Select your language

Suggested languages for you:
Log In Start studying!
StudySmarter - The all-in-one study app.
4.8 • +11k Ratings
More than 3 Million Downloads
Free
|
|

All-in-one learning app

  • Flashcards
  • NotesNotes
  • ExplanationsExplanations
  • Study Planner
  • Textbook solutions
Start studying

Phytochromes

Save Save
Print Print
Edit Edit
Sign up to use all features for free. Sign up now
Phytochromes

Have you ever noticed how plants tend to grow away from the shadows and towards the light? How do plants know where light is available? What kind of physiological changes occur when a plant is exposed to different wavelengths of light?

Here, we will discuss how pigments called phytochromes help plants detect red and far-red wavelengths of light. We will also discuss the role of phytochromes in plants’ response to light and the overall development of plants.

Phytochrome definition

The ability of a plant to detect light in its surroundings is crucial to its competitiveness and survival. Through photoreceptors, plants are able to recognize and react to blue, red, and far-red light wavelengths. These photoreceptors are made up of chromoproteins, which are made up of a protein and a light-absorbing pigment known as a chromophore.

Phytochromes are a family of chromoproteins that are sensitive to red and far-red light. In their dark state, phytochromes are found in the cytoplasm where they are synthesized; however, upon light activation, they are translocated to the nucleus.

Phytochromes are present in plants, cyanobacteria and fungi.

The cytoplasm refers to the fluid that fills a cell in which subcellular structures are suspended.

The nucleus is a membrane-bound organelle that houses the cell's genetic information.

Phytochrome function in plants

Phytochromes have two forms:

  • Pr (phytochrome red) absorbs red light (approximately 667 nm). The absorption of red light converts Pr to Pfr, the active form of the phytochrome protein.

  • Pfr (phytochrome far-red) absorbs far-red light (approximately 730 nm). The absorption of far-red light converts Pfr to Pr, the inactive state of the phytochrome protein.

The absorption of red or far-red light alters the chromophore's structure, which affects the conformation and activity of the phytochrome protein to which it is attached.

The two photo-interconvertible forms are collectively known as the phytochrome system. The phytochrome system acts like a "biological switch" that detects and responds to the color, intensity, and duration of light: the exposure to red light can cause the phytochrome to initiate or “switch on” physiological activities, whereas the exposure to far-red light causes it to inhibit or “switch off” physiological activities (Fig. 1).

Pfr–the active form of phytochrome–can either directly activate other molecules in the cytoplasm or be transported to the nucleus, where it initiates or inhibits specific gene expression.

Gene expression is the process by which genetic information in DNA is used to produce RNA, which is then used to synthesize proteins.

Phytochrome responses

Phytochromes regulate various plant responses to light that are crucial to their development including seed germination, shade avoidance, and photoperiodism.

Phytochrome and seed germination

Seeds are basically plants in their embryonic stage while enclosed in a seed coat. For seeds to germinate, they must be in favorable soil moisture and temperature conditions. In nature, seeds detect and respond to environmental cues–such as light–that tell them seasonal conditions to initiate germination. What role does phytochrome play in this process?

Plants produce phytochrome in its inactive form (Pr), and when seeds are stored in the dark, the pigment almost completely remains in the Pr form. Sunlight contains both red and far-red light, however the Pfr conversion is faster than the Pr conversion.

So, in the presence of sunlight, the Pfr to Pr ratio rises. This means that the germination of seeds exposed to adequate sunlight is triggered by the production and accumulation of Pfr.

Scientists in the 1930s exposed water-swollen seeds to a few minutes of single-colored light of varying wavelengths before storing them in the dark. Then after two days, researchers counted the number of seeds that germinated under each color of light. They reported that more seeds germinated under red light (at approximately 660 nm) while seeds germinated under far-red light had lower germination percentage compared to dark controls.

With this, what do you think happens when lettuce seeds are exposed to a flash of red light followed by a flash of far-red light, or when far-red light is followed by red light? The reaction of the seeds is determined by the last flash of light: the effects of red and far-red light are reversible.

When lettuce seeds are exposed to red light, Pr is transformed to Pfr, which promotes the cellular responses that lead to germination. On the other hand, the exposure of red-illuminated seeds to far-red light will cause Pfr to revert to Pr, suppressing the germination response.

So why does seed germination respond so strongly to light? Many varieties of seeds, especially smaller ones, germinate only when the light environment and other variables are near ideal due to low nutrient availability.

In the case of lettuce, their seedlings cannot grow long enough before they run out of fuel so if their seeds were to germinate even a centimeter under the soil surface, the seedling will be unable to receive sunlight and would eventually die. As such, such seeds can lay dormant for years until the light conditions become more favorable to ensure the survival of the seedlings.

Phytochrome and shade avoidance

Full, unfiltered light from the sun contains significantly more red light than far-red light. Because chlorophyll absorbs strongly in the red part of the visible spectrum but not in the far-red region, any plant that is shaded by another plant will be exposed to red-depleted, far-red-enriched light.

