Lab 6: Plant Signaling

Auxin

Auxin is a plant hormone (IAA) produced in apical meristems, and it regulates plant growth, cell elongation, and directional responses to environmental stimuli like light and gravity.

Meristems & Apical Meristem

Meristems are regions of active cell division, and apical meristems are the main growth points where auxin is produced (at the tip in dicots and base in monocots).

Tropism

A tropism is a directional growth response in plants, where growth occurs toward or away from a specific stimulus such as light or gravity.

Phototropism

Phototropism is the growth response to light, where shoots typically exhibit positive phototropism by growing toward the light source.

Mechanism of Phototropism

In phototropism, auxin redistributes to the shaded side of the plant, causing increased cell elongation on that side and bending the plant toward light.

Phototropism Key Details

The greatest bending occurs just below the apical meristem, and removing the tip prevents the response because the auxin source is lost.

Auxin Effects (Shoots vs Roots)

Auxin stimulates cell elongation in shoots but inhibits elongation in roots, which explains why the same hormone causes opposite growth responses.

Gravitropism

Gravitropism is the growth response to gravity, where roots show positive gravitropism (downward growth) and shoots show negative gravitropism (upward growth).

Roots vs Shoots Responses

Roots grow downward and away from light, while shoots grow upward and toward light, allowing the plant to access both nutrients and sunlight.

Functional Significance of Tropisms

Tropisms are beneficial because shoots grow upward to capture light for photosynthesis, while roots grow downward to absorb water and nutrients.

Strength of Tropisms

Gravitropism is generally stronger than phototropism, meaning gravity often has a greater influence on growth direction than light.

Dicots vs Monocots

Dicots bend at the tip where the apical meristem is located, while monocots bend at the base due to differences in growth region location.

Auxin Redistribution

Auxin redistributes unevenly in response to stimuli like light or gravity, and this unequal distribution causes differential growth and bending.

Apical Dominance

Apical dominance occurs when auxin from the apical meristem suppresses lateral bud growth, allowing the plant to prioritize vertical growth.

Apical Dominance Mechanism

Auxin produced at the tip travels downward and inhibits axillary buds, but removing the tip removes this inhibition and allows lateral growth.

Apical Dominance Experiment Design

In the apical dominance experiment, plants with the tip removed are compared to plants with the tip removed and auxin added back to test auxin’s role.

Apical Dominance Variables

In this experiment, the independent variable is auxin presence, while the dependent variable is the growth of axillary (lateral) buds.

Apical Dominance Expected Results

If auxin is absent, lateral buds will grow, but if auxin is added back, lateral bud growth will be suppressed again.

Ethylene

Ethylene is a plant hormone that promotes senescence, fruit ripening, flower fading, and leaf abscission.

Senescence

Senescence is the aging process in plants, during which structures like leaves gradually lose function and prepare for detachment.

Leaf Abscission

Leaf abscission is the process by which a leaf detaches from the plant at a specialized region called the abscission zone.

Abscission Zone

The abscission zone is the region at the base of a leaf where separation occurs, and it becomes sensitive to ethylene when auxin levels decrease.

Auxin vs Ethylene in Abscission

High auxin levels keep leaves attached, but when auxin decreases, ethylene can trigger the abscission process.

Stages of Leaf Abscission

Leaf abscission occurs in stages: high auxin keeps the leaf healthy, reduced auxin initiates senescence, and ethylene causes final detachment.

Leaf Abscission Experiment

In the abscission experiment, auxin-treated leaves remain attached longer, while untreated leaves fall sooner, showing auxin delays leaf drop.

Environmental Effects on Abscission

Factors like age, drought, temperature, and day length reduce auxin levels, which promotes leaf abscission.

Petiole

The petiole is the stalk that attaches the leaf to the stem, and it is the structure where abscission occurs.

Plant Hormones (General)

Plant hormones are chemical signals that coordinate growth, development, and responses to environmental stimuli.