Phytochromes

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.

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.

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.

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.

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.