Plant Responses to Internal & External Signals

For the readings, pay special attention to the diagrams

Review of Signal Transduction PathwaysReception: signal is detected by some sort of

receptorTransduction: signal is “carried” from the

receptor to the nucleusThis is where you have secondary messengers

transfer the signalProtein kinase, orThe cGMP pathway (like the cAMP pathway)

Response: activates different transcriptional factors or enzymes

Example: Potatoes

Potatoes grow beneath the soil in a dark environment (they produce many short stems that lack leaves in the hope that the stem will break through the soil surface)The adaptations to grow in the dark is known as

etiolation

When a stem breaks through the surface, leaves expand, roots elongate, and the plant produces chlorphyllThis process is called de-etiolation

Reception

A photoreceptor, phytochrome, detects that light after the shoot breaks through the soilPhytochromes are not located on the

cell surface, but are instead located in the cytoplasm

When light strikes the phytochrome, it causes a conformational change that causes a transduction pathway to begin

Transduction

Reception can be from a VERY weak light source, therefore you need to “amplify” the signal through transductionThe phytochrome, when activated, causes an

increase in the concentration of cGMP (like cAMP) and calcium ion.

cGMP activates protein kinases (through phosphorylation)

The increased calcium concentration with the activation of kinases leads to a response

Response

Many transcriptional factors are activated in de-etiolationSome are activated by phosphorylationSome are activated by cGMPSome are activated by calcium

In addition to transcriptional factors, post-translational enzymes are also activated to modify the created proteins

Most of the proteins created are associated with photosynthesis and chlorophyll production

PLANT HORMONES

How hormones coordinate growth, development and response to environment

Plant Hormones

Hormones are chemical signals that coordinate the various parts of an organismA hormone is a compound produced in one part

of the organism which is then transported to other parts of the organism, where it triggers responses in target cells and tissues

Many hormones are effective in VERY small concentrations

Many times, hormone concentrations are dependent on environmental stimuli

Example with Light

Example of the action of hormonesAuxin is a hormone that induces a plant to move

towards or away from a stimuli, tropismStimulus: LightResponse: the plants growth pattern will cause

the growth shoot to move towards or away from the lightPhototropism: growth towards the lightNegative phototropism: growth away from

the light

Plant Hormones

There are 5 major classes of plant hormones, each with specific functions:AuxinCytokininsGibberellinsEthylene Abscisic acid

NOTE: Many hormones interact with each other to enhance or inhibit their activities

Auxin

Found:In the embryo of seeds, meristems of apical

buds and young leavesFunction:

Stimulates stem elongation and root growth (causes the root cells to elongate)

Stimulates development of fruitInvolved in phototropism and gravitropism,

response of a plant to the effects of gravity

Cytokinins

Found:Made in the roots and transported to other

organs of the plantFunction:

Affect the growth and differentiation of rootsStimulates cell division and growth (in

conjunction with auxins)Stimulates germination, growth from a seedDelay senescence, or the aging of the plant

Gibberellins

Found: in meristems of apical buds and roots, young leaves and embryos

Function:Promote seed and bud

germination, stem elongation and leaf growth

Stimulate flowering and fruit development

Affect root growth and differentiation

EthyleneFound: in tissues of ripe

fruit, nodes of stems, and aging leaves and flowers

Function:Opposes some of the

effects of auxin (feedback)

Promotes fruit ripeningSenescence (aging) is

at least party caused by ethylene

“One bad apple spoils the whole bunch”

Abscisic Acid

Found: in leaves, stems, roots, and green fruit

Function: Induces seed dormancy

Anti-gibberellin

Inhibits cell growth Anti-cytokinin

Inhibits fruit ripening Anti-ethylene

Closes stomata during water stress, allowing many plants to survive droughts

PLANT RESPONSES

How plants respond to various factors

Tropisms

Tropisms are growth responses that result in curvatures of whole plant organs toward or away from a stimuli

