Excitability and Response in Algae and Plants

Overview: What “Excitability” Means in Algae and Plants

Algae and plants do not have nerves, muscles, or brains, but many of their cells can still sense changes and respond in a coordinated way. In this context:

• Excitability = the ability of a cell or tissue to perceive a stimulus and undergo a rapid, measurable change (electrical, chemical, or structural).
• Response = the resulting change in behavior, physiology, or growth of the cell or whole organism.

In contrast to animals, plant responses are often:

• Slower and growth-based (tropisms) or
• Localized and mechanical (leaf folding, trap closing)
  but they can still involve fast electrical signals that spread through tissues.

This chapter focuses on how algae and plants:

1. Perceive stimuli,
2. Generate electrical signals, and
3. Translate those signals into specific responses.

Stimulus Perception in Algae and Plants

Plant and algal cells detect external changes through specialized molecules and structures in their membranes and organelles. Only what is specific to algae/plants is emphasized here.

Light Sensing

Many algae and land plants are photoautotrophs and depend critically on light. They possess:

• Photoreceptors (light-sensitive proteins), such as:
• Phytochromes – primarily sensitive to red/far-red light
• Cryptochromes – sensitive to blue/UV-A light
• Phototropins – blue light receptors that mediate phototropism and stomatal opening
• In many motile algae:
• Eyespots (stigma) – pigmented structures combined with photoreceptors that allow light direction sensing, enabling phototaxis (movement toward or away from light).

Light detection can trigger:

• Changes in gene expression (e.g., switching from dark to light growth),
• Adjustments in photosynthetic apparatus,
• Directional growth (phototropism) in land plants,
• Swimming orientation in algae.

Mechanical and Touch Sensing

Plant cells are enclosed by cell walls, but they still sense mechanical stimuli, such as:

• Touch (e.g., insect landing, contact with support),
• Bending by wind,
• Pressure and tension in tissues.

Specialized mechanosensitive ion channels in membranes open when the membrane is stretched or bent, allowing ions (e.g., $Ca^{2+}$) to flow in. This can initiate:

• Rapid movements (e.g., leaf folding in Mimosa pudica),
• Coiling of tendrils in climbing plants,
• Triggering of traps in carnivorous plants (e.g., Dionaea muscipula, Venus flytrap),
• Long-term growth changes (thigmomorphogenesis: altered growth due to repeated mechanical stimulation).

Chemical and Nutrient Sensing

Algae and plant roots, leaves, and even pollen tubes detect:

• Nutrients (nitrate, phosphate, ammonium, potassium),
• Toxic substances (heavy metals, salt),
• Signaling molecules from other organisms (e.g., root exudates, pheromone-like signals from fungi, herbivore saliva components).

Perception usually involves:

• Receptor proteins (e.g., receptor-like kinases) at the plasma membrane,
• Transporters that change their activity in response to substrate concentration.

Responses include:

• Directed growth of roots toward nutrient-rich zones (chemotropism),
• Establishment of symbioses (e.g., with mycorrhizal fungi),
• Activation of defense pathways after herbivore attack (e.g., synthesis of toxins or repellents).

Gravity and Orientation

Land plants have specialized structures for gravitropism; some algae also orient themselves with respect to gravity or buoyancy.

• Statocytes in roots and shoots contain statoliths (dense starch-filled plastids) that sediment within the cell depending on orientation.
• This sedimentation is sensed by mechanosensitive proteins, leading to unequal distribution of growth regulators and thus directional growth (roots downwards, shoots upwards).

Water Status and Osmotic Conditions

Cells monitor:

• Internal turgor pressure,
• External osmotic potential (salt concentration),
• Water availability in soil or surrounding medium (in algae).

Water-related sensing leads to:

• Stomatal movements (opening/closing),
• Osmotic adjustments (e.g., synthesis of compatible solutes),
• Root architecture changes to explore wetter soil zones.

Electrical Excitability in Plants and Algae

Although they lack neurons, numerous algae and plant cells show bioelectrical phenomena, including:

• Membrane potentials (differences in voltage across the plasma membrane),
• Action potentials (fast electrical changes),
• Variation potentials (slower, longer-lasting potentials associated with damage or stress),
• Local ionic changes (especially in $Ca^{2+}$ concentration).

Resting Membrane Potential in Plant and Algal Cells

Plant and algal plasma membranes maintain a resting potential, often more negative inside than typical animal nerve cells (commonly around $-120$ to $-160$ mV).

Key features:

• A proton pump (H\^+-ATPase) exports $H^+$ ions from the cell, creating:
• An electrical gradient (inside negative),
• A proton concentration gradient (outside more acidic).
• Other ion channels and transporters use this gradient to move ions (e.g., $K^+$, $Cl^-$, $Ca^{2+}$).

The membrane potential is fundamental for:

• Nutrient uptake,
• Turgor regulation,
• Signal generation and propagation.

Plant and Algal Action Potentials

In some cells, a strong enough stimulus can trigger a rapid, all-or-nothing depolarization followed by repolarization: a plant action potential.

