Plant Tropisms and Phytohormone Regulation

Vocabulary flashcards covering key terms related to plant tropic responses and the roles of phytohormones such as auxin, cytokinin, gibberellin, and ethylene.

Tropism

A directional growth response of a plant organ to an external stimulus; positive tropism is toward the stimulus, negative tropism is away.

Phototropism

Growth response to light; plant shoots usually exhibit positive phototropism, bending toward the light source.

Gravitropism (Geotropism)

Growth response to gravity; shoots are negatively geotropic (grow upward) and roots are positively geotropic (grow downward).

Phytohormone

A plant hormone that acts as a chemical messenger controlling growth, development and responses to stimuli, often active at its site of production.

Auxin (Indole-3-acetic acid, IAA)

A major phytohormone that regulates cell elongation, apical dominance, root and shoot growth, and tropic responses; effect depends on concentration.

Auxin efflux carrier (PIN3 protein)

A plasma-membrane glycoprotein that actively transports auxin out of the cell, establishing concentration gradients for polar auxin transport.

Acid Growth Hypothesis

Model proposing that auxin activates proton pumps, acidifying the cell wall, which activates expansins that loosen cellulose cross-links, allowing cell elongation.

Expansin

A cell-wall protein activated by low pH that disrupts hydrogen bonding between cellulose microfibrils and polysaccharides, loosening the wall for expansion.

Apical dominance

Phenomenon where auxin from the shoot apex suppresses growth of axillary buds, promoting elongation of the main stem.

Cytokinin

Phytohormone synthesized in roots; moves upward to stimulate cell division and lateral bud growth, acting antagonistically to auxin.

Auxin–cytokinin ratio

Relative concentrations of auxin and cytokinin that determine whether cells develop into roots, shoots, or remain inhibited in axillary buds.

Gibberellin

Phytohormone that increases the rate of cell division and elongation, promoting stem and internode growth.

Ethylene (Ethene)

A gaseous phytohormone that promotes fruit ripening and various developmental processes.

Positive feedback (ethylene in ripening)

Process where ethylene triggers fruit ripening, and ripening fruits produce more ethylene, synchronizing maturation.

Phototropin receptor

Light-sensitive protein in plant cell membranes that detects lateral light and initiates auxin redistribution for phototropic bending.

Coleoptile

Protective sheath surrounding the emerging shoot in monocots; a primary site of auxin production during early growth.

Differential growth

Unequal cell elongation on opposite sides of an organ, causing bending toward or away from a stimulus.

Statolith

Dense, starch-filled organelle that settles to the lower side of plant cells, helping detect gravity for gravitropic responses.

Proton pump (H⁺-ATPase)

Membrane enzyme activated by auxin to export H⁺ ions, lowering pH in the cell wall and enabling cell expansion.

PIN-mediated polar auxin transport

Directional movement of auxin through tissues driven by coordinated placement of PIN efflux carriers on one side of each cell.

Positive phototropism

Curvature of plant shoots toward light due to higher auxin concentration on the shaded side, leading to greater elongation there.

Negative geotropism

Growth of shoots opposite the direction of gravity, resulting in upward growth away from the ground.