I Involucradas principalmente en expansi´

Auxinas

I

Primer grupo de reguladores de crecimiento estudiados en plantas

(Darwin y Darwin, 1880)

I

Involucradas principalmente en expansi´

on celular, tropismos y

dominancia apical

Formas en la planta

I

Naturales:

I Acido 3-indolac´´ etico (forma m´as abundante en la planta)(IAA,

Indol-3-acetic acid)

I _{Acido 4-cloro-3-indolac´}´ _etico I Acido fenilac´´ etico

I Acido indol-3-but´ırico´ (IBA,Indol-3-butyric acid) I

Sint´

eticas (principales):

I Acido 1-naftal´´ en-ac´etico(NAA,1-naphtalenacetic acid)

I Acido 2,4-dicloro-fenoxi-ac´´ etico(2,4D)

I Acido 2-metoxi-3,6-diclorobenzoico´ (Dicamba)

I Acido 4-amino-3,5,6-tricloro-picol´ınico´ (Picloram)

Auxinas

OH O

OH O O

Cl Cl

Ac. 3-indol-acético

Ac. 2-nafatalén-acético

Ac. 2,4 dicloro-fenoxi-acético Ac. 2-metoxi-3,6-diclorobenzoico

Lugares de s´ıntesis

I

Tejidos en crecimiento activo

I Meristemo apical del tallo (sitio principal)

I Puntas y m´argenes de las hojas en desarrollo

I Frutos en crecimiento

Bios´ıntesis de auxinas

I

A partir del tript´

ofano

I V´ıa de la triptamina

I V´ıa del indol-3-piruvato

I V´ıa del indol-3-acetonitrilo

I V´ıa de la indolacetamida (bacterias)

Bios´ıntesis de auxinas

Triptamina Indol-3-piruvato Indolacetamida

C

Indol-3-acetonitrilo

Ac. 3-indol-acético Ac. indol-3-glicerol-fosfato

Distribuci´

on dentro de la c´

elula

I

Principalmente en el citosol y cloroplasto

I

Libre (activo) o formando complejos con otras mol´

eculas

I Esteres´

I Mio-inositol

I Glucosa

I Amino´acidos

I Glucanos

Formas activas y conjugadas

IAA - glucosa

IAA - Alanina

IAA - Leucina

IAA - Metil

IAA

IAA - mio-inositol

oxIAA -glucosa

oxIAA -Aspartato

Unión reversible

(almacenamiento)

Transporte en la planta

I

Desde ´

organos de s´ıntesis hacia otros ´

organos/c´

elulas

I

Formas conjugadas son m´

oviles en el floema

I

Movimiento c´

elula a c´

elula

I Principalmente en par´enquima vascular, mayoritariamente del xilema

I Modelo quimio-osm´otico: gradiente de protones (ingreso) / potencial

de membrana (egreso)

I Ingreso

I Pasivo

I Activo (2H+_{-IAA simporte)}

I V´ıa predominante dependiendo de la concentraci´on de H+

I Egreso

I _{Principalmente pasivo a trav´es de transportadores (prote´ınas PIN)}

I Localizados selectivamente en las paredes distales del gradiente de

Transporte polar de auxinas

Cytosol ATP H+ IAA– ATP H+ ATP H+

1. IAA enters the cell either passively in the undissociated form (IAAH) or by secondary active cotransport in the anionic form (IAA–).

2. The cell wall is maintained at an acidic pH by the activity of the plasma membrane H+ -ATPase.

3. In the cytosol, which has a neutral pH, the anionic form (IAA–) predominates.

4. The anions exit the cell via auxin anion efflux carriers that are concentrated at the basal ends of each cell in the longitudinal pathway.

FIGURE19.13The chemiosmotic model for polar auxin transport. Shown here is one cell in a column of auxin-transporting cells. (From Jacobs and Gilbert 1983.) Shoot

apex

Seedling Hypocotyl

Apical end (A) Excised section Invert

A (donor) B (receiver) Transport into receiver takes place Basal end (B) B (donor) A (receiver) Transport into receiver is blocked Agar donor block containing radiolabeled auxin

FIGURE19.11The standard method for measuring polar auxin transport. The polarity of transport is independent of orientation with respect to gravity.

