Plant Biology: Auxin and Gravitropism
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Questions and Answers

What is the role of auxin in root bending?

  • It decreases soil moisture
  • It promotes leaf development
  • It inhibits root growth
  • It influences asymmetric response (correct)

Root gravitropism was absent in early plant evolution.

False (B)

What effect does encountering an obstacle have on root tips?

Roots rapidly respond and adjust their growth direction.

The elimination of asymmetric auxin response and root bending is observed in _______ mutant.

<p>eir1 (pin2)</p> Signup and view all the answers

Match the following terms with their descriptions:

<p>Gravitropism = Growth response to gravity Thigmotropism = Growth response to touch Auxin = Plant hormone influencing growth Mutant = Organism with a genetic alteration</p> Signup and view all the answers

What is the role of statoliths in seed plants?

<p>They help in the sedimentation response to gravity. (D)</p> Signup and view all the answers

Thigmotropism involves the differential growth of plants in response to gravity.

<p>False (B)</p> Signup and view all the answers

What is the significance of PIN proteins in plant gravitropism?

<p>They direct auxin distribution to promote differential growth.</p> Signup and view all the answers

Statoliths first appear in the ______ of seed plants.

<p>root cap</p> Signup and view all the answers

Match the plant type with its statolith characteristics:

<p>Moss (Physcomitrella) = No statoliths Fern (Ceratopteris) = Statoliths in root cap Lycophyte (Selaginella) = Statoliths above the meristem Seed plants = Statoliths in root cap and associated with fast gravitropic response</p> Signup and view all the answers

Which of the following is true regarding the evolution of gravitropism?

<p>Gravitropism existed in Charophytes prior to seed plants. (C)</p> Signup and view all the answers

The process of cell elongation in shoots is inhibited by more auxin on the basal side.

<p>False (B)</p> Signup and view all the answers

How does thigmotropism affect roots when they encounter obstacles?

<p>Roots grow around the obstacle.</p> Signup and view all the answers

Asymmetric growth in shoots involves changes to ______.

<p>Ca2+</p> Signup and view all the answers

Which PIN proteins are specialized for gravitropism in plants?

<p>PIN2 and PIN3 (A)</p> Signup and view all the answers

Which photoreceptors are primarily responsive to red and far-red light?

<p>Phytochromes (C)</p> Signup and view all the answers

Blue light is primarily detected by phytochromes.

<p>False (B)</p> Signup and view all the answers

What is the significance of the experiments performed by Darwin and his son regarding phototropism?

<p>They demonstrated that a substance from the tip of the coleoptile is responsible for its bending towards light.</p> Signup and view all the answers

The photoreceptors known as __________ respond to blue and UVA light.

<p>phototropins</p> Signup and view all the answers

Match the following photoreceptors with their light responsiveness:

<p>Phytochromes = Red, far-red light Cryptochromes = Blue, UVA light Phototropins = Blue, UVA light Chlorophyll = Photosynthesis</p> Signup and view all the answers

What happens to a coleoptile when its tip is covered?

<p>It shows no bending. (D)</p> Signup and view all the answers

Phototropism is the movement of plant shoots away from light.

<p>False (B)</p> Signup and view all the answers

Identify the term for the proteins that have a photosensory domain interacting with chromophores.

<p>photoreceptors</p> Signup and view all the answers

What substance did Frits Went isolate that caused coleoptiles to bend towards light?

<p>Auxin (C)</p> Signup and view all the answers

Briggs found that more auxin accumulates on the illuminated side of a maize seedling.

<p>False (B)</p> Signup and view all the answers

What is the term used to describe the movement of auxin from the lighted side to the shaded side of a plant?

<p>Phototropism</p> Signup and view all the answers

In phototropism, auxin causes cell elongation on the _____ side of the plant.

<p>shaded</p> Signup and view all the answers

In the Cholodny-Went hypothesis, auxin redistribution occurs leading to _____ in the shaded side.

<p>cell elongation (D)</p> Signup and view all the answers

What analysis method shows higher auxin responses on the unilluminated side of the plant?

<p>DR5:GFP reporter gene</p> Signup and view all the answers

Match the researchers with their contributions regarding auxin.

