Plant Responses to Internal and External Signals (bio 39)

Comprehensive vocabulary flashcards covering Concept 39: Plant Responses to Internal and External Signals, including plant hormones, light responses, and defense mechanisms.

Dodder (Cuscuta)

A parasitic, nonphotosynthetic flowering plant that locates host plants using chemical scents and exploits them using haustoria.

Haustoria

Feeding appendages used by parasitic plants like dodder to tap into a host plant's phloem.

Etiolation

Morphological adaptations for growing in darkness, characterized by ghostly pale stems, unexpanded leaves, and short, stubby roots.

De-etiolation

Also known as greening, the process where a plant shoot reaches light and undergoes profound changes such as stem elongation slowing, leaf expansion, and chlorophyll production.

Phytochrome

A class of photoreceptor pigments that function in light detection during de-etiolation and exist in two photoreversible forms: $P_r$ and $P_{fr}$.

Aurea mutant

A tomato mutant with reduced levels of phytochrome that exhibit less greening than wild-type tomatoes when exposed to light.

Second messengers

Small molecules and ions, such as $Ca^{2+}$ and cyclic GMP (cGMP), that amplify a signal and transfer it from a receptor to other proteins.

Calclum ions ($Ca^{2+}$)

A second messenger in phytochrome signaling that can increase in cytosolic concentration by 100-fold, reaching levels around $10^7\,M$ upon activation.

Protein kinases

Enzymes that modify other proteins by the phosphorylation of specific amino acids, often acting in kinase cascades to link stimuli to cellular responses.

Protein phosphatases

Enzymes that dephosphorylate specific proteins, acting as a "switch-off" process in signal transduction pathways.

Plant growth regulator

Organic compounds, either natural or synthetic, that modify or control one or more specific physiological processes within a plant.

Auxin (IAA)

Indoleacetic acid, a hormone produced in shoot tips that stimulates stem elongation, regulates plant architecture, and mediates phototropism.

Tropism

Any growth response that results in plant organs curving toward or away from stimuli.

Phototropism

The growth of a shoot toward (positive) or away from (negative) light, directed by differential cell elongation on opposite sides of the shoot.

Polar transport

The unidirectional movement of auxin from the tip to the base of a plant, mediated by the polar distribution of auxin transport proteins.

Acid growth hypothesis

A model proposing that auxin stimulates proton ($H^+$) pumps, lowering cell wall pH to activate expansins that loosen the wall fabric for elongation.

Expansins

Proteins activated by low pH that break cross-links between cellulose microfibrils and other cell wall constituents.

Cytokinins

A class of hormones that stimulate cytokinesis (cell division) and work with auxin to influence cell differentiation.

Callus

A mass of undifferentiated plant cells that forms when the ratio of cytokinins to auxin is at specific levels in tissue culture.

Apical dominance

The ability of an apical bud to suppress the development of axillary buds, controlled by sugar and hormones like auxin and cytokinins.

Gibberellins (GA)

Hormones synthesized in young roots and leaves that stimulate stem elongation, fruit growth, and seed germination.

$\alpha$-amylase

A digestive enzyme synthesized in the aleurone of grain seeds in response to gibberellins to mobilize stored nutrients during germination.

Abscisic acid (ABA)

A hormone that inhibits growth, promotes seed dormancy, and triggers rapid stomatal closure during drought stress.

Ethylene

A gaseous hormone produced in response to stresses, ripening fruit, and programmed cell death; it also mediates the triple response.

Triple response

An ethylene-induced growth maneuver consisting of a slowing of stem elongation, a thickening of the stem, and horizontal growth.

Senescence

The programmed death of certain cells, organs, or an entire plant, often associated with a burst of ethylene.

Brassinosteroids

Steroids similar to cholesterol that induce cell elongation and division at concentrations as low as $10^{-12}\,M$.

Jasmonates

Fatty acid-derived molecules like methyl jasmonate (MeJA) that play critical roles in plant defense against herbivores and pathogens.

Strigolactones

Xylem-mobile chemicals that stimulate seed germination, suppress adventitious roots, and help establish mycorrhizal associations.

Photomorphogenesis

Key events in plant growth and development that are triggered by light, including detection of light direction, intensity, and wavelength.

Action spectrum

A graph depicting the relative effectiveness of different wavelengths of radiation in driving a particular physiological process.

Circadian rhythms

Cycles with a frequency of approximately 24 hours that are controlled by an internal biological clock.

Photoperiodism

A physiological response to the relative lengths of night and day, such as the timing of flowering.

Short-day plant

A plant that requires a light period shorter than a critical length (or a continuous dark period longer than a critical length) to flower.

Long-day plant

A plant that flowers only when the light period is longer than a certain number of hours (or the dark period is shorter than a critical length).

Vernalization

The use of pretreatment with cold to induce flowering in certain plant species.

Florigen

A protein (specifically the FT protein) that travels from leaf cells to the shoot apical meristem to trigger the transition to a flowering state.

Gravitropism

A plant's growth response to gravity, which can be positive (roots growing downward) or negative (shoots growing upward).

Statoliths

Dense cytoplasmic plastids containing starch grains that settle to the lower portions of cells to help the plant detect gravity.

Thigmomorphogenesis

Changes in plant form that result from mechanical perturbation, such as rubbing or wind.

Thigmotropism

Directional growth of a plant in response to touch, such as the coiling of tendrils around a support.

Action potentials

Electrical impulses in plants that resemble nerve impulses in animals, such as those used by the Venus flytrap to close its trap.

Heat-shock proteins

Proteins synthesized by plant cells above certain temperatures (like $40^{\circ}C$) that help protect other proteins from denaturation.

PAMP-triggered immunity

A localized immune response in plants initiated by the recognition of pathogen-associated molecular patterns, such as bacterial flagellin.

Phytoalexins

Broad-spectrum antimicrobial chemicals produced by plants that have fungicidal and bactericidal properties.

Effector-triggered immunity

An immune response involving hundred of disease resistance (R) genes that detect specific pathogen effectors.

Hypersensitive response

A local defense mechanism involving programmed cell death at and near an infection site to restrict the spread of a pathogen.

Systemic acquired resistance

A general, plant-wide defense response against a diversity of pathogens, instigated by the signaling molecule methylsalicylic acid.