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Biology library

Course: Biology library > Unit 34

Lesson 1: Plant responses to light
Science
Biology library
Plant biology
Plant responses to light
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Phototropism & photoperiodism

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Phototropism, plant growth towards or away from light, and photoperiodism, regulation of flowering and other developmental transitions by day/night length.

Key points

  • Plants have a variety of developmental, physiological, and growth responses to light—sometimes only to particular wavelengths of light.
  • In phototropism a plant bends or grows directionally in response to light. Shoots usually move towards the light; roots usually move away from it.
  • In photoperiodism flowering and other developmental processes are regulated in response to the photoperiod, or day length.
  • Short-day plants flower when day length is below a certain threshold, while long-day plants flower when day length is above a certain threshold.
  • In many plants, photoperiodism is controlled by the overlap between the day length cue and the plant's internal circadian rhythms.

Introduction

Almost all plants can photosynthesize, and photosynthesis is key to these plants' survival: it lets them make sugar molecules that serve as fuel and building materials. But plants respond to light—sometimes, to specific wavelengths of light—in other ways as well. These non-photosynthesis-related responses allow plants to adjust to their environment and optimize growth.
For instance, some types of seeds will germinate only when they receive a sufficient amount of light—along with other cues. Other plants have ways to detect if they are in the shade of neighboring plants based on the quality of light they receive. They can increase their upward growth to outcompete their neighbors and get a bigger share of sunshine.
Plant responses to light depend, logically enough, on the plant’s ability to sense light. Light sensing in plants involves special molecules called photoreceptors, which are made up of a protein linked to a light-absorbing pigment called a chromophore. When the chromophore absorbs light, it causes a change in the shape of the protein, altering its activity and starting a signaling pathway. The signaling pathway results in a response to the light cue, such as a change in gene expression, growth, or hormone production.
In this article, we will focus on two examples of plant responses to light and explore how these responses allow plants to match their growth to their environments:
  • Phototropism is a directional response that allows plants to grow towards, or in some cases away from, a source of light.
  • Photoperiodism is the regulation of physiology or development in response to day length. Photoperiodism allows some plant species to flower—switch to reproductive mode—only at certain times of the year.
Let's take a look at how these light responses work!

Phototropism

One important light response in plants is phototropism, which involves growth toward—or away from—a light source. Positive phototropism is growth towards a light source; negative phototropism is growth away from light.
Shoots, or above-ground parts of plants, generally display positive phototropism—they bend toward the light. This response helps the green parts of the plant get closer to a source of light energy, which can then be used for photosynthesis. Roots, on the other hand, will tend to grow away from light.start superscript, 1, end superscript

