Plant Sensory Systems (Updated L.O.)

How do plants detect and respond to environmental stimuli?

• Use sensory cells to detect external stimuli and transduce them into internal signals

• Then transmitted through cell-to-cell signaling molecules (such as hormones)

• Target cells then alter gene expression or growth patterns to produce an appropriate response.

Explain how stems and leaves can move in response to physical stimuli.

• Via thigmotropism and thigmonastic movements, where touch is detected by mechanoreceptors that trigger electrical and hormonal signaling pathways

• In directional responses like tendrils wrapping, differential growth occurs

• In rapid movements like the Venus flytrap, changes in membrane potential and turgor pressure cause quick, non-directional motion

How do cells process sensory information in plants?

Sensory receptors detect stimuli and initiate signal transduction pathways involving phosphorylation cascades or second messengers that amplify the signal. This allows small external signals to produce large intracellular responses.

How do plant cells respond to hormones?

Hormones bind to specific receptors either on the cell surface or inside the cell, triggering signal transduction pathways that alter gene expression or protein activity. Different cells respond differently depending on receptor types and signaling components.

What is signal transduction in plants?

The process by which an external stimulus is converted into an internal cellular response through a series of molecular events such as receptor activation, phosphorylation cascades, and second messenger production.

Step by step: process of signal transduction pathway

1. Signal arrives to the cell

2. Receptor protein in or on the cell wall changes in response to that signal

3. Receptor or associated protein catalyzes the phosphorylation reaction

4. Phosphorylated protein triggers either a phosphorylation cascade, or releases secondary messengers (like Ca2+)

5. Phosphorylated proteins or second messengers initiate the response

6. Can activate or repress transcription, activate or repress translation, or change ion flow via a channel or pump

What is phototropism?

Phototropism is the growth of a plant toward light, primarily blue light, which maximizes photosynthesis. It is a directional growth response caused by differential cell elongation.

Step-by-step: how does phototropism occur?

1. Blue light is detected by phototropins in the shoot tip

2. Triggers a signaling cascade that redistributes auxin to the shaded side of the plant.

3. Auxin activates proton pumps that lower cell wall pH

4. Loosens the wall and allows cells on the shaded side to elongate more, causing bending toward the light.

How does blue light trigger phototropic responses?

Blue light activates phototropins, which undergo phosphorylation and initiate signal transduction pathways. These pathways alter auxin distribution and trigger growth responses such as bending and stomatal opening.

What is the role of auxin in phototropism?

Auxin accumulates on the shaded side of a plant shoot, where it stimulates cell elongation by activating proton pumps and loosening the cell wall. This unequal growth causes the plant to bend toward the light.

What is the acid-growth hypothesis?

States that auxin stimulates proton pumps to export H⁺ into the cell wall, lowering pH and activating enzymes that loosen cellulose fibers. This allows water uptake and cell expansion, leading to growth.

What did Darwin’s experiments demonstrate about phototropism?

Light is detected at the tip of the plant, but bending occurs below the tip, indicating a signaling molecule transmits the response. This provided early evidence for hormone-mediated growth.

What were the specific conclusions of Darwin’s experiment?

Light responsible for triggering phototropism is sensed at the coleoptile tip. The cells at the tip sense the light, a hormone travels from the tip down the coleoptile, cells lower in the coleoptile respond to the hormone and cause bending.

What is the Cholodny-Went hypothesis?

Proposes that auxin is redistributed to one side of the plant in response to light, causing asymmetric growth. This differential elongation leads to bending toward the light source.

What is phytochrome?

A light-sensitive pigment that detects red and far-red light and regulates processes such as seed germination, stem elongation, and flowering. It acts as a molecular switch between two conformations.

How does phytochrome switch between forms?

Exists as Pr and Pfr forms, where red light converts Pr to Pfr and far-red light converts Pfr back to Pr. This reversible switching allows plants to respond to changing light conditions.

How does phytochrome regulate seed germination?

Pfr is the active form that promotes germination, while Pr is inactive and inhibits it. The final light exposure determines which form predominates and thus whether germination occurs.

How does phytochrome affect stem elongation and flowering?

In shaded environments rich in far-red light, phytochrome remains in the inactive Pr form, promoting stem elongation to reach light. It also regulates flowering by responding to day length and light conditions.

What is photoreversibility?

