Chapter 39. Plant Responses to Internal and External Signals. Chapter 39. Response to stimuli. Plants, being rooted to the ground must respond to whatever environmental change comes their way
Plant Responses to Internal and External Signals
After a week’s exposure tonatural daylight. The potatoplant begins to resemble a typical plant with broad greenleaves, short sturdy stems, andlong roots. This transformationbegins with the reception oflight by a specific pigment,phytochrome.Potato Example
Before exposure to light. Adark-grown potato has tall,spindly stems and nonexpandedleaves—morphologicaladaptations that enable theshoots to penetrate the soil. Theroots are short, but there is littleneed for water absorptionbecause little water is lost by theshoots.
Plasma membraneReception … Transduction … Response
Reception: Internal and external signals are detected by receptors (proteins that change in response to specific stimuli)
Transduction: Second messengers transfer and amplify signals from receptors to proteins that cause specific responses
Response: Results in regulation of one or more cellular activities. In many cases this involves the increased activity of certain enzymes
2 One pathway uses cGMP as asecond messenger that activatesa specific protein kinase.The otherpathway involves an increase incytoplasmic Ca2+ that activatesanother specific protein kinase.
3 Both pathwayslead to expression
of genes for proteinsthat function in thede-etiolation(greening) response.
1 The light signal isdetected by thephytochrome receptor,which then activatesat least two signaltransduction pathways.
Ca2+Greening…an example of signal transduction
In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.
Darwin and Darwin (1880)
In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin)but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical.Darwin’s experiments with Phototropisms
In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side.
The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark.
Excised tip placed
on agar block
Growth-promotingchemical diffusesinto agar block
Agar blockwith chemicalstimulates growth
Control(agar blocklackingchemical)has noeffect
Offset blockscause curvature
Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin.Went’s experiment
by low pH, separate cellulose microfibrils from
cross-linking polysaccharides. The exposed cross-linking
polysaccharides are now more accessible to cell wall enzymes.
4 The enzymatic cleavingof the cross-linking
polysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand.
2 The cell wallbecomes moreacidic.
1 Auxinincreases theactivity ofproton pumps.
5 With the cellulose loosened,
the cell can elongate.
CytoplasmCell elongation in response to auxin
2The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzes
starch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.)
1 After a seedimbibes water, theembryo releasesgibberellin (GA)
as a signal to thealeurone, the thinouter layer of theendosperm.
3 Sugars and other nutrients absorbedfrom the endospermby the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.
dark and exposed to varying ethylene concentrations. Their growthwas compared with a control seedling not treated with ethylene.
All the treated seedlings exhibited the tripleresponse. Response was greater with increased concentration.
Ethylene concentration (parts per million)
Ethylene induces the triple response in pea seedlings,with increased ethylene concentration causing increased response.
Phototropic effectiveness relative to 436 nm
Time = 0 min.
Time = 90 min.Action Spectra
Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light.
The graph below shows phototropic effectiveness (curvature per photon) relativeto effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light.
The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.
Far-redSeed Germination Experiment
During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light.
The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposedto red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right).
Red light stimulated germination, and far-red light inhibited germination.The final exposure was the determining factor. The effects of red and far-red light were reversible.
An increase of red light during the day causes Pfr to accumulate, while the amount of Pr accumulates in dim light
The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants.
CONCLUSIONCritical Night Length
During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants.
(a) “Short-day” plantsflowered only if a period ofcontinuous darkness waslonger than a critical darkperiod for that particularspecies (13 hours in thisexample). A period ofdarkness can be ended by abrief exposure to light.
(b) “Long-day” plantsflowered only if aperiod of continuousdarkness was shorterthan a critical darkperiod for thatparticular species (13hours in this example).
To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted toa plant that had not been induced.
Plant subjected to photoperiod
that does not induce flowering
Plant subjected to photoperiod
that induces flowering
Both plants flowered, indicating the transmission of a flower-inducingsubstance. In some cases, the transmission worked even if one was a short-day plantand the other was a long-day plant.
CONCLUSIONTest for presence of a flowering hormone
Does a flowering hormone exist (florigen)?
(b) Experimental root (nonaerated)
(a) Control root (aerated)Flooding
Synthesis andrelease ofvolatile attractants
2Recruitment of Predatory animals
(a) If an Avr allele in the pathogen corresponds to an R allelein the host plant, the host plant will have resistance,making the pathogen avirulent.R alleles probably code forreceptors in the plasma membranes of host plant cells. Avr allelesproduce compounds that can act as ligands, binding to receptorsin host plant cells.
Figure 39.30aAvirulent pathogen
Signal molecule (ligand)
from Avr gene product
Plant cell is resistant
Plant cell becomes diseased
No R allele;
plant cell becomes diseased
No R allele;
plant cell becomes diseased
(b) If there is no gene-for-gene recognition because of one ofthe above three conditions, the pathogen will be virulent,causing disease to develop.Virulent pathogen
4 Before they die,infected cellsrelease a chemicalsignal, probablysalicylic acid.
3 In a hypersensitiveresponse (HR), plantcells produce anti-microbial molecules,seal off infectedareas by modifyingtheir walls, andthen destroythemselves. Thislocalized responseproduces lesionsand protects otherparts of an infectedleaf.
5 The signal is
distributed to the
rest of the plant.
6 In cells remote fromthe infection site,the chemicalinitiates a signaltransductionpathway.
2 This identification
step triggers a
7 Systemic acquired
resistance isactivated: theproduction ofmolecules that helpprotect the cellagainst a diversityof pathogens forseveral days.
1 Specific resistance is
based on the
binding of ligands
from the pathogen
to receptors in
R-Avr recognition and