Spring Break HW

7/25/2019 Spring Break HW

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Rebecca Zhang

April 17, 2014

AP Biology

Mrs. Istrico

Capbell BI!"!#$ %e, AP &'ition

Chapter 38: Angiosperm Reproduction and Biotechnology

(lo)ers o* +eceit

Most plantanial relationships are -t-alistic, eaning that the anial, 'isperses a plants see's

an' also proi'es the plant )ith )ater, ineral n-trients, an' protection. In ret-rn the anial eats a

portion o* the plants see's an' *r-it. (lo)ering plants, or angiospers, can se/-ally repro'-ce )ith

spatially 'istant ebers o* their o)n species.

38.1 Flowers, double ertili!ation, and ruits are uni"ue eatures o the angiosperm lie cycle

he li*e cycles o* plants are characterie' by an alternation o* generations, in )hich -lticell-lar

haploi' an' 'iploi' generations tae t-rns pro'-cing each other. he 'iploi' plant, the sporophyte,

pro'-ces haploi' spores by eiosis. heses spores 'ii'e by itosis, giing rise to the -lticell-lar

gaetophytes, the ale an' *eale haploi' plants that pro'-ce gaetes. (ertiliation, the *-sion o*gaetes, res-lts in 'iploi' ygotes, )hich 'ii'e by itosis an' *or ne) sporophytes.

In angiospers, the sporophyte is the 'oinant generation3 It is larger, ore conspic-o-s, an'

longerlie' than the gaetophyte. !er the co-rse o* see' plant eol-tion, gaetophytes becae

re'-ce' in sie an' )holly 'epen'ent on the sporophyte *or n-trients. Angiosper gaetophytes are the

ost re'-ce' o* all plants, consisting o* only a *e) cells.

he ey 'erie' traits o* the angiosper li*e cycle can be reebere' as thethree (s*lo)ers,

'o-ble *ertiliations, an' *r-its. ince angiospers, along )ith gynospers, are see' plants, ano)le'ge o* see' str-ct-re an' *-nction is also critical to -n'erstan'ing the angiosper li*e cycle.

(lo)er tr-ct-re an' (-nction

(lo)ers, the repro'-ctie shoots o* angiosper sporophytes, are typically copose' o* *o-r

)horls or o'i*ies leaes calle' *loral organs. 5nlie egetatie shoots, *lo)ers are 'eterinate sheets6

they cease gro)ing a*ter the *lo)er an' *r-it are *ore'.(loral organssepals, petals, staens, an' carpelsare attache' to a part o* the ste calle' the

receptacle. taens an' carpels are repro'-ctie organs, )hereas sepals an' petals are sterile. epals,

)hich enclose an' protect -nopene' *loral b-'s, are -n-s-ally ore lea*y in appearance than the other

*loral organs. Petals are typically ore brightly colore' than sepals an' a'ertise the *lo)er to insects an'

other pollinator.A staen consists o* a stal calle' the *ilaent an' a terinal str-ct-re calle' the anther6 )ithin

the anther are chabers calle' icrosporangia pollen sacs8 that pro'-ce pollen. A carpel has an oary at

its base an' along, slen'er nec calle' the style.At the top o* the style is a generally sticy str-ct-re calle'

the stiga that capt-res pollen. 9ithin the oary are one or ore o-les6 the n-ber o* o-les 'epen's

on the species. he ter pistil is soeties -se' to re*er to a single carpel or t)o or ore *-se' carpels.

Coplete *lo)ers hae all *o-r basic *loral organs. oe species hae incoplete *lo)ers,

lacing sepals, petals, staens or carpels. oe incoplete *lo)ers are sterile, lacing *-nctional staensan' carpels6 others are -nise/-al, lacing only one. oe *lo)ers are borne singly, )hile others are

arrange' in sho)y cl-sters calle' in*lorescences.

Male Gametophytes

he ale gaetophyte begins its 'eelopent )ithin the sporangia pollen sacs8 o* the anther.

9ithin the sporangia are icrosporocytes, each o* )hich )ill *or *o-r haploi' icrospores thro-gh

eiosis. &ach icrospore can gie rise to a haploi' ale gaetophyte. A icrospore 'ii'es once by

itosis an' pro'-ces a generatie cell an' a t-be cell. he generatie cell )ill eent-ally *or sper.


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+-ring at-ration o* the ale gaetophyte, the generatie cell passes into the t-be cell. he t-be

cell, enclosing the generatie cell, pro'-ces the pollen t-be, )hich 'eliers sper to the egg. his is a

pollen grain, an iat-re ale gaetophyte. his t)ocelle' str-ct-re is encase' in a thic, ornate,

'istinctie, an' resistant )all. A pollen grain becoes a at-re gaetophyte )hen the generatie cell

'ii'es by itosis to *or t)o sper cells.In ost species, this occ-rs a*ter the pollen grain lan's on the stiga o* the carpel an' the pollen

t-be begins to *or. he pollen t-be gro)s thro-gh the long style o* the carpel an' into the oary, )hereit releases the sper cells in the icinity o* the ebryo sac.

Female Gametophytes

!-les, each containing a single sporangi-, *or )ithin the chabers o* the oary. !ne cell in

the sporangi- o* each o-le, the egasporocyte, gro)s an' then goes thro-gh eiosis, pro'-cing *o-r

haploi' egaspores. In any angiospers, only one egaspore s-ries. his egaspore 'ii'es

by itosis three ties )itho-t cytoinesis, *oring in one cell )ith eight haploi' n-clei. Mebranes

partition this ass into a -lticell-lar *eale gaetophytethe ebryo sac.

hree cells sit at one en' o* the ebryo sac3 t)o synergi' cells *laning the egg cell. hesynergi's *-nction in the attraction an' g-i'ance o* the pollen t-be. At the other en' o* the egg sac are

three antipo'al cells o* -nno)n *-nction. he other t)o n-clei, the polar n-clei, share the cytoplas o*

the large central cell o* the ebryo sac. he o-le no) consists o* the ebryo sac an' the s-rro-n'ing

integ-ents, layers o* protectie tiss-e *ro the sporophyte that )ill eent-ally 'eelop into the see'coat.

