What is a plant sensory system?

The plant sensory system is a sophisticated system that responds to environmental stimuli such as light,
temperature, gravity, and touch. Research predicts that plant hormones and signaling mechanisms are
responsible for this regulation system in plants.

Plants response to external stimuli

Animals respond to external stimuli by moving from one place to the other. The plants, unlike animals, cannot move from one location to another due to their rootedness. As a result, they use hormones and plant signaling pathways to respond to various environmental factors like light, temperature, touch, gravity, and so on. Auxin, cytokinins, gibberellins, abscisic acid, and ethylene are examples of the former, while phototropism, gravitropism, and thigmotropism are examples of the latter. Plant intelligence has also evolved to produce a chemical signal in response to unpleasant stimuli such as predator attacks, physical damage, and diseases.

Hormones in plants

According to research, plant cells have the potential to respond to external events by producing growth regulators known as hormones. These hormones can alter all areas of a plant's life, from germination to blooming, to fruiting to senescence, where they act locally or by getting transmitted to other sections to trigger a reaction. Auxin, cytokinins, gibberellins, abscisic acid, and ethylene are the major types of plant hormones.

Auxin

Scientists Peter Boysen-Jensen and Arpad Paal demonstrated that the presence of auxin, a chemical molecule causes a plant's shoot to respond to light. Auxin is the plant's master growth regulator. It is responsible for controlling apical dominance, cell elongation, and light and gravity responses. It also regulates other processes such as seed germination, flowering, and fruiting, both directly and indirectly.

Cytokinins

Cytokinins dominate cell division (cytokinesis) in plants. It is found in growing tissues such as in roots, embryos, and fruits, where cell division occurs. It also stimulates cell differentiation and delays aging to promote mitosis.

Gibberellins

Gibberellins are a family of plant hormones that control shoot elongation, seed germination, and fruit and flower development. They also delay senescence in leaves and fruits by causing disruption dormancy in seeds of plants, which need to get exposed to cold or light to germinate.

Abscisic acid

Abscisic acid induces abscission of the leaves, inhibits stem elongation, and dormancy in lateral buds and seeds in response to adverse environmental conditions such as dehydration, freezing temperatures, or decreased day lengths. Its activity counteracts many of the growth-promoting effects of gibberellins and auxins. It is also referred as an antagonist of the hormone gibberellin.     

Ethylene

Ethylene is a gaseous hormone that causes plants to ripen their fruit, wilt their flowers, and lose their leaves. Ethylene is abundantly produced by aging tissues (mainly older leaves) and stem nodes.

Signaling pathways that regulate plant sensory systems

The following part shows how different signaling pathways affect plant responses to light, gravity, temperature, and other factors.

Response to light

Phototropism or phototropic response refers to a plant's response to light. Plants are capable of detecting three different wavelengths of light, such as blue light, red light, and far-red light, through the use of photoreceptors. These receptors are covalently bonded to a light-absorbing pigment called a chromophore, which is together known as chromoproteins or phototropin. Red light causes stem elongation in plants, whereas blue light promotes stem bending. The red light also indicates that a plant is under the shade of another plant.

Phytochromes are the chromoproteins responsible for detecting red/far-red light, where Pr phytochromes red, 667 nm and Pfr phytochrome far-red, 730 nm are two interconvertible variants of these phytochromes. The inactive form is Pr, while the active form is Pfr. When the Pr form absorbs red light, it quickly transforms into the Pfr form. When Pfr absorbs far-red light, it returns quickly to Pr form. This phytochrome system acts as a light switch in plants.

The phytochrome system also controls the opening and closing of leaves, stomatal gas exchange, chloroplast mobility within plant cells, seed germination, and photoperiodism, which occurs when plants response to day length by flowering.

Response to gravity

The plant response to far-red gravity is known as gravitropism. It ensures that the sprout faces the sun and the roots penetrate in the soil. Negative gravitropism refers to the upward growth of the shoot (against gravity), whereas positive gravitropism refers to the downward growth (in favor of gravity) of the roots. For instance, statoliths, also known as amyloplasts (starch granules), are a specific type of plastids found in shoots and specialized cells of root cap that settles downward in response to gravity.

Response to temperature and water

Plants develop heat shock proteins when exposed to sudden high temperatures, and acquires thermotolerance when subjected to steady high-temperature swings. Low temperatures also cause plants to increase lipid concentration in plasma membranes, produce antifreeze proteins, and to create ice crystals in extracellular regions rather than intracellular spaces. Leaves of the plants begin to fall off in the autumn, signaling the onset of the cold season.