Compared with animal neurons:

• Amplitude: smaller or comparable (tens to ~100 mV),
• Speed: slower (from mm/s to several cm/s, versus m/s to >100 m/s in many neurons),
• Ions involved: more diverse; $Ca^{2+}$ and $Cl^-$ often play larger roles, $Na^+$ less dominant.

General stages:

1. Resting state – membrane strongly negative inside.
2. Depolarization – stimulus opens ion channels (e.g., $Ca^{2+}$ or $Cl^-$ channels); membrane becomes less negative or briefly positive.
3. Repolarization – activation of $K^+$ efflux channels and reactivation of $H^+$ pumps restore negative potential.
4. Refractory-like period – a short period where it is more difficult to trigger another full action potential.

Different plant tissues can generate action potentials, including:

• Sieve elements of the phloem,
• Excitable cells in sensitive leaves and trap structures,
• Some algae (e.g., giant algal cells used historically in electrophysiology).

Action potentials allow rapid long-distance communication in response to local stimuli (e.g., wounding, strong light, touch).

Variation Potentials

Plants also display variation potentials (also called slow wave potentials):

• Typically induced by wounding, burning, or severe stress,
• Slower in onset and more prolonged than classical action potentials,
• Amplitude and shape depend strongly on the type and severity of the disturbance,
• Often associated with hydraulic changes (pressure waves in xylem) and chemical signals (e.g., reactive oxygen species).

Variation potentials are important for:

• Systemic activation of defense programs,
• Coordinated physiological adjustments (e.g., stomatal closure distant from the damage site).

Calcium as a Universal Signal

$Ca^{2+}$ plays a central role in excitability of plant and algal cells:

• External stimuli (light, touch, wounding, chemical signals) often cause localized or propagating $Ca^{2+}$ waves, due to:
• Influx of $Ca^{2+}$ from the external medium,
• Release from internal stores (e.g., vacuole, endoplasmic reticulum).
• The pattern of $Ca^{2+}$ increase (amplitude, duration, frequency, spatial spread) encodes information (“calcium signatures”).

Downstream, $Ca^{2+}$-binding proteins (e.g., calmodulin, CDPKs – calcium-dependent protein kinases) translate the $Ca^{2+}$ signal into changes in gene expression, metabolism, or ion channel activity.

Specific Examples of Excitable Responses in Algae

Phototaxis and Photophobic Responses

Motile unicellular and colonial algae (e.g., certain green algae) show:

• Positive phototaxis – movement toward low or moderate light intensities,
• Negative phototaxis – movement away from damaging high light,
• Photophobic responses – abrupt changes in swimming direction when light intensity suddenly changes.

Mechanism outline:

• Light is detected by photoreceptors in the eyespot.
• Changes in illumination cause rapid modulation of ion channels in the flagellar membrane, often involving $Ca^{2+}$.
• Altered flagellar beating pattern leads to different swimming direction or speed.

These behaviors allow algae to position themselves at optimal depths or microenvironments for photosynthesis.

Chemotaxis and Environmental Sensing

Some algae respond to gradients of chemicals (e.g., nutrients, toxins, mating signals) via chemotaxis:

• Surface receptors or transporters detect concentration differences.
• Signal transduction pathways modulate internal $Ca^{2+}$ and other second messengers.
• Flagellar motility is adjusted, changing direction or swimming pattern.

This enables algae to:

• Seek nutrient-rich zones,
• Avoid harmful substances,
• Find partners for sexual reproduction (in species with gamete attraction).

Large Algal Cells as Experimental Models

Certain giant algal cells (e.g., from the genus Chara) have been used to study excitability:

• They show clear action potentials propagating along the cell.
• These action potentials can be triggered by mechanical or electrical stimuli.
• Conductivity and ionic mechanisms in these cells helped reveal basic principles of plant and algal bioelectricity.

Specific Examples of Excitable Responses in Plants

Rapid Movements in “Sensitive” Plants

Some plants translate electrical excitability to visible, rapid movements.

Leaf Folding in *Mimosa pudica*

Mimosa pudica (the sensitive plant) folds its leaflets and droops its petiole when touched, shaken, or heated.

Key features:

• Stimulus perception: touch cells and mechanosensitive channels in the leaf.
• Electrical signal: action potentials are generated and travel along phloem and specialized cells.
• Motor organs (pulvini) at leaflet bases and petiole bases change turgor:
• $K^+$ and other ions are redistributed between cells,
• Water follows osmotically,
• Cells on one side lose turgor and collapse, causing bending.

The sequence from stimulus to full leaf movement can occur within seconds.

Venus Flytrap and Other Carnivorous Plants

The Venus flytrap (Dionaea muscipula) closes its trap when trigger hairs are touched.

Distinctive aspects:

• A single touch produces an electrical signal but does not close the trap.
• Typically two or more touches within a short time are required to sum signals and reach the threshold for trap closure.
• Action potentials propagate across the trap lobe, leading to rapid changes in turgor and tissue elasticity, snapping the trap shut.
• Additional action potentials during prey struggle regulate:
• Degree of closure,
• Secretion of digestive enzymes.