Pollination

Pollination, )hich brings ale an' *eale gaetophytes together, is the *irst step in the chain o*

eents that lea's to *ertiliation. oe plants, s-ch as grasses an' any trees, release large :-antities o*

pollen on the )in' to copensate *or the ran'oness o* this 'ispersal echanis. At certain ties o* the

year, the air is loa'e' )ith pollen, as anyone plag-e' by pollen allergies can attest. oe a:-atic plants

rely on )ater to 'isperse pollen. Most angiospers interact )ith insects or other anials that trans*er

pollen 'irectly bet)een *lo)ers.

Coevolution

he ;oint eol-tion o* t)o interacting species, each in response to selection ipose' by the other,

is calle' coeol-tion. Many species o* *lo)ering plants hae coeole' )ith speci*ic pollinators. leels, )hich also occ-rs '-ring anial gaete *-sion.

In another siilarity to anials, plants establish a bloc to polyspery, the *ertiliation o* an egg

by ore than one sper cell. In plants, a 'eposition o* cell )all aterial ay echanically ipe'e

sper. In aie, this barrier is establishe' )ithin 4? secon's a*ter the initial sper *-sion )ith the egg.


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ee' +eelopent, (or, an' (-nction

A*ter 'o-ble *ertiliation, the o-le 'eelops into a see', an' the oary 'eelops into a *r-it

enclosing the see'.

Endosperm Development

&n'osper 'eelopent -s-ally prece'es ebryo 'eelopent. A*ter 'o-ble *ertiliation, the

triploi' n-cle-s o* the o-les central cell 'ii'es, *oring a -ltin-cleates-percellhaing a ilyconsistency. he en'osper becoes -lticell-lar )hen cytoinesis partitions the cytoplas bet)een

n-clei.

Cell )alls *or, an' the en'osper becoes soli'. Cocon-tilis an e/aple o* li:-i'

en'osper, an' cocon-teatis an e/aple o* soli' en'osper. he )hite *l-**y part o* popcorn is also

soli' en'osper. In grains an' ost other onocot species, the en'osper is rich in n-trients, )hich it

proi'es to the 'eeloping ebryo. In ost e-'icots, the *oo' reseres o* the en'osper are copletely

e/porte' to the cotyle'ons be*ore the see' copletes its 'eelopent, an' conse:-ently the at-re see'

lacs en'osper.

As the ebryo 'eelops, the see' stocpiles proteins, oils, an' starch. Initially, these n-trients are

store' in the en'osper. "ater in see' 'eelopent in any species, the storage *-nction is taen oer by

the s)elling storage leaes, cotyle'ons, o* the ebryo itsel*.Embryo Development

he *irst itotic 'iision o* the ygote splits the *ertilie' egg into a basal cell an' a terinal cell,

)hich gies rise to ost o* the ebryo. he basal cell contin-es to 'ii'e, pro'-cing a threa' o* cells, the

s-spensor, that anchors the ebryo to its parent. he s-spensor *-nctions in the trans*er o* n-trients to the

ebryo *ro the parent an', in soe plant species, *ro the en'osper. As the s-spensor elongates, it

p-shes the ebryo 'eeper into the n-tritie an' protectie tiss-es.

he terinal cell 'ii'es seeral ties an' *ors a spherical proebryo attache' to the

s-spensor. Cotyle'ons begin to *or as b-ps on the proebryo. A e-'icot, )ith its t)o cotyle'ons, is

heartshape' at this stage. !nly one cotyle'on 'eelops in onocots.

A*ter the cotyle'ons appear, the ebryo elongates. Cra'le' bet)een the cotyle'ons is the

ebryonic shoot ape/ )ith the apical eriste o* the ebryonic shoot. At the opposite en' o* theebryo a/is is the ape/ o* the ebryonic root, also )ith a eriste. A*ter the see' gerinates, the apical

eristes at the tips o* the shoot an' root s-stain priary gro)th as long as the plant lies.

Structure of the Mature Seed

+-ring the last stages o* at-ration, a see' 'ehy'rates -ntil its )ater content is only abo-t ?

1?@ o* its )eight. he ebryo stops gro)ing an' enters 'orancy -ntil the see' gerinates. he ebryo

an' its *oo' s-pply are enclose' by a protectie see' coat *ore' by the integ-ents o* the o-le.

+orancy ay be ipose' by the presence o* an intact see' coat, or the ebryo ay be 'orant.

In the see' o* a coon bean, a e-'icot, the ebryo consists o* an elongate str-ct-re, the

ebryonic a/is, attache' to *leshy cotyle'ons. Belo) the point at )hich the *leshy cotyle'ons are

attache', the ebryonic a/is is calle' the hypocotyl. he hypocotyl terinates in the ra'icle, or

ebryonic root. he portion o* the ebryonic a/is aboe )here the cotyle'ons are attache', an' belo)the *irst pair o* iniat-re leaes, is the epicotyl. he epicotyl, yo-ng leaes, an' shoot apical eriste

are collectiely calle' the pl--le.

he cotyle'ons o* the coon bean are pace' )ith starch be*ore the see' gerinates beca-se

they absorbe' carbohy'rates *ro the en'osper )hen the see' )as 'eeloping. o)eer, the see's o*

soe e-'icot species, s-ch as castor beans Ricin-s co-nis8, retain their *oo' s-pply in the en'osper

an' hae ery thin cotyle'ons. he cotyle'ons absorb n-trients *ro the en'osper an' trans*er the to

the ebryo )hen the see' gerinates.


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he ebryo o* a onocot has a single cotyle'on. Mebers o* the grass *aily, incl-'ing aie

an' )heat, hae a specialie' cotyle'on calle' a sc-tell-. he sc-tell- is ery thin, )ith a large

s-r*ace area presse' against the en'osper, *ro )hich the sc-tell- absorbs n-trients '-ring

gerination. he ebryo o* a grass see' is enclose' by t)o sheathes3 a coleorhia, )hich coers the

yo-ng root, an' a coleoptile, )hich coers the yo-ng shoot.

ee' +orancy3 An A'aptation *or o-gh ies

As a see' at-res, it 'ehy'rates an' enters a 'orancy phase, a con'ition o* e/treely lo)

etabolic rate an' s-spen'e' gro)th an' 'eelopent. he con'itions re:-ire' to brea 'orancy an'

res-e gro)th an' 'eelopent ary aong species. oe see's gerinate as soon as they are in a

s-itable enironent. !thers reain 'orant -ntil soe speci*ic enironental c-e ca-ses the to brea

'orancy.

