Chapter 39: Plant Responses to Internal and External Signals

Vocabulary from Campbell's Biology

Etiolation

Plant morphological adaptations for growing in darkness.

De-Etiolation

The changes a plant shoot undergoes in response to sunlight; also known informally as greening.

Second Messengers

A molecule that relays messages in a cell from a receptor to a target where an action within the cell takes place.

Hormone

In multicellular organisms, one of many types of secreted chemicals that are formed in specialized cells, travel in body fluids, and act on specific target cells in other parts of the organism, changing the target cells' functioning.

Tropism

A growth response that results in the curvature of whole plant organs toward or away from stimuli due to differential rates of cell elongation.

Phototropism

The bending of a plant or other organism in response to light, either toward the source of light (positive) or away from it (negative).

Auxin

A term that primarily refers to indoleacetic acid (IAA), a natural plant hormone that has a variety of effects, including cell elongation, root formation, secondary growth, and fruit growth.

Expansins

Plant enzyme that breaks the cross-links (hydrogen bonds) between cellulose microfibrils and other cell wall constituents, loosening the wall's fabric.

Cytokinins

Any of a class of related plant hormones that retard aging and act in concert with auxin to stimulate cell division, influence the pathway of differentiation, and control apical dominance.

Gibberellin

Any of a class of related plant hormones that stimulate growth in the stem and leaves, trigger the germination of seeds and breaking of bud dormancy, and (with auxin) stimulate fruit development.

Abscisic Acid (ABA)

A plant hormone that slows growth, often antagonizing the actions of growth hormones. Two of its many effects are to promote seed dormancy and facilitate drought tolerance.

Ethylene

A gaseous plant hormone involved in responses to mechanical stress, programmed cell death, leaf abscission, and fruit ripening.

Triple Response

A plant growth maneuver in response to mechanical stress, involving slowing of stem elongation, thickening of the stem, and a curvature that causes the stem to start growing horizontally.

Senescence

The programmed death of certain cells or organs or the entire organism.

Brassinosteroids

A steroid hormone in plants that has a variety of effects, including inducing cell elongation, retarding leaf abscission, and promoting xylem differentiation.

Jasmonates

Any of a class of plant hormones that regulate a wide range of developmental processes in plants and play a key role in plant defense against herbivores.

Strigolactones

Any of a class of plant hormones that inhibit shoot branching, trigger the germination of parasitic plant seeds, and stimulate the association of plant roots with mycorrhizal fungi.

Photomorphogenesis

Effects of light on plant morphology.

Action Spectrum

A graph that profiles the relative effectiveness of different wavelengths of radiation in driving a particular process.

Blue-Light Photoreceptors

Any of several classes of light-absorbing molecules that have physiological effects when activated by blue light.

Phytochromes

Plant pigments that absorb mostly red and far-red light and regulate many plant responses, such as seed germination and shade avoidance.

Circadian Rhythms

A physiological cycle of about 24 hours that persists even in the absence of external cues.

Photoperiodism

A physiological response to photoperiod, the interval in a 24-hour period during which an organism is exposed to light. An example of this response is flowering.

Short-Day Plant

A plant that flowers (usually in late summer, fall, or winter) only when the light period is shorter than a critical length.

Long-Day Plant

A plant that flowers (usually in late spring or early summer) only when the light period is longer than a critical length.

Day-Neutral Plant

A plant in which flower formation is not controlled by photoperiod or day length.

Vernalization

The use of cold treatment to induce a plant to flower.

Florigen

A flowering signal, probably a protein, that is made in leaves under certain conditions and that travels to the shoot apical meristems, inducing them to switch from vegetative to reproductive growth.

Gravitropism

A response of a plant or animal to gravity.

Statoliths

In plants, a specialized plastid that contains dense starch grains and may play a role in detecting gravity.

Thigmomorphogenesis

A response in plants to chronic mechanical stimulation, resulting from increased ethylene production. An example is thickening stems in response to strong winds.

Thigmotropism

A directional growth of a plant in response to touch.

Action Potentials

An electrical signal that propagates (travels) along the membrane of a neuron or other excitable cell as a nongraded (all-or-none) depolarization.

Biotic

Pertaining to the living factors—the organisms—in an environment.

Abiotic

Nonliving; referring to the physical and chemical properties of an environment.

Heat-Shock Proteins

A protein that helps protect other proteins during heat stress. They are found in plants, animals, and microorganisms.

Pathogen-Associated Molecular Patterns (PAMPs)

A molecular sequence that is specific to a certain pathogen.

Effectors

A pathogen-encoded protein that cripples the host's innate immune system.

Hypersensitive Response

A plant's localized defense response to a pathogen, involving the death of cells around the site of infection.

System Acquired Resistance

A defensive response in infected plants that helps protect healthy tissue from pathogenic invasion.

Salicylic Acid

A signaling molecule in plants that may be partially responsible for activating systemic acquired resistance to pathogens.

Herbivory

A +/- ecological interaction in which an organism eats part of a plant or alga.