Other carnivorous plants (e.g., Drosera, sundews) also use electrical signals to coordinate tentacle bending and secretion, though movements are often slower.

Stomatal Movements and Long-Distance Signals

Stomata are pores in the epidermis of leaves, each surrounded by two guard cells. Their opening and closing involves both ion transport and electric changes.

Stimuli affecting stomata include:

• Light (especially blue light),
• CO₂ concentration,
• Humidity and soil water status,
• Hormones (e.g., abscisic acid during drought),
• Systemic signals from stress or damage elsewhere in the plant.

In response:

• Guard cells adjust the activity of $K^+$ channels, anion channels, $H^+$-ATPases, etc.
• Membrane potential changes guide the direction of ion flow.
• Changes in turgor open or close the pore.

Additionally, long-distance electrical and chemical signals (e.g., from wounded leaves) can induce stomatal closure in undamaged leaves, reducing water loss or spread of pathogens.

Systemic Wound and Herbivory Responses

When an herbivore chews a leaf or a leaf is mechanically damaged:

• Local perception:
• Mechanical damage and herbivore-derived chemicals (e.g., components of saliva) are detected.
• Local cells undergo depolarization, $Ca^{2+}$ influx, and production of reactive oxygen species and hormones (e.g., jasmonates).
• Travel of signals:
• Electrical signals (action and variation potentials) move through vascular tissues.
• Hydraulic and chemical signals travel through xylem and phloem.
• Distant responses:
• Gene expression changes in undamaged leaves,
• Synthesis of defense compounds (e.g., toxins, digestibility-reducing proteins),
• Emission of volatile signals that can attract predators of the herbivores or warn neighboring plants.

Thus, excitability underlies complex, whole-plant defense coordination.

Tropisms: Directional Growth Responses

Although tropisms are growth-based and occur over longer time scales, they often start with rapid ionic and electrical changes at the site of perception.

Examples:

• Phototropism:
• Blue light detected by phototropins on one side of the shoot.
• Asymmetric ion fluxes and growth regulator distribution lead to faster growth on the shaded side, bending the shoot toward light.
• Gravitropism:
• Movement of statoliths in statocytes triggers differential ion transport and growth responses on opposite sides of the organ.

These tropic responses rely on:

• Early membrane potential changes,
• Local $Ca^{2+}$ signals,
• Subsequent activation of signaling cascades that ultimately affect growth patterns.

Integration of Signals and Forms of Response

Excitability in algae and plants is rarely due to a single stimulus or single pathway. Instead, cells and tissues integrate multiple environmental and internal signals.

Cross-Talk of Signal Types

• Electrical signals (action potentials, variation potentials),
• Chemical signals (hormones, reactive oxygen species, peptides),
• Hydraulic signals (changes in tension/pressure in xylem),
• $Ca^{2+}$ waves and other second messengers

interact to produce a coordinated response.

For instance, a leaf experiencing drought:

1. Senses water deficit (osmotic/mechanical cues),
2. Generates electrical and $Ca^{2+}$ signals,
3. Increases abscisic acid levels,
4. Induces stomatal closure and gene expression changes to conserve water.

Local vs. Systemic Responses

• Local responses: occur at the point of stimulus (e.g., local leaf closure, cell wall strengthening, local defense compound accumulation).
• Systemic responses: occur throughout the organism (e.g., broad changes in metabolism, systemic acquired resistance, systemic stomatal behavior).

Algae, especially unicellular ones, mainly exhibit whole-cell responses, as a single cell constitutes the entire individual. Multicellular algae and land plants show both local and distant responses, mediated by internal conduction pathways.

Reversible vs. Irreversible Responses

• Reversible:
• Leaf folding/unfolding in Mimosa,
• Trap opening/closing in carnivorous plants,
• Stomatal opening/closing,
• Rapid turgor changes.
• Irreversible or long-lasting:
• Growth direction changes (tropisms),
• Developmental shifts (e.g., transition to flowering induced by day length),
• Structural defense enhancements (e.g., increased lignification, formation of trichomes).

Both types often start with the same basic excitability mechanisms—perception and bioelectrical/chemical signaling—but differ in which downstream processes are activated.

Summary of Key Points

• Algae and plants are excitable organisms even without nerves; they perceive environmental stimuli and respond through electrical, chemical, and mechanical changes.
• Stimulus perception uses specialized receptors for light, mechanical forces, chemicals, gravity, and water status.
• Electrical phenomena in plant and algal cells include membrane potentials, action potentials, variation potentials, and complex $Ca^{2+}$ signaling.
• Algae show excitable behavior in movements such as phototaxis and chemotaxis, often mediated by eyespots and changes in flagellar activity.
• Plants use excitability to drive:
• Rapid movements (leaf folding, trap closure),
• Dynamic regulation of stomata,
• Systemic wound and defense responses,
• Directional growth (tropisms).
• Integration and cross-talk between electrical, chemical, and hydraulic signals allow algae and plants to exhibit coordinated, adaptive behaviors in response to changing environments.