ee' 'orancy increases the chances that gerination )ill occ-r at a tie an' place ost

a'antageo-s to the see'ling. (or e/aple, see's o* any 'esert plants gerinate only a*ter a s-bstantial

rain*all, th-s ens-ring eno-gh )ater to coplete 'eelopent. 9here nat-ral *ires are coon, any

see's re:-ire intense heat to brea 'orancy, allo)ing the to tae a'antage o* open space )ith

re'-ce' copetition. 9here )inters are harsh, see's ay re:-ire e/ten'e' e/pos-re to col' to brea

'orancy. all see's s-ch as lett-ce re:-ire light *or gerination an' brea 'orancy only i* they are

b-rie' near the s-r*ace. !ther see's re:-ire a cheical attac as they pass thro-gh an anials 'igestietract be*ore they can gerinate.

he length o* tie that a 'orant see' reains iable an' capable o* gerinating aries *ro a

*e) 'ays to 'eca'es or een longer, 'epen'ing on the species an' on enironental con'itions. Most

see's are '-rable eno-gh to last *or a year or t)o -ntil con'itions are *aorable *or gerination. h-s, the

soil has a pool o* nongerinate' see's that ay hae acc--late' *or seeral years. his is one reason

egetation reappears so rapi'ly a*ter a *ire, 'ro-ght, *loo', or other enironental 'isr-ption.

ee' #erination an' ee'ling +eelopent

#erination o* see's 'epen's on ibibition, the -ptae o* )ater '-e to the lo) )ater potential o*

the 'ry see'. Ibibition ca-ses the e/pan'ing see' to r-pt-re its see' coat an' triggers etabolic changes

in the ebryo that enable it to res-e gro)th. (ollo)ing hy'ration, enyes begin 'igesting the storage

aterials o* the en'osper or cotyle'ons, an' the n-trients are trans*erre' to the gro)ing regions o* theebryo. he *irst organ to eerge *ro the gerinating see' is the ra'icle, the ebryonic root.


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*-se' carpels an' are calle' siple *r-its. oe siple *r-its are *leshy, lie a peach, )hereas others are

'ry, lie a pea po' or n-t.

An aggregate *r-it res-lts *ro a single *lo)er that has ore than one carpel, each *oring a

sall *r-it. he *r-itlets are cl-stere' together on a single receptacle. he stra)berry is an aggregate

*r-it, consisting o* an enlarge' receptacle ebe''e' )ith tiny onesee'e' *r-its.

A -ltiple *r-it 'eelops *ro an in*lorescence, a gro-p o* *lo)ers tightly cl-stere' together.9hen the )alls o* the oaries thicen, they *-se together an' *or one *r-it, as in a pineapple.

In soe angiospers, other *loral parts as )ell as the oaries contrib-te to the *r-it. -ch *r-its

are calle' accessory *r-its. In apples, the *leshy part o* the siple *r-it is 'erie' ainly *ro the s)ollen

receptacle6 only the core o* the apple *r-it 'eelops *ro the oary.

he *r-it -s-ally ripens abo-t the sae tie its see's are copleting their 'eelopent. (or a 'ry

*r-it s-ch as a soybean po', ripening is little ore than the aging an' 'rying o-t o* *r-it tiss-es, allo)ing

the *r-it to open an' release the see's. he ripening o* *leshy *r-its is ore elaborate, )ith its steps

controlle' by the cople/ interactions o* horones.

Ripening res-lts in an e'ible *r-it that seres as an enticeent to the anials that help sprea' the

see's. hep-lp

o* the *r-it becoes so*ter as a res-lt o* enyes 'igesting coponents o* the cell)alls. he color changes *ro green to re', orange, or yello). he *r-it becoes s)eeter as organic aci's

or starch olec-les are conerte' to s-gar, )hich ay reach a concentration as high as 20@ in a ripe

*r-it.


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Rebecca Zhang

April 1, 2014

AP Biology

Mrs. Istrico

Capbell BI!"!#$ %e, AP &'ition

Chapter 3#: $lant Responses to %nternal and &'ternal (igns

ti-li an' a tationary "i*e

oe plants open an' close their *lo)ers at partic-lar ties o* the 'ay6 these ties are

pres-ably )hen their insect pollinators are ost actie. he passage o* tie is only one o* any

enironental *actors that a plant -st sense in or'er to s-rie an' repro'-ce s-ccess*-lly. Plants are

*ar *ro inaniate ob;ects. A plants orphology an' physiology are t-ne' to its s-rro-n'ings by

cople/ interactions bet)een enironental sti-li an' internal signals.

At the organisal leel, plants an' anials respon' to enironental sti-li by ery 'i**erent

eans. Roote' in one location *or li*e, a plant generally respon's to enironental c-es by a';-sting its

pattern o* gro)th an' 'eelopent. As a res-lt, plants o* the sae species ary in bo'y *or -ch ore

than 'o anials. Be*ore plants can initiate gro)th responses to enironental signals, they -st 'etect

the change in their enironent. At the cell-lar leel, the processes by )hich plants an' anials perceieenironental changes are e:-ally cople/ an' o*ten hoologo-s.

3#.1 (ignal transduction pathways lin) signal reception to response

All organiss, incl-'ing plants, hae the ability to receie speci*ic enironental signals an'

respon' to the in )ays that enhance their s-rial an' repro'-ctie s-ccess. "ie anials, plants hae

cell-lar receptors that they -se to 'etect iportant changes in their enironent. hese changes ay be

an increase in the concentration o* a gro)th horone, an in;-ry *ro a caterpillar -nching on leaes, or

a 'ecrease in 'ay length as )inter approaches.

(or a sti-l-s to elicit a response, certain cells in the organis -st possess an appropriate

receptor, a olec-le that is a**ecte' by the speci*ic sti-l-s. 5pon receiing a sti-l-s, a receptor

initiates a speci*ic series o* biocheical steps, a signal trans'-ction path)ay, that co-ples reception o* the

sti-l-s )ith response o* the organis. Plants are sensitie to a )i'e range o* sti-li, each initiating a

speci*ic signal trans'-ction path)ay.Plant gro)th patterns ary 'raatically in response to the presence ers-s the absence o* light.

