Plant Responses to Internal and External Signals

Dodder (Cuscuta)

• Parasitic, nonphotosynthetic flowering plant that relies entirely on its host for nutrients and survival.

• 4Seedling actively searches for a suitable host plant, and if none is found within approximately one week, it dies due to lack of resources.

• Tendrils coil tightly around the host plant, and over several days, specialized structures called haustoria penetrate the host's tissues to tap into the phloem for nutrient acquisition.

• The number of coils formed by the dodder tendrils around the host is influenced by the nutritional status of the host plant.

• Employs chemical cues to locate potential hosts, exhibiting a sophisticated form of plant-to-plant communication.

Plant Interactions with the Environment

• Plants perceive environmental conditions such as sunlight and nutrient availability through various sensory mechanisms.

• Signal transduction pathways in plants share similarities with those found in animals, indicating a conserved evolutionary history.

• Plants respond to environmental signals by altering their growth and development patterns rather than through physical movement.

• Capable of adjusting to seasonal changes to optimize their survival and reproductive success.

• Engage in a wide array of interactions with other organisms, utilizing intricate transduction pathways to mediate these relationships.

Signal Transduction Pathways

• Employ signal transduction pathways to respond to changes in their environment, thereby enhancing their chances of survival.

• Example of a potato sprouting in a cupboard illustrates the plant's ability to respond to darkness by initiating growth.

Etiolation

• Etiolation represents a set of adaptations that enable plants to grow in complete darkness.

• A young potato plant, for instance, can sprout underground, utilizing stored resources to initiate growth.

• This process conserves both water and energy, allowing the plant to allocate resources efficiently.

• Stems elongate rapidly to facilitate the plant's emergence from the soil before its tuber reserves are fully depleted.

De-etiolation (Greening)

• Upon reaching light, the shoot undergoes de-etiolation, marked by a slowdown in stem elongation, expansion of leaves, elongation of roots, and production of chlorophyll.

Signal Transduction

• Phytochrome acts as a receptor for light signals, initiating a cascade of events.

• Mutant studies have been instrumental in elucidating the intricacies of cell signal processing in plants.

General Model for Signal Transduction Pathways

• A stimulus triggers the pathway by interacting with a receptor, which then activates relay proteins and generates second messengers.

• The signal is transmitted along the pathway, ultimately leading to specific cellular responses.

• Receptors can be located either on the cell surface or within the cell, depending on the nature of the signal.

Reception

• Receptors play a crucial role in detecting signals and undergoing conformational changes in response to stimulus.

• Phytochrome, situated in the cytoplasm, functions as the receptor for de-etiolation.

• The aurea tomato mutant, characterized by reduced phytochrome levels, highlights the importance of this receptor in plant development.

Transduction

• Receptors exhibit sensitivity to even weak signals, enabling plants to respond to subtle environmental cues.

• Second messengers, such as calcium ions (Ca^{2+}) and cyclic GMP (cGMP), amplify the signal, enhancing the plant's response.

• Phytochrome activation leads to the opening of Ca^{2+} channels, resulting in increased cytosolic Ca^{2+} levels.

• Phytochrome undergoes a conformational change, activating guanylyl cyclase, which produces cGMP.

• Both Ca^{2+} and cGMP are essential for eliciting a complete response to the initial stimulus.

Response

• Second messengers modulate various cellular activities to bring about the appropriate response.

• Transcriptional regulation and post-translational modification are key mechanisms involved in this process.

Post-Translational Modification of Preexisting Proteins

• Phosphorylation, mediated by kinases, alters protein activity, thereby influencing cellular processes.

• Kinases activate transcription factors, promoting gene expression.

• Protein phosphatases, on the other hand, deactivate proteins, providing a means of regulating protein activity.

• Maintaining a balance between kinases and phosphatases is essential for proper cellular function.

Transcriptional Regulation

• Transcription factors play a central role in controlling gene transcription, thereby regulating the plant's response to stimuli.

• De-etiolation factors are activated by phosphorylation via cGMP or Ca^{2+}, leading to changes in gene expression.

• Some mutants exhibit light-grown morphology even in the dark due to defects in repressor proteins, underscoring the importance of transcriptional regulation.