Principales procesos regulados por auxinas

I

Expansi´

on celular

I

Fototropismo y gravitropismo

Expansi´

on celular

I

Incremento de la extensibilidad de la pared celular

I

A trav´

es de:

I Activaci´on de H+-ATPasas

I Activaci´on de la s´ıntesis y secreci´on del gen codificante de

H+-ATPasas

I

ATPasas acidifican el apoplasto activando expansinas que relajan la

tensi´

on entre los componentes de la pared celular

Expansi´

on celular

Taiz y Zeiger, 2006 ATP H+ ATP H+ ATP H+ ATP ATP H+ H+ H+ H+ ATP H+ ATP H+ Protein processing Rough ER Golgi body

ATP

ATP

Expansin Plasma

membrane CELL WALL

NUCLEUS

Promoter H+_-ATPase

gene

Activation Activation hypothesis:

Auxin binds to an auxin-binding protein (ABP1) located either on the cell surface or in the cytosol. ABP1-IAA then interacts directly with plasma membrane H+_{-ATPase to}

stimulate proton pumping (step 1). Second messengers, such as calcium or intracellular pH, could also be involved.

ABP1 IAA ABP1 IAA IAA Synthesis hypothesis: IAA-induced second messengers activate the expression of genes (step 2) that encode the plasma membrane H+_-ATPase

(step 3). The protein is synthesized on the rough endoplasmic reticulum (step 4) and targeted via the secretory pathway to the plasma membrane (steps 5 and 6). The increase in proton extrusion results from an increase in the number of proton pumps on the membrane. Second messengers 1 3 2 4 5 6 mRNA

H+_-ATPase

in vesicle membrane Activation hypothesis

Expansi´

on celular

Taiz y Zeiger, 2006 Hemicelluloses Pectins Rhamnogalacturonan I(a pectin)

Structural protein Cellulose

microfibril

O O O

O O HO H H H H H H H H H O O OH O CH2OH

CH2OHO

O O

HO OH H

H

H

H H CH2OH O

HO OH H

O O O

O O

O O O

O O

O O O

O O HOH H H H H OH O CH2OH

O

O O O

O O

Cell wall

Cellulose microfibril

Hemicelluloses bound to the surface and entrapped within the microfibril

Crystalline domains 4 nm

(1→4)b-Glucan chains

Cellobiose repeating unit b-1→4 Glycosidic linkage

Expansi´

on celular

Sampedro y Cosgrove, 2005

K+

Tropismos

I

Fototropismo

I Mayor expansi´on celular en el lado oscuro

I Transporte de auxinas lateral hacia el lado oscuro

I Se˜nales percibidas por fototropinas

I

Gravitropismo

I Auxinas involucradas pero no est´a claro c´omo

Fototropismo

Intact seedling

(curvature) Tip of coleoptileexcised (no curvature) Opaque cap on tip (no curvature) Darwin (1880) Light 4-day-old oat seedling Coleoptile Seed 1 cm

Roots _{Boysen-Jensen (1913)}

Mica sheet inserted on dark side (no curvature)

Mica sheet inserted on light side (curvature)

Tip removed Gelatin between tip and coleoptile stump Normal phototropic curvature remains possible

From experiments on coleoptile phototropism, Darwin concluded in 1880 that a growth stimulus is produced in the coleoptile tip and is transmitted to the growth zone.

In 1913, P. Boysen-Jensen discovered that the growth stimulus passes through gelatin but not through water-impermeable barriers such as mica.

Fototropismo

Taiz y Zeiger, 2006 45°

Went (1926)

Tip removed Tip replaced

on one side of coleoptile stump

Growth curvature develops without a unilateral light stimulus

Coleoptile tips

on gelatin Tips discarded; gelatincut up into smaller blocks

Coleoptile bends in total darkness; angle of curvature can be measured Each gelatin

block placed on one side of coleoptile stump

IAA in gelatin block (mg/L)

C u rv at u re (d eg re es ) ₂₀ 15 10 5

0.05 0.10 0.15 0.20 0.25 0.30 Number of coleoptile

tips on gelatin

C u rv at u re (d eg re es ) ₂₀ 15 10 5

0 2 4 6 8 10

Paál (1919)

Fototropismo

Taiz y Zeiger, 2006 Coleoptile tip completely divided by thin piece of mica; no redistribution of auxin observed.

