Ch 37 + 39: Plant Responses to Internal and External Signals

What is a signal?

It is something that conveys information

What are the basic steps for a signal eliciting a response/the signal transduction pathway?

1. Reception - the signal is received

2. Transduction - the signal is transferred and amplified

3. Response - the signal activates a cellular response

What do Cytokinins do in plants?

• it stimulates cell division, cell differentiation, and apical dominance

  • They are involved in regulating the growth and development of plant cells.

Where is cytokinin produced in plants?

• In actively growing tissues like roots, embryos, and fruits

How do cytokinins interact with Auxin in plants?

• Many effects of Cytokinins rely on Auxin.

  • Example: Cytokinin alone causes cells to grow large but not divide.

  • Equal amounts of Cytokinin and Auxin lead to undifferentiated cells.

  • More Cytokinin promotes shoot growth, while more Auxin promotes root growth.

What is abscisic acid?

• ABA

• a type of plant hormone that slows growth

• has two main effects = seed dormancy and drought tolerance

Where is abscisic acid produced?

• in most plant tissues

What causes seed dormancy?

• High levels of ABA that only go away when conditions are favorable for the seed to grow

• often increased during seed maturation

• Example: Heavy rain can wash out ABA, allowing seeds to germinate.

What are some factors that can inactivate ABA in seeds?

ABA can be inactivated by:

1. Heavy rain for desert plants

2. Prolonged cold exposure (seeds remain dormant through winter, then start to germinate as temp rises and water becomes more available)

3. Exposure to light (in seeds initially inside dark fruit, seed outside fruit gets light exposure which inactivates ABA and then begins germination)

In what biomes might plants have low levels of ABA? Why?

• in coastal swamps. germination starts when the seeds are on plant so they are ready to grow when they fall and can cont. to grow

How does ABA help plants respond to water stress?

ABA is produced in water-stressed roots and transported through the xylem.

• This acts as an "early warning system" for the plant.

• Water-stressed leaves also produce ABA to signal the plant to respond to drought conditions.

What is the order for plant signaling during drought stress?

• * External signal = drought

• 1.) Internal signal causes ABA to accumulate in the leaves

• 2.) Guard cell potassium channels will open

• 3.) Stomata closes to prevent water loss

• 4.) Transpiration decreases

What is ethylene?

• a type of hormone produced by plants

When is ethylene produced?

• in response to stress.

  • Examples of stress include drought, flooding, mechanical pressure, injury, and infection.

How does a plant respond to mechanical stress/physical pressure?

• produced when the seedling tip pushes against an obstacle.

  • This triggers a triple response:

    1. Stem elongation

    2. Thickening of the stem

    3. Horizontal growth

What is senescence in plants, and how is it related to Ethylene?

• It is the programmed death of certain cells, organs, or the entire plant.

  • A burst of Ethylene triggers the onset of senescence.

  • ex.) leaves dying in the fall

What is leaf abscission, and how is ethylene involved?

• it is the loss of leaves.

  • When Ethylene levels are higher than Auxin levels, abscission occurs.

  • Aging leaves produce less Auxin, which leads to the promotion of abscission by Ethylene.

How does Ethylene affect fruit ripening?

• It causes softening and sweetening of the fruit.

• Ripening triggers the production of more Ethylene, creating a chain reaction.

• Ethylene is a gas, so its signal spreads from fruit to fruit.

Based on what you know about plant hormones, which of the following strategies will not produce the desired effect?

A. You have a small tree and would like to encourage branching, so you cut the tips off the main shoots

B. A florist wishes to keep leaves and flowers fresh longer so dips the cut flower stems in a solution with cytokinins

C. Plants are harvested for retail and are sprayed with a solution of auxin prior to shipping

D. Unripe fruit is continuously flushed with ethylene-free air while it is shipped from farm to supermarket

C. Plants are harvested for retail and are sprayed with a solution of auxin prior to shipping because drying out would be the main concern and ABA is needed to increase drought intolerance

What are some external signals a plant might receive and what do they cause the plant to do?

• light

• gravity

• touch

• drought/flooding

• time (day length and seasons)

• Cause the plant to produce internal chemical signals

What role does light play in plant growth and development?

• Light cues many key events in plant growth and development.

  • Light also signals the passage of days and seasons.

What is the first step of the signal transduction pathway?

• Reception

  • The signal is detected by a receptor.

  • Receptor proteins change shape in response to specific stimuli (signals).

  • Usually embedded in the cell membrane but sometimes in the cytoplasm.

What happens during the transduction step of signal transduction?

• The change in receptor activates second messengers.

• Some second messengers include calcium or cyclic GMP.

• One signal can activate many second messengers, leading to amplification.

What occurs during the response step of signal transduction?

• Second messengers activate response proteins.

• This leads to the regulation of one or more cellular activities

How do plants communicate between different parts of the plant?

• Plants use chemicals for communication.

• Often produced in response to external signals.

How are chemical signals transported in plants?

• Chemical signals are transported through the xylem and phloem.

  • Xylem and phloem are part of the plant's vascular system.

  • In animals, chemical signals are transported through the circulatory system.

What is a hormone in the context of plants?

• A hormone is a signaling molecule.

  • Produced in low concentrations in one part of the plant and transported to other parts.

  • Also referred to as plant growth regulators.

What do plant hormones affect?

Plant hormones impact cell division, elongation, and differentiation.