<p>Frits Went = Isolated auxin Briggs = Examined auxin distribution between sides Cholodny-Went = Proposed hypothesis for phototropism Haga and Sakai = Studied auxin response using genes</p> Signup and view all the answers

Frits Went's experiments involved removing tips from oat coleoptiles and placing them on _____ for 1 hour.

<p>agar</p> Signup and view all the answers

Which of the following functions do phytochromes NOT control?

<p>Nutrient uptake (A)</p> Signup and view all the answers

Phytochrome is involved in the greening of plants during photomorphogenesis.

<p>True (A)</p> Signup and view all the answers

What factor interacts with phytochrome to activate transcription?

<p>PIF3</p> Signup and view all the answers

Phytochrome is important for _________ avoidance, which involves vertical leaves and stem elongation.

<p>shade</p> Signup and view all the answers

Match the following phytochrome functions with their descriptions:

<p>Photomorphogenesis = Influences leaf opening and stem elongation Shade avoidance = Adjusts plant growth in response to light levels Zeitgeber = Regulates circadian rhythms in plants Flowering = Determines plant reproductive timing</p> Signup and view all the answers

What wavelengths of light are enriched in light transmitted through plants?

<p>Far-red (B)</p> Signup and view all the answers

The interaction between phytochrome and cryptochrome has no impact on flowering time.

<p>False (B)</p> Signup and view all the answers

Which two genes increase their activity at dawn due to the action of phytochrome?

<p>CCA1 and LHY</p> Signup and view all the answers

What effect do short days (SD) have on tuber formation in potatoes?

<p>Promotion of tuber formation (C)</p> Signup and view all the answers

Long days induce tuber formation in potatoes.

<p>False (B)</p> Signup and view all the answers

What is the role of phytochrome (PHYB) in tuber formation?

<p>Phytochrome (PHYB) represses tuber formation.</p> Signup and view all the answers

Grafting a wild type shoot onto a phyB mutant root __________ tuber formation.

<p>prevents</p> Signup and view all the answers

What happens to phyB mutants when grown under long days?

<p>They produce tubers (C)</p> Signup and view all the answers

The signal of phytochrome (PHYB) is transmissible through grafting.

<p>True (A)</p> Signup and view all the answers

What induces bud dormancy in plants?

<p>It is usually affected by environmental conditions such as day length.</p> Signup and view all the answers

Flashcards

Gravitropism

The ability of roots to grow towards gravity.

Thigmotropism

The response of a plant to touch or physical contact.

Auxin

A plant hormone that plays a role in root growth and development.

Obstacle Avoidance

The process when a plant grows in response to a physical obstacle.

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EIR1 (PIN2)

A gene that plays a role in auxin signaling and root bending.

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Phytochrome

A plant pigment that absorbs red and far-red light, and regulates plant responses to light.

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Photomorphogenesis

The process by which plants adjust their growth and development in response to light.

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Etiolated

A state where a plant has elongated stems, undeveloped leaves, and no chlorophyll, typically found in the absence of light.

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Shade Avoidance

The process by which plants respond to the perceived presence of nearby plants, typically by elongating stems, leading to taller growth.

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Autophosphorylation

A biochemical process where a protein adds a phosphate group to itself.

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Photoisomerization

A process where a molecule changes its shape due to light absorption.

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PIF3

A type of transcription factor that interacts with phytochrome to regulate gene expression.

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Circadian Rhythm

A rhythmic cycle in organisms that is synchronized with the 24-hour day-night cycle.

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What are photoreceptors?

Photoreceptors are specialized proteins that detect light. They have two main parts: a photosensory domain that absorbs light and an output domain that initiates a response based on the light absorbed.

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What are chromophores?

Chromophores are light-absorbing pigments found within the photosensory domain of photoreceptors. They are essential for capturing light energy.

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What is the function of the output domain?

The output domain of a photoreceptor is responsible for triggering a response within the plant based on the light absorbed by the photosensory domain.

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What are phytochromes?

Phytochromes are photoreceptors that mainly respond to red and far-red light. They play a crucial role in regulating plant growth, development, and flowering.

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What are phototropins?

Phototropins are photoreceptors that respond to blue and UVA light. They are primarily involved in phototropism, the bending of a plant towards a light source.

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What are cryptochromes?

Cryptochromes are photoreceptors that respond to blue and UVA light. They play a role in regulating various plant processes, including flowering and circadian rhythms.

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What is phototropism?

