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 small, nonprotein, water soluble molecule or ion, such as a calcium ion (Ca₂) or cyclic AMP, that relays a signal to a cell’s interior in response to a signaling molecule bound by a signal receptor protein.

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 body, changing the target cells’ functioning. Hormones are thus important in long-distance signaling.

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

Growth of a plant shoot toward or away from light.

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.

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.

gibberellins

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.

brassinosteroids

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

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.

strigolactones

A class of plant hormone that inhibits shoot branching, triggers the germination of parasitic plant seeds, and stimulates the association of plant roots with mycorrhizal fungi.

ethylene

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

expansins

Plant enzyme that breaks the crosslinks (hydrogen bonds) between cellulose microfibrils and other cell wall constituents, loosening the wall’s fabric.

apoptosis

A type of programmed cell death, which is brought about by activation of enzymes that break down many chemical components in the cell.

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 growth phase in a plant or plant part (as a leaf) from full maturity to death.

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

A type of light receptor in plants that initiates a variety of responses, such as phototropism and slowing of hypocotyl elongation.

phytochromes

A type of light receptor in plants that mostly absorbs red light and regulates many plant responses, such as seed germination and shade avoidance.

circadian rhythyms

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

photoperiodism

A physiological response to photoperiod, the relative lengths of night and day. An example of photoperiodism 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.

abiotic

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

biotic

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

heat-shock proteins

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

virulent

Describing a pathogen against which an organism has little specific defense.

avirulent

Describing a pathogen that can mildly harm, but not kill, the host.

gene-for-gene recognition

A widespread form of plant disease resistance involving recognition of pathogen-derived molecules by the protein products of specific plant disease resistance genes.

hypersensitive response

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

systematic 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.

Environmental stress: Drought

Major Response: ABA production, reducing water loss by closing stomata

Environmental stress: Flooding

Major Response: Formation of air tubes that help roots survive oxygen deprivation

Environmental stress: Salt

Major Response: Avoiding osmotic water loss by producing solutes tolerated at high concentrations

Environmental stress: Heat

Major Response: Synthesis of heat-shock proteins, which reduce protein denaturation at high temperatures

Environmental stress: Cold

Major Response: Adjusting membrane fluidity; avoiding osmotic water loss; producing antifreeze proteins