Plant Hormones

Plant Hormones There are five major types of plant

hormones: Gibberelins Cytokinins Ethylene Abcisic Acid Auxins

The structure and function of each type of hormone will be described

Gibberellins

Overview

Gibberellins (GAs) regulate and influence: cell elongation seed germination dormancy flowering sex expression enzyme induction leaf and fruit senescence.

Germination

Signal starch hydrolysis through inducing the synthesis of the enzyme α-amylase in the aleurone cells

Gibberellins produced in the scutellum diffuse to the aleurone cells where they stimulate the secretion α-amylase

α-Amylase then hydrolyses starch into glucose

Gibberellins cause higher levels of transcription of the gene coding for the α-amylase enzyme

Gibberellins: Chemical Structure

Gibberelins have complex ring structures Typically contain carboxylic acid groups Many specific gibberelins exist

Numeric naming system (i.e. GA#) May be classified into two structural

types: C-19 Gibberelins (19 carbon) C-20 Gibberelins (20 carbon)

Gibberellins: Chemical Structure

Type 1: 19 Carbon Gibberelins

OH

O

O

O

A C-19 Gibberellin GA9

Gibberellins: Chemical Structure

Type 2: 20 Carbon Gibberelins

O

HO

O

HO

A C-20 Gibberellin

GA12

Cytokinins

Cytokinins Cytokinins

Found in a variety of plants and have many functions

Synthesized in meristematic tissues in roots and transported to aboveground organs

Regulate growth and development of tissue primarily by promoting cell division Involved in germination, shoot differentiation, leaf

senescence Interacts with other plant hormones for some

functions

Cytokinins Function

Regulates apical dominance and lateral root initiation

Slows down senescence (plant aging) and chlorophyll degradation in aging leaves

Regulates growth of dicot seedlings in the dark (in combination with ethylene)

Involved in development of sex organs and male sterility

Synthesized in meristematic tissues in roots and transported to aboveground organs

Cytokinins

Cytokinins contain adenine:

Two structure types: Isoprenoid

Isoprene structural units: Aromatic

Contain aromatic groups

N

NNH

N

Cytokinins: Isoprenoid

N

NNH

N

HNCH2

H2C

OH

CH3

Zeatin

N

NNH

N

HNCH2

H2C

OH

CH3

H

H

Dihydrozeatin

Isoprene units

adenine

Cytokinins: Aromatic

N

NHN

NHN

benzyladenine

adenine

Aromatic group

Cytokinins: Aromatic

CH2

NH

N

NN

HN

HO

orthotopolin

adenine

Aromatic group

Ethylene

EthyleneEthylene

Universally produced by all plants Angiosperms, Gymnosperms, Ferns, Mosses,

Liverworts Also found in some fungi, yeast and

bacteria Important roles in:

Abscission Germination Senescence Stress response to pathogens

Ethylene and Fruit Ripening

Helps fruits go through color change, softening of walls, conversion of starch to sugar

Ethylene is produced in low amounts throughout plant life some “climacteric” plants

have sudden peaks in ethylene synthesis which signals ripening changes

Ethylene gas is sprayed on fruit crops to ripen at same time

Some stress situations trigger ethylene production exposure to heat/cold physical damage attack by fungal or bacterial pathogens flooding that limits oxygen

Similar to Abscisic acid’s stress response

Ethylene and Stress

Growth and Messaging

Ethylene and growth Promotes root growth and root hair growth Can cause asymmetric growth in stems and

leaves Ethylene regulates seedlings’ horizontal growth

& apical hook formation…= “Triple response” of seedlings grown in dark

Can act as second messenger Auxin, cytokinin can cause ethylene production

in seedlings

Ethylene’s “triple response”Ethylene’s “triple response”

Apical hook formation

Ethylene: Chemical Structure

Ethylene is a very small, simple molecule compared to other plant hormones

Two carbons sharing a double bond Ethylene is a gas at room temperature

C C

H

H

H

H

Ethylene

Abscisic Acid

Abscisic Acid (ABA)Abscisic Acid (ABA)

Found universally in plants and algae Many functions! Important roles in:

plant development bud & seed dormancy Germination cell division leaf senescence Abscission cellular response to stress

Abscisic Acid

Acts as a general inhibitor of growth and metabolism Inhibits growth in hypocotyls, epicotyls,

leaves, coleoptiles Seed dormancy

ABA promotes seed dormancy so plant seeds can withstand desiccation

ABA as a Stress Hormone ABA increases with various

environmental or biological plant stresses Excess heat, pests, excess salt

and/or dehydration Wilted plants have high levels of

ABA In a drought, ABA increases in

some plants, causing the stomata to close, preventing water loss

ABA can also produces osmolytes that protect cell membranes from dehydration

Abscisic Acid Chemical Structure

Abscisic acid is a carboxylic acid

Abscisic Acid

CH3

OH

O

CH3H3C

O OH

Carboxylic acid

Auxins

It’s All in the Name

“Auxins” from the Greek word αυξανω = "I grow or increase".

