BIO 131 Chapter 37: Plant Sensory Systems, Signals, and Responses

37.1 - 37.7

The ability to gather information and respond to it is important because plants:

* need to find the nutrients and things they need to survive
* need to avoid the things that will hurt them—predation, parasitism, harmful environmental conditions
* need to be able to physically interact with their environment
* need to find partners for sexual reproduction

Process by which plants gather, process, and respond to environmental stimuli:

* sensory cells receive an external signal and change it to an intracellular signal
* sensory cells send a signal to target cells in other parts of the body that can respond to the information
* target cells receive the signal and change their activity in a way that produces an appropriate response

Signal receptors

* specialized proteins that change shape in response to an environmental stimulus (such as being struct by a particular wavelength of light, having pressure applied, or binding to a particular type of molecule)
* shape change converts an external signal to an intracellular signal which primes the sensory cell for action
* most are located in the plasma membrane, but may be located in the cytosol as well

Signal transduction

* the process activated when a receptor in the plasma membrane binds a signal
* 2 fundamental components, which may interact to modify cell response

Phosphorylation cascade

* signal transduction pathway triggered when the change in the receptor protein’s shape leads to the transfer of a phosphate group from ATP to the receptor or one of its associated proteins
* activates proteins involved in chains of phosphorylated proteins

Second messengers

* are produced when receptor proteins trigger the production of intracellular signals or their release from storage areas
* includes calcium ions (Ca2+) which are stored in the vacuole, smooth ER, or cell wall

Hormones

* organic compounds produced in small amounts in one part of the plant and transported to the target cells, where they cause a physiological response
* usually are small molecules in plants present in very low concentrations
* may be transmitted by specialized membrane transport proteins (e.g. auxin), in xylem/phloem sap, or by simple diffusion from the originating cell
* it is common for different types of hormones to interact with each other and modulate the cell’s response

Coleoptile

* a modified leaf that forms a sheath protecting the emerging shoots of young grasses
* used in the first experiments on plants’ response to light

Coleoptile experiments

* plants exposed to light excluding blue light (filtered through potassium dichromate solution) did not bend toward the light, suggesting that bending only occurs toward light including blue wavelengths
* noted no photosynthetic response without the presence of blue wavelengths as well

Phototropism

* the directed movement of plants in response to light
* plants bend due to elongation of cells on the shaded side of the plant

PHOT 1

* membrane protein in the tips of emerging shoots
* gains a phosphate group in response to blue light ( is a blue light receptor) and subsequently triggers the phototropic response
* coded for by *PHOT1,* which was discovered by analyzing mutant *Arabidopsis* individuals that lack phototropic response

PHOT2

* second blue-light receptor found

Phototropins

* photoreceptors that detect blue light and initiate phototropic response
* trigger signal transduction cascades that result in at least two other responses

Blue light responses

* shoot bending
* chloroplast movements inside leaf cells
* opening of stomata (and consequently photosynthesis)

Chloroplast movements

* put chloroplasts in positions to optimize light absorption
* in high light, the chloroplasts move to the sides of cells to shade each other
* in low light, they spread out to absorb more light

Early experiments (phototropism)

* removal or covering of coleoptile tips resulted in no phototropism
* covering below the tips in the area where bending occurs did not affect phototropism
* suggests that sensory cells respond to blue light by releasing a hormone produced in the tips of the shoot and transported to site of bending

Phototropism experiments (Boysen-Jensen)

* cut off oat tips and replacing them with porous agar or nonporous mica slabs under the tip
* only plants with agar tips showed normal phototropism
* suggests that the phototropic signal was a chemical that could diffuse and that the chemical was water-soluble (because the agar is water based)

Auxin

* first plant hormone ever discovered
* promotes cell elongation in the shoot

Indoleacetic Acid (IAA)

* most common naturally occurring hormone of the auxin class
* was difficult to identify because it is present in such low concentrations