(or e/aple, a potato, a o'i*ie' -n'ergro-n' ste, can spro-t shoots *ro its a/illary b-'s. hese

shoots are ghostly pale an' hae long, thin stes6 -ne/pan'e' leaes6 an' re'-ce' roots. !nce a shoot

reaches the s-nlight, its orphology an' biocheistry -n'ergo pro*o-n' changes, collectiely calle' 'e

etiolation, or greening. he elongation rate o* the stes slo)s, the leaes e/pan', the roots start to

elongate, an' the shoot pro'-ces chlorophyll.

he 'eetiolation response is an e/aple o* ho) a plant receies a signalin this case, light

an' ho) this reception is trans'-ce' into a response, greening. t-'ies o* -tants hae proi'e' al-able

insights into the roles that ario-s olec-les play in the three stages o* cellsignal processing3 reception,

trans'-ction, an' response.

Reception

he receptor *or 'eetiolation in plants is calle' a phytochroe. 5nlie any receptors, )hichare in the plasa ebrane, this phytochroe is in the cytoplas.

he iportance o* this phytochroe )as con*ire' thro-gh inestigations o* a toato -tant,

calle' a-rea, that greens less than )il'type toatoes )hen e/pose' to light. In;ecting a''itional

phytochroe *ro other plants into a-rea lea* cells an' e/posing the to light pro'-ce' a noral 'e

etiolation response. hese e/perients in'icate that phytochroe *-nctions in light 'etection '-ring 'e

etiolation.

rans'-ction


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Receptors s-ch as phytochroe are sensitie to ery )ea enironental an' cheical signals.

(or e/aple, e/pos-re to only a *e) secon's o* oonlight slo)s ste elongation in 'argro)n oa

see'lings.

hese )ea signals are apli*ie' by secon' essengerssall, internally pro'-ce' cheicals

that trans*er an' apli*y the signal *ro the receptor to proteins that ca-se the speci*ic response. In the

'eetiolation response, each actiate' phytochroe ay gie rise to h-n're's o* olec-les o* a secon'

essenger, each o* )hich ay lea' to the actiation o* h-n're's o* olec-les o* a speci*ic enye."ight ca-ses phytochroe to -n'ergo a con*orational change that lea's to increases in the

leels o* the secon' essengers cyclic #MP an' Ca2>. Changes in c#MP leels can lea' to ionic changes

)ithin the cell by in*l-encing the properties o* ion channels. Cyclic #MP also actiates speci*ic protein

inases, enyes that phosphorylate an' actiate other enyes.

Response

5ltiately, a signal trans'-ction path)ay lea's to the reg-lation o* one or ore cell-lar actiities.

hese responses to sti-lation -s-ally inole the increase' actiity o* certain enyes thro-gh t)o

echaniss3 by transcriptional reg-lation o'i*ying the transcription o* R.

he echanis by )hich a signal prootes a ne) 'eelopental co-rse ay 'epen' on the

actiation o* positie transcription *actors proteins that increase transcription o* speci*ic genes8 ornegatie transcription *actors proteins that 'ecrease transcription8.

oe Arabi'opsis -tants hae a lightgro)n orphology e/pan'e' leaes an' short, st-r'y

stes8 )hen gro)n in the 'ar. hese -tants are not green beca-se the *inal step in chlorophyll

pro'-ction re:-ires light. he -tants hae 'e*ects in a negatie transcription *actor that inhibits thee/pression o* other genes norally actiate' by light. 9hen the negatie *actor is eliinate' by

-tation, the bloce' path)ay becoes actiate'. ence, these -tants, e/cept *or their pale color,

appear to hae been gro)n in the light.

DeEtiolation !Greening" Proteins+-ring the 'eetiolation response, a ariety o* proteins are either synthesie' or actiate'. hese

proteins incl-'e enyes that *-nction in photosynthesis 'irectly or that s-pply the cheical prec-rsors

*or chlorophyll pro'-ction.

!ther proteins a**ect the leels o* plant horones that reg-late gro)th. he leels o* t)o

horones a-/ins an' brassinosteroi's8 that enhance ste elongation 'ecrease *ollo)ing phytochroe

actiationhence, the re'-ction in ste elongation that accopanies 'eetiolation.

3#.* $lant hormones help coordinate growth, de+elopment, and responses to stimuli


8/18

orones are cheical signals that are pro'-ce' in in-te ao-nts in one part o* the bo'y, are

transporte' to other parts o* the bo'y, bin' to speci*ic receptors, an' trigger responses in target cells an'

tiss-es.

Plants an' anials 'i**er in their responses to horones. Plants 'ont hae bloo' or a circ-latory

syste to transport horonelie signal olec-les. oe plant horones act only locally. oe signal

olec-les in plants, s-ch as s-crose, typically occ-r at concentrations that are h-n're's o* tho-san's

ties higher than the concentration o* a typical horone.


9/18

oe o* the a;or classes o* plant horones are a-/ins, cytoinins, gibberellins,

brassinosteroi's, abscisic aci', an' ethylene. Plant horones ten' to be relatiely sall olec-les that are

transporte' *ro cell to cell across cell )alls, a path)ay that blocs the oeent o* large olec-les.

Plant horones are pro'-ce' at ery lo) concentrations. ignal trans'-ction path)ays apli*y the

horonal signal any*ol' an' connect it to a cells speci*ic responses.

In general, plant horones control plant gro)th an' 'eelopent by a**ecting the 'iision,

elongation, an' 'i**erentiation o* cells. oe horones also e'iate shorterter physiological responseso* plants to enironental sti-li. &ach horone has -ltiple e**ects, 'epen'ing on its site o* action, its

concentration, an' the 'eelopental stage o* the plant. It is horonal balance, rather than horones

acting in isolation, that control the gro)th an' 'eelopent o* plants.

#u$in

he ter a-/in is -se' *or any cheical s-bstance that prootes the elongation o* coleoptiles,

altho-gh a-/ins act-ally hae -ltiple *-nctions in angiospers. he nat-ral a-/in occ-rring in plants is

in'oleacetic aci' IAA8, b-t seeral other copo-n's also hae a-/in actiity.

In gro)ing shoots, a-/in is transporte' -ni'irectionally, *ro the shoot ape/ 'o)n to the base.

he spee' at )hich a-/in is transporte' 'o)n the ste *ro the shoot ape/ is abo-t 10 Fhr, a rate that

is too *ast *or 'i**-sion b-t slo)er than translocation in the phloe. A-/in sees to be transporte'

'irectly thro-gh parenchya tiss-e, *ro one cell to the ne/t. his -ni'irectional transport o* a-/in is

calle' polar transport an' has nothing to 'o )ith graity.A-/in traels -p)ar' i* a ste or coleoptile is place' -psi'e 'o)n. he polarity o* a-/in

transport is '-e to the polar 'istrib-tion o* a-/in transport protein in the cells. Concentrate' at the basalen' o* the cells, a-/in transporters oe the horone o-t o* the cell an' into the apical en' o* the

neighboring cell.