They were the first of the major plant hormones to be discovered.

Overview

essential for cell growth affects both cell division and cellular

expansion. may promote axial elongation (as in

shoots), lateral expansion (as in root swelling), or isodiametric expansion (as in fruit growth)

auxin-promoted cellular expansion occurs in the absence of cell division.

auxin-promoted cell division and cell expansion may be closely sequenced within the same tissue (root initiation, fruit growth)

Important Functions

coordination of many growth and behavioral processes in the plant life cycle

stimulate or inhibit the expression of specific genes.

coordinate development at all levels in plants, from the cellular level through organs and ultimately the whole plant.

Master Hormone

indole-3-acetic acid (IAA). the most important member of the

auxin family the most potent native auxin generates the majority of auxin

effects in intact plants

Working Together

patterns of active transport are complex

typically act in concert with, or in opposition to other plant hormones

auxins and other plant hormones nearly always interact to determine patterns of plant development.

Auxin Shared Functions

stimulates cell elongation by stimulating wall loosening factors, such as elastins, to loosen cell walls (with gibberellins)

stimulates cell division (with cytokinins) applied to callus, rooting can be

generated (with cytokinin) xylem tissues can be generated (with

cytokinins)

More Auxin Shared Functions

promotes femaleness in dioecious flowers (with ethylene)

inhibits or promotes leaf and fruit abscission (with ethylene)

stimulate cell division in the cambium andin tissue culture (with cytokinins)

Auxin Functions

Stimulate cell elongation

stimulate differentiation of phloem and xylem

Stimulate root initiation on stem cuttings and lateral root development in tissue culture

mediate the tropistic response of bending in response to gravity and light

suppresses growth of lateral buds

delay leaf senescence

More Auxin Functions

can induce fruit setting and growth in some plants

involved in assimilate movement toward auxin, possibly by an effect on phloem transport

delay fruit ripening promote flowering in Bromeliads stimulate growth of flower parts stimulate the production of ethylene at high

concentrations inhibit growth by closing the stoma during

water stress.

Auxins: Chemical Structure

Many naturally occurring auxins exist, along with many synthetic auxins used in agriculture

Most naturally occurring auxins contain an indole ring group or a phenyl group

Auxins (natural and synthetic) are carboxylic acids

Halides are also seen in both natural and synthetic auxins

Naturally Occurring Auxins

NH

O

OH

indole-3-acetic acid

NH

Cl

OH

O

4-chloroindole-3-acetic acid

Carboxylic acid

=IAA, the most important member of the auxin family

Naturally Occurring Auxins

phenylacetic acid

OH

O

Synthetic Auxins

NH

O

OH

indole-3-butyric acid

napthalene acetic acid

O

OH

Synthetic Auxins

O

Cl

Cl

OHO

2,4-dichlorophenoxy-acetic acid

Ether linkage

halogens

SourcesSources Wikipedia, Auxin, 2010, http://en.wikipedia.org/wiki/Auxin Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston:

Benjamin-Cummings Company, 2001. Delker, C., Raschke, A. and Quint, M., 2008, Auxin dynamics: the

dazzling complexity of a small molecule’s message, Planta, vol 227, 929-941.

Gibberellins: A Short History, from http://www.plant-hormones.info, the home since 2003 of a website developed by the now-closed Long Ashton Research Station

Wikipedia, Gibberellin, 2010, http://en.wikipedia.org/wiki/Gibberellin Koning, Ross E. 1994. Auxins. Plant Physiology Information Website.

http://plantphys.info/plant_physiology/auxin.shtml. (4-7-2010). Litwak, G. 2005. Plant hormones. Elsevier Academic Press: San Diego,

CA. Raghavan, V. 1997. Molecular embryology of flowering plants.

Cambridge University Press. New York, NY. Srivastava, LM. 2002. Plant growth and development: hormones and

environment. Elsevier Science: San Diego, CA. http://www.plant-hormones.info/auxins.htm the home since 2003 of a

website developed by the now-closed Long Ashton Research Station

Photo credits: http://humankinetics.files.wordpress.com/2009/07

/fresh-fruit.jpg http://www.nature.com/emboj/journal/v22/n6/

thumbs/7595043f4.jpg http://plantphys.info/plant_physiology/images/

tripleresponse.gif http://farm4.static.flickr.com/

3657/3513022448_e7bb1c305e_m.jpg http://www.hiltonpond.org/images/

FreezeHackberry01.jpg