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PLANT HORMONES Cheat Sheet (DRAFT) by

Plant HORMONES/phytohormones, are chemical messengers that are produced by plants and regulate various physiological and developmental processes

This is a draft cheat sheet. It is a work in progress and is not finished yet.

HORMONE

Greek: hormon
to excite
Naturally occurring, signalling molecules that exert a profound influence on physio­logical processes
Produced in tiny amounts by one part of an organism's body and transp­orted to other parts, where it binds to a specific receptor and triggers responses in target cells and tissues.

Phytoh­ormones

Principal means of interc­ellular commun­ication within plants
Produced within plants, and are effective at extremely low concen­tra­tions
Transp­orted to different parts of the plants to perform various physio­logical functions
Plant hormones control growth, flowering, fruiting, aging, and even death.
Effect of a particular hormone is concen­tration dependent; hormones may have different effects at different concen­tra­tions.
Like animal hormones, plant hormones affect target cells via receptor proteins.
Plants regulate levels of hormones by altering precur­sors, transport, inacti­vation, breakdown, or storage.

TYPES OF PHYTOH­ORMONES

auxin
gibber­ilins
cytokinins
Ethylene
Abscissic Acid
Brassi­nos­teroids
Strigo­lac­tones

AUXIN

AUXIN: THE GROWTH HORMONES
Auxin was the first plant hormone to be discov­ered.
Greek word auxein
means "to increase or to grow.“
Enlarg­ement of plant cells.
Indole­-3-­acetic acid (IAA)
is the most widely distri­buted natural auxin
Promotes production of
Shoot apical meristems
 
young leaves
 
root tips
 
germin­ating seeds
 
fruits

PHYSIO­LOGICAL EFFECTS OF AUXIN

Auxins promote cell elongation of stems and coleop­tiles
Coleoptile
is the pointed protective sheath covering the emerging shoot of monocots
Photot­ropism is mediated by the lateral redist­rib­ution of auxin
Gravit­ropism involves lateral redist­rib­ution of auxin
Auxin promotes apical dominance
Auxin promotes the formation of lateral and advent­itious roots
Auxin delays the onset of leaf abscision
When the level of auxin declines, a special layer of cells — the abscission layer — forms at the base of the petiole.
Auxin promotes fruit develo­pment

Gibber­ellins

Gibber­ellins: Regulators of Plant Height
discovered by
Ewiti Kurosawa
Causes
Internodal elongation known as the ‘bakanae’ or ‘foolish seedling’ disease of rice
Isolated from
fungus (Gibbe­rella fujikuroi)
Stimulate stem elongation
Promotes PRODUCTION of:
Meristems of apical buds and roots,
 
young leaves
 
developing seeds

PHYSIO­LOGICAL EFFECTS OF GIBBER­ELLINS

Gibber­ellins Stimulate Stem Growth in Plants.
Gibber­ellin applic­ation results in bolting (stem growth)
"­Foolish rice" seedlings, suffer from an overdose of gibber­ellins normally found in plants in lower concen­tra­tions
Gibber­ellins promote fruit set and parthe­nocarpy
GA promote early seed develo­pment and germin­ation.
Gibber­ellins mobilize nutrients during seed germin­ation

Cytokinins : Regulators of Cell Division

Discovered in the search for?
factors that stimulate plant cells to divide
The most common natural cytokinin is?
Zeatin,
because it was discovered first in?
(Zea mays)

PHYSIO­LOGICAL EFFECTS OF CYTOKININS

Auxin :Cytokinin regulates root and short initiation in callus tissues
Cytokinin stimulates the Growth Of Axillary Buds
Cytokinins Delay Leaf Senescence
Leaf senescence is delayed in a transgenic tobacco plant containing a cytokinin biosyn­thesis gene, ipt. The ipt gene is expressed in response to signals that induce senesc­ence.
 

Ethylene: The Gaseous Hormone

Discovered in the early
1900s
as a ?
fruit ripening
Not required for?
normal vegetative growth
Synthe­sized primarily in?
in response to stress and may be produced in large amounts by tissues undergoing senescence or ripening
Promotes production of?
Fruit ripening
 
Senescence
 
Leaf abscission
 
Wounds and stress

PHYSIO­LOGICAL EFFECTS OF Ethylene

Ethylene stimulates fruit ripening.
Fruits that ripen in response to ethylene exhibit a charac­ter­istic respir­atory rise called climac­teric before the ripening phase
Ethylene triggers ripening, and ripening triggers more ethylene produc­tion-a rare example of positive feedba­ckm­ech­anism
As apples ripen, they release ethylene. Over- ripened apples release the hormone in high amounts, causing other apples stored nearby to ripen faster and rot sooner.
Ethylene promotes senescence and leaf abscis­sion.
 
auxin from the leaf prevents abscission
 
the amount of auxin from the leaf decreases and the ethylene level rises
 
Synthesis of enzyme that hydrolyze the cell wall polysa­cch­arides, resulting in cell separation and leaf abscission
Ethylene instigates triple response
slowing of stem elongation
 
thickening of the stem
 
Curvature that causes the stem to start growing horizo­ntally
Ethylene regulates epinasty

Abscisic Acid

Abscisic Acid: A Seed Maturation and Antistress Signal
Accumu­lates as a response to stressful enviro­nmental condit­ions, such as dehydr­ation, cold temper­atures, or shortened day lengths

PHYSIO­LOGICAL EFFECTS OF AA

Abscissic acid induces seed and bud dormancy
ABA induces dormancy in seeds by blocking germin­ation and promoting the synthesis of storage proteins
 
ABA accumu­lates in dormant buds as an adaptive feature in cold climates
ABA Closes Stomata in Response to Water Stress
ABA binding leads to influx of Calcium and the opening of potassium channel
 
Potassium ions exits the guard cells and water follows. Guard cells become flaccid, closing the stomatal aperture

Strigo­lac­tones

Strigo­lac­tones are signaling compounds made by plants.
2 Main functions:
as endogenous hormones to control plant develo­pment
 
as components of root exudates to promote symbiotic intera­ctions between plants and soil microbes.
Some plants that are parasitic on other plants have establ­ished a third function, which is to stimulate germin­ation of their seeds when in close proximity to the roots of a suitable host plant.

Brassi­nos­teroids

Brassi­nos­teroids (BRs) as a class of steroid plant hormones partic­ipate in the regulation of numerous develo­pmental processes, including root and shoot growth, vascular differ­ent­iation, fertility, flowering, and seed germin­ation.
Brassi­nos­teroids (BR) and gibber­ellins (GA) promote seed germin­ation of these species and counteract the germin­ati­on-­inh­ibition by abscisic acid (ABA).