Altho-gh a-/in a**ects seeral aspects o* plant 'eelopent, one o* its chie* *-nctions is to

sti-late the elongation o* cells in yo-ng shoots. he apical eriste o* a shoot is a a;or site o* a-/in

synthesis. As a-/in oes *ro the ape/ 'o)n to the region o* cell elongation, the horone sti-lates

cell gro)th, bin'ing to a receptor in the plasa ebrane.

A-/in sti-lates cell gro)th oer only a certain concentration range*ro abo-t 10to 104

M. At higher concentrations, a-/ins ay inhibit cell elongation, probably by in'-cing the pro'-ction o*

ethylene, a horone that generally acts as an inhibitor o* elongation.

Accor'ing to the aci' gro)th hypothesis, in a shoots region o* elongation, a-/in sti-lates theplasa ebranes proton p-ps, increasing the oltage across the ebrane an' lo)ering the p in

the cell )all.

"o)ering the p actiates e/pansin enyes that brea the crosslins bet)een cell-lose

icro*ibrils an' other cell )all constit-ents, th-s loosening the )all. Increasing the ebrane potential

enhances ion -ptae into the cell, )hich ca-ses the osotic -ptae o* )ater, increasing t-rgor an'

elongating the loose)alle' cell.A-/in rapi'ly alters gene e/pression, ca-sing cells in the region o* elongation to pro'-ce ne)

proteins )ithin in-tes. oe o* these proteins are shortlie' transcription *actors that repress or

actiate the e/pression o* other genes. A-/in sti-lates a s-staine' gro)th response o* aing the

a''itional cytoplas an' )all aterial re:-ire' by elongation.

A-/in is -se' coercially in the egetatie propagation o* plants by c-ttings. reating a

'etache' lea* or ste )ith rooting po)'er containing a-/in o*ten ca-ses a'entitio-s roots to *or nearthe c-t s-r*ace.

A-/in is also inole' in the branching o* roots. !ne Arabi'opsis -tant that e/hibits e/tree

proli*eration o* lateral roots has an a-/in concentration 17*ol' higher than noral. ynthetic a-/ins,

s-ch as 2,4'initrophenol 2,4+8, are )i'ely -se' as selectie herbici'es. Monocots, s-ch as aie or

t-r*grass, can rapi'ly inactiate these synthetic a-/ins. +icots cannot actiate the synthetic a-/ins,

ho)eer, an' they 'ie *ro a horonal oer'ose. h-s, spraying cereal *iel's or t-r* )ith 2,4+

eliinates 'icot broa'lea*8 )ee's s-ch as 'an'elions.


10/18

A-/in also a**ects secon'ary gro)th by increasing cabial actiity an' in*l-encing the

'i**erentiation o* cabial initials. +eeloping see's synthesie a-/in, )hich prootes the gro)th o* *r-it.

ynthetic a-/ins spraye' on toato ines in'-ce the 'eelopent o* see'less toatoes )itho-t

pollination.

Cyto%ininsCytoinins sti-late cytoinesis, or cell 'iision. Cytoinins )ere originally 'iscoere' in the

1%40s by Dohannes an !erbee, )ho *o-n' that he co-l' sti-late the gro)th o* plant ebryos bya''ing cocon-t il to his c-lt-re e'i-. A 'eca'e later, (ole oog an' Carlos !. Miller in'-ce'

c-lt-re' tobacco cells to 'ii'e by a''ing 'egra'e' saples o* +


11/18

In stes, gibberellins sti-late cell elongation andcell 'iision. !ne hypothesis proposes that

gibberellins sti-late cell )allloosening enyes, *acilitating the entry o* e/pansin proteins. In a

gro)ing ste, a-/ins, by aci'i*ying the cell )all an' actiating e/pansins, an' gibberellins, by*acilitating the penetration o* e/pansins, act in concert to proote elongation.

he e**ects o* gibberellins in enhancing ste elongation are ei'ent )hen ')ar* arieties o*

plants are treate' )ith gibberellins. A*ter treatent )ith gibberellins, soe ')ar* pea plants gro) to

noral height. I* gibberellins are applie' to )il'type plants, ho)eer, there is o*ten no response, perhapsbeca-se these plants are alrea'y pro'-cing the optial 'ose o* the horone.

he ost 'raatic e/aple o* gibberellinin'-ce' ste elongation is bolting, the rapi' *oration

o* the *loral stal. In any plants, both a-/ins an' gibberellins -st be present *or *r-it to set.

I* hopson see'less grapes are spraye' )ith gibberellin '-ring 'eelopent, the interno'es o*

the grape b-nch elongate, allo)ing ore space *or each grape. he e/tra space prootes air circ-lation

bet)een the grapes an' aes it har'er *or yeast an' other icroorganiss to in*ect the *r-its.

he ebryo o* a see' is a rich so-rce o* gibberellins. A*ter )ater is ibibe', the release o*

gibberellins *ro the ebryo signals the see' to brea 'orancy an' gerinate. oe see's that re:-ire

special enironental con'itions to gerinate, s-ch as e/pos-re to light or lo) teperat-res, brea

'orancy )hen they are treate' )ith gibberellins. #ibberellins s-pport the gro)th o* cereal see'lings by

sti-lating the synthesis o* 'igestie enyes that obilie store' n-trients.

'rassinosteroidsBrassinosteroi's are steroi's that are cheically siilar to cholesterol an' the se/ horones o*

anials. Brassinosteroi's in'-ce cell elongation an' 'iision in ste segents an' see'lings atconcentrations as lo) as 1012M. hey also retar' lea* abscission an' proote /yle 'i**erentiation.

heir e**ects are so :-alitatiely siilar to those o* a-/ins that it too seeral years *or plant physiologists

to recognie that brassinosteroi's )ere not types o* a-/ins.

Doann Chory an' her colleag-es proi'e' ei'ence *ro olec-lar biology that brassinosteroi's

are plant horones. A brassinosteroi''e*icient#rabidopsis-tant has orphological *eat-res siilar to

those o* lightgro)n plants een )hen gro)n in the 'ar. he -tation a**ects a gene that norally co'es

*or an enye siilar to one inole' in steroi' synthesis in aalian cells. he -tant )as restore'

to noral by the e/periental application o* brassinosteroi's.

