Phototropism – Definition, Types, Mechanism, Importance, Examples

What is Phototropism?

• Phototropism refers to the directional growth exhibited by photosynthesizing organisms, predominantly plants, in response to light stimuli. This phenomenon is crucial for autotrophic organisms, such as plants, which rely on photosynthesis to synthesize their own nutrients.
• Through the intricate process of photosynthesis, these organisms transform light, water, and carbon dioxide (CO2) into essential sugars, facilitating both energy production and growth. Given that plants are sessile by nature, they lack the ability to relocate in search of optimal light conditions.
• As a compensatory mechanism, they employ phototropism to optimize light absorption through their leaves. This adaptive response ensures that they harness maximum light energy for photosynthesis, thereby promoting their survival and growth. There are two primary types of phototropism: positive and negative. Positive phototropism is characterized by growth towards a light source, a behavior commonly observed in plant shoots.
• Conversely, negative phototropism, sometimes termed “aphototropism,” results in growth away from the light. It’s noteworthy that plant roots predominantly exhibit negative phototropism, though they also demonstrate gravitropism, a growth response to the Earth’s gravitational force. At a cellular level, the mechanism underlying phototropism involves the hormone auxin.
• Cells located on the side of the plant distal to the light source have a higher concentration of auxin. This differential auxin distribution leads to elongation of these cells, causing the plant to bend towards the light. Phototropism is one among various tropisms in plants, which are growth responses to specific external stimuli. It’s essential to differentiate between negative phototropism and skototropism.
• While the former pertains to growth away from light, the latter specifically denotes growth towards darkness. The majority of plant shoots display positive phototropism and adjust their chloroplasts within leaves to optimize photosynthetic efficiency.
• However, certain vine shoot tips manifest negative phototropism, enabling them to approach and climb solid, dark objects. The synergistic effects of both phototropism and gravitropism ensure that plants orient and grow in directions most beneficial to their survival and growth.

Definition of Phototropism

Phototropism is the directional growth of an organism, especially plants, in response to a light stimulus.

Phototropism Lecture Video

Phototropism Discovery – Early Experiments

The phenomenon of phototropism, where plants grow directionally in response to light, has been a subject of scientific intrigue for centuries. Pioneering experiments by eminent scientists have paved the way for our understanding of this intricate process.

1. Charles Darwin’s Experiment (1880): Charles Darwin, in collaboration with his son, embarked on a series of experiments to elucidate the mechanisms underlying phototropism. Using canary grass and oat coleoptiles as their subjects, they observed a distinct bending of seedlings towards sunlight. Their innovative approach involved covering the tips of the oat coleoptiles, which inhibited their phototropic response. However, when the lower portion of the coleoptiles was shielded, the phototropic bending persisted. From these observations, Darwin deduced that the coleoptile tips were highly sensitive to light and orchestrated the bending towards it. Furthermore, he postulated that the middle section of the coleoptiles played a role in acidifying the cell wall by activating protons, leading to a decrease in pH. This acidification activated enzymes called expansins, which rendered the cell wall less rigid, facilitating the bending motion.
2. Boysen Jensen’s Experiment (1913): Building on Darwin’s foundational work, Boysen Jensen conducted a series of experiments to further probe the intricacies of phototropism. He initiated his study by severing the tip of seedling coleoptiles and reintroducing it, separated by a thin gelatin layer. Remarkably, the stem still exhibited a curvature towards the light source. In a subsequent experiment, Jensen introduced a mica sheet below the coleoptile tip on its shaded side, which did not inhibit the phototropic curvature. However, when the mica sheet was positioned on the illuminated side, the curvature was notably absent. Jensen’s experiments led him to conclude that a material substance, which could traverse both the gelatin and mica barriers, was instrumental in inducing the phototropic curvature. This substance was later identified as auxins, a class of plant hormones.

These early experiments, conducted with meticulous precision and innovative methodologies, laid the groundwork for our contemporary understanding of phototropism. They underscore the relentless pursuit of knowledge by scientists and the intricate mechanisms plants employ to optimize their growth in response to environmental stimuli.

