Chapter 39 ~Chapter 39 ~ Plant Responses to Plant Responses to Internal and External Internal and External SignalsSignals

Signal Transduction Pathways

ReceptionReception Transduction – w/ the help of second messengersTransduction – w/ the help of second messengers Response Response

Generally increase activity of certain enzymesGenerally increase activity of certain enzymes Enzyme activationEnzyme activation

Stimulation of transcription (making Stimulation of transcription (making enzymes)enzymes)

Activation of existing enzymesActivation of existing enzymes

Ex. Etiolation – plant growth in the dark

Etiolation of potatoes – begin growth undergroundEtiolation of potatoes – begin growth underground Reception – leads to de-etiolation (greening)Reception – leads to de-etiolation (greening)

Phtyochrome – receptor that picks up lightPhtyochrome – receptor that picks up light TransductionTransduction

Activation of second messengers – cyclic GMP and Activation of second messengers – cyclic GMP and calcium ionscalcium ions

ResponseResponse Production of enzymes that lead to expansion of Production of enzymes that lead to expansion of

leaves and production of chlorophyllleaves and production of chlorophyll

Plant hormones Hormone: chemical signals that Hormone: chemical signals that

coordinate parts of an organism; coordinate parts of an organism; produced in one part of the body and produced in one part of the body and then transported to other parts of the then transported to other parts of the body; low concentrationsbody; low concentrations

Tropism: movement toward or away Tropism: movement toward or away from a stimulusfrom a stimulus Positive vs NegativePositive vs Negative Caused by cell elongation on the Caused by cell elongation on the

opposite sideopposite side Frits Went - experiments w/ Frits Went - experiments w/

phototropism led to discovery of phototropism led to discovery of chemical responsible chemical responsible Hormone: auxinHormone: auxin

Others: gravitropism, thigmotropismOthers: gravitropism, thigmotropism

Auxin

IAA (indoleacetic acid)IAA (indoleacetic acid) Location:Location: seed embryo; meristems of apical buds and seed embryo; meristems of apical buds and

young leavesyoung leaves Function:Function: stem elongation; root growth, stem elongation; root growth,

differentiation, branching; fruit development; apical differentiation, branching; fruit development; apical dominance; tropismsdominance; tropisms

May be used as a herbicide on some plants – death May be used as a herbicide on some plants – death due to hormone overdosedue to hormone overdose

Cytokinins Zeatin – most common varietyZeatin – most common variety Location:Location: roots (and actively growing tissues) roots (and actively growing tissues) Function:Function: root growth and differentiation; cell division and root growth and differentiation; cell division and

growth; germination; delay senescence (aging); apical growth; germination; delay senescence (aging); apical dominance (terminal bud suppresses the development of dominance (terminal bud suppresses the development of axillary buds)axillary buds)

Works w/ auxin – higher levels of auxin = root growth; Works w/ auxin – higher levels of auxin = root growth; higher levels of cytokinins = shoot growthhigher levels of cytokinins = shoot growth

Gibberellins GAGA33

Location:Location: meristems of apical buds and roots, meristems of apical buds and roots, young leaves, embryoyoung leaves, embryo

Function:Function: germination of seed and bud; stem germination of seed and bud; stem elongation; leaf growth; flowering (bolting); elongation; leaf growth; flowering (bolting);

fruit development; root growth and differentiationfruit development; root growth and differentiation

Abscisic acid ABAABA Location:Location: leaves, stems, roots, green fruit leaves, stems, roots, green fruit Function:Function: inhibits growth; closes stomata inhibits growth; closes stomata

during stress; counteracts breaking of dormancyduring stress; counteracts breaking of dormancy

Ethylene Gaseous hormoneGaseous hormone Location:Location: ripening fruit tissue; stem nodes; aging leaves ripening fruit tissue; stem nodes; aging leaves

and flowersand flowers Function:Function: fruit ripening; oppositional to auxin (leaf fruit ripening; oppositional to auxin (leaf

abscission); promotes/inhibits: growth/development of roots, abscission); promotes/inhibits: growth/development of roots, leaves, and flowers; senescence (aging); apoptosis leaves, and flowers; senescence (aging); apoptosis (programmed cell death); mechanical stress (programmed cell death); mechanical stress

Daily and Seasonal Responses Circadian rhythmCircadian rhythm (24 hour periodicity) (24 hour periodicity) PhotoperiodismPhotoperiodism (phytochromes) (phytochromes) Short-day plantShort-day plant: light period shorter than a critical length to flower : light period shorter than a critical length to flower

(flower in late summer, fall, or winter; poinsettias, chrysanthemums)(flower in late summer, fall, or winter; poinsettias, chrysanthemums) Long-day plantLong-day plant: light period longer than a critical length to flower : light period longer than a critical length to flower

(flower in late spring or early summer; spinach, radish, lettuce, iris)(flower in late spring or early summer; spinach, radish, lettuce, iris) Day-neutral plantDay-neutral plant: unaffected by photoperiod (tomatoes, rice, : unaffected by photoperiod (tomatoes, rice,

dandelions)dandelions) Critical night lengthCritical night length controls flowering controls flowering

Phytochromes

Plant pigment that Plant pigment that measures length of measures length of darkness in a darkness in a photoperiod (red light)photoperiod (red light)

PPrr (red absorbing) (red absorbing) 660nm660nm

PPfrfr (far-red absorbing) (far-red absorbing) 730nm730nm

Plant Defenses – Against Herbivores Physical – thornsPhysical – thorns Chemical – Chemical –

Distasteful or toxic compoundsDistasteful or toxic compounds Symbiosis Symbiosis

Caterpillars eat leavesCaterpillars eat leaves Damaged leaf releases chemicals to attract parsitoid Damaged leaf releases chemicals to attract parsitoid

waspswasps Wasps lay eggs inside caterpillarWasps lay eggs inside caterpillar Larva eat their way out – killing hostLarva eat their way out – killing host

Early warning system to plants close byEarly warning system to plants close by

Plant Defenses – Against Pathogens

Virulent pathogen – plant has little defense against the pathogenVirulent pathogen – plant has little defense against the pathogen Avirulent pathogen – pathogens do mild harm, but don’t kill the Avirulent pathogen – pathogens do mild harm, but don’t kill the

plantplant Epidermis – first line defenseEpidermis – first line defense Gene-for-gene recognition – R (resistance) genes in plant Gene-for-gene recognition – R (resistance) genes in plant

recognize certain pathogens that are coded w/ an avirulence recognize certain pathogens that are coded w/ an avirulence (Avr) gene(Avr) gene Both must be present to avoid infectionBoth must be present to avoid infection

Elicitors – stimulate the production of phytoalexins Elicitors – stimulate the production of phytoalexins (antimicrobial compounds) and PR (pathogenesis-related) (antimicrobial compounds) and PR (pathogenesis-related) proteinsproteins