De-etiolation (“Greening”) Proteins

• Enzymes involved in photosynthesis and chlorophyll production are upregulated during de-etiolation.

• Plant hormones such as auxin and brassinosteroids decrease in abundance following phytochrome activation, contributing to the overall response.

Plant Hormones

• Plant hormones are signaling molcules present at very low concetrations that can have large effects.

• Plant growth regulators that can modify physiological processes.

• Active at low concentrations.

• Exhibit multiple effects and influence a variety of hormones in a combined process.

• Responses depend on hormone amounts and ratios.

• Interactions control growth/development.

• Major types: auxin, cytokinins, gibberellins, abscisic acid, ethylene, brassinosteroids, jasmonates, strigolactones.

Auxin

• Auxin is a chemical messenger in plants.

Tropism

• Growth response to stimuli.

Phototropism

• Growth toward/away from light.

• Positive/negative phototropism.

• Differential cell growth on shoot sides.

• Darwin's experiments: tip senses light, transmits signal.

• Boysen-Jensen experiments: mobile chemical signal.

• Higher auxin concentration on darker side.

• Auxin promotes coleoptile elongation.

• IAA is major natural auxin.

• Polar transport: unidirectional, tip to base.

• Auxin stimulates cell elongation, regulates architecture.

Role of Auxin in Cell Elongation

• Stimulates cell elongation in young shoots.

• High concentrations inhibit via ethylene production.

Acid Growth Hypothesis

• Proton pumps crucial in growth response to auxin.

• Auxin stimulates proton pumps, lowers pH in cell wall.

• Expansins loosen wall fabric.

• Increased membrane potential enhances ion uptake, increases turgor.

• Auxin alters gene expression for sustained growth.

Auxin’s Role in Plant Development

• Polar transport controls spatial organization.

• Reduced auxin flow indicates unproductive branch.

• Establishes phyllotaxy.

• Directs leaf vein patterns.

• Controls vascular cambium activity.

• Regulates angiosperm female gametophytes.

Practical Uses for Auxins

• IBA used in vegetative propagation.

• Synthetic auxins (e.g., 2,4-D) as herbicides.

• Induces fruit development in greenhouse tomatoes.

Cytokinins

• Modified adenine forms.

• Stimulate cytokinesis.

• Zeatin is common cytokinin.

• Effects on cell division, differentiation, apical dominance, aging.

Control of Cell Division and Differentiation

• Produced in growing tissues (roots, embryos, fruits).

• Transported via xylem sap.

• Act with auxin to stimulate cell division, influence differentiation.

• Cytokinins alone have no effect.

• Cytokinin to auxin ratio controls differentiation.

• Callus develops shoots with increased cytokinin, roots with increased auxin.

Control of Apical Dominance

• Apical bud suppresses axillary buds via sugar, auxin, cytokinins, strigolactones.

• Auxin inhibits axillary buds indirectly.

• Strigolactones repress axillary bud growth.

• Cytokinin antagonizes auxin/strigolactone, allows limited growth.

Anti-aging Effects

• Inhibit protein breakdown, stimulate RNA/protein synthesis, mobilize nutrients.

Gibberellins (GAs)

• Fungus causes hyperelongation.

• Plants produce gibberellins.

Stem Elongation

• Produced in young roots/leaves.

• Stimulate stem/leaf growth via cell elongation/division.

• Act with auxin to promote elongation.

• Bolting: rapid floral stalk growth.

Fruit Growth

• Auxin & gibberellins needed for fruit development.

• Sprayed on Thompson seedless grapes for elongation.

Germination

• Embryo releases gibberellins after water imbibed.

• Stimulate digestive enzyme synthesis.

Abscisic Acid (ABA)

• Slows growth.

• Antagonizes growth hormones.

Seed Dormancy

• Ensures germination in favorable conditions.

• ABA levels increase during maturation.

• Ratio of ABA to gibberellins determines dormancy/germination.

Drought Tolerance

• ABA accumulates in leaves, closes stomata.

Ethylene

• Produced in response to stress, ripening, programmed cell death, high auxin.

Triple Response to Mechanical Stress

• Slowing, thickening, curvature of stem.