History of Phototropism

Phototropism, the growth movement of plants in response to light, has been a subject of scientific inquiry for centuries. The understanding of this phenomenon has evolved over time, with contributions from various scientists who have provided distinct perspectives and experimental evidence. Here is a chronological account of the significant milestones in the history of phototropism:

1. Theophrastus (371-287 BC): Often referred to as the “Father of Botany,” Theophrastus was among the first to observe phototropism. He proposed that the bending of plants towards light was a result of fluid removal from the illuminated side of the plant’s stem.
2. Franscis Bacon (1561-1626): Bacon postulated that the phototropic response was a consequence of wilting, a perspective that diverged from Theophrastus’s fluid removal theory.
3. Robert Sharrock (1630-1684): Sharrock introduced a novel perspective, suggesting that plants bend due to the influence of fresh air, a theory that emphasized the role of the environment in plant growth.
4. John Ray (1628-1705): Building on environmental factors, Ray hypothesized that plants positioned near windows bend towards the cooler temperature, attributing phototropism to temperature gradients.
5. Charles Darwin (1809-1882): In a significant leap in understanding, Darwin proposed that a specific substance produced at the plant’s tip was responsible for its curvature. His experiments with oat coleoptiles laid the foundation for future research on phototropism.
6. Nikolai Cholodny and Frit’s Went (1926): Collaboratively, Cholodny and Went delved deeper into Darwin’s hypothesis. Through their research, they discovered elevated levels of a particular substance that migrated to the shaded side of the plant stem, furthering the understanding of the phototropic response.
7. Kenneth Thimann (1904-1977): Thimann’s contributions were pivotal in the field of plant physiology. He successfully isolated and identified the substance responsible for phototropism, naming it “Auxin.” This discovery cemented the role of auxins in plant growth and movement.

The journey of understanding phototropism has been marked by continuous exploration, with each scientist building upon the discoveries of their predecessors. This collaborative and cumulative approach has provided a comprehensive understanding of the intricate mechanisms governing plant growth in response to light.

How Does Phototropism Work? – Mechanism of Phototropism

Phototropism is a complex physiological process in plants, driven by intricate molecular mechanisms that enable them to orient towards a light source. This phenomenon is orchestrated by a series of signaling molecules, genes, and hormones that collectively guide the plant’s growth direction in response to light.

1. Light Sensing and Coleoptile’s Role: The coleoptile, the plant’s apex, plays a pivotal role in sensing light. It is within the central region of the coleoptile that the curvature, indicative of phototropic response, manifests.
2. Cholodny–Went Hypothesis: A foundational theory in understanding phototropism, the Cholodny–Went hypothesis posits that when a plant is exposed to asymmetrical light, the hormone auxin migrates towards the shaded side. This migration results in the elongation of cells on the shaded side, causing the plant to curve towards the light.
3. Role of Auxins: Auxins are central to the phototropic response. They activate proton pumps, leading to a decrease in pH within the cells on the plant’s shaded side. This acidification triggers enzymes called expansins, which weaken the cell wall’s hydrogen bonds, rendering it more flexible. Concurrently, the heightened proton pump activity causes an influx of solutes into these cells, amplifying the osmotic gradient. This results in water entering the cells, increasing turgor pressure, and ultimately driving the phototropic movement.
4. PIN Genes and Auxin Transport: PIN genes encode proteins that are instrumental in phototropism. These proteins, particularly PIN3, are auxin transporters responsible for determining auxin’s spatial distribution. The phototropins, upon receiving light, potentially inhibit the PINOID kinase (PID), promoting PIN3 activity. This cascade leads to an asymmetric auxin distribution, culminating in the stem’s asymmetric cell elongation.
5. Phototropins and Their Expression: Phototropins, specifically phot1 and phot2, are expressed predominantly in the coleoptile’s upper region. While phot2 mutants exhibit typical phototropic responses, the absence of both phot1 and phot2 nullifies any phototropic reaction. The expression levels of these phototropins vary with the plant’s age and light intensity, underscoring their dynamic role in phototropism.
6. Chloroplast Rearrangement: Mature leaves house chloroplasts vital for photosynthesis. These chloroplasts reposition themselves in varying light conditions to optimize photosynthesis. Genes like NPH1 and NPL1 play a role in this rearrangement and, by extension, in phototropism.
7. Involvement of AGC Kinases: Recent research highlights the role of multiple AGC kinases in phototropism. These kinases, including PINOID, D6PK, and their homologs, modulate auxin transport and are activated by PDK1.1 and PDK1.2. These kinases appear to operate at different stages of the phototropic response, with some having a broader phosphorylation spectrum than others.

In summary, phototropism is a multifaceted process, underpinned by a series of molecular events. From light sensing to hormonal gradients and cellular adjustments, each step is meticulously coordinated to ensure plants grow optimally in response to light stimuli.

