30.6 Plant Sensory Systems and Responses

Plant Responses to Light and Gravity

Red and blue light in plant growth

Plants use specific photoreceptors to detect different wavelengths of light, and each wavelength triggers distinct growth responses.

Red light is absorbed by phytochrome photoreceptors. It stimulates seed germination by breaking dormancy, promotes stem elongation and leaf expansion, and enhances flowering in certain long-day plants like spinach by regulating photoperiodism (the plant's ability to measure day length).

Blue light is absorbed by cryptochrome and phototropin photoreceptors. It plays several roles:

• Stimulates phototropism (growth toward light), which optimizes light capture for photosynthesis
• Regulates stomatal opening, facilitating $CO_2$ uptake and transpiration
• Influences chloroplast movement within cells to maximize light absorption
• Inhibits stem elongation, promoting compact, sturdy growth

A useful contrast: red light tends to promote elongation and flowering, while blue light fine-tunes positioning and gas exchange.

Gravitropism for plant orientation

Gravitropism is a plant's growth response to gravity, and it works in opposite directions for roots and shoots.

• Roots show positive gravitropism: they grow downward, toward gravity. This anchors the plant and positions roots to absorb water and nutrients.
• Shoots show negative gravitropism: they grow upward, away from gravity. This positions leaves to capture sunlight for photosynthesis.

The mechanism depends on statoliths, which are dense starch-filled granules found in root cap cells and endodermal cells. Here's how it works:

1. When a plant is tilted, statoliths settle to the lowest side of the cell due to their density.
2. This settling triggers a redistribution of auxin to the lower side.
3. In roots, the higher auxin concentration on the lower side inhibits cell elongation, causing the root to bend downward.
4. In shoots, the higher auxin concentration on the lower side promotes cell elongation, causing the shoot to bend upward.

The same hormone (auxin) produces opposite effects in roots versus shoots because root cells and shoot cells have different sensitivity thresholds to auxin concentration.

Plant Hormones and Responses to Stimuli

Hormones in plant development

Five major hormone classes coordinate plant growth, development, and stress responses.

Auxins (the most common natural form is indole-3-acetic acid, or IAA):

• Stimulate cell elongation and division
• Promote apical dominance by inhibiting lateral bud growth, which shapes overall plant architecture
• Mediate both phototropism and gravitropism

Cytokinins:

• Promote cell division and differentiation
• Delay leaf senescence (aging), keeping leaves photosynthetically active longer
• Interact with auxins to balance shoot and root growth. A high cytokinin-to-auxin ratio favors shoot growth; a high auxin-to-cytokinin ratio favors root growth.

Gibberellins:

• Stimulate stem elongation and leaf expansion
• Promote seed germination and fruit development

Abscisic acid (ABA):

• Triggers stomatal closure during water stress, conserving water
• Induces seed and bud dormancy, helping plants survive unfavorable conditions
• Generally acts as a growth inhibitor when the plant is under stress

Think of ABA as the "stress hormone." While other hormones promote growth, ABA puts the brakes on when conditions are poor.

Ethylene:

• Promotes fruit ripening and leaf abscission (the dropping of leaves)
• Induces senescence and various stress responses
• Unique among plant hormones because it's a gas, allowing it to diffuse between cells and even between neighboring plants

Thigmo responses of plants

Plants respond to touch and mechanical stimulation through three types of thigmo responses:

• Thigmotropism is a directional growth response to physical contact. Tendrils of climbing plants like peas coil around support structures, allowing the plant to grow vertically without investing in a thick, rigid stem.
• Thigmonasty is a non-directional, reversible movement triggered by touch. The Venus flytrap is the classic example: when prey contacts trigger hairs on the leaf surface, the trap snaps shut in a fraction of a second. Unlike thigmotropism, the direction of the stimulus doesn't determine the direction of the response.
• Thigmogenesis refers to touch-induced changes in overall growth patterns. Plants exposed to repeated wind or mechanical stress develop thicker, shorter stems with greater mechanical strength. This is why trees in windy environments tend to be stockier than sheltered ones.

Plant defenses against predators

Plants can't run from herbivores, so they've evolved a range of physical, chemical, and systemic defenses.

Physical defenses:

• Thorns, spines, and prickles deter herbivores (roses are a familiar example)
• Thick cuticles and bark limit pathogen entry and reduce water loss

Chemical defenses:

• Secondary metabolites like alkaloids and terpenes deter or poison herbivores. Nicotine in tobacco, for instance, is toxic to many insects.
• Volatile compounds released by damaged plants can attract predators of the herbivore (such as parasitic wasps), providing an indirect defense.

Wound responses follow a sequence when tissue is damaged:

1. The plant compartmentalizes the damaged area to prevent infection from spreading.
2. It produces defensive compounds like lignin and suberin to seal wounds and reinforce cell walls.
3. It activates systemic acquired resistance (SAR), a whole-plant defense response that primes undamaged tissues against future attack. SAR involves signaling molecules (like salicylic acid) traveling from the wound site to distant tissues.

Plant Sensory Systems and Environmental Responses

Circadian rhythm and plant behavior

Plants have an internal biological clock that runs on roughly a 24-hour cycle, even in the absence of external light cues. This circadian rhythm regulates the timing of stomatal opening, photosynthetic activity, gene expression, and flowering. By anticipating daily changes (such as dawn and dusk) rather than just reacting to them, plants can prepare metabolic processes in advance, giving them a competitive advantage.

Signal transduction in plant responses

When a plant detects an environmental stimulus, it converts that signal into a cellular response through signal transduction pathways. The general process works like this:

1. A receptor on or in the cell detects the stimulus (light, hormone, touch, etc.).
2. The receptor activates second messengers inside the cell (common ones include calcium ions and cyclic AMP).
3. Second messengers amplify the signal and activate transcription factors.
4. Transcription factors turn on or off specific genes, producing the appropriate response.

This cascade allows a single external signal to trigger a large, coordinated cellular response.

Plant stress responses

When plants face adverse conditions like drought, high salinity, extreme temperatures, or pathogen attack, they activate a suite of physiological and biochemical changes.

• Drought stress triggers ABA production, leading to stomatal closure and reduced water loss.
• Salt stress prompts the plant to synthesize compatible solutes (like proline) that help maintain cell turgor without toxic ion buildup.
• Temperature extremes induce production of heat shock proteins or antifreeze proteins that stabilize cellular structures.
• Pathogen attack activates defense genes and can trigger SAR (described above) or localized cell death (the hypersensitive response) to isolate the infection.

These stress responses involve complex gene regulation and often overlap, since multiple stresses frequently occur together in natural environments.