#bscisic #cid

Abscisic aci' ABA8 )as 'iscoere' in'epen'ently in the 1%G0s, )hen one research gro-pst-'ying b-' 'orancy an' another inestigating lea* abscission isolate' ABA. Ironically, ABA is no

longer tho-ght to play a priary role in either b-' 'orancy or lea* abscission, b-t it is an iportant

plant horone )ith a ariety o* *-nctions.

ABA generally slo)s gro)th. !*ten ABA antagonies the actions o* the gro)th horones

a-/ins, cytoinins, gibberellins, an' brassinosteroi's. It is the ratio o* ABA to one or ore gro)th

horones that 'eterines the *inal physiological o-tcoe.

!ne a;or e**ect o* ABA on plants is see' 'orancy. ee' 'orancy has great s-rial al-e

beca-se it ens-res that see's gerinate only )hen there are optial con'itions o* light, teperat-re, an'oist-re.

"eels o* ABA ay increase 100*ol' '-ring see' at-ration, lea'ing to inhibition o*

gerination an' in'-cing the pro'-ction o* special proteins that help see's )ithstan' the e/tree

'ehy'ration that accopanies at-ration.Many types o* 'orant see's gerinate )hen ABA is reoe' or inactiate'. (or e/aple, the

see's o* soe 'esert plants brea 'orancy only )hen heay rains )ash ABA o-t o* the. !ther see's

re:-ire light or prolonge' e/pos-re to col' to inactiate ABA.

he ratio o* ABA to gibberellins 'eterines )hether the see' reains 'orant or gerinates.

he a''ition o* ABA to see's that are abo-t to gerinate aes the 'orant again. Inactiate' ABA or

lo) leels o* ABA can lea' to precocio-s gerination. A aie -tant )hose see's gerinate )hile still

on the cob lacs a *-nctional transcription *actor re:-ire' *or ABA to in'-ce e/pression o* certain genes.


12/18

ABA is the priary internal signal that enables plants to )ithstan' 'ro-ght. 9hen a plant begins

to )ilt, ABA acc--lates in leaes an' ca-ses stoata to close rapi'ly, re'-cing transpiration an'

preenting *-rther )ater loss. ABA ca-ses an increase in the opening o* potassi- channels in the plasa

ebrane o* g-ar' cells, lea'ing to a assie loss o* potassi- *ro the cells.

he accopanying osotic loss o* )ater lea's to a re'-ction in g-ar' cell t-rgor, an' thestoatal pores close. In soe cases, )ater shortages can stress the root syste early, lea'ing to the

transport o* ABA *ro roots to leaes an' *-nctioning as an early )arning syste.M-tants that areprone to )ilting are o*ten 'e*icient in ABA pro'-ction.

Ethylene

In 1%01, +iitry


13/18

9hen a lea* *alls in a-t-n, the breaing point is an abscission layer near the base o* the petiole.

he parenchya cells here hae ery thin )alls, an' there are no *iber cells aro-n' the asc-lar tiss-e.

he abscission layer is *-rther )eaene' )hen enyes hy'rolye polysacchari'es in the cell )alls. he

)eight o* the lea*, )ith the help o* the )in', ca-ses a separation )ithin the abscission layer. Be*ore the

lea* *alls, a layer o* cor *ors a protectie scar on the t)ig si'e o* the abscission layer, preentingpathogens *ro ina'ing the plant.

A change in the balance o* ethylene an' a-/in controls abscission. An aging lea* pro'-ces lessan' less a-/in, an' this aes the cells o* the abscission layer ore sensitie to ethylene. As the

in*l-ence o* ethylene preails, the cells in the abscission layer pro'-ce enyes that 'igest the cell-lose

an' other coponents o* cell )alls.

&thylene plays a role in the cons-ption o* ripe *r-its by anials, )hich help 'isperse the see's

o* *lo)ering plants. Iat-re *r-its are tart, har', an' green b-t becoe e'ible at the tie o* see'

at-ration, triggere' by a b-rst o* ethylene pro'-ction. &nyatic brea'o)n o* cell )all coponents

so*tens the *r-it, an' the conersion o* starches an' aci's to s-gars aes the *r-it s)eet. he pro'-ction

o* ne) scents an' colors helps a'ertise *r-its ripeness to anials, )hich eat the *r-its an' 'isperse the

see's.

A chain reaction occ-rs '-ring ripening3 &thylene triggers ripening, an' ripening triggers ore

ethylene pro'-ctiona rare e/aple o* positie *ee'bac on physiology. Beca-se ethylene is a gas, the

signal to ripen sprea's *ro *r-it to *r-it. (r-its can be ripene' :-icly by storing the in a plastic bag,)here they acc--late ethylene gas, or by increasing ethylene leels in coercial pro'-ction.

Alternatiely, to preent preat-re ripening, apples ay be store' in bins *l-she' )ith carbon

'io/i'e, )hich preents ethylene *ro acc--lating an' inhibits the synthesis o* ne) ethylene.

#enetic engineering o* ethylene signal trans'-ction path)ays has potentially iportant

coercial applications a*ter harest. (or e/aple, olec-lar biologists hae bloce' the transcription

o* one o* the genes re:-ire' *or ethylene synthesis in toato plants. hese toato *r-its are pice' )hile

green an' are in'-ce' to ripen on 'ean' by the a''ition o* ethylene gas.

ystes Biology an' orone Interactions

Plant responses o*ten inole the interactions o* any horones an' their signal trans'-ction

path)ays. he st-'y o* horone interactions can be a cople/ proble. (or e/aple, *loo'ing

'eep)ater rice lea's to a ?0*ol' increase in internal ethylene leels an' a rapi' increase in ste

elongation. (loo'ing also lea's to an increase in sensitiity to gibberellin, )hich is e'iate' by a'ecrease in ABA leels. h-s, ste elongation is the res-lt o* interaction aong three horones an' their

signal trans'-ction chains.

Many plant biologists proote a systesbase' approach, )hich attepts to 'iscoer an'

-n'erstan' biological properties eerging *ro the interactions o* any syste eleents. 5sing genoic

techni:-es, biologists can i'enti*y all the genes in a plant.

hree plants are alrea'y se:-ence'3 the research plant Arabi'opsis( rice, an' cotton)oo'. !negoal o* systes biology is to o'el a liing plant pre'ictably. he ability to o'el a liing plant )ill

enable scientists to pre'ict the res-lt o* a genetic anip-lation )itho-t een setting *oot in the laboratory.