Cholodny-Went Hypothesis

* proposed that phototropism results from an asymmetric distribution of auxin
* predicts that auxin produced in the tip of coleoptiles is transported at higher concentrations down the shaded side compared to the illuminated side
* also predicts that the asymmetrical distribution of auxin causes cells on the shaded side to elongate more than cells on the illuminated side, causing the shoot to bend toward the light

Auxin receptors (cell elongation)

* TIR1 (intracellular receptor) and ABP1 (binding protein that is part of a receptor complex in the plasma membrane)
* auxin binding to these receptors in stem cells triggers a signaling pathway that ultimately increases the activity of membrane proton pumps

Acid-growth hypothesis

* the idea that proton pumps (acidification of cell wall) are responsible for cell elongation
* as protons are pumped out of the cell, an electrochemical gradient is established
* the gradient (negative inside, positive outside cell) favors the entry of cations such as K+ into the cell
* as cation solute concentrations increase inside the cell, water follows via osmosis
* when cell walls on one side of a stem are acidified in response to auxin, bending results through asymmetrical elongation

Requirements for plant cell expansion

* cell wall must loosen to allow for an increase in cell size
* water must enter the cell and generate turgor pressure on the cell wall to trigger an increase in cell volume

Expansins

* cell wall proteins which “unzip” the hydrogen bonds that form between cellulose microfibrils and other polymers in the cell wall, loosening the structure
* activated when proton pumping lowers the pH of the cell wall to 4.5

Red light (\~ 660-700nm) signals

* drive photosynthesis, just as blue light does

Far-red (710+ nm) signals

* far-red wavelengths are not absorbed strongly by photosynthetic pigments, so they tend to pass through leaves
* far-red wavelengths are therefore prominent in light that is filtered through leaves before reaching the forest flower, and are indicative of shade to a plant

Red/far-red “switch”

* since red indicates light and far-red indicates shade, the light sensed can act as a “switch” for cellular processes


* e.g. red light promotes seed germination, while far-red light inhibits it

Photoreversibility hypothesis

* proposes that one pigment exhibits switching behavior and has two conformations, one of which can absorb red and the other of which can absorb far-red
* hypothesized to occur because light absorption makes the photoreceptor pigment change shape

Phytochrome

* specialized light receptor which is hypothesized to exhibit photoreversibility in response to different wavelengths
* has been shown to play a role in plant growth responses controlled by light, including seed germination, etiolation, and circadian rhythms

P.(fr)

* phytochrome conformation induced by red light
* active conformation

P.(r)

* phytochrome conformation induced by far red light
* inactive conformation

Etiolation

* phenomenon in which shaded plants develop a pale yellow color, lengthen their stems, and grow long, thin, and spindly
* plants growing in shade decide to invest more energy in growing taller and less energy in leaf development as they try to reach the light

Plant circadian rhythms

* the leaves of some species open and close in slow but regular fashion that is determined by changes in light levels
* closely tied to the plant’s molecular clock, which is reset each morning

Flower formation

* develops from modified shoots
* begins when an apical meristem stops making energy-harvesting stems and leaves and begins to produce the modified stems and leaves that make up flowers
* plant commits to investing energy in sexual reproduction
* switch that triggers flowering in response to changes in day length also shows red/far-red photoreversibility

Photoperiodism

* any response by an organism that is based on photoperiod
* important in plants because it allows individuals to respond to seasonal changes in climate

Photoperiod

* the relative lengths of day and night

Long-day (short-night) plants

* bloom midsummer, when days are longest and nights are shortest
* flower only when days are longer than a species-specific length, usually between 10-16 hours
* radishes, lettuce, spinach, corn, irises, etc

Short-day (long-night) plants

* bloom in spring, late summer, or fall when days are shortest and nights are longest
* flower only if days are shorter than a species-specific length
* asters, chrysanthemums, poinsettias

Day-neutral plants

* flower without regard to photoperiod
* roses, snapdragons, tomatoes, cucumbers, many weeds