How do plant hormones trigger responses?

• Plant hormones bind to specific receptors at target cells and tissues.

  • This triggers a response within the plant.

What is Auxin and the type typically in plants?

• typically IAA or indoleacetic acid in plants

• An important hormone for plant tropism.

• Promotes elongation of plant cells.

• Produced in shoot and root tips, then transported to other cells.

• Controls spatial organization of the plant. (determines where and how dif parts of the plant grow)

• The first plant hormone discovered.

Why do plant cells elongate faster on the shaded side?

• The shaded side receives more auxin.

  • Auxin promotes cell elongation, causing faster growth on the shaded side, leading to tropism (e.g., phototropism).

What does more auxin in a plant mean?

apical dominance to promote new leaf growth, the axillary buds don’t develop

What happens if the tip of the plant is removed or covered?

• No tropism occurs.

  • If the tip is removed or opaquely covered, the plant does not respond to tropism (e.g., no bending towards light).

  • means that the tip produces a chemical to help plant grow (IAA/Auxin)

How does auxin affect cell elongation?

• Auxin stimulates cell elongation by binding to receptors and activating proton pumps.

  • The proton pumps activate expansins (enzymes), which loosen the cell walls.

  • Increased water intake raises turgor pressure and wall plasticity, promoting cell elongation.

  • Auxin also alters gene expression to sustain growth.

How do Cytokinins affect axillary bud growth?

• cytokinins influence axillary bud growth based on the balance with Auxin.

  • More Auxin suppresses axillary bud growth. (Increases apical growth)

  • More Cytokinin promotes axillary bud growth.

What happens when ABA is inactivated in a plant?

it typically triggers seed germination.

• Inactivation of ABA signals the plant that conditions are favorable for growth.

• For example, heavy rain can wash away ABA, or exposure to light can inactivate it, both of which allow seeds to germinate.

• This process ensures that seeds germinate at the right time, like when environmental conditions are optimal (e.g., after rain or light exposure).

What are some internal signals a plant might receive?

• Hormones

• Environmental Stress Responses

• Nutrient Signals

• Circadian Rhythms

• Cellular Signals (Secondary Messengers)

What do plants detect about light?

Plants detect the presence, direction, intensity, and wavelength of light.

How do plants detect light?

Plants detect light through photoreceptors.

What type of light are plants most sensitive to?

Plants are most sensitive to blue and red light.

What do blue-light photoreceptors detect and why are they important?

• detect blue light.

  • They are important for phototropism (growth towards light) and the light-induced opening of stomata.

What do red-light photoreceptors (phytochromes) detect and why are they important?

• detect red light.

  • They are important for seed germination, shade avoidance, and de-etiolation (the process where a seedling shifts from growing in the dark to light conditions).

How do both blue-light and red-light photoreceptors work together?

Both types of photoreceptors help with timing daily and seasonal activities in plants.

In general, light-sensitive germination is more pronounced in small seeds compared with germination of large seeds. Why?

A. Smaller seeds have smaller internal resources and lower levels of ABA, which contributes to seed dormancy. So small seeds are more sensitive to all germination factors.

B. Small seeds require light for germination to ensure germination only when close to the soil surface, because they have relatively smaller internal resources.

C. Small seeds have a hypersensitive de-etiolation (greening) response to light

D. Smaller seeds have thinner coats so they are more sensitive to light reception

B. Small seeds require light for germination to ensure germination only when close to the soil surface, because they have relatively smaller internal resources.

After the statoliths and auxin redistribute, what happens in the root and shoot? Select the best answer.

A. The mass at the bottom of the root pulls it downward, but in the shoot, the mass gets buoyed up by water pressure

B. The auxin triggers branching in both regions.

C. Auxin triggers cell elongation in shoots, but not roots, so shoots bend up and roots bend down.

D. Auxin dissolves the statoliths, releasing materials which can promote root and shoot growth

Auxin triggers cell elongation in shoots, but not roots, so shoots bend up and roots bend down.

What is the plant's response to gravity called?

• The plant's response to gravity is called gravitropism.

  • Gravitropism allows plants to grow in response to gravitational pull.

How do plants detect gravity?

• by the settling of statoliths.

  • Statoliths are dense cytoplasmic components that move within cells, helping the plant sense gravity.

What happens to Auxin (IAA) in response to gravity?

• Auxin (IAA) accumulates near the statoliths.

  • The distribution of Auxin helps direct growth, with more Auxin promoting elongation on the lower side of the plant's stem or root in response to gravity.

What is thigmonasty in plants?

rapid leaf movement in response to touch.

• Some plants undergo rapid leaf movement when touched, such as the sensitive plant (Mimosa pudica).

What is thigmotropism in plants?

the response of plants to touch.

• Vines and other climbing plants grow in response to touch, which enables them to cling to structural supports.

How does drought stress affect plants?

• occurs when evapotranspiration exceeds water uptake.

  • ABA is produced in mesophyll and root cells, triggering stomatal closure.

  • Auxin stimulates root growth to try to access more water.

How does flooding stress affect plants?

• occurs when soil pores fill with water instead of air, leading to too little oxygen (O2).

  • In some plants, ethylene triggers the death of root cortex cells, creating "air tubes" to help with oxygen transport to other parts of the plant.

What is thigmomorphogenesis?

• when exposure to wind can alter plant growth form, making trees shorter and stockier.