Phototropism is the growth of a plant in response to a light stimulus. Positive phototropism is when the plant bends towards the light source.

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What did Darwin's experiments show about phototropism?

Darwin's experiments with canary grass and oat coleoptiles showed that the tip of the coleoptile was responsible for perceiving light and initiating bending. This suggested that a signal was transported from the tip to the lower part, causing elongation.

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Phototropism

The bending of a plant's stem towards a light source.

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Auxin Redistribution

The process by which auxin moves from the illuminated side of a plant stem to the shaded side, leading to unequal growth and bending towards the light.

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Went's Experiment

Frits Went's experiment where he demonstrated that auxin is responsible for phototropism. He isolated auxin from oat coleoptiles and showed that its distribution influences bending.

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Cholodny-Went Hypothesis

The hypothesis that explains phototropism by stating that auxin is redistributed from the illuminated side to the shaded side of a plant stem, promoting cell elongation on the shaded side and causing bending towards the light.

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Briggs's Experiment

The discovery that auxin accumulates on the un-illuminated side of a plant stem, leading to increased cell elongation on that side and bending towards the light.

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DR5:GFP Reporter Gene

A reporter gene, which in this context, helps visualize the distribution of auxin within a plant by expressing a fluorescent protein (GFP) where auxin is present.

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Auxin Distribution Visualization

A method of studying auxin distribution in plants by observing the pattern of fluorescent protein expression driven by the DR5:GFP reporter gene.

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Statoliths

Specialized cells in the root cap of plants that contain dense starch granules. These granules sediment in response to gravity, triggering a cascade of events that lead to differential growth and gravitropic responses.

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PIN proteins

A family of proteins involved in transporting auxin, a plant hormone that regulates growth and development.

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Fast gravitropic response

A type of gravitropic response that occurs quickly, within minutes or hours, in response to a change in gravity. This is typically observed in seed plants.

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Statolith distribution in different plant groups

Statoliths are not found in mosses (Physcomitrella), but they are present in ferns (Ceratopteris) and lycophytes (Selaginella) in the root cap and above the meristem.

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Evolutionary adaptations for fast gravitropic response in seed plants

PIN amplification, specialized PIN proteins (PIN2, PIN3) for gravitropism, and statoliths in the root cap are all adaptations specific to seed plants, related to their fast gravitropic response.

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Gravitropism's evolutionary history

Gravitropism is an ancient response, dating back to charophytes, which have a rhizoid (root-like structure) and protonema (a thread-like structure).

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Auxin redistribution in gravitropism

The redirection of auxin transport within a plant cell, specifically targeting the bottom (basal) side of the cell in response to gravity. This distribution of auxin then influences cell elongation, causing roots to grow downwards and shoots to grow upwards.

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Bud Dormancy

The state of a plant bud where growth is temporarily halted, allowing for survival during adverse conditions like cold winters or hot summers.

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Photoperiod Influence on Tuber Formation

Photoperiod, or the length of the day, plays a crucial role in regulating tuber formation in potatoes.

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Long Photoperiod and Tuber Formation

Long photoperiods (long days) suppress tuber formation in potatoes.

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Phytochrome B's Role in Tuber Formation

The plant hormone phytochrome B (PHYB) plays a role in repressing tuber formation in potatoes.

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PHYB Signal Transmission

The signal from PHYB that inhibits tuber formation travels from the shoot to the root.

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PHYB Mutants and Tuber Formation

Potato plants with a mutation in the PHYB gene produce tubers even under long days.

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Grafting and PHYB Signal Origin

If a wild-type shoot (with functional PHYB) is grafted onto a PHYB mutant root, tuber formation is prevented. This indicates that the PHYB signal originates from the shoot.

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Grafting and PHYB Signal Transmissibility

If a PHYB mutant shoot is grafted onto a wild-type root, tubers are formed under long days. This indicates that the PHYB signal is transmissible from the shoot to the root.