Far-red light transforms phytochrome in shaded leaves to the Pr (inactive) form which induces the plant to allocate more of its resources into growing taller so that it can absorb more direct sunlight.

In contrast, the closest non-shaded or even less-shaded plants have more red light. When leaves exposed to these areas absorb direct sunlight, the Pfr (active) form of the phytochrome is activated, promoting branching and slowing down vertical growth.

Plant shoots utilize the phytochrome system to grow away from the shadow and toward the light where it can take in more energy for nutrient production. Because light competition is so intense in a dense plant community, the phytochrome system provides plants with an evolutionary advantage.

Phytochrome and photoperiodism

In addition to helping plants detect light, phytochrome helps plants determine the time of day or year, a property called photoperiodism.

The Pr/Pfr ratio at dawn can be used by a plant to calculate the length of the day/night cycle. Furthermore, by collecting this information over several days, a plant can compare the length of the previous night to several other nights in the past.

  • If the plant tracks increasingly shorter nights, then it can sense that spring is approaching.

  • If the plant tracks increasingly longer nights, then it can sense that autumn is approaching.

Detecting the length of day or night along with temperature and water availability enables plants to determine the time of year and modify their physiology accordingly. It is through this mechanism that the flowering of plants, the production of seeds, and the dormancy of buds occurs seasonally.

Some ground-level woodland plants flower early on in spring so that they can produce seeds before the leaf canopy fully emerges and reduces the amount of light that will pass and reach the forest floor.

On the other hand, trees and perennial plant species in northern latitudes respond to shortening day length by inducing cold hardiness and bud dormancy in order to prepare for the incoming freezing winter temperatures.

Role of phytochrome in plant development

Phytochromes equip plants with the ability to detect specific wavelengths of light, providing them with spatial (like where there are gaps in the canopy through which light can pass) and temporal (like what time of the year it is) information about their environment. Such information is crucial for the timing of growth and developmental changes: from seed germination and seedling establishment to flowering and reproduction.

By providing them with this information and catalyzing various biological responses, phytochromes enable plants to optimize the amount of light energy they are able to take in and turn into nutrients.

Phytochromes - Key takeaways

  • Phytochromes are a family of chromoproteins that are sensitive to red and far-red light. Phytochromes have two forms: Pr (phytochrome red) and Pfr (phytochrome far-red).
  • Pr (phytochrome red) absorbs red light (approximately 667 nm). The absorption of red light converts Pr to Pfr, the active form of the phytochrome protein.
  • Pfr (phytochrome far-red) absorbs far-red light (approximately 730 nm). The absorption of far-red light converts Pfr to Pr, the inactive state of the phytochrome protein.
  • In their dark state, phytochromes are found in the cytoplasm where they are synthesized; however, upon light activation, they are translocated to the nucleus.
  • Phytochromes regulate various plant responses to light that are crucial to their development including seed germination, shade avoidance, and photoperiodism.

References

  1. Zedalis, Julianne, et al. Advanced Placement Biology for AP Courses Textbook. Texas Education Agency.
  2. Reece, Jane B., et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
  3. Mathews, S. (2006). Phytochrome-mediated development in land plants: Red light sensing evolves to meet the challenges of changing light environments. Molecular Ecology, 15 (12), 3483-3503. https://doi.org/10.1111/j.1365-294X.2006.03051.x
  4. Klose C, Viczián A, Kircher S, Schäfer E, Nagy F. Molecular mechanisms for mediating light-dependent nucleo/cytoplasmic partitioning of phytochrome photoreceptors. New Phytol. 2015 May;206(3):965-71. doi: 10.1111/nph.13207. Epub 2014 Dec 15. PMID: 26042244; PMCID: PMC4406131.
  5. Chen M, Chory J, Fankhauser C. Light signal transduction in higher plants. Annu Rev Genet. 2004;38:87-117. doi: 10.1146/annurev.genet.38.072902.092259. PMID: 15568973.

Frequently Asked Questions about Phytochromes

Phytochromes are a family of chromoproteins that are sensitive to red and far-red light. The phytochrome system acts like a "biological switch" that detects and responds to the color,  intensity, and duration of light: the exposure to red light can cause the phytochrome to initiate or “switch on” physiological activities, whereas the exposure to far-red light causes it to inhibit or “switch off” physiological activities.

In their dark state, phytochromes are found in the cytoplasm; upon light activation, they are translocated to the nucleus.

Through photoreceptors, plants are able to recognize and react to blue, red, and far-red light wavelengths. These photoreceptors are made up of chromoproteins, which are made up of a protein and a light-absorbing pigment known as a chromophore. Phytochromes are a family of chromoproteins that are sensitive to red and far-red wavelengths of light.

Phytochromes have two forms:

 

  • Pr (phytochrome red) absorbs red light (approximately 667 nm). The absorption of red light converts Pr to Pfr, the active form of the phytochrome protein. 

  • Pfr (phytochrome far-red) absorbs far-red light (approximately 730 nm). The absorption of far-red light converts Pfr to Pr, the inactive state of the phytochrome protein.

