Chapter 8 - Plant Hormones

CHAPTER 8 PLANT HORMONES Biochemistry and Metabolism

Plants Hormones :  Means intercellular comunication within plants is mediated by the action of chemical messengers 

Signal molecules that individually or cooperatively direct the development of individual cells, or /@

 Carry information between cells and thus coordinate growth and development  Naturally occurring organic substances that at low concentration will influence physiological process  Site of synthesis not clearly localized  More diffuse, cannot always localized to discrete organs

Types of plant hormones  5 recognized groups of plant hormones : Auxins; Gibberellins; Cytokinins; Abscisic acid; and Ethylene  More recently a 6 groups : Brassinosteroids; Polyamines; Jasmonic acid; and Salicylic acid

(i) AUXIN  

First plant hormone to be discovered Play a major role : i. in the regulation of plant cell elongation ii. in the growth responses of plants to undirectional stimulus, it known as tropism

 Natural auxins :  Indole-3-acetic acid (IAA)  4-chloroindole-3-acetic asid  Phenylacetic acid  Indole-3-butyric acid (IBA)

Synthetic auxins:  Naphthaleneacetic acid (NAA)  2-Methoxy-3,6-dichloro-benzoic acid (dicamba)  2,4-Dichlorophenoxyacetic acid (2,4-D)  2,4,5-Trichlorophenoxy-acetic acid (2,4,5-T)  Amount of IAA present depend on a number of factors :



(i) Type of tissue (ii) Age of tissues (iii) Stage of growth Concentrations exceeding the optimum characteristically result in reduced growth

 Auxin concentration is high enough ⇨ growth inhibited compared with control  Exogenous hormone ⇨ do not show a significant response  Endogenous hormone ⇨ content at intact tissues enough to support elongation  Effect of exogenous supplied auxin only in tissues that have been removed from the auxin supply (such as excised segments of stem and coleoptile)

Auxins : Chemistry, Metabolic and Transport 

1870s ; Darwin and son Francis studies plant growth phenomena of the growth stimulus in Avena sativa (oat) coleoptile

 Auxin regulated cell enlargement in excised tissues such as coleoptile  If the tip of a coleoptile removed ⇨ coleoptile growth ceased. They found that the tip of the coleoptile preceived light  If their covered the tips  With metal – No growth toward light  With glass – Growing towards light  # coleoptile growth towards light is controlled by coleoptile tip

 Went’s discovery : Growth promoting substance from excised coleoptile tips would diffuse into agar block  Blocks could the used to restore growth in decapitated coleoptile  If agar block containing auxin was placed on one side of coleoptile stump ⇨ coleoptile bent away from the side containing the block  Curvature occurred because: i. the increase in auxin on one side stimulated cell elongation; and ii. decrease in auxin on the other side caused a descrease in the growth rate Refer to fig 16.1 page 400 (Taiz /Zieger)- Plant Physiology

Transport of auxins  Depend on the developmental stage of theorgan/tissue of plant  Transport of hormone into or out of tissue/organ influence level of active hormone within tissue/organ  Hormone (Auxin) transport of plant is the polarity of movement  Polar transport expressed as movement in one direction  This indicate that polarity is not driven by external forces (gravity etc.) but the property of cells themselves

 When movement is preferentially away from the morphological apex toward the morphological base of the transporting tissue, the direction of movement described as basipetal  Movement in the opposite direction, that it toward the morphological apex referred as acropetal  When stem or coleoptile is inverted, original direction is maintained # Refer to Fig 16.7 page 349 textbook

Fig. 16.7 Polarity in auxin transport in Avena sativa coleoptile segment Donor block

14

C-IAA

14C-IAA

A B

Receiver block

B A

A

B

A B

B A

14C-IAA

14C-IAA

Normal orientation

Inverted sections

Physiologial Effects of Auxin  Auxin has a variety of effect s on plant growth and morphogenesis such as :  promote the elongation growth of stem and coleoptile (however, it inhibit root elongation)  promote cell division in stems (but inhibit in lateral buds)  Development of fruit

