CAMPBELL & REECE

CHAPTER 39

Plant Responses to Internal & External

Signals

Stimuli & a Stationary Life

observation about plants:1 part of plant sends signals to anothersense gravity & direction of lightresponds to environmental stimuli & internal

signals

Stimuli & the Stationary Life

animals have behavioral responses to stimuliplants generally respond to environmental

cues by adjusting its patterns of growth & development as result: plants of same species vary in body

form much more than animals

all organisms have ability to receive specific environmental & internal signals & respond to them in ways that enhance survival & reproductive success.

Plant cells have cellular receptors used to detect important changes in their environment Change in daylight hours Insect eating their leaves

Signal Transduction Pathways

@ cellular level plants & all other eukaryotes are surprisingly similar in signaling mechanisms

in order to respond to any stimuli cell must have receptor molecule that is sensitive to & affected by specific stimuli

Etiolation / De-etiolation

morphological adaptations for growing in the dark:

plant in dark allocates as much nrg as possible to elongation of stems break ground b/4 exhausts nutrients in tubers

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.

Signal Transduction

Step 1:Reception

proteins that change shape in response to specific stimuli

usually a weak signal binds to a receptor causing it to undergo a conformational change

Signal Transduction

Step 2:Transduction

amplification of message thru 2nd messengers

2nd messengers: small molecules or ions transfer signal from receptor to other

proteins that carry out the response

Signal Transduction

Step 3: Response usually involves increasing activity of an

enzyme by:1. post-translational modification of pre-

existing proteins2. transcriptional regulation

Signal Transduction in Plants

Post-Translational Modification of Pre-Existing Proteins

mostly involves phosphorylation of specific a.a. alters protein’s hydrophobicity & activity cAMP & Ca++ activate protein kinases

which then phosphorylates another protein phosphorylation of a transcription factor

Protein phosphatases dephosphorylate specific protein = off switch for activated proteins

Transcriptional Regulation

changing transcription factors turns genes on (or off) involves transcription factors or

repressors probably mechanism used for

developmental changes

De-Etiolation (“Greening”) Proteins

proteins involved in making chlorophyll precursors or certain plant hormones are either synthesized or activated

Plant Hormones

hormone (Gr): to excite chemical messengers produced in 1 part

of organism & transported to other parts bind to specific receptors in target cells trigger responses

some scientists consider plant hormones as plant growth regulators to describe organic cpds (natural or synthetic) that modify or control 1 or more specific physiological processes in a plant

Plant Hormones

control every aspect of plant growth & development to some degree

Phototropism

Tropism: any growth response that results in plant organs curving toward or away from stimuli

Phototropism: growth towards light (+ phototropism) or away from light (- phototropism)

Phototropism

Phototropism & Auxin

Auxin

Auxin

produced in shoot apical meristems & young leaves

high levels found in developing fruits & seedsFunctions:stimulates cell elongationpromotes formation of lateral & adventitious

rootsregulates development of fruitenhances apical dominancefunctions in phototropism & gravitropismpromotes vascular differentiation

Abscisic Acid (ABA)

can be made in all plant cells, found in all plant tissues

Functions:inhibits growthpromotes stomata closure during drought

stresspromotes seed dormancy, leaf senescencepromotes desiccation tolerance

ABA

Cytokinins

mostly made in roots transported upFunctions:regulate cell division in shoots & rootsmodify apical dominancepromotes lateral bud growthpromotes movement of nutrients to sink

tissuesstimulates seed germinationdelays leaf senescence

Gibberellins

found in meristems of apical buds & rootsyoung leaves, & developing seeds are

primary sites of productionFunctions:stimulate stem elongation, pollen

development, pollen tube growth, fruit growth, & seed development & germination

regulate sex determination & transition from juvenile to adult phases

Gibberellins

Brassinosteroids

found in all plant tissues, several types, act locally

Functions:in shoots promote cell divisionpromote root growth when low

concentrations, when high retard growthpromote xylem differentiationinhibit phloem differentiationpromote seed germination & pollen tube

formation

Brassinosteroids

Strigolactones

carotenoid-derived made in roots in response to low phosphate levels or high auxin flow from shoots

Functions:promote seed germinationcontrol of apical dominancecontrols mycorrhizal fungi attraction to root

Strigolactones

Ethylene

gas, produced by most parts of plantamt increases as plant ages or during

ripening of fruit or if plant wounded or stressed

Functions:promotes ripening of most fruitsenhances rate of senescencepromotes root & root hair formationpromotes flowering in pineapple family