3#.3 Responses to light are crucial or plant success

"ight is an especially iportant enironental *actor in the lies o* plants. It is re:-ire' *or

photosynthesis, an' it c-es any ey eents in plant gro)th an' 'eelopent. he e**ects o* light on

plant orphology are )hat plant biologists call photoorphogenesis."ight reception also allo)s plants to eas-re the passage o* 'ays an' seasons. Plants 'etect the

presence, 'irection, intensity, an' )aelength o* light. A graph calle' an action spectr- 'epicts the

relatie e**ectieness o* 'i**erent )aelengths o* ra'iation in 'riing a partic-lar process.

he action spectr- o* photosynthesis has t)o peas, one in the re' an' one in the bl-e,

atching the absorption peas o* chlorophyll. Action spectra can be -se*-l in the st-'y o*any process

that 'epen's on light.


14/18

Action spectra reeal that re' an' bl-e light are the ost iportant in reg-lating a plants

photoorphogenesis. Researchers i'enti*ie' t)o a;or classes o* light receptors3 bl-elight

photoreceptors an' phytochroes that absorb ostly re' light.Bl-e"ight Photoreceptors

he action spectra o* any plant processes 'eonstrate that bl-e light is e**ectie in initiating

'ierse responses. he biocheical i'entity o* the bl-elight photoreceptor )as so el-sie that it )as

calle' cryptochroe. In the 1%%0s, olec-lar biologists analying Arabi'opsis -tants *o-n' threecopletely 'i**erent types o* pigents that 'etect bl-e light.

Cryptochroes are olec-lar relaties o* +


15/18

eening an' raise the in the orning. hese oeents contin-e een i* the plants are ept in constant

light or constant 'arness.

-ch physiological cycles )ith a *re:-ency o* abo-t 24 ho-rs that are not 'irectly pace' by any

no)n enironental ariable are calle' circa'ian rhyths. Circa'ian rhyths are coon to all

e-aryotic li*e.Beca-se organiss contin-e their rhyths een )hen place' in ine sha*ts or orbite' in

satellites, the rhyths 'o not appear to be triggere' by any s-btle b-t perasie enironental signal.All research th-s *ar in'icates that the oscillator *or circa'ian rhyths is internal. his internal

cloc, ho)eer, is entraine' to a perio' o* precisely 24 ho-rs by 'aily signals *ro the enironent. I* an

organis is ept in a constant enironent, its circa'ian rhyths 'eiate *ro a 24ho-r perio' to *ree

r-nning perio's ranging *ro 21 to 27 ho-rs.

A lea'ing hypothesis *or the olec-lar echanis -n'erlying biological tieeeping is that it

'epen's on the synthesis o* a protein that reg-lates its o)n pro'-ction thro-gh *ee'bac control. his

protein ay be a transcription *actor that inhibits transcription o* the gene that enco'es *or the

transcription *actor itsel*. he concentration o* this transcription *actor ay acc--late '-ring the *irst

hal* o* the circa'ian cycle an' then 'ecline '-ring the secon' hal* '-e to sel*inhibition o* its o)n

pro'-ction.

Researchers hae recently -se' a noel techni:-e to i'enti*y cloc -tants in Arabi'opsis.

Molec-lar biologists splice' the gene *or l-ci*erase the enye responsible *or biol-inescence in*ire*lies8 to the prooter o* certain photosynthesisrelate' genes that sho) circa'ian rhyths in

transcription. 9hen the biological cloc t-rne' on the prooter o* the photosynthesis genes in

Arabi'opsis, it also sti-late' the pro'-ction o* l-ci*erase, an' the plant glo)e'. his enable'

researchers to screen plants *or cloc -tations, seeral o* )hich are 'e*ects in proteins that norallybin' photoreceptors. hese -tations ay 'isr-pt a light'epen'ent echanis that sets the biological

cloc.

he &**ect o* "ight on the Biological Cloc

Beca-se the *reer-nning perio' o* any circa'ian rhyths is longer or shorter than the 24ho-r

'aily cycle, the rhyths eent-ally becoe 'esynchronie' )ith the nat-ral enironent )hen 'enie'

enironental c-es. -ans e/perience this type o* 'esynchroniation )hen )e cross seeral ties

ones in an airplane, lea'ing to the phenoenon calle' ;et lag. &ent-ally, o-r circa'ian rhyths becoe

resynchronie' )ith the e/ternal enironent.Plants are capable o* reestablishing their circa'ian synchroniation. Both phytochroe an' bl-e

light photoreceptors can entrain the circa'ian rhyths o* plants.

he phytochroe syste inoles t-rning cell-lar responses o** an' on by eans o* the P r P*r

s)itch. In 'arness, the phytochroe ratio shi*ts gra'-ally in *aor o* the Pr*or, in part *ro the

synthesis o* ne) Prolec-les an', in soe species, by the slo) biocheical conersion o* P*rto Pr. 9hen

the s-n rises, the P*rleel s-''enly increases by rapi' photoconersion o* Pr. his s-''en increase in P*reach 'ay at 'a)n resets the biological cloc.

Interactions bet)een phytochroe an' the biological cloc enable plants to eas-re the passage

o* night an' 'ay. he relatie lengths o* night an' 'ay change oer the co-rse o* the year, e/cept at the

e:-ator. Plants -se this change to a';-st their actiities in synchrony )ith the seasons.Photoperio'is an' Responses to Reasons

he appropriate appearance o* seasonal eents, s-ch as see' gerination, *lo)ering, an' theonset an' breaing o* b-' 'orancy, is o* critical iportance in the li*e cycles o* ost plants. he

enironental sti-l-s that plants -se ost o*ten to i'enti*y the tie o* year is the photoperio', the

relatie lengths o* night an' 'ay. A physiological response to the photoperio', s-ch as *lo)ering, is calle'

photoperio'is.

!ne o* the earliest cl-es to ho) plants 'etect the progress o* the seasons cae *ro a -tant

ariety o* tobacco st-'ie' by 9. 9. #arner an' . A. Allar' in 1%20. his ariety, Marylan' Maoth,

'oes not *lo)er in s-er as noral tobacco plants 'o, b-t in )inter.