The ability of phytochrome to switch between Pr and Pfr forms depending on light exposure. This allows plants to dynamically adjust growth based on environmental light.

What is gravitropism?

Gravitropism is the growth response of plants to gravity, with roots growing downward (positive gravitropism) and shoots growing upward. This ensures proper orientation for nutrient uptake and light exposure.

Step-by-step: how does gravitropism occur in roots?

1. The root tip is moved into a horizontal position

2. Auxin is actively redistributed by amyloplasts (gravity-sensing cells) in the root cap

3. Causes the redistribution of auxin to the bottom/lower side of the root

4. High auxin levels inhibit growth on that lower side, causing the top side to grow and bend/curve downward

What are statoliths and their role in gravitropism?

Dense, starch-filled organelles that settle under gravity and activate pressure receptors in root cells. This initiates the signaling pathway for gravitropic responses.

How does auxin affect root growth during gravitropism?

In roots, high auxin concentrations inhibit cell elongation, while lower concentrations promote growth. This differential effect causes bending as one side grows faster than the other.

How do physical forces affect plant growth patterns?

Mechanical stimuli such as wind trigger thigmomorphogenesis, leading to shorter, thicker plants with stronger cell walls. This adaptation increases resistance to physical stress.

What is thigmomorphogenesis?

A change in plant growth pattern in response to mechanical stress, resulting in sturdier and more compact growth. It involves altered gene expression and cell wall strengthening.

What is thigmotropism?

Directional growth in response to touch, such as tendrils wrapping around objects. This allows plants to climb and access light more efficiently.

What are thigmonastic movements?

Rapid, non-directional responses to touch, such as the Venus flytrap snapping shut. These movements involve electrical signaling and rapid changes in cell pressure.

How do plant hormones regulate growth and responses?

They coordinate growth and responses by acting as signaling molecules that regulate gene expression and cellular activity. Different hormones can interact to produce complex responses.

What is the role of auxin in plants?

• Involved in cell elongation and apical dominance

• Promotes differentiate of xylem and phloem

• Helps to define long axis of body (responses to phototropism and gravitropism)

• Produced in shoot apical meristems and young leaves

• The first plant hormone to be isolated and characterized

What is apical dominance and how is auxin involved?

Apical dominance is the suppression of lateral bud growth by auxin produced in the shoot tip. Removing the apical bud reduces auxin levels and allows lateral buds to grow.

What is the role of cytokinins?

• Promote cell division in the presence of auxin

• Promote chloroplast development and break lateral bud dormancy

• Delay senescence (AKA aging)

• Produced in root apical meristems and many other tissues

How do gibberellins affect plant growth?

• Promote stem growth via both cell elongation and division

• Promote seed germination

• Produced in apical meristems, immature seeds, and anthers (pollen-producing organs)

What is the relationship of gibberellins and production of alpha-amylase?

1. Germination begins in the seed as it absorbs water

2. gibberellins diffuse from the embryo to the aleurone layer

3. Cells in this layer respond by releasing digestive enzymes like alpha-amylase

4. Such enzymes digest starch, releasing sugars and other molecules that a growing plant needs

How does abscisic acid (ABA) affect plants?

• Inhibits bud growth and seed germination

• Induces closure of stomata in response to water stress

• Acts as a stress hormone analogous to cortisol in humans

• Produced in almost all cells

How do gibberellins and ABA differ?

Gibberellins promote growth and germination, while ABA inhibits growth and induces dormancy. These hormones act antagonistically to balance plant development.

What are brassinosteroids and their function?

• Promote cell elongation in stems and leaves

• Structurally related to steroid hormones in animals

• Produced in almost all tissues

How do brassinosteroids affect plant body size?

Stimulate cell expansion and division, leading to increased plant size. Deficiency results in dwarf plants, demonstrating their importance.

What is ethylene and its role in plants?

• Exists in a gaseous form (gas hormone)

• Involved in fruit ripening

• Induces senescence of fruits, flowers, and leaves

• Produced in all organs when plants are under stress

How does ethylene relate to senescence?

Promotes it by triggering breakdown of cellular components and accelerating aging processes. It also regulates leaf drop and fruit ripening.

How do hormones interact to regulate plant responses?

Often interact synergistically or antagonistically to fine-tune growth and responses to environmental stimuli. This coordination allows plants to adapt to changing conditions.