Plant Hormones 101

MUPGRET Workshop

What are hormones?

“a group of naturally occurring organic compounds that influence physiological processes at low concentrations.”

“a substance that is transferred from one part of an organism to another.”

From: Plant Hormones: Biosynthesis, Signal Transduction, Action! 2005. Peter Davies, ed.

From: Went and Thimann. 1937. Phytohormones.

Hormones

Can be synthesized locally or transported to their site of action.

They differ from mammalian hormones in this respect.

The first hormone

Auxin Fits the original definition of a

hormone. Transported from site of synthesis to site

of action.

Sachs

Hypothesized root forming and flowering forming compounds move through the plants and cause morphological changes in 1880.

Darwin

First to observe phototropism, the bending of a plant to light.

Also in 1880. Hypothesized that redistribution of a

chemical within the young seedling caused it to bend toward the light.

Auxin

Darwin’s compound that caused phototrophism.

Isolated several years later by extracting the chemicals that diffused from cut coleoptiles into blocks of agar.

More about this compound tomorrow when Dr. Hagen talks.

Classical Plant Hormones

Auxin Gibberellins Cytokinins Abscisic Acid Ethylene

New generation plant hormones

Brassinosteriods Salicylic acid Peptides

Polyamines

http://www.plant-hormones.info/Index.htm

Auxin

Indole-3-acetic acid (IAA) and its conjugates.

Synthesized from tryptophan or indole in leaf primodia, young leaves and developing seed.

IAA Structure

Auxin Transport

In vascular cambium and procambial strands.

Possibly between epidermal cells. Transport to root may occur through

phloem.

Auxin Stimulates

Cell enlargement Stem growth Cell division in

cambium Phloem and xylem

differentiation

Root initiation Root branching Phototropism Floral organ growth

Auxin functions to

Delay leaf senescence Promote apical dominance Delay fruit ripening

Auxin mutant

Brachytic 2 (br2) mutant in maize.

Gibberellin

Family of 125 compounds.

Gibberellin acid (GA3)is the most common.

Synthesized in young shoots and developing seed.

Gibberellins

Chloroplast is the initial site of synthesis.

Transported in phloem and xylem.

Gibberellin promotes

Stem elongation Cell division and elongation in stems Germination if cold or light treatment is

required. Enzyme production, eg. α-amylase. Fruit set

Gibberellin mutant

•Dwarf 8 (D8) mutant in maize.

•The same gene in wheat is responsible for the Green Revolution.

•Introduction of this gene into cultivated wheat earned Norman Borlaug the Nobel Prize.

Cytokinins

Adenine derivatives.

Zeatin is the most common.

Synthesized in root tips and developing seed.

Transported in xylem.

Cytokinin promotes

Cell division if auxin is present. Photomorphogenesis, eg. Crown gall

formation Lateral bud growth Leaf expansion by cell enlargement Stomatal opening Chloroplast development

Cytokinin delays

Leaf senescence

Ethylene

H2C=CH2 Gas Synthesized from

methionine Most tissues can

synthesize ethylene in response to stress.

Chemical Structure

Ethylene

Transport occurs by diffusion. Not absolutely required for growth.

Mutants with non-functional ethylene gene develop normally.

Ethylene causes

The triple response in dark Reduced stem elongation Stem thickening Lateral growth

Ethylene stimulates

Defense response to wounding or disease

Release from dormancy Shoot growth and differentiation Root growth and differentiation Adventitious root formation Leaf and fruit abscission

Ethylene stimulates

Flower opening Fruit ripening

Abscisic Acid

Synthesized from glyceraldehyde-3-phosphate through carotenoid pathway.

Synthesized in roots, mature leaves, and seeds.

Abscisic Acid

Synthesis increases in response to drought.

Transported from roots in xylem. Transported from shoots in phloem.

ABA stimulates

Stomatal closure Root growth under water stress. Storage protein synthesis in seeds. Breaking dormancy Defense response

ABA inhibits

Shoot growth under water stress.

Abscisic Acid Mutant

viviparous 5 (vp5) mutant in maize.

Polyamines

Aliphatic amines. Putrescine, spermidine

and spermine are most common.

Effective at low concentrations.

Mutants have abberant development.

Spermidine structure

H2N

HN NH2

Brassinosteroids

Sixty steroidal compounds.

Effective at low concentrations.

Brassinolide structure

Brassinosteriods stimulate

Cell division Cell wall loosening Vascular differentiation Ethylene biosynthesis

Brassinosteroids

Required for fertility Inhibit root development and growth

Jasmonates

Jasmonic acid is most common

Methyl esters Scented

Methyl jasmonate

Jasmonate function

Induce tuberization Important in plant defense Inhibit growth Inhibit germination Promote senescence Promote pigmentation

Salicylic Acid

Synthesized from phenylalanine.

Promotes production of pathogenesis related proteins.

Can reverse effects of ABA in some cases.

Signal Peptides

Small molecules that can be transported throughout the plant and effect development.

Involved in defense response Help determine cell fate Involved in self-incompatibility Involved in nodule formation in

legumes