Chapter 31: Plant Responses to Internal and External Signals

Vernalization

A pretreatment with cold to induce flowering. For example winter wheat will not flower unless it has been exposed to several weeks of temp below 10°c

What is the difference between how plants and animals respond to their environment?

Although they both compete for resources and sense and recognize environmental stimuli in similar ways, animals respond to that stimuli by moving, while plants respond by altering their growth and development

Tropism

Any response resulting in curvature of organs towards or away from a stimulus.

The role of auxin in cell elongation:

As auxin moves down to the region of cell elongation, the hormone stimulates cell growth, probably by binding to a receptor in the plasma membrane. Only stimulates growth at a specific concentration, any higher and ethylene, a hormone that hinders growth is produced

Auxin's Role in Plant Development:

Auxin carries information about the development, size, and environment of branches. This flow of information controls branching patterns. A reduced flow of auxin from a branch, indicates that the branch is not being sufficiently productive, telling the cell that new branches are needed elsewhere, releasing lateral buds below the branch from dormancy making them grow. Also establishes phyllotaxy when polar auxin transport generates local peaks in auxin concentrations, determining sites of leaf primordium formation

Auxin's Role in Gene Expression

Auxin causes cells in the region of elongation to produce new proteins within minutes. Some of them are short lived transcription factors that repress or activate expression of other genes

Hypersensitive Response

Causes localized cell and tissue death near the infection site. Induces production of phytoalexins and PR proteins, which attack the specific pathogen. Stimulates changes in the cell wall that confine the pathogen.

Systemic Acquired Resistance

Causes plant-wide expression of defense genes. Protects against a diversity of pathogens. Provides a long-lasting response.

Plant Hormones

Chemical signals that modify or control one or more specific physiological processes within a plant organ. Produced in low concentrations, and interaction between different hormones also control growth and development

Cold

Cold can decrease membrane fluidity. Plants alters lipid composition of membranes as a response

Circadian Rhythms

Cycles that are about 24 hours long and are governed by an internal "clock", biological clock. In plants, it can cause some of them, like legumes, to lower their plants during the evening and raise them in the morning whether or not there is light. Some other plants process oscillate only during the day whether or not light is present. Circadian rhythms may depend on a protein synthesis regulated through feedback inhibition. Photochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues. Also sunlight is required to resynchronize their biological clock

Statoliths

Dense cytoplasmic components. Plants detect gravity by settling of statoliths

Action Spectrum

Depicts relative response of a process to different wavelengths.

Photomorphogenesis

Effects of light on plant morphology

Flooding

Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding

Expansins

Enzymes that loosen the wall's fabric. Activated when the proton pump lowers the pH in the cell wall.

The Triple Response to Mechanical Stress

Ethylene induces the triple response which is a slowing of stem elongation, a thickening of the stem, and a curvature that causes it to grow horizontally. After the mechanical stress is resolved the stem resumes vertical growth. This allows a growing shoot to avoid obstacles

Critical Night Length:

In the 1940's, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length.

Abscisic Acid (ABA)

Increases during seed maturation and inhibits germination until optimal conditions are met. ABA is the primary internal signal that enables plants to withstand drought by promoting stomatal closing.

Gene-for-Gene Recognition

Involves recognition of effector molecules by the protein products of specific plant disease resistance (R) genes. An R protein recognizes a corresponding molecule made by the pathogen's Avr gene, this then activated the plant's defenses including the hypersensitive response and systemic acquired resistance.

Senescence

Is the programmed death of cells or organs. A burst of ethylene is always associated with senescence.

Biotic stresses

Living. Include herbivores and pathogens

Phytochromes and Seed Germination

Many seeds remain dormant until light conditions are optimal. Red light stimulates seed growth, and far red light inhibits it

Salicylic Acid

Methylsalicylic acid travels from an infection site to remote areas of the plant where it is converted to salicylic acid, which initiates pathogen resistance

Etiolation

Morphological adaptations for growing in darkness, collectively.

Abiotic stresses

Nonliving. Include drought, flooding, salt stress, heat stress, and cold stress

The effect of different color light on coleoptiles

Only light wavelengths below 500 nm (blue and violet) induce curvature of coleoptiles

Phytochromes

Photo receptors that absorb mostly red light. Regulate seed germination and shade avoidance. Exist in two states Pr and Pfr. Red light causes conversion of Pr to Pfr. Far red light causes conversion of Pfr to Pr.

Action potentials

Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses, called action potentials. These impulses resemble nerve impulses in animals, though the action potentials of plants are thousands of times slower.

Auxin

Refers to any chemical that promotes elongation of coleoptiles. Produced in shoot tips and transported from cell to cell down the stem, moves only from tip to base, at the rate of 1cm/hr.

Thigmomorphogenesis

Refers to changes in form that result from mechanical disturbance.

Darwin- Darwin experiment

Removed and covered parts of grass coleoptiles to determine what part senses light. One plant had the tip removed, one had the tip covered with an opaque cap, one had the tip covered by a transparent cap, and the other had the site of the curvature covered by an opaque shield. They concluded that phototropism occurs when the tip is illuminated.