Phototropism involves a mobile signal

In 1880, Charles Darwin and his son Francis published a paper in which they described the bending of grass seedlings towards light. Specifically, they examined this response in very young plants that had just sprouted whose leaves and shoots were still covered by a sheath called the coleoptile.
light source (drawn as candle) and a coleoptile in a pot. The pictures shows a straight coleoptile becoming bent toward the light as time passes. The bending is caused by cells closer to the light expanding less than the plant cells facing away from the light.
The father-and-son team analyzed the bending response using experiments in which they covered either the tip or the lower part of the coleoptile.start superscript, 1, end superscript Through these experiments, they found that light was perceived at the coleoptile's tip. However, the response—bending, at a cellular level, unequal elongation of cells—took place well below the tip. They concluded that some kind of signal must be sent downwards from the coleoptile’s tip towards its base.
light source (drawn as candle) and a coleoptile in a pot with a metal cap covering the very top of the coleoptile. The picture shows a straight coleoptile remaining straight when the metal cap is covering the tip.
In 1913, Danish physiologist Peter Boysen-Jensen followed up on this work by showing that a chemical signal produced at the tip was indeed responsible for the bending response:
  • He first cut off the tip of a coleoptile, covered the cut section with a block of gelatin, and replaced the tip. The coleoptile was able to bend normally when it was exposed to light.
  • When he tried the experiment again using an impermeable flake of mica instead of gelatin, the coleoptile lost the ability to bend in response to light.
light source (drawn as candle) and a coleoptile in a pot . The coleoptile on the left has a permeable piece of gelatin separating the tip from the rest of the coleoptile, and it bends toward the light. The coleoptile on the right has a impermeable piece of mica separating the tip from the rest of the coleoptile, and it does not bend toward the light.
Only the gelatin—which allowed a chemical signal to travel through its pores—could allow the necessary communication between tip and base.
Through a variation on this experiment, Boysen-Jensen was also able to show that the mobile signal traveled on the shaded side of the seedling. When the mica plate was stuck in on the illuminated side, the plant could still bend towards the light, but when it was stuck in on the shaded side, the bending response no longer occurred. The results of this experiment also implied that the signal was a growth stimulant rather than a growth repressor since the phototropic response involved faster cell elongation on the shaded side than on the lit side.
light source (drawn as candle) and a coleoptile in a pot . The coleoptile on the left has a piece of mica separating the part of the tip further from the light from the rest of the coleoptile, and it does not bend toward the light. The coleoptile on the right has a impermeable piece of mica separating the part of the tip closer to the light from the rest of the coleoptile, and it bends toward the light.

Phototropins and auxin

Today, we know that proteins called phototropins are the main photoreceptors responsible for light detection during phototropism—the name is a handy reminder of their role! Like other plant photoreceptors, phototropins are made up of a protein bound to a light-absorbing organic molecule, called the chromophore. Phototropins absorb light in the blue range of the spectrum. When they absorb light, they change shape, become active, and can change the activity of other proteins in the cell.
When a coleoptile is exposed to a source of light, phototropin molecules on the illuminated side absorb lots of light, while molecules on the shady side absorb much less. Through mechanisms that are still not well understood, these different levels of phototropin activation cause a plant hormone called auxin to be transported unequally down the two sides of the coleoptile.
Close up of tip of coleoptile showing the plant hormone auxin (pictured as red dots) concentrated toward tip. When light hits one side of the coleoptile, the phototropins are more active on the side with light, causing the auxin to flow down the shady side. The side of the coleoptile with less auxin has less elongated cells, and the side with more auxin has more elongated cells, causing the tip to bend toward the light.
More auxin is transported down the shady side, and less auxin is transported down the illuminated side. Auxin promotes cell elongation, causing the plant to grow more on the shady side and bend in the direction of the light source.

Photoperiodism

Some types of plants require particular day or night lengths in order to flower—that is, to transition to the reproductive phase of their life cycle.
  • Plants that flower only when day length drops below a certain threshold are called short-day plants. Rice is an example of a short-day plant.squared
  • Plants that flower only when day length rises above a certain threshold are called long-day plants. Spinach and sugar beets are long-day plants.squared
By flowering only when day or night lengths reach a certain threshold, these plants are able to coordinate their flowering time with changes in the seasons.
Not all plants are short-day or long-day. Some plants are day-neutral, meaning that flowering does not depend on day length. Also, flowering is not the only trait that can be regulated by photoperiod—day length—although it's the one that's gotten the most attention from researchers. Tuber formation in potatoes, for instance, is also under photoperiodic control, as is bud dormancy in preparation for winter in trees growing in cold areas.cubed

What is the plant actually measuring?