Photoperiodic flowering experiments

* interrupting the night period with a flash of light changed the flowering response, which allowed researchers to link the phytochrome switch to flowering
* noted that clock protein levels rise during the day and trigger the expression of the gene *CONSTANS (CO)*

CO protein

* transcription factor that affects the production of a flowering hormone
* stabilized by phytochrome when it is activated by light, allowing it to accumulate in cells
* stimulates the production of the flowering hormone in long-day plants and inhibits the production of the flowering hormone in short-day plants

Flowering hormone experiments

* exposing just one leaf on the plant to flowering conditions led to the whole plant flowering—suggests that the signal to flower comes from the leaves and travels to the apical meristem
* grafting leaves from plants exposed to flowering conditions resulted in plants which had not been exposed flowering—suggests that some product created in the transplanted leaf must travel to the recipient plant meristem, which would then trigger a developmental switch

*FLOWERING LOCUS T* (*FT*)

* gene known to promote flowering when activated
* expressed in the leaf vascular tissue
* research indicates that its protein product may be the flowering hormone

Gravitropism

* ability to move in response to gravity

Root cap (gravitropism)

* if removed, roots stop responding to gravity—suggests that gravity-sensing occurs somewhere inside the root cap
* cells in the center contain starch granules which are critical for regulating gravitropic response

Statolith Hypothesis (2 ideas)

* starch-storing organelles (amyloplasts) are pulled to the bottom of root cap cells by the force of gravity, since starch is denser than water
* the position of the amyloplasts activates sensory proteins in the plasma membrane which initiate the gravitropic response

Auxin redistribution (gravitropism)

* normal conditions: auxin flows down the middle of the root, toward the perimeter, then away from the root cap
* tipped root: sensory receptors trigger changes in the position of auxin transport proteins that redistribute auxin
* auxin is redistributed so that the lower part of the root receives increased concentrations and the upper portion receives lower concentrations
* high auxin concentrations inhibit growth root, so the differences in auxin concentrations trigger differential growth that results in root bending downward

Plant response to wind/touch

* receptor cells transduce mechanical force into an internal signal
* signal results in transcription of a large suite of genes being turned up or down in response to touch or mechanical stimuli such as wind
* results in altered growth which is NOT directional, only resulting in sturdier plants

Thigmomorphogenesis

* phenomenon in which some of proteins transcribed in response to wind/touch stiffen cell walls
* results in shorter and stockier plants

Thigmotropism

* a plant’s response to touch moving in a specific direction
* can be moderately fast (e.g. climbing tendrils)

Thigmonastic movements

* rapid, nondirectional movements such as a venus flytrap snapping shut
* occur when a touch receptor cell transduces the mechanical signal into an electrical signal by allowing ion flow across the membrane, resulting in a change in membrane potential

Action potential

* the characteristic propagation of electrical signals that travels between a sensory receptor and a “motor cell” (cells which change shape by swelling and pushing venus flytrap shut)
* generated by sensory hairs inside venus flytrap leaves
* moves across plant tissues via plasmodesmata

Apical dominance

* growth is restricted to the main stems and lateral buds remain dormant
* is broken if apical buds die or are removed, but can be experimentally maintained by applying auxin to the shoot’s cut surface after bud is removed—confirms role of auxin and suggests that the tip cells send a constant stream of information down to other organs and tissues

Auxin transport

* polar/unidirectional (only hormone to be so)
* occurs down the stem and through the root via parenchyma cells in the ground and vascular tissue
* enters apical end of cells via specialized membrane proteins, diffuses to other end of the cell, then transported out by proteins located only in the basal part of the cell
* creates a strong auxin gradient down the long axis of the plant

Auxin (Overall roles)

* control growth via apical dominance, phototropism, and gravitropism
* promote fruit development when produced by seeds
* involved in abscission when concentrations fall
* essential for proper differentiation of xylem and phloem cells in the vascular tissue and development of the vascular cambium
* stimulates development and growth of adventitious roots
* hypothesized to signal where cells are located relative to the root-shoot axis and response to conditions affecting the root-shoot axis (gradient)