Five models of auxin distribution in phototropism

Auxin, a pivotal plant hormone, plays a central role in the phototropic response of plants, guiding their growth direction in relation to light. In 2012, Sakai and Haga presented an in-depth exploration of the differential auxin concentrations on the illuminated and shaded sides of the stem, which underpin this response. Using Arabidopsis thaliana as a model organism, they proposed five distinct models elucidating the dynamics of auxin transport during phototropism:

1. Deactivation Model: The first model posits that direct exposure to light results in the deactivation of auxin on the illuminated side of the plant. Consequently, the shaded side retains active auxin, promoting its growth and causing the plant to bend towards the light source.
2. Inhibition of Biosynthesis Model: The second model suggests that light acts as an inhibitor of auxin biosynthesis on its exposed side. This leads to a relative decrease in auxin concentration on the illuminated side compared to the shaded side, which remains unaffected.
3. Horizontal Auxin Flow Model: According to the third model, auxin flows horizontally from both the illuminated and shaded sides of the plant. When exposed to light, there is an enhanced flow of auxin from the illuminated side to the shaded side. This increased auxin concentration on the shaded side fosters its growth, resulting in the plant’s curvature towards the light.
4. Inhibition of Basipetal Flow Model: The fourth model illustrates that light exposure inhibits the basipetal (downward) flow of auxin on the illuminated side. This causes auxin to predominantly flow down the shaded side, promoting its growth.
5. Combined Flow Model: The fifth model integrates elements from both the third and fourth models. It envisions a primary vertical auxin flow from the plant’s apex towards its base, with a secondary horizontal flow branching out to both sides of the plant. Exposure to light inhibits the horizontal flow towards the illuminated side, resulting in an asymmetric auxin distribution. Sakai and Haga’s research indicates that this fifth model aligns most closely with the observed auxin distribution patterns and the subsequent phototropic response in hypocotyls.

In conclusion, these models offer a comprehensive understanding of the intricate auxin dynamics during phototropism. They underscore the adaptability of plants and their sophisticated mechanisms to optimize light absorption for growth and survival.

Types of Phototropism

Phototropism is a growth movement in plants that is instigated by light stimuli. Depending on the direction of growth in relation to the light source, phototropism is categorized into two primary types:

1. Positive Phototropism: This type of phototropism is characterized by the movement or orientation of plant parts, particularly the shoot system, towards the light source. Such a response ensures that the plant maximizes its exposure to light, which is essential for processes like photosynthesis. In essence, positive phototropism facilitates the plant’s ability to harness light energy more efficiently.
2. Negative Phototropism: Contrary to positive phototropism, negative phototropism involves the movement or orientation of plant parts, specifically the root system, away from the light source. This response ensures that roots grow deeper into the soil, seeking nutrients and anchoring the plant securely. By moving away from light, the roots are better positioned to fulfill their primary functions in nutrient absorption and anchorage.

In summary, the dual nature of phototropism, encompassing both positive and negative responses, highlights the plant’s adaptive strategies to optimize its growth and survival in varying light conditions.

Role of Phototropins and Auxins in Phototropism

Phototropism is a growth response in plants that is directed by light stimuli. Two primary molecules, phototropins and auxins, play pivotal roles in this process, ensuring that plants optimize their growth in relation to light sources.

1. Phototropins: Phototropins are specialized photoreceptors that predominantly absorb light in the blue spectrum. These molecules are particularly sensitive to variations in light intensity and direction. There are two primary types of phototropins in higher plants: Phot1 and Phot2. Their activity is contingent on the intensity of blue light:
   • Under low-intensity blue light, Phot1 is predominantly active.
   • However, when exposed to high-intensity blue light, both Phot1 and Phot2 function in tandem, exhibiting redundant activity.
   The primary roles of phototropins encompass a range of physiological processes, including stomatal opening, photosynthetic exchange, chloroplast movement, and the expansion of leaf-blades and cotyledons. Their activity varies based on light exposure: they are more active and absorb more light in regions with higher light intensity, while in shaded or darker regions, their activity diminishes.
2. Auxins: Auxins are plant growth hormones, with a high concentration typically found in the plant’s tip, known as the coleoptile. The distribution of auxins is influenced by the activity of phototropins. When a plant is exposed to sunlight, the unequal distribution of phototropins leads to an asymmetric transport of auxins. Specifically, there is a greater migration of auxins towards the darker side of the coleoptile compared to the illuminated side.This differential auxin distribution has a direct impact on growth. The elevated auxin concentration on the darker side promotes cell elongation, resulting in enhanced growth in that region. Consequently, the plant exhibits a bending movement towards the light source, optimizing its exposure to light for processes like photosynthesis.

In essence, the coordinated actions of phototropins and auxins underpin the phototropic response in plants, ensuring that they adapt and grow optimally in varying light conditions.

