Plants grow toward the sun primarily due to a phenomenon known as phototropism, a growth response in which a plant orients itself in relation to light. This process involves a complex interplay of hormonal activities, molecular mechanisms, and cellular responses that enable plants to maximize their exposure to sunlight, essential for photosynthesis.
The study of phototropism dates back to the late 19th century, with significant contributions from Charles Darwin, who first noted that the tips of plant shoots are responsible for sensing light. Darwin and his son Francis conducted experiments demonstrating that the tip of the coleoptile, a sheath protecting young shoots, is crucial for phototropic responses. They hypothesized that a signal is transmitted from the tip to the growth region of the plant.
Types of Phototropic Movement
- Positive phototropism: Growing in the direction of a light source, as found in the plant shoot.
- Negative phototropism (Aphotropism): Growing away from light, as observed in plant roots.
Following Darwin’s observations, Danish scientist Peter Boysen-Jensen furthered the research by demonstrating that the signal responsible for phototropism could be transmitted through a permeable agar block but not through an impermeable mica sheet[1], indicating the involvement of a diffusible substance. This was a pivotal step in identifying the chemical nature of the signal. In the 1920s, Dutch scientist Frits Went identified this diffusible substance as auxin[2],
a plant hormone that regulates growth. Went’s experiments involved placing agar blocks on decapitated coleoptiles, which resulted in bending, proving that auxin distribution is crucial for phototropism. Auxins, particularly indole-3-acetic acid (IAA)[3], were found to promote cell elongation on the shaded side of the plant, causing it to bend toward the light source. Auxins play a critical role in various growth processes, including phototropism,
by influencing gene expression and cellular activities. The perception of light by photoreceptors, such as phototropins, initiates a signaling cascade that leads to the redistribution of auxin within the plant tissues. Phototropins absorb blue light and undergo conformational changes, triggering the phototropic response. On a molecular level, the Aux/IAA and ARF (Auxin Response Factor)[4] signaling modules are central to auxin signal transduction.
Aux/IAA proteins repress the activity of ARFs, which are transcription factors that regulate the expression of auxin-responsive genes. Light perception leads to the degradation of Aux/IAA proteins, freeing ARFs to activate target genes that promote differential growth.
At the cellular level, auxin transport and its uneven distribution are crucial for establishing organ patterns and guiding growth responses. The Polar auxin transport system, mediated by PIN proteins, ensures that auxin is directionally transported from cell to cell, creating gradients that dictate growth directions. This organized transport is essential for the plant’s overall architecture and response to environmental cues.
Phototropism and auxin signaling influence various aspects of plant growth and development, including:
- Wound Response: Auxin accumulation at wound sites promotes cell division and differentiation, aiding in tissue repair.
- Root Growth and Development: Auxin regulates root architecture by controlling the formation of lateral roots and root hairs.
- Apical Dominance: Auxin produced by the apical bud inhibits the growth of lateral buds, maintaining the plant’s vertical growth.
- Fruit Growth and Development: Auxin promotes fruit set and growth, influencing fruit size and shape.
- Flowering: Auxin interacts with other hormones to regulate flowering time and development.
Auxin also plays a role in ethylene biosynthesis, a hormone involved in fruit ripening and senescence. Synthetic auxins, such as 2,4-D and NAA, are used as herbicides due to their ability to disrupt normal plant growth. These compounds mimic natural auxins but cause uncontrolled growth, leading to the death of the target plants.
Examples of Phototropism
- Sunflowers: Young sunflower plants display phototropism by bending towards the sun during the day to maximize light exposure for photosynthesis.
- Houseplants: Indoor plants, such as pothos or philodendrons, often grow towards windows or light sources.
- Grass Seedlings: Grass seedlings show positive phototropism by bending towards the light source as they emerge from the soil.
- Dandelions: Dandelion flowers track the sun during the day, exhibiting phototropism to ensure their flowers get optimal light.
- Bean Plants: Bean plants grown near a window will lean towards the light source.
- Tomato Plants: Tomato plants in a garden will grow towards the direction of sunlight.
- Corn: Corn plants bend towards light sources when they are young, optimizing their growth for sunlight capture.
- Ivy: Ivy plants growing on a shaded wall will extend their growth towards areas where sunlight is available.
- Sunflower Seeds: Germinating sunflower seeds exhibit phototropism as their shoots bend towards light to ensure optimal growth conditions.
- Morning Glory Vines: These vines will grow towards light sources to ensure they can climb and spread effectively.
The growth of plants toward the sun, driven by phototropism, involves a complex interplay of hormonal activities, molecular signaling, and cellular responses.
Footnotes
- Scientists discovered that this growth direction is controlled by a chemical signal. In an experiment, they placed a jelly-like substance called agar on cut plant tips, and the plants still grew towards light, showing that the signal could move through the agar. However, when they used a solid mica sheet instead, the plants did not bend towards the light because the signal couldn’t pass through the mica. This showed that the signal is a chemical that needs to move through something porous, like agar, but can’t pass through a solid barrier like mica . [Back]
- Auxins are plant hormones that act like growth signals, telling different parts of the plant how to grow and develop. They help plants bend toward light, grow roots, and produce fruits. Auxins are mainly found in the tips of shoots and roots, where they regulate cell growth by making cells on the darker side of the plant elongate, causing the plant to bend towards the light. They are essential for many growth processes, including the direction of root growth and the overall shape of the plant. [Back]
- Indole-3-acetic acid (IAA) is a natural type of auxin, which is a plant hormone that helps control how plants grow. It tells the plant’s cells when to grow and divide, playing a key role in how plants bend towards light, develop roots, and form fruits. IAA is produced mainly in the tips of young shoots and is transported to other parts of the plant, ensuring that growth happens in the right places. [Back]
- Aux/IAA and ARF are like a plant’s on/off switch for growth. Auxin, a plant hormone, tells the plant how to grow. ARF (Auxin Response Factor) is the “on” switch that turns on genes to make the plant grow. Aux/IAA is the “off” switch that stops ARF from working. When there is a lot of auxin, it breaks down the Aux/IAA “off” switch, letting ARF turn on the growth genes. This helps the plant grow towards light, among other things. [Back]
Further Reading
Sources
- ScienceDaily “How do plants grow toward the light? Scientists explain mechanism behind phototropism” https://www.sciencedaily.com/releases/2013/05/130528105946.htm
- The Pratical Planter “Why Plants Grow Towards Light (And the Role of Auxins)” https://thepracticalplanter.com/why-plants-grow-towards-light/
- Wikipedia “Phototropism” https://en.wikipedia.org/wiki/Phototropism
- NAGWA “Lesson Explainer: Plant Hormones” https://www.nagwa.com/en/explainers/142193981979/
- Wikipedia “Auxin” https://en.wikipedia.org/wiki/Auxin
- Khan Academy “Phototropism & photoperiodism” https://www.khanacademy.org/science/biology/plant-biology/plant-responses-to-light-cues/a/phototropism-photoperiodism
- ThoughtCo. “Understanding Plant Tropisms” https://www.thoughtco.com/plant-tropisms-4159843
- Science Buddies “Plants on the Move! Experiments with Phototropism” https://www.sciencebuddies.org/science-fair-projects/project-ideas/PlantBio_p041/plant-biology/plants-movement-phototropism
- Science Facts “Phototropism” https://www.sciencefacts.net/phototropism.html