When seeds are exposed to water, gibberellin gets activated, signaling the gene to create amylase, which is an enzyme that transforms starch contained in the seed to sugars, allowing the seed to germinate. Seeds release abscisic acid in response to drought, which inhibits gibberellins and prevents seed germination. When a mature plant is exposed to drought, it produces abscisic acid, which helps the stomata close and limit water loss from the leaves. However, if the plant is subjected to prolonged dryness, it suffers from cell death by surrendering leaves and stems, and eventually dies.

Response to mechanical stimuli

Plant movement in reaction to touch or mechanical stimulation is referred to as thigmotropism. Slow thigmotropism and fast thigmotropism are two types of thigmotropism that plants can exhibit. Slow thigmotropism is governed by auxin, which is used by vines that grow around for support. Auxin causes stem elongation in response to contact in this case. The Venus fly trap plant species closes its leaves very fast to entrap an insect, and the mimosa plant that closes its leaves quickly when touched, are the plants that follow fast thigmotropism.

Thigmomorphogenes is a process in which a plant exhibits delayed developmental changes in its structure after being exposed to mechanical stimulation for lengthy periods.

Plants' response to predators and physical injuries

Barriers such as bark, thorns, modified leaves, waxy cuticles, and so on are the first line of protection in plants. If this barrier is breached by predators such as herbivores, the plant resorts to a secondary line of defense by creating secondary metabolites. These metabolites are chemical molecules produced in plants in the form of poisons or enzymes for the sole purpose of protection and are not necessary for plant growth. For instance, plants generate an alkaloid known as quinine, which has a bitter taste and deters herbivores from eating them. Similarly, glycol cyanide in the cassava root that releases cyanide only when the herbivore consumes it. Plants respond to pathogens such as fungi bacteria, and viruses by creating toxins as phytoalexins, which kill the pathogen locally. Moreover, a hypersensitive response is triggered if the illness spreads. Pathogen-specific receptors in plant cells recognize specific pathogen molecules and trigger the hypersensitive response and results in the formation of methyl salicylate (MeSA), which activates the systemic acquired response (SAR). To prevent the spread of the infection, this systemic response activates transcription of pathogenesis-resistance genes. If the plant tissue is injured beyond repair owing to pathogenic infection or injury, the plant induces abscission of the damaged tissue.

Common Mistakes

Sensory movements in plants are often mistaken for motor movements by students. Kinins and myosins are used by plant motor systems to transfer cellular components and it does not involve the use of plant sensory systems.

Related concepts

• Plant growth
• Plant stem growth
• Vascular plants
• Plant neurobiology
• Transport of water and solutes in plant

Context and Applications

This subject is important in professional exams for undergraduate, graduate, and post-graduate studies, particularly in

• Bachelor of Science in Botany
• Master of Science in Botany
• Master of Science in Plant physiology
• Research in Agriculture

Practice Problems

1. To which of the following external stimuli do plants respond along with light, temperature, water, and touch?

1. Gravity
2. Birds
3. Humans
4. Insects

Answer- Option a

Explanation- Plants respond to gravity through a process known as gravitropism. It guarantees that the sprout is exposed to the sun and the roots are able to enter the soil.

2. Which of the following hormones is also involved in the regulation of plant sensory systems along with auxin, cytokinins, gibberellins, and abscisic acid?

1. Ethyl alcohol
2. Ethylene
3. Methylene
4. Methyl alcohol

Answer- Option b

Explanation- Ethylene is a gaseous hormone produced by plants that causes fruit to ripen, flowers to wilt, and leaves to fall off.

3. Besides Venus flytrap which other plant species display fast thigmotropism?

1. Senna
2. Rice
3. Potato
4. Mimosa

Answer- Option d

Explanation- The mimosa plant closes its leaves quickly when touched through a process known as fast thigmotropism.

4. Which of the following are the two interconvertible variants of phytochromes seen in plants?

1. Pr red and Pfr far-red
2. Pq red and Pqr far-red
3. Pr blue and Pfr far-blue
4. Pq blue and Pqr far-blue

Answer- Option a

Explanation: Pr phytochromes red, 667 nm, and Pfr phytochrome far-red, 730 nm are two interconvertible forms of phytochromes that detect red/far-red light in plants.

5. What activates the systemic acquired response (SAR) in plants?

1. Ethyl salicylic acid
2. Ethyl salicylate
3. Methyl salicylate (MeSA),
4. Methyl salicylic acid

Answer- Option c

Explanation: Plant cells sense specific pathogen chemicals through pathogen specific receptors and activate the hypersensitive response, which leads to the synthesis of methyl salicylate (MeSA) and further initiates the systemic acquired response (SAR).