Importance of Phototropism

1. Optimal Light Absorption: Phototropism ensures that plants grow towards the light, maximizing their exposure to sunlight. This is crucial for photosynthesis, the process by which plants convert light energy into chemical energy to fuel their growth.
2. Enhanced Growth and Development: By orienting themselves towards light sources, plants can optimize their growth rates and overall health. Adequate light is essential for various physiological processes in plants.
3. Reproductive Success: For flowering plants, optimal light exposure can enhance flower and fruit production. This can lead to a higher chance of successful pollination and seed dispersal.
4. Survival in Competitive Environments: In densely populated areas, plants compete for sunlight. Phototropism allows plants to grow in directions where light is most abundant, giving them a competitive advantage over other plants.
5. Efficient Energy Utilization: By growing towards light sources, plants can ensure they are using their energy reserves efficiently. This is especially important for young plants that have limited energy stored in their seeds.
6. Adaptation to Changing Environments: Phototropism allows plants to adapt to changing light conditions. For instance, if a larger plant or structure shades a plant, phototropic responses can help it reorient its growth to access available light.
7. Support for Climbing Plants: Climbing plants, like vines, use phototropism to grow towards structures they can cling to. Once they reach these structures, they can grow upwards, getting closer to light sources.
8. Root Development: Negative phototropism in roots ensures that they grow away from light and deeper into the soil. This helps in anchoring the plant and accessing essential nutrients and water.
9. Protection from Harmful Radiation: While plants need sunlight for photosynthesis, excessive exposure to UV radiation can be harmful. Phototropism can help regulate the amount of light a plant receives, protecting it from potential light-induced damage.
10. Evolutionary Advantage: Over time, the ability to respond to light has provided plants with an evolutionary advantage, allowing them to thrive in diverse environments with varying light conditions.

In summary, phototropism plays a vital role in the life cycle of plants, influencing their growth, health, reproductive success, and overall survival. It is a testament to the intricate ways in which plants interact with and adapt to their environment.

Phototropism Examples

1. Sunflowers (Helianthus annus): Sunflowers are quintessential examples of phototropism in the plant kingdom. These plants exhibit a unique behavior known as solar tracking. Throughout the day, sunflower heads follow the sun’s trajectory from the East to the West. Remarkably, during nighttime, they reorient themselves from West to East, preparing to track the sun once again at dawn. This behavior is not merely a fascinating display but is rooted in the plant’s survival strategy. Sunflowers have a heightened requirement for sunlight, which is essential for their growth, fruiting, and flowering. This increased dependency on sunlight likely drives their daily solar tracking activity, ensuring they capture maximum light for photosynthesis and other vital processes.
2. Pilobolus Fungi: While phototropism is commonly associated with plants, certain fungi also exhibit this phenomenon. The genus Pilobolus, particularly the species Pilobolus crystallinus, offers a captivating example. These fungi are saprobic, deriving nutrients from decaying organic matter. Specifically, P. crystallinus thrives on the feces of herbivorous animals. To ensure its survival and propagation, this fungus has developed a unique spore dispersal mechanism. It explosively propels its spores from the sporangiophore into the air. These spores then adhere to vegetation, which, when consumed by herbivores, allows the spores to traverse the animal’s digestive system and eventually land in their feces. Phototropism plays a crucial role in this process. The fungus directs its spores towards light, targeting areas where there might be gaps in vegetation. This strategic orientation increases the likelihood of the spores landing on vegetation away from dung, enhancing the chances of ingestion by animals.

These examples underscore the diverse and adaptive ways in which organisms utilize phototropism, whether it’s a plant seeking sunlight or a fungus aiming for optimal spore dispersal.

Quiz Practice

What is phototropism?
a) Movement of plants in response to gravity
b) Movement of plants in response to touch
c) Movement of plants in response to light
d) Movement of plants in response to water

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Which part of the plant is primarily responsible for sensing light in phototropism?
a) Roots
b) Stems
c) Leaves
d) Coleoptile tip

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Which hormone plays a significant role in phototropism?
a) Gibberellin
b) Cytokinin
c) Auxin
d) Abscisic acid

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What type of phototropism is exhibited when plant shoots grow towards light?
a) Positive phototropism
b) Negative phototropism
c) Geotropism
d) Hydrotropism

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Which plant is popularly known for its phototropic behavior of tracking the sun?
a) Rose
b) Tulip
c) Sunflower
d) Marigold

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What happens to the auxin distribution when one side of a plant is exposed to light?
a) It is evenly distributed on both sides.
b) It concentrates on the side exposed to light.
c) It moves away from the light-exposed side.
d) It degrades rapidly.

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Which scientist is credited with early experiments on phototropism using oat coleoptiles?
a) Robert Sharrock
b) Charles Darwin
c) John Ray
d) Franscis Bacon

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Roots generally show which type of phototropism?
a) Positive phototropism
b) Negative phototropism
c) No phototropism
d) Both positive and negative phototropism

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Phototropins are responsible for absorbing which range of light spectrum?
a) Red
b) Green
c) Blue
d) Yellow

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What is the primary function of phototropism?
a) To help plants capture prey
b) To help plants move away from light
c) To help plants maximize light absorption for photosynthesis
d) To help plants reproduce

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