39 plant responses to environmental challenges. 39 plant responses to environmental challenges 39.1...
TRANSCRIPT
39Plant 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 canavanine-containing 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, slow-growing 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).