16/18

In lightreg-late' chabers, the Marylan' Maoth *lo)ere' only i* the 'ay length )as 14

ho-rs or shorter, )hich e/plaine' )hy it 'i' not *lo)er '-ring the longer 'ays o* the s-er. #arner an'

Allar' the Marylan' Maoth a short'ay plant beca-se it re:-ire' a light perio'shorterthan a critical

length to *lo)er. !ther e/aples o* short'ay plants are chrysanthe-s, poinsettias, an' soe soybean

arieties."ong'ay plants *lo)er only )hen the light perio' is longerthan a critical n-ber o* ho-rs.

&/aples incl-'e spinach, ra'ish, lett-ce, iris, an' any cereals.+ayne-tral plants *lo)er )hen they reach a certain stage o* at-rity, regar'less o* the 'ay

length. &/aples incl-'e toatoes, rice, an' 'an'elions.

In the 1%40s, researchers 'iscoere' that it is act-ally night length, not 'ay length that controls

*lo)ering an' other responses to photoperio'. Research 'eonstrate' that the cocleb-r, a short'ay

plant, *lo)ere' i* the 'aytie perio' )as broen by brie* e/pos-res to 'arness, b-t not i* the nighttie

perio' )as broen by a *e) in-tes o* 'i light.

hort'ay plants are act-ally longnight plants, re:-iring a ini- length o* -ninterr-pte'

'arness. Cocleb-r is act-ally -nresponsie to daylength, b-t it re:-ires at least ho-rs o* continuous

dar%nessto *lo)er.

iilarly, long'ay plans are act-ally shortnight plants. A long'ay plant gro)n in photoperio's

o* long nights that )o-l' not norally in'-ce *lo)ering )ill *lo)er i* the perio' o* contin-o-s 'arness

is interr-pte' by a *e) in-tes o* light.Altho-gh the critical *actor is night length, the ters long'ay an' short'ay are ebe''e' *irly

in the ;argon o* plant physiology. "ong'ay an' short'ay plants are 'isting-ishe' not by an absol-te

night length b-t by )hether the critical night length sets a a/i- or ini- n-ber o* ho-rs o*

'arness re:-ire' *or *lo)ering.Re' light is the ost e**ectie color in interr-pting the nighttie portion o* the photoperio'.

Action spectra an' photoreersibility e/perients sho) that phytochroe is the actie pigent. I* a *lash

o* re' light '-ring the 'ar perio' is *ollo)e' ie'iately by a *lash o* *arre' light, the plant 'etects no

interr-ption o* night length, th-s 'eonstrating re'F*arre' photoreersibility.

Plants eas-re night length ery acc-rately. oe short'ay plants )ill not *lo)er i* night is een

one in-te shorter than the critical length. oe plants species al)ays *lo)er on the sae 'ay each year.

Altho-gh soe plants re:-ire only a single e/pos-re to the appropriate photoperio' to begin

*lo)ering, others re:-ire seeral s-ccessie 'ays o* the appropriate photoperio'. oe plants respon' tophotoperio' only i* pretreate' by another enironental sti-l-s. (or e/aple, )inter )heat )ill not

*lo)er -nless it has been e/pose' to seeral )ees o* teperat-res belo) 100C, calle' ernaliation

be*ore e/pos-re to the appropriate photoperio'. he -se o* pretreatent )ith col' to in'-ce *lo)ering is

calle' ernaliation.

# Flo)ering *ormone+

Altho-gh b-'s pro'-ce *lo)ers, it is the leaes that 'etect photoperio' an' trigger *lo)ering. I*een a single lea* receies the appropriate photoperio', all b-'s on a plant can be in'-ce' to *lo)er, een

i* they hae not e/perience' this signal. Plants that lac leaes )ill not *lo)er een i* e/pose' to the

appropriate photoperio'.

he *loral sti-l-s can oe across across a gra*t *ro an in'-ce' plant to a nonin'-ce' plantan' trigger *lo)ering in the latter. he sti-l-s appears to be the sae *or short'ay an' long'ay plants,

'espite 'i**ering photoperio' con'itions re:-ire' *or leaes to sen' this signal. he *lo)ering signal,calle' *lorigen, resiste' i'enti*ication *or 70 years. A gene calle' ("!9&RI


17/18

3#. $lants respond to attac)s by herbi+ores and pathogens

Plants 'o not e/ist in isolation b-t interact )ith any other species in their co-nities. oe o*

these interspecies interactions are -t-ally bene*icial.

Most interspecies interactions are not bene*icial to the plant. As priary pro'-cers, plants are at

the base o* ost *oo' )ebs an' are s-b;ect to attac by a )i'e ariety o* planteating herbioro-s8anials. Plants are also s-b;ect to attacs by pathogenic ir-ses, bacteria, an' *-ngi. Plants co-nter these

threats )ith 'e*ense systes that 'eter herbiory an' preent in*ection or cobat pathogens that in*ectthe plant.

+e*enses Against erbiores

erbioryanials eating plantsis a stress *or plants in any ecosyste. Plants resist

herbiory )ith both physical 'e*enses, s-ch as thorns, an' cheical 'e*enses, s-ch as the pro'-ction o*

'istaste*-l or to/ic copo-n's. oe plants recr-itpre'atory anials that help 'e*en' the against

speci*ic herbiores. hese olatile olec-les can also *-nction as an early )arning syste *or nearby

plants o* the sae species.

Iris Eappers an' her colleag-es, at 9ageningen 5niersity in the


18/18

an' restricts the sprea' o* a pathogen. A*ter cells at the in*ection site o-nt a cheical 'e*ense an' seal

o** the area, they 'estroy theseles.

Pathogen inasions can also pro'-ce cheical signals that tell )here o* in*ection to the )hole

plant. he res-lting systeic ac:-ire' resistance is associate' )ith the systeic e/pression o* soe

'e*ense genes, incl-'ing those that enco'e *or PR proteins.AR is nonspeci*ic, proi'ing protection against any pathogens *or 'ays. he ost liely

can'i'ate *or the signal obilie' *ro the in*ection site to elicit AR is salicylic aci' A8. A issynthesie' at high leels aro-n' the in*ection site. A ay then be carrie' by the phloe, )hich

acc--lates thro-gho-t the plant at lo)er leels. &/ternal application o* A in'-ces the pro'-ction o*

PR proteins an' resistance to pathogens.

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