Hormones that Affect Seed Germination

Plant Responses to Light

Photomorphogenesis: the effects of light on plant morphology

effects of light on plants includes: photosynthesis triggers key events in plant growth &

development allows plants to measure the passage of

days & seasons

Plant Responses to Light

plants detect the presence or absence intensity direction wavelength (color) of light

Action Spectrum

depicts the relative effectiveness of different wavelengths of radiation in driving a particular process

useful in studying any process that depends on light (phototropism included)

by comparing action spectra with absorption spectra of pigments close correspondence for a given pigment suggests the pigment is the photoreceptor mediating the response

2 Major Classes of Light Receptors

action spectra reveal that red & blue light most important colors in regulating a plant’s photomorphogenesis

Major Classes of light receptors:1. Blue-light photoreceptors2. Phytochromes

Blue-Light Photoreceptors

Functions:phototropismopening of stomataslowing of hypocotyl elongation that occurs

when seedling breaks ground

Blue-Light Photoreceptors

plants use 3 or more photoreceptors to detect blue light

1. Cryptochrome similar to DNA repair enzymes inhibits stem elongation

2. Phototropin protein kinase mediates phototrophic

curvature3. Zeaxanthin

with #2 stoma opening

Phytochromes as Photoreceptors

act like molecular “on/off” switchesred light turns them onfar-red light turns them offregulate:

shade avoidance germination of many seeds

Photoreversible States of Phytochrome

Phytochromes & Shade Avoidance

provides plant with information about quality of light

during daylight hrs amt of red & far-red light ~= plants use ratio of the 2 to determine

quality of light ex: tree under canopy getting more far-red

than red light uses its resources to grow taller/ tree getting mostly red light will use resources to grow bushier

Circadian Rhythms

cycles with ~ 24 frequencies not affected by any known environmental variables

Photoperiodism

regulates time of flowering in many species:1. Short-Day Plants

require a night longer than some critical value to flower

ex: mums, poinsiettias, some varieties of tobacco, soy beans

2. Long-Day Plants need night length shorter than some

critical value to flower ex: spinach, radish, lettuce, irises, many

cereals

Photoperiodism

3. Day-Neutral Plants unaffected by photoperiod

flower when reach certain maturity ex: tomatoes, rice, dandelions

Photoperiodism

some plants require 1 single exposure to photoperiod required to flower

others need several successive days of required times

& others only respond to photoperiod if previously exposed to some environmental stimulus (period of cold or warm weather)

Vernalization: period of cold b/4 flowering

Vernalization

Florigen

flowering signal, probably a protein, made in leaves under certain conditions

travels shoot apical meristems inducing them to switch from vegetative reproductive growth

Gravitropism

bending of an organ in response to gravityroots show + gravitropismshoots show – gravitropismstratoliths (starch-filled plastids) enable roots

to detect gravity

Thigmotropism

growth response to touch changes in plant form due to mechanical

pertubationplants very sensitive to mechanical stressex:

measuring leaf length affects its future growth

Thigmotropism

rapid leaf movements involve transmission of electrical impulses called action potentials resemble nervous system action potentials but thousands times slower

Environmental Stresses

Environmental Stress Major Response

DROUGHT ABA production, reducing water loss by closing stomata

FLOODING Formation of air tubes that help roots survive O2 deprivation

SALT Avoiding osmotic water loss by producing solutes tolerated @ high concentrations

HEAT Synthesis of heat-shock proteins , which reduce protein denaturation @ high temperatures

COLD Adjusting membrane fluidity, avoiding osmotic water loss, producing antifreeze proteins

Responses to Attacks by Herbivores

plants release chemicals that are distasteful or toxic

ex: canavanine an unusual a.a., similar to

arginine insect that eats plant incorporates this a.a.

in place of arginine which adversely affects protein shape alters functions insect dies

jackbean

Defenses Against Herbivores

some plants able to attract predatory animals that help defend plant against herbivores

Ex: parasitoid wasps inject their eggs into caterpillars eating

plant eggs hatch inside & larvae eat their way

out

Defenses Against Herbivores

chemicals released in response to herbivores can also function as early warning system for nearby plants of same species

in response neighboring plants release biochemical responses making them less vulnerable to attack

ex: strawberries, lima beans

Defenses Against Pathogens

Hypersensitive Response: seals off infection & destroys both pathogen & infected host cells in region is localized & specific infected area release antimicrobial

molecules use methlysalicyclic acid as signaling

chemical to rest of plant rest of plant then activates systemic

acquired resisitance

System Acquired Resistance

nonspecific protection against diverse pathogens