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Study Notes

External Factors and Plant Growth

  • Tropisms are movements of plant parts in response to external stimuli. Examples include gravitropism (gravity), phototropism (light), hydrotropism (water), and thigmotropism (touch).
  • Positive tropisms are movements toward the stimulus, while negative tropisms are movements away from the stimulus.
  • Gravitropism is the response of plants to gravity. Shoots display negative gravitropism, and roots display positive gravitropism.
  • Changes in the direction of gravity in roots result in a change of auxin response on the side of the root toward gravity. This results in decreased cell elongation on the lower side, which causes the root to curve towards gravity.
  • PIN1 is apically positioned in stele cells, PIN4 is positioned basally in the quiescent center, PIN3 is laterally positioned in the columella cells, and PIN2 is basally positioned in the lateral root cap, cortex, and epidermis.
  • PIN3 is important for root and shoot gravitropism; pin3 mutants responding to gravity in both roots and hypocotyls.
  • PIN3 is expressed in the starch sheath of stems and the root columella, which are the gravity sensing cells.
  • Statoliths, which are amyloplasts, sediment to the bottom of statocytes in the root cap when the direction of gravity changes. This relocation of statoliths causes a shift in actin filaments, which targets vesicles containing PIN3 towards the basal side, resulting in increased auxin on that side and inhibition of the cell elongation in the root and promotion of cell elongation in the shoot.
  • Starch biosynthetic mutants (pgm-1) show reduced responses to gravity. Auxin maximum shift (DR5:GFP) was not observed toward the lateral root cap in these mutants.
  • Distribution of auxin depends on actin-dependent localization of vesicles carrying auxin efflux carriers. Sedimentation of amyloplasts causes a shift in actin filaments, which targets vesicles to the basal side.
  • The evolution of "fast" gravitropic response occurred in seed plants. Statoliths first appeared in the root cap of seed plants.
  • Thigmotropism is the differential growth of plants in response to touch. Asymmetric growth in shoots involves changes to calcium, though the molecular mechanisms are unclear. Roots growing around obstacles such as compacted soil or rock resume their positive gravitropism response.
  • Roots rapidly respond to obstacles (e.g., scalpel blades) encountered by the root tip. Asymmetric auxin response correlates with root bending.
  • Root bending depends on asymmetric auxin response, which is affected by the presence of eir1 and pin2 mutants.
  • Plant responses to light vary via phytochromes (red/far-red), cryptochromes (blue), and phototropins (blue).
  • Shoots display positive phototropism (movement towards light), with leaf-type structures moving along the light gradient.
  • Photoreceptors are proteins with photosensory domains that interact with chromophores (light-absorbing pigments).
  • Phytochromes are also needed for responses to red and far-red light wavelengths, and are relevant for germination and flowering responses.
  • Phototropism is the movement of shoots toward light, while coleoptiles bend towards the light if the tip is not covered.

Circadian Rhythms

  • Circadian rhythms are approximately daily rhythmic phenomena that control many plant responses to light.
  • Examples of processes that are regulated by circadian rhythms are leaf folding and unfolding, and photosynthetic activity.

Vernalization

  • Some plants require a cold treatment (vernalization) to flower.
  • This cold treatment causes changes to plant morphology that result in flowering.
  • Vernalization is a mechanism that allows plants to respond to seasonal cues.
  • The cold treatment can be mimicked, which suggests various effects on plant morphology and gene expression.
  • Vernalization response varies based on the ecotype of relevant plants.
  • Some ecotypes require vernalization (winter annuals) while other do not (summer annuals)

Photoperiodism

  • Photoperiodism is the response of plants to seasonal changes in the day/night cycle.
  • Short-day plants flower when the light period is shorter than a certain critical length (e.g., Chrysanthemums, poinsettias, and strawberries).
  • Long-day plants flower when the light period is longer than a certain critical length (e.g., some potatoes, wheat, spinach, lettuce).
  • Day-neutral plants flower regardless of day length (e.g., cucumber, sunflower, rice, and pea).
  • Different regions of the world result in different photoperiodic experiences of a given species, depending on latitude.
  • Phytochrome (and cryptochrome) act as the main component in the long-day pathway.
  • Plants use photoreceptors, like phytochrome and cryptochrome, to measure the photoperiod.
  • A short-day plant's flowers are controlled by the plant's internal clock and the length of the light period.

Other

  • Tuber formation in potato plants occurs primarily in response to short days.
  • Phytochrome is needed to regulate tuber formation and is transmissible, between species.

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Explore the intricate role of auxin in root bending and gravitropism through this quiz. From the significance of PIN proteins to the effects of encountering obstacles, test your knowledge on plant responses to gravity and touch. Ideal for students studying plant biology.

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