Phototropism
Darwin and Phototropism
Introduction to Charles Darwin's foundational contributions to plant biology, particularly his early observations with his son, Francis Darwin:
They demonstrated the phenomenon of phototropism, which is the directional growth or bending of plants toward a light source.
They highlighted the critical importance of the coleoptile, a protective sheath enclosing the embryonic shoot of grasses (like oats or maize) in seedlings, as the primary site for perceiving light stimuli in plant responses to light.
Their initial findings, published in "The Power of Movement in Plants" (1880), suggested that the bending response was mediated by a diffusible chemical substance produced in the very tip of the coleoptile, which then influenced growth in the region below the tip.
Experiment by Boys and Jensen (1910-1913):
They further investigated Darwin's hypothesis, proposing that a mobile, diffusible signaling substance from the coleoptile tip was responsible for inducing cell elongation, leading to the bending.
Their methodical approach involved:
Decapitated barley coleoptiles by carefully removing their tips.
Subsequently, they replaced the removed tips with various materials to test permeability:
When a permeable material like gelatin was inserted between the tip and the base, allowing for chemical diffusion.
When an impermeable material like mica was inserted, physically blocking any substance from moving downwards.
Results unequivocally showed:
Bending toward light occurred successfully when gelatin was used, providing strong evidence that a diffusible substance from the tip was indeed responsible for transmitting the growth signal.
Conversely, no bending occurred when mica was used, confirming that physical separation prevented the signal transmission and thereby inhibited the phototropic response, underscoring the necessity of the diffusible substance.
Fritz Went's Contribution
Further pivotal investigation into the nature of the diffusible substance was carried out by Fritz Went (1926-1928):
He developed an ingenious method where he incubated excised coleoptile tips on small agar blocks, thereby collecting and concentrating the diffusible growth-promoting substance secreted from the tips.
His subsequent findings revolutionized the understanding of plant hormones:
Agar blocks inoculated with these tips, when placed off-center (asymmetrically) on decapitated coleoptiles in the dark, consistently induced pronounced cell elongation on the side where the block was placed. This differential growth caused the decapitated coleoptile to bend away from the side with the agar block, mimicking a phototropic response.
This experiment conclusively demonstrated that the agar blocks contained a substance that promoted growth and that asymmetrical application led to differential growth.
This led to the groundbreaking first identification and isolation of a specific plant growth hormone, which he named auxin (Indole-3-acetic acid or IAA is the most common naturally occurring auxin).
Auxin's multifaceted role:
It was identified as a major regulator not only for phototropism but also for other crucial plant processes such as cell elongation, gravitropism (growth in response to gravity), apical dominance (the inhibition of lateral bud growth by the apical bud), and the initiation of adventitious roots.
Mechanism of Auxin Action
Explanation of how auxin affects cell elongation primarily focuses on the 'acid growth hypothesis', though the concept itself is generally not assessed in detail for basic exams.
Auxin stimulates proton (H^+) pumps in the plasma membrane, actively pumping H^+ ions into the cell wall.
This acidification activates expandin enzymes within the cell wall, which then loosen the cell wall fibers, making the wall more extensible.
With the cell wall now loosened, the turgor pressure within the cell can cause the cell to absorb water and expand, leading to irreversible cell elongation.
Action Spectrum for Phototropism
Discussion of the specific pigments involved in mediating the phototropic response:
Initially, there was speculation that chlorophyll (due to its light-absorbing properties) or phytochrome (due to its known role in light perception) might be the photoreceptors.
However, experiments determining the action spectrum for phototropism revealed that the actual pigments responsible exhibit peak absorption in the blue light range (approximately 400-500 nm), with specific peaks around 450 nm.
Blue light is thus critically important for initiating the phototropic response, activating specific blue-light photoreceptors known as phototropins, which then trigger a signaling cascade leading to differential growth.
Phytochrome and Plant Development
Introduction to phytochrome, a family of photoreceptors crucial for various aspects of plant development:
It plays a central role in numerous plant phenomena, including flowering induction, seed germination (especially for light-sensitive seeds), and shade avoidance responses (e.g., stem elongation).
Phytochrome exists in two photoreversible forms:
Phytochrome red (P_r): This is the inactive form synthesized in the cytoplasm. It absorbs red light (peak absorption around 660 nm).
Phytochrome far-red (P{fr}): This is the active form. When Pr absorbs red light, it rapidly transforms into P{fr} (660 ext{ nm } Pr o P_{fr}, a process that occurs quickly in sunlight).
Conversely, when P{fr} absorbs far-red light (peak absorption around 730 nm), it rapidly reverts back to Pr (730 ext{ nm } P{fr} o Pr). In darkness, P{fr} also slowly reverts to Pr over several hours.
Responses predominantly mediated by the P_{fr} form:
P{fr} is generally the biologically active form of phytochrome, initiating a signaling pathway. Its presence or absence, and the ratio of Pr to P_{fr}, dictate various physiological effects, including the stimulation or prevention of flowering, promotion or inhibition of seed germination, and control of seedling de-etiolation.
Phytochrome's intricate relationship with biological clocks:
Plants utilize phytochrome, in conjunction with their internal circadian rhythms, to precisely sense and respond to changes in day/night length, a process known as photoperiodism.
This mechanism allows for the accurate differentiation of short-day and long-day plants based on the duration of light and — more importantly — darkness they experience.
Importance of Critical Night Length
It is recognized that plants measure the critical uninterrupted night length rather than the critical day length to determine their flowering response.
This can be strikingly demonstrated through laboratory experiments where manipulation of the dark period, such as interrupting a long night with a brief flash of light, can decisively control flowering in photoperiodic plants.