Although we classify plants as short-day or long-day, in some cases, plants may actually be measuring the length of the night. That is, it can be the length of the period of continuous darkness, not the length of the period of continuous light, that determines whether or not the plant flowers.
This is particularly true of short-day plants, whose photoperiodic response is often tightly linked to the length of the night. Typical short-day plants share the following characteristics:start superscript, 2, comma, 4, comma, 5, end superscript
  • They flower when the day is short and the night is long.
  • They do not flower when the day is long and the night is short.
  • They do not flower when the long night is interrupted by a brief period of light.
  • They do not flower when the long day is interrupted by a brief period of dark.
Graph showing the relative hours of daylight vs. night (out of 24 hours) that will cause a short day plant to flower. If the night length is 16 continuous hours, the critical length, the plant will flower.
Image credit: diagram based on similar diagram in Thomas and Vince-Pruestart superscript, 5, end superscript
What exactly does all that tell us? The pattern in the diagram above means that short-day plants measure the length of the night—the continuous period of darkness—and not the length of the day—the continuous period of light. That is, a short-day plant will only flower if it gets uninterrupted darkness for a length of time that meets or exceeds its flowering threshold. If there is an interruption to the darkness, such as a brief period of light, the plant will not flower, even though the continuous period of light—day—is still short.
The situation changes a bit when we consider long-day plants. Some long-day plants do measure the length of the night, like the short-day plants in the diagram above. However, unlike short-day plants, these long-day plants need the period of darkness to be shorter than or equal to a critical length! Long-day plants that measure the night length are said to be dark-dominant because it's the period of continuous darkness that's important for flowering.
Many other long-day plant species, however, seem to measure the length of the day, not the night, in determining when to flower. These plants are said to be light-dominant.start superscript, 6, end superscript Scientists think that the majority of long-day plant species are actually light-dominant, while the majority of short-day plant species are dark-dominant.start superscript, 6, end superscript

How does the plant determine day or night length?

This is a question plant biologists have been wondering about for decades! Many models have been suggested over the years, but today, most biologists think photoperiodism—at least, in many species—is the result of interactions between a plant's "body clock" and light cues from its environment. Only when the light cues and the body clock line up in the right way will the plant flower.
This model is called the external coincidence model of photoperiodism. Its name highlights that an external cue—day length—has to coincide in a certain way with the plant's internal rhythms in order to trigger flowering. These rhythms are circadian rhythms, patterns in gene expression or physiology that repeat on a 24-hour cycle and are driven by the plant's internal body clock.
How the external coincidence model works is best understood for the long-day plant Arabidopsis, a relative of mustard. In this plant, levels of a specific mstart text, R, N, A, end text that encodes a flowering induction protein rise and fall on a circadian cycle, with mstart text, R, N, A, end text levels going up sharply in the evening.start superscript, 2, comma, 7, end superscript
When there is no light in the evening, the high levels of mstart text, R, N, A, end text don't get the plant very far. That's because the flowering induction protein is usually broken down as soon as it's made. If, however, there's light in the evening—a long day—photoreceptors are activated by the light and jump in to save the protein from degradation. The protein can then build up and trigger flowering.start superscript, 2, comma, 7, comma, 8, end superscript
Graph showing changing mRNA levels over 24 hours. Areas where the plant is exposed to light are highlighted in yellow. When light overlaps with high levels of mRNA, photoreceptors are activated by the light and protect the flowering induction protein, which can lead to flowering.
Image credit: based on similar diagrams in Lagercrantzsquared, Figure 4; Kimballstart superscript, 1, end superscript; and Valverde et al., Figure 4start superscript, 6, end superscript
Thanks to this molecular system, the plant flowers only when the days are long—when light extends late enough to overlap with the high mstart text, R, N, A, end text expression.

Other models of photoperiodism

Although it seems likely that many plant species use some type of external coincidence model to control flowering and other photoperiod-regulated processes, different plants have different genes and "wiring". It's possible that some plant species have fundamentally different ways of measuring photoperiod and linking this information to developmental changes.
For instance, an older model of photoperiodism, the phytochrome hourglass model, does not depend on overlap between circadian rhythms and photoperiod length. Instead, it suggests that phytochromes could act as a clock to measure the length of the night. Although this model is no longer widely accepted, it could potentially be valid for certain types of plants.

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