes to lose fluidity.

Freezing may cause ice crystals to form, damaging membranes.

Concept 28.3 Plants Adapt to Environmental Stresses

In hot environments, plant have adaptations similar to xerophytes—hairs and spines to dissipate heat and leaf forms that intercept less sunlight.

Heat shock response—heat shock proteins are made in response to abscisic acid.

Includes chaperonins, which help other proteins maintain structure and avoid denaturation.

Concept 28.3 Plants Adapt to Environmental Stresses

Cold-hardening—acclimation by exposure to cooler temperatures over many days.

An increase in unsaturated fatty acids in cell membranes allows them to retain fluidity and function normally.

Proteins similar to heat shock proteins are also produced.

Concept 28.3 Plants Adapt to Environmental Stresses

Ice crystals inside cells can kill them by puncturing organelles and plasma membranes.

Ice crystals outside cells draw water from cells and dehydrate them.

Freeze-tolerant plants have antifreeze proteins that slow the growth of ice crystals.

Concept 28.3 Plants Adapt to Environmental Stresses

Saline (salty) environments:

High concentrations of Na+, K+, Ca2+, and Cl–

Deserts, coastal marshes, agricultural land that becomes saline from irrigation and use of chemical fertilizers (salinization).

Figure 28.13 Salty Soil

Concept 28.3 Plants Adapt to Environmental Stresses

Saline environments have very negative water potential, so plants must have an even more negative water potential.

Sodium ions can be toxic—they inhibit enzymes and protein synthesis.

Concept 28.3 Plants Adapt to Environmental Stresses

Halophytes—plants adapted to saline environments

Take up Na+ and Cl– into roots and transport them to the central vacuoles of leaf cells.

This makes water potential in the plant more negative than the soil solution, allowing uptake of water.

Concept 28.3 Plants Adapt to Environmental Stresses

Some halophytes have salt glands that excrete salts.

Salts accumulate on leaves until washed away or blown away.

Negative water potential in the salt-laden leaves also promotes water flow from the roots by transpiration–cohesion–tension.

Figure 28.14 Excreting Salt

Concept 28.3 Plants Adapt to Environmental Stresses

Some plants can tolerate heavy metals that are toxic to other plants.

Heavy metals can occur naturally in soils, or as a result of mining operations.

Hyperaccumulators store large quantities of metals such as arsenic, cadmium, nickel, aluminum, and zinc.

Concept 28.3 Plants Adapt to Environmental Stresses

Adaptations of hyperaccumulators:

• Increased ion transport into the roots

• Increased rates of translocation of ions to the leaves

• Accumulation of ions in vacuoles in the shoot

• Resistance to the ions’ toxicity

Concept 28.3 Plants Adapt to Environmental Stresses

Phytoremediation is a form of bioremediation using plants to clean up pollution in soils.

Hyperaccumulators are used; or their genes are used to create transgenic plants that grow rapidly and are otherwise adapted to the polluted environment.

The plants are then harvested and disposed of to remove the contaminants.

Figure 28.15 Phytoremediation

Answer to Opening Question

In the Green Revolution, wheat plants were bred to have R genes (Sr24 and Sr31) that conferred resistance to existing strains of wheat rust.

The first strain of Ug99 had Avr genes that form elicitor proteins that do not bind to Sr31.

In 2006, a new variant of Ug99 was found with additional Avr genes that allow the pathogen to overcome Sr24 gene resistance as well.

Figure 28.16 Overcoming Resistance to Pathogens

Answer to Opening Question

90% of the wheat grown in the world has no resistance to the new strain of Ug99.

Occasional plants that show resistance are sent to a laboratory in Minnesota where they are tested for Avr and R genes.

Seeds from thousands of varieties of wheat from all over the world are being grown and examined for R genes.