Gravitropism

Response to gravity. Roots show positive gravitropism by growing downward; shoots show negative gravitropism by growing upward

Salt stress

Salt can lower the water potential of the soil solution and reduce water uptake. Produces solutes tolerated at high concentrations, which keeps the water potential of cells more negative than that of the soil solution

Boysen-Jensen Experiment

Separated the tip from the rest of the plant with gelatin (permeable) and mica (impermeable). Concluded that phototropism occurs when tip is separated by a permeable membrane, which means that transportation of something down the plant results in curvature.

Brassinosteroids

Steroids that induce cell elongation and division in stem segments and seedlings. Slows leaf abscission and promotes xylem and phloem differentiation

Florigen

The flowering signal, a flowering inducing signal is transmitted across grafts to induce flowering in both long and short day plants

Phytochromes and Shade Avoidance

The phytocrome system also provides the plant with information about the quality of light. Shaded plants receive more far red light, this shifts the phytochrome ratio in favor of Pr, inducing vertical growth of the plant

Blue light photoreceptors

Various blue-light photoreceptors control phototropism (movement in response to light), stomatal opening, and hypocotyl elongation

De-etiolation

When a shoot reaches light, the plant reverses the morphological adaptions that no longer are required, and undergoes changes to grow in the light

Leaf Abscission

A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls. Before the leaf falls all essential elements are sucked from it, allowing the plant to recycle them. Then the leaf detaches form the stem at the abscission layer near the base of the petiole

Virulent pathogen

A pathogen is one that a plant has little specific defense against

Avirulent pathogen

A pathogen is one that may harm but does not kill the host plant

Photoperiodism

A physiological response to a photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year. Red light can interrupt the night portion of a photoperiod, but an equal amount of red and far red light has no effect.

Heat

Excessive heat can denature proteins so heat shock proteins are produce, which help protect other proteins from the heat

Defense against Pathogens

First line of defense is the barrier created by the epidermis and periderm. If that doesn't stop the infection, a chemical attack is employed which destroys the pathogen and prevents its spread, this is enhanced by the plants ability to recognize certain pathogens

Day-Neutral

Flowering is controlled by plant maturity, not photoperiod.

Thigmotropism

Growth in response to touch. Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

Phototropism

Growth of a plant towards or away from light. Shoots have positive phototropism, growth towards light. Roots have negative phototropism, growth away from light

Coleoptile

Hollow, cylindrical sheath that surrounds primary leaf of a germinating monocot seed

Ethylene

Hormone produced in response to stress such as drought, flooding, mechanical pressure, injury, or infection. Also produced during fruit ripening, making the fruit attractive to animals which can spread the seeds, and programmed cell death and in response to high concentration of auxin.

Cytokinins

Hormones that affect cell division, differentiation, apical dominance, and aging. Produced in actively growing tissues. Acts with auxin to stimulate cell division, as auxin elongates the cell, cytokinins force the cell to divide. Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues.

Gibberellins

Hormones that affect stem elongation, fruit growth, and seed germination. Produced in young roots and leaves. They stimulate stem and leaf growth by enhancing cell elongation and cell division. Acts with auxin to enhance fruit growth. A release of gibberellins from the embryo signals seeds to germinate

Acid Growth Hypothesis

Hypothesis for how auxin enables cell elongation. States that in shoot regions of elongation, auxin stimulates the plasma membrane's proton (H+) pump, increasing membrane voltage and lowers intracellular pH. This activates expansins that break hydrogen bonds between cellulose microfibers, loosening the cell wall's fabric. Increasing membrane potential enhances ion uptake causing osmotic uptake and increased turgor. Increased turgor and cell wall plasticity enables the cell to grow

Went's Experiment

Identified how a growth-promoting chemical causes coleoptile to grow towards light. He removed the tip of coleoptiles and placed them on agar cubes, which absorbed the growth chemical. On the control he placed a cubed that lacked chemicals, another with a cube of the chemical placed on the center, and two other with the cube offset to increase concentration on one side. Concluded that the growth promoting chemical must be concentrated unevenly on one side for the coleoptile to curve (always away from the side with the chemical)

Defenses against Herbivores

Plants counter excessive herbivory with physical defenses, such as thorns and trichomes, and chemical defenses, such as distasteful or toxic compounds. Some plants even "recruit" predatory animals that help defend against specific herbivores

Drought

Plants respond to stress by closing stomata, reducing exposed surface area, and shedding leaves

Long-Day Plants

Plants that flower when a light period is no longer than a certain number of hours. Are governed by whether the critical night length sets a maximum number of hours of darkness. Needs a short uninterrupted night

Short-Day Plants

Plants that flower when a light period is shorter than a critical length. Are governed by whether the critical night length sets a minimum number of hours of darkness. Needs a long uninterrupted night

Polar Transport of auxin

Polar transport is transport unrelated to gravity. Auxin movement is caused by the polar distribution of auxin transport proteins in the cells