Senescence

• Ethylene associated with apoptosis during senescence.

Leaf Abscission

• Ethylene to auxin ratio controls abscission.

• Enzymes digest cell walls.

Fruit Ripening

• Ethylene triggers ripening.

• Softening via breakdown, starches to sugars.

More Recently Discovered Plant Hormones

• Brassinosteroids: induce cell elongation/division, slow abscission, promote xylem differentiation.

• Jasmonates: defense and development, regulate nectar secretion, ripening, pollen production, flowering, germination, root growth, tuber formation, mycorrhizal symbioses, tendril coiling, cross-talk with phytochrome and other hormones

• Strigolactones: stimulate germination, suppress root formation, establish mycorrhizal associations, control apical dominance.

Responses to Light

• Light triggers photomorphogenesis.

• Plants detect presence, direction, intensity, wavelength.

• Red/blue light regulate photomorphogenesis.

Blue-Light Photoreceptors

• Initiate phototropism, stomata opening, hypocotyl elongation slowing.

• Cryptochromes inhibit stem elongation.

• Phototropin mediates stomatal opening, chloroplast movements, phototropism.

Phytochrome Photoreceptors

• Regulate germination, shade avoidance.

• Pr absorbs red, converts to Pfr.

• Pfr absorbs far-red, converts to Pr.

• Pfr triggers developmental responses.

Phytochrome and Seed Germination

• Red light increases germination, far-red inhibits.

• Last flash determines response.

• Sunlight contains both, Pfr increases in sunlight.

Phytochromes and Shade Avoidance

• Phytochrome system indicates light quality.

• Shade shifts ratio in favor of Pr.

Biological Clocks and Circadian Rhythms

• Cyclic variations ilda 24 hours, not directly controlled by environment.

• Daily signals entrain clock to 24 hours.

• Circadian rhythms deviate from 24 hours in constant environments.

• Free-running periods: deviation from 24 hours.

The Effect of Light on the Biological Clock

• Light entrains clock to 24 hours.

• Pfr increases at dawn, resets clock.

• Phytochrome and clock measure night/day passage.

Critical Night Length

• Flowering controlled by night length.

• Short-day plants sensitive to small changes.

• Biological clock and phytochrome determine season.

Photoperiodism and Responses to Seasons

• Physiological response to night/day lengths.

Other Stimuli Responses

• Light, gravitropism, mechanical stimuli, environmental stresses, defense.

Gravitropism

• Roots: positive, shoots: negative.

• Ensures root grows into soil, shoot toward sunlight.

Statoliths

• Detect gravity.

• Plastids with starch grains.

• Trigger calcium redistribution, lateral auxin transport.

Mechanical Stimuli

• Thigmomorphogenesis: changes in form.

• Touch specialists: responsiveness integral.

Thigmotropism

• Directional growth to touch.

Environmental Stresses

• Abiotic factors affect crop yields.

Drought

• Conserve water by reducing transpiration.

• ABA increases, stomata close.

Flooding

• Oxygen deprivation causes ethylene production, cell death.

Salt Stress

• Water deficit due to excess salt.

• Halophytes: survive via salt glands.

Heat Stress

• Stomata close to conserve water, sacrificing cooling.

• Heat-shock proteins protect from heat.

Cold Stress

• Ice forms, reducing water potential.

• Solutes increase, unsaturated fatty acids maintain fluidity.

• Antifreeze proteins hinder ice crystal growth.

Attacks by pathogens and herbivores

• Interactions with herbivores, viruses, bacteria, fungi.

Defenses Against Pathogens

• Epidermis/periderm first defense.

• PAMP-triggered immunity, effector-triggered immunity.

PAMP-Triggered Immunity

• Recognize pathogen-associated molecular patterns (PAMPs).

Effector-Triggered Immunity

• Use disease resistance (R) genes.

Hypersensitive Response

• Local cell/tissue death.

• Produces enzymes/chemicals against pathogen.

Systemic Acquired Resistance

• Plant-wide defense gene expression.

• Nonspecific, lasts for days.

• Methylsalicylic acid converted to salicylic acid.

• Curbed by seed stockpiling.

Defenses Against Herbivores

• Physical (thorns,