Cytokinins

* the group of plant hormones that promote cell division (strong evidence that it does so by activating genes that keep the cell cycle going)
* mostly synthesized in root apical meristems and transported up into the shoot system via xylem
* can also be synthesized in young fruits/seeds/buds and other developing organs
* derived from adenine

Zeatin

* cytokinin found in most species
* naturally occurring adenine derivative

Cytokinin receptors

* group of closely related proteins located primarily in the endoplasmic reticulum
* activate genes that regulate cell division when cytokinins bind

Dormancy

* temporary state of reduced metabolic activity

Gibberellins

* large family of closely related compounds which stimulate growth in plants
* discovered and isolated from quickly-growing rice seedlings with “foolish seedling disease” caused by Gibberella fungal infection
* most species produce several different gibberellins

Abscisic acid (ABA)

* molecule which inhibits growth
* involved in stomatal closing

α-amylase

* digestive enzyme that breaks the bonds between the sugar subunits of starch
* released during germination from the aleurone layer and diffuses into the endosperm—digestion of starch released sugars that can be used by the growing embryo
* production and release is increased by gibberellic acid (GA) and decreased by ABA

ABA stomatal closing experiment

* did not water half of the experiment plant
* stomata began to close even when the measured water potential in the un-watered leaves remained the same as fully watered plants
* suggests that roots from the dry side of the pot were signaling drought stress even though the leaves were not actually experiencing a water shortage yet
* ABA levels in roots of dry side of plant were measured to be very high relative to watered side and relative to control plants, suggesting ABA is produced in roots and transported to leaves to serve as early warning for drought stress

Guard cell closing and opening

* based on activity of proton pumps and ion transporters in the plasma membrane
* creation of osmotic gradients result in the movement of water; guard cells open when turgid and close when flaccid

Guard cell opening mechanism

* photoreceptors are stimulated by blue light
* large numbers of protons are pumped out of each cell, resulting in strong electrochemical gradient
* gradient favors entry of K+ and Cl- into the cell
* water follows by osmosis
* guard cells swell and stomata open

Guard cell closing (ABA)

* ABA inhibits guard cell proton pumps
* chloride and other anion channels are opened, resulting in change in membrane potential
* change in membrane potential causes potassium ion channels to open
* large amounts of K+ leave cells
* water follows by osmosis
* loss of turgor, closing of the pore

Brassinosteroids

* sugars in these molecules trigger growth spurts
* similar to testosterone and estradiol
* promote growth; key regulator of overall plant body size

Brassinosteroids vs. animal steroid hormones

* animal: often lipid soluble (bind to receptors in cytosol)
* brassinosteroids: often hydrophilic (bind to membrane receptors)

Senescence

* a regulated process of aging and eventual death of an entire organism or organs such as fruits or leaves
* regulated by complex interactions between several hormones in response to light, temperature, and other factors

Ethylene

* hormone most strongly associated with senescence
* gas at normal temperatures
* synthesized from the amino acid methionine
* involved in fruit ripening, flowers fading, and leaf abscission
* influences plant growth
* stress hormone induced by drought and other conditions
* induces an increase in cellular respiration

Ripening

* characterized by a conversion of stored starch to sugar, removal or destruction of protective toxins, degradation of cell walls (softening the fruit) and chlorophyll, and production of pigments and aromas that signal ripeness
* thought to be an adaptation that enhances the fruit’s attractiveness to animals that disperse seeds

Leaf abscission

* auxin is produced in healthy leaves and transported through the petiole—when leaves produce less auxin (in response to environmental conditions or age), cells in a region of the petiole called the abscission zone become more sensitive to ethylene in the tissue
* increased ethylene sensitivity activates enzymes that weaken the walls of cells in the abscission zone
* chlorophyll in the leaf is broken down, and nutrients are withdrawn and stored in parenchyma in the stem
* eventually, cell walls break down enough that the leaf falls

Crosstalk

* the interaction between the signal pathways triggered by different hormones
* molecular mechanism responsible for integrating information from many sensory cells and signals