Phytochromes have two forms:

 

  • Pr (phytochrome red) absorbs red light (approximately 667 nm). The absorption of red light converts Pr to Pfr, the active form of the phytochrome protein. 

  • Pfr (phytochrome far-red) absorbs far-red light (approximately 730 nm). The absorption of far-red light converts Pfr to Pr, the inactive state of the phytochrome protein.

Final Phytochromes Quiz

Question

Phytochromes are sensitive to what wavelengths of light?

Show answer

Answer

Red

Show question

Question

The absorption of red light converts ____, the active form of the phytochrome protein. 

Show answer

Answer

Pr to Pfr

Show question

Question

What is the inactive form of phytochrome?

Show answer

Answer

Pr

Show question

Question

Where is phytochrome synthesized?

Show answer

Answer

Cytoplasm

Show question

Question

Where is the active form of phytochrome translocated to?

Show answer

Answer

Nucleus

Show question

Question

The absorption of ___ converts Pfr to Pr, the inactive state of the phytochrome protein.

Show answer

Answer

far-red light

Show question

Question

Why is the phytochrome system like a biological switch?

Show answer

Answer

The phytochrome system acts like a "biological switch" that detects and responds to the color,  intensity, and duration of light: the exposure to red light can cause the phytochrome to initiate or “switch on” physiological activities, whereas the exposure to far-red light causes it to inhibit or “switch off” physiological activities

Show question

Question

How does phytochrome initiate seed germination?

Show answer

Answer

Plants produce phytochrome in its inactive form (Pr), and when seeds are stored in the dark, the pigment almost completely remains in the Pr form. Sunlight contains both red and far-red light, however the Pfr conversion is faster than the Pr conversion. 


So, in the presence of sunlight, the Pfr to Pr ratio rises. This means that the germination of seeds upon exposure to adequate sunlight is triggered by the production and accumulation of Pfr.

Show question

Question

How does phytochrome inhibit seed germination?

Show answer

Answer

When seeds that have been exposed to red light are then exposed to far-red light,  Pfr will revert to Pr, suppressing the germination response.

Show question

Question

Why does seed germination respond so strongly to light? 

Show answer

Answer

Many varieties of seeds, especially smaller ones, germinate only when the light environment and other variables are near ideal due to low nutrient availability. 

Seeds can lay dormant for years until the light conditions become more favorable to ensure the survival of the seedlings.


Show question

Question

Any plant that is shaded by another plant will be exposed to ___.

Show answer

Answer

red-depleted, far-red-enriched light

Show question

Question

Why does exposure to far-red light induce plants to grow taller?

Show answer

Answer

Far-red light transforms phytochrome in shaded leaves to the Pr (inactive) form which induces the plant to allocate more of its resources into growing taller so that it can absorb more direct sunlight. 

Show question

Question

How does the phytochrome system enable shade avoidance?

Show answer

Answer

Far-red light transforms phytochrome in shaded leaves to the Pr (inactive) form which induces the plant to allocate more of its resources into growing taller so that it can absorb more direct sunlight. In contrast, the closest non-shaded or even less-shaded plants have more red light. When leaves exposed to these areas absorb direct sunlight, the Pfr (active) form of the phytochrome is activated, promoting branching and slowing down vertical growth. In this way, plant shoots utilize the phytochrome system to grow away from the shadow and toward the light where it can take in more energy for nutrient production


Show question

Question

What do we mean when we say that phytochromes are photo-interconvertible?

Show answer

Answer

Photo-interconvertible means that a phytochrome can convert and revert between two forms when exposed to specific wavelengths of light. The Pr (inactive) form of a phytochrome can convert into Pfr (active) form when exposed to red-light. When the Pfr (active) form is exposed to far-red light, it reverts to its Pr (inactive) form.

Show question

Question

Why are phytochromes important for plant development?

Show answer

Answer

Phytochromes equip plants with the ability to detect specific wavelengths of light, providing them with spatial and temporal information about their environment. Such information is crucial for the timing of growth and developmental changes: from seed germination and seedling establishment to flowering and reproduction.

Show question

60%

of the users don't pass the Phytochromes quiz! Will you pass the quiz?

Start Quiz

Discover the right content for your subjects

No need to cheat if you have everything you need to succeed! Packed into one app!

Study Plan

Be perfectly prepared on time with an individual plan.

Quizzes

Test your knowledge with gamified quizzes.

Flashcards

Create and find flashcards in record time.

Notes

Create beautiful notes faster than ever before.

Study Sets

Have all your study materials in one place.

Documents

Upload unlimited documents and save them online.

Study Analytics

Identify your study strength and weaknesses.

Weekly Goals

Set individual study goals and earn points reaching them.

Smart Reminders

Stop procrastinating with our study reminders.

Rewards

Earn points, unlock badges and level up while studying.

Magic Marker

Create flashcards in notes completely automatically.

Smart Formatting

Create the most beautiful study materials using our templates.

Sign up to highlight and take notes. It’s 100% free.