Plant Responses

Plant Responses to Stimuli

Unlike animals, plants cannot just uproot themselves and move to another location when responding to environmental stimuli. Instead, plants have hormones (chemical messengers) and other sophisticated mechanisms that detect stimuli such as light, gravity, and water and send signals to initiate physiological changes in response to these stimuli.

Plant Responses to Light

The ability to detect light in the environment is essential for a plant's competitiveness and survival.

Plants have photoreceptors that can detect and respond to at least three wavelengths of light:

1. Blue light

2. Red light

3. Far-red light

Photoreceptors consist of chromoproteins. A chromoprotein is comprised of a protein attached to a light-absorbing pigment via a covalent bond.

Plant response to blue light: phototropism

Some plants respond to environmental changes by developing their stems, roots, or leaves toward or away from the stimulus; such responses are called tropisms.

Plants tend to grow toward a light source because they need light energy to produce sugars.

The chromoproteins responsible for regulating phototropism are called phototropins. In addition to phototropism, phototropins also regulate other plant responses including the opening and closing of leaves, the movement of chloroplasts within cells, and the opening of stomata for gas exchange during photosynthesis.

The process by which phototropins cause plants to bend toward a light source are summarized as follows:

• Phototropins called phot1 and phot2 in the apical meristem detect blue light triggering the accumulation of a plant hormone called auxin (also known as indole acetic acid) on the shaded side of the plant.

• Auxin stimulates cell elongation by pumping protons from the cells to the space between the plasma membrane and the cell wall, causing the cells to expand.

• Because cell expansion takes place only on the shaded side of the stem, the plant bends toward the light source.

Figure 1 below shows how auxin regulates plant response to light.

Plant response to red light / far-red light: stem elongation, germination, photoperiodism

While blue light promotes bending, red light promotes stem elongation. Red light–as opposed to far-red light–promotes stem elongation because to a plant, red light means full sun, while far-red light means it is being shadowed out by another plant. This is due to the fact that unfiltered, full sunshine includes significantly more red light than far-red light.

Chlorophyll absorbs more strongly in the red part of the visible spectrum than in the far-red region, so a plant's ability to distinguish between red light and far-red light enables it to grow away from shaded areas toward light.

The chromoproteins that detect red and far-red light are called phytochromes. Phytochromes have two forms:

1. Pr (phytochrome red) which is capable of absorbing red light, and
2. Pfr (phytochrome far-red) which is capable of absorbing far-red light.

When Pr absorbs red light, it changes into Pfr, and when Pfr absorbs far-red light, it quickly changes back to Pr. The absorption of red or far-red light alters the structure of the chromophore, affecting the conformation and activity of the phytochrome protein to which it is attached.

In short, phytochrome activity is initiated by red light and inhibited by far-red light. The two forms of phytochrome–collectively called the phytochrome system–act as a biological switch.

Phytochrome promotes plant growth toward red light via cytokinin (a hormone that promotes cell division) and gibberellin (a hormone that stimulates stem elongation). Cytokinin is triggered by the Pfr form of phytochrome, promoting cell division in apical meristems exposed to red light.

In many plant species, the phytochrome system also controls seed germination.

Seeds may become dormant after fertilization so that they germinate at a time and place where the seedling has a better chance of survival.

For some plant species, exposure to red light indicates that the seed is in a suitable place for access to sunlight following germination. Some seeds may not germinate in the dark, where the phytochrome is in the Pr. The conversion of Pr into Pfr promotes the transcription of amylase (an enzyme that changes starch reserves in the seed into simple sugars), initiating seed germination.

It is important to note that not all plant species need light to germinate. Some seeds germinate through a light-independent process that is regulated by a plant hormone called gibberellin.

Plant response to gravity

Shoots typically sprout up from the earth, while roots grow down into the ground whether exposed to light or in complete darkness. When given enough time, a plant lying on its side in the dark will eventually develop upward shoots. This is because of gravitropism.

Gravitropism is the tendency of roots to grow down into the soil and the tendency of branches to grow upwards toward the sun due to the force of gravity. Gravitropism can be negative or positive:

• Negative gravitropism refers to the upward growth of the shoot apical tip.

• Positive gravitropism refers to the downward growth of the roots.

Gravitropism is regulated by auxins and amyloplasts. As mentioned before, auxins are plant hormones that promote cell elongation. Amyloplasts, on the other hand, are cell organelles containing heavy starch granules that fall to the bottom of the cell in response to gravity.

When amyloplasts fall to the bottom of the cell, they come into contact with the endoplasmic reticulum (ER) which releases calcium ions. In turn, the calcium ions signal the cells to transport auxin to the bottom of the cell.

This means that when the plant is tilted, the amyloplasts move, causing auxin to accumulate in what the plant perceives to be the new bottom of the root, that is, in the direction of gravity. Amyloplasts can be found in shoots and in the root cap.

Auxin affects the growth of roots and shoots differently:

• In roots, a high concentration of auxin suppresses cell elongation, so cells grow slowly on the lower side while cells grow normally on the upper side, causing the root to bend toward the high concentration of auxin which is downward.

• In shoots, a higher concentration of auxin promotes cell elongation causing the shoot to bend away from the region with greater auxin concentration. As such, gravitropism causes shoots to grow upward.

Plant responses to water limitations

Water consumption causes the hormone gibberellin to signal the transcription of the gene encoding amylase, an enzyme that changes starch reserves in the seed into simple sugars, initiating seed germination.

When the plant lacks water, germination is inhibited by abscisic acid (also known as ABA), a hormone that suppresses the function of gibberellins. Thus, gibberellins and abscisic acid have contradictory functions that work hand-in-hand to regulate germination in response to stimuli like water.

Additionally, in the absence of water, abscisic acid also causes stomata (pores in leaves) to close, preventing gas exchange and inhibiting photosynthesis. If the stomata of a plant remain closed for too long, the plant begins to die in localized regions (in leaves and stems, for instance). This process is regulated by the hormone ethylene, which has the ability to induce localized cell death.

Other plant responses related to growth

There are other responses to stimuli that affect the growth and development of plants. Here we will discuss two: apical dominance and leaf abscission.

Apical dominance

Many plants grow at a single apex which is dominant over other stems. Apical dominance means that the growth of a single apex exceeds those of other stems. Apical dominance is controlled by the presence of auxin at the apical meristem.

Other growth-regulating hormones, like cytokinins, require auxin to function. Cytokinins increase cell division only in the presence of auxin. Together, auxin and cytokinins promote cell growth. Because auxin is only present in the apical bud and not the lateral buds, plant growth takes place only in the apical bud.

Leaf abscission

Some plants shed their leaves in response to seasonal changes (based on temperatures, light, water, or other environmental stimuli). This process is known as leaf abscission, and it is controlled by interactions between auxin and ethylene.

During the growth season, the leaf generates a lot of auxins, which inhibits ethylene activity; however, when the seasons change, the leaf produces less auxin. Lower auxin levels allow ethylene to commence senescence (maturing) and, eventually, programmed cell death at the point of leaf attachment to the stem, enabling the leaf to fall off in a regulated way without causing harm to the remainder of the plant.