There are three major stimuli that induce tropismsLight (Phototropism)Gravity (Gravitropism)Touch (Thigmotropism)

Phototropism

Phototropism is the growth of a shoot towards lightThis is primarily

due to the action of auxin

Auxin elongates the cells on the non-light side

Light Receptors: There are 2 main types of photoreceptors

Blue-light photoreceptor: These receptors absorb mostly blue light

May be responsible for opening stomata, and inhibit hypocotyl elongation in seedlings breaking ground

Phytochromes: These receptors absorb mostly red light

Responsible for de-etiolation, seed germination, and “avoid” shade

Circadian Rhythms

The production of enzymes, hormones and other processes oscillate during the dayThis is due to many environmental factors

Light levels, temperature, humidityThere are other processes that occur with a

frequency of every 24 hours that are not dependent on environment: circadian rhythms

Biological Clocks/Circadian Rhythms

A physiological cycle with a frequency of about 24 hours is called a circadian rhythm

Even without external, environmental cues, circadian rhythms persist in humans and in all eukaryotes

Example: jet lag in humans, leaf position in bean plants

It is believed these are due to some internal biological clock that regulate these processes (these work independent of the day/night cycle)

Photoperiodism

A physiological response to day length (differs in winter, summer, spring, and fall) is known as photoperiodism Short-day plants

Require a shorter light periodFlower in later summer/fall/winterExample: poinsettias

Long-day plantsRequire a longer light periodFlower in late spring/early summerExample: spinach

Day-neutral plantsAre unaffected by photoperiodExample: tomatoes

But it’s actually the night that matters!! (if there is even a little sunlight during the “night” the flowers will not bloom)

Other Factors that Affect Flowering

In addition to photoperiod, some plants need additional environmental cues to induce floweringExample: Some plants need to be exposed to

critical temperature rangesVernalization: the need to be exposed to long

periods of “cold” temperatures to induce flowering (this occurs in winter wheat)

Missing Flower Hormone

It is believed that the photoperiod is detected by some chemical signal located in the leaves, florigen (not yet found)If all of the leaves are removed

from the plant, it is no longer affected by photoperiod

Plant Defenses

Plants defend themselves against herbivores in several waysPhysical defenses, such as thornsChemical defenses, such as

producing distasteful/toxic

compoundsCan use chemicals to attract

insects to help defend the plant

• Wasps

Plant Defenses

Chemical warning systems:When there is an infestation by

insects, plants can release a chemical signal that causes other plants to activate “defense” genes to counteract the infesting organisms

Plant Defenses

Defense against pathogens is also important for plant survivalFirst line of defense is the plant’s “skin”

Plant dermis, cuticle, barkIf a plant becomes infected, they release a series of

chemicals that destroy the pathogen (much like our immune system)If a pathogen is able to “avoid” or “suppress” a plants

defenses, the pathogen is said to be virulentMany times the pathogen weakens, but does not kill,

the plant so that the pathogen may survive• This condition is called avirulent

Pathogen Detection

At the genetic level, plant disease resistance can begin with gene-for-gene recognitionThe plant is able to “recognize” the protein products of

the pathogen and able to mount a defense against the disease

Another “detection” method are molecules called elicitorsExample: oligosaccharins are molecules that derive

from damage cell wallsThey can also mount a defense against disease

Plant Response to Pathogen

Once warned, plants can release chemicals that can fight an invaderPhytoalexins are a group of compounds that are

antimicrobial (the equivalent of our B and Tcells)There are also a general group of proteins (PR,

or pathogenesis-related, proteins) that are antimicrobial or act as messengers to activate further defense

If the pathogen is avirulent, then there may be a more aggressive, localized, response called hypersensitive response

General Defense

A hypersensitive response may produce a chemical signal that alerts the rest of the plant As a result, more phytoalexins and PR

proteins can be released to produce a nonspecific defense, system acquired resistance (SAR)

A hormone that is thought to produce this resistance is salicylic acid