Coleoptile tip partly divided by thin piece of mica; lateral redistribution of auxin occurs. (A) Dark _{Corn coleoptile tip}

excised and placed on agar Agar block Curvature angle (degrees) (B) Unilateral light

No destruction of auxin

Undivided agar block Divided agar block

Unilateral light does not cause the photodestruction of auxin on the illuminated side.

Auxin is transported laterally to the shaded side in the tip. (D)

(C)

11.5 11.2

8.1 15.4 25.8

Gravitropismo

Taiz y Zeiger, 2006 Fotos (c) M.B. Wilkins

Gravitropismo

Taiz y Zeiger, 2006

M M C C P P Root tip Amyloplast Root tip Vertical orientation Horizontal orientation Uniform pressure on ER Unequal pressure on ER (C)

Amyloplasts tend to sediment in response to reorientation of the cell and to remain resting against the ER. When the root is oriented vertically, the pressure exerted by the amylo-plasts on the ER is equally distributed.

In a horizontal orient-ation the pressure on the ER is unequal on either side of the vertical axis of the root.

FIGURE19.30 The perception of gravity by statocytes ofArabidopsis. (A) Electron micrograph of root tip, showing apical meristem (M), columella (C), and periph-eral (P) cells. (B) Enlarged view of a columella cell, showing the amyloplasts resting on top of endoplasmic reticulum at the bottom of the cell. (C) Diagram of the changes that occur during reorientation from the vertical to the horizontal position. (A, B courtesy of Dr. John Kiss; C based on Sievers et al. 1996 and Volkmann and Sievers 1979.)

(A)

(B)

Statolith

Statolith

Gravitropismo

Taiz y Zeiger, 2006

(B)

(A)Vertical orientation

Hor izont al or ient ation

Elongation zone (flavonoid synthesis) IAA IAA IAA IAA IAA IAA IAA Root cap Cortex

Root cap cell (enlarged)

Statoliths Stele

IAA

1. IAA is synthesized in the shoot and transported to the root in the stele.

2. When the root is vertical, the statoliths in the cap settle to the basal ends of the cells. Auxin transported acropetally in the root via the stele is distributed equally on all sides of the root cap. The IAA is then transported basipetally within the cortex to the elongation zone, where it regulates cell elongation.

3. In a horizontal root the statoliths settle to the side of the cap cells, triggering polar transport of IAA to the lower side of the cap. 6. The decreased auxin

concentration on the upper side stimulates the upper side to grow. As a result, the root bends down.

4. The majority of the auxin in the cap is then transported basipetally in the cortex on the lower side of the root. 5. The high concentration

of auxin on the lower side of the root inhibits growth.

Dominancia apical

I

Auxinas directamente involucradas en dominancia apical

I

Mecanismo a´

un no dilucidado

I (Estudio de caso al final de la unidad)

I

Posible interacci´

on con alg´

un otro factor en los meristemos laterales

Dominancia apical

Taiz y Zeiger, 2006 FIGURE19.36 Auxin

sup-presses the growth of axillary

buds in bean (Phaseolus

vulgaris) plants. (A) The axillary buds are suppressed in the intact plant because of apical dominance. (B) Removal of the terminal bud releases the axil-lary buds from apical domi-nance (arrows). (C) Applying IAA in lanolin paste (contained in the gelatin capsule) to the cut surface prevents the outgrowth of the axillary buds. (Photos ©M. B. Wilkins.)

(A) (B)

Dominancia apical

Otros efectos

I

Retraso de la absici´

on de las hojas

I

Promoci´

on del crecimiento de frutas j´

ovenes

I

Desarrollo de ra´ıces laterales y adventicias

Efectos herbicidas

I

Auxinas de dif´ıcil o lenta degradaci´

on

I

Promoci´

on de crecimiento insostenible

I _{Adem´as promoci´on de s´ıntesis de etileno a altas concentraciones}

I

Menor sensibilidad en monocotiled´

oneas

I Propiedades herbicidas selectivas

I Monocots tienen mecanismos para conjugar r´apidamente estos

compuestos

I

Ejemplos

I 2,4D

I Dicamba

I Picloram

OH O O

Cl Cl

Dicamba 2,4D

ácido 2,4-diclorofenoxiacético Picloram