Regular exposure to specific alternating light and dark conditions alters the Pr/P{fr} ratio, thereby influencing gene expression and ultimately controlling flowering responses.
Short-Day and Long-Day Plants
Classification of plants based on their specific flowering requirements in relation to photoperiod:
Short-day plants (SDP), also known as long-night plants, require a long, uninterrupted period of darkness (i.e., short days) to flower. If the critical night length is interrupted, flowering may be inhibited (e.g., chrysanthemums, poinsettias, rice).
Long-day plants (LDP), also known as short-night plants, require a short period of darkness (i.e., long days) to flower. If the critical night length is extended or interrupted in a way that creates a shorter perceived night, flowering is promoted (e.g., spinach, iris, wheat).
Day-neutral plants (DNP) are unaffected by the length of the day or night and flower once they reach a certain developmental stage or size, regardless of photoperiod (e.g., corn, tomatoes, dandelions).
Characteristics of short-day (long-night) and long-day (short-night) plant reactions to light manipulation:
Flashing red light during the middle of a long night can crucially influence flowering by converting Pr to P{fr}. For short-day plants, this interruption effectively "shortens" the perceived night, often inhibiting flowering. For long-day plants, this same interruption can promote flowering by shortening the perceived night.
Experimental Manipulations of Phytochrome
Overview of sophisticated experiments involving precise control of light exposure:
Alternate exposures to red (R) and far-red (FR) light pulses were used to exquisitely adjust plant responses, particularly concerning seed germination and flowering.
In light-sensitive seeds, a final exposure to red light stimulates germination because it initiates PfrPfr formation. Conversely, a concluding pulse of far-red light prevents germination by promoting PrPr formation.
These experiments provided definitive proof of phytochrome's photoreversibility and its role as a molecular switch, noting plant behavior based on the precise timing and type of light exposure (R vs. FR) and demonstrating how the last light exposure dictates the biological outcome.
Glossary of Key Terms
Phototropism: The directional growth or bending of plants toward a light source.
Action Spectrum: A graph illustrating the effectiveness of different wavelengths of light in driving a specific biological process (e.g., phototropism), which helps identify the photoreceptors involved.
Blue Light: The specific range of light (approximately 400-500 nm, with strong peaks around 450 nm) that is critically important for initiating the phototropic response in plants, activating specific blue-light photoreceptors known as phototropins.
Darwin & Darwin Experiments: Early observations by Charles and Francis Darwin (1880) demonstrating the phenomenon of phototropism. They showed that the coleoptile tip is the primary site for light perception and suggested a diffusible chemical substance produced there mediated the bending response.
Boysen-Jensen Experiment: Experiments conducted between 1910 and 1913 that confirmed Darwin's hypothesis. They demonstrated that a permeable material (like gelatin) inserted below a decapitated coleoptile tip allowed bending toward light, while an impermeable material (like mica) blocked the signal, thus proving a diffusible substance transmits the growth signal.
Went Experiments: Pivotal investigations by Fritz Went (1926-1928) where he collected a growth-promoting substance from excised coleoptile tips using agar blocks. Asymmetrical placement of these agar blocks on decapitated coleoptiles induced differential cell elongation and bending, leading to the identification and isolation of auxin.
Auxin: The first identified plant growth hormone, discovered and isolated by Fritz Went (specifically Indole-3-acetic acid or IAA). It is a major regulator for diverse plant processes including cell elongation, phototropism, gravitropism, apical dominance, and the initiation of adventitious roots.
Photoperiodism: The physiological response of plants to the relative lengths of day and night, specifically the duration of the uninterrupted night length, which dictates crucial life cycle events such as flowering and seed germination.
Flowering: The process of reproductive development in plants, often precisely regulated by photoperiodism (the interplay of light and dark durations) and mediated by various plant hormones, especially phytochrome.
Seed Germination: The initial process by which a seed sprouts and develops into a seedling. This process can be significantly influenced by light quality (red vs. far-red light) and the activity of phytochrome, particularly in light-sensitive seeds.
Phytochrome: A family of photoreceptors essential for various aspects of plant development and environmental sensing. It exists in two interconvertible forms, phytochrome red (Pr) and phytochrome far-red (P{fr}), which absorb red and far-red light, respectively, to regulate processes like flowering, seed germination, and shade avoidance.
Red Light: Light with a peak absorption around 660 nm. The absorption of red light rapidly converts the inactive phytochrome red (Pr) form into the biologically active phytochrome far-red (P{fr}) form. A final pulse of red light often promotes germination or other phytochrome-mediated responses.
Far-Red Light: Light with a peak absorption around 730 nm. The absorption of far-red light rapidly converts the active phytochrome far-red (P{fr}) form back into the inactive phytochrome red (Pr) form. A final pulse of far-red light often inhibits germination or reverses red light-induced effects.
Short-Day (Long-Night) Plants (SDP): Plants that require a single, continuous period of darkness exceeding a specific critical length (i.e., short days with long nights) to flower. If this critical night length is interrupted by light, flowering may be inhibited.
Long-Day (Short-Night) Plants (LDP): Plants that require a period of darkness shorter than a specific critical length (i.e., long days with short nights) to flower. If the critical night length is
Long-Day (Short-Night) Plants (LDP): These plants, also known as short-night plants, are characterized by their requirement for a period of uninterrupted darkness that is shorter than a specific critical length in order to initiate flowering. Essentially, they thrive and flower when exposed to long days and consequently, short nights. Conversely, if the critical night length is extended (making the nights longer than their threshold) or if a longer night period is interrupted by even a brief flash of light (which effectively shortens the perceived dark period), such conditions promote their flowering. Examples include spinach, iris, and wheat.