Pathogens

* disease-causing agents
* bacteria, viruses, fungi, etc

Physical defense (plants)

* cuticle—good barrier to pathogens
* thorns, spines, trichomes, etc help protect from damage by herbivores

Chemical defense (plants)

* lace tissues with secondary metabolites
* this type of defense is effective but expensive in terms of ATP and materials invested
* mostly inducible

Secondary metabolites

* molecules closely related to compounds in key synthetic pathways but not found in all plants
* poison herbivores
* insect-repellents—lemon, peppermint, basil, sage oils
* insecticides—pine pitch, chrysanthemum pyrethroids
* disrupt nervous systems—tannins, opium, caffeine, cocaine, nicotine, THC, etc

Inducible defenses

* responses to attacks and infections that are triggered by the presence of a threat (not present all the time)
* found in both plants and animals

Plant pathogens

* tend to infect only one or a few host species (coevolution)
* enter mostly through stomata or wounds

“Surveillance” proteins

* receptor proteins that can bind to residues left by pathogenic cells
* signal to the plant that a pathogen is present, triggering HR and SAR through signal cascades

Hypersensitive response (HR) characteristics

* stomatal closure (prevent more pathogens from entering the plant)
* production of toxin molecules targeted to the pathogen
* reinforcement of neighboring plant cell walls to limit movement of the pathogen
* rapid suicide of cells in the infected region within hours of the start of an infection; results in brown spots in leaves that accumulate over the growing season

Coronatine

* small compound used by *Pseudomonas* pathogens to force stomatal re-opening after shutting during HR
* allows more bacteria to enter the leaf

Systemic Acquired Resistance (SAR)

* slower, more widespread of events triggered by a hormone produced at the infection site
* primes cells throughout the root or shoot system over several days for resistance to assault by a pathogen

SAR hormone

* acts globally (throughout plant) as well as locally (point of infection)
* results in the expression of a large suite of genes called the pathogenesis-related (PR) genes, which produce proteins that help prevent an infection from spreading

Methyl salicylate (MeSA)

* hypothesized to be the SAR hormone
* derived from salicylic acid
* levels increase dramatically after infection, particularly in the phloem sap leaving infection sites
* reduction correlates with reduction or abolishment in SAR response
* addition in lower leaves of tobacco plants leads to SAR in upper untreated leaves

Proteinase inhibitors

* found in many seeds and some storage organs such as potato tubers
* block enzymes responsible for digesting proteins—ingestion causes herbivores to become sick and avoid them

Proteinase inhibitor experiment

* allowed herbivorous beetles to attack one leaf on several potato plants
* concentrations of inhibitor in non-attacked leaves on the attacked plants averaged over three times the concentrations in control plants

Systemin

* hormone produced in tomato plants and closely related species which induces the production of proteinase inhibitors
* peptide hormone (18 amino acids long)

Systemin signal transduction pathway

* systemin is released from damaged cells
* systemin travels through the phloem and binds to membrane receptors on target cells
* activation of receptors triggers series of chemical reactions that synthesize another hormone (jasmonic acid)
* Jasmonic acid activates a cytosolic signal pathway that induces the production of at least 15 new gene products (including proteinase inhibitors)

“Talking Trees” Hypothesis

* when an insect starts eating a plant, volatile compounds evaporate from a leaf’s surface and travel through the air
* nearby plants sense the chemicals and respond by increasing their own defenses, even if the chemicals come from a totally different species

Parasitoid

* organism that is free-living as an adult but parasitic as a larva
* tend to be similar in size to their hosts and therefore must consume and kill their hosts to complete development
* particularly common when insect outbreaks occur in croplands

Parasitoid experiments

* insect-damaged leaves produced 11 molecules that were not produced by undamaged leaves OR leaves mechanically damaged by scissors/tools
* female wasps preferred to fly toward insect-damaged leaves over mechanically damaged leaves
* supports the hypothesis that plants produce pheromones that recruit wasps in response to attack by caterpillars