 Effect of auxin depend of factor, including (i) developmental stage of tissue or organ (ii) concentration of auxin (iii) type of auxin (natural @ synthetic) (iv) involvement of other plant hormones (v) use of intact versus excised tissue for experiment

1. Cell elongation/enlargement  Stimulate cell elongation in excised tissue (coleoptile)  Auxin concentration response curve show increasing response with increasing concentration of auxin an optimum concentration is reached  Concentration exceeding the optimum ⇨ rduced growth  Concentration high ⇨ growth inhibited

2. Increases the extensibility of the cell wall  Increase in cell wall extensibility in coleoptile and young developing stem  Auxin acts at the plasma membrane @ within the cell  Acid growth hypothesis proposed that auxin activates ATPproton pump located in plasma membrane.  Acid-growth hypothesis to explains auxin stimulated plant cell elongation and enlargement  According to the hypothesis, auxin causes responsive cells to extrude proton (H+)actively into the cell wall regions ⇨ decrease pH activates wall-loosening enzymes that promote the breakage of cell wall bond ⇨ increase wall extensibility

3. Growth responses to directional stimuli  Auxin mediate effect of light and gravity on plant growth  Plant oriented to the environment ⇨ leaves (houseplant) facing to window; roots growing toward the earth  Tropic responses i.e. growth in response to directional light ⇨ phototropism or gravity ⇨ gravitropism  Toward light ⇨ +ve phototropism  Away from light ⇨-ve phototropism

4. Inhibits the growth of lateral buds @ Apical Dominance  Removal of shoot apex, stimulate growth one @ more lateral bud  Auxin from apical bud inhibits growth of lateral (axillary) bud

5. Promote the formation of lateral roots

 Elongation growth of root inhibited by auxin, but initial of lateral (branch) root and adventitious root is stimulated by high auxin level  The dividing cell by auxin form a root apex and lateral root grow through root cortex and epidermis  Adventitious root develop from a part of the plant @ other than the normal form of root branching

6. Delay the onset of leaf abscission  Process of shedding organs such as leaves, flower and fruit; known as abscission  Abscission occurs in region called abscission zone; located near the base petiole.  Organ ages ⇨ cell wall in abscission layer weaken and separate  Leaf aging called leaf senescence. The cell wall in abscission layer digested which cause them to become soft and weak  The leaf break off at the abscission layer due to stress  Abscission dependent on concentration of auxin on either side of the abscission layer  As leaf ages release of auxins decline, inducing the ethylene synthesis ⇨ abscission stimulated

Environmental stress eg. Water stress, nutrient deficiency reduce IAA Ethylene produced at the abscission layer senescence

Cell wall expand and suberized Abscission layer

cellulase & pectinase are produced Middle lamela degraded Cells separated, leaves/fruits shedd

7. Regulates fruit development  Auxin produced in pollen, endosperm and embryo of developing seeds  Involve in the initial stimulus for fruit growth from pollination  After fertilization, fruit growth depend on auxin produced in developing seed  In some species ⇨seedless fruits produced naturally @ by treating the unpollinated flower with auxin called parthenocarpy  Ethylene known to influence fruit development  The effect of auxin on fruiting mediated through the promotion of ethylene synthesis

GIBBERELLINS  Gibberellins members of terpenoids  Gibberellins; assigned as GA  most important in higher plants  GA3 extracted from fungal culture  3 principal sites of gibberellin biosynthesis : (i) Developing seeds (developing endosperm, cotyledon, scutellum) (ii) Developing fruits (iii) Young leaves of developing apical bud and elongating shoots (iv) Apical region of root

PHYSIOLOGICAL ACTION OF GIBERELLINS 1.Stimulate hyperelongation of intact stems  Occurs especially in dwarf and rosette plants  Promote elongation in intact plants rather than excised tissues  Study on dwarf mutants of Oryza sativa, Zea mays, Phaseolus vulgaris  These mutants exhibit the normal phenotype when treated with GA3  GA treated enhanced internode elongation

 The role of gibberellin in stem elongation come from the study of rosette plants  Rosette plants ⇨ compact growth habit, closely spaced leaves  Failure of internode elongation may result from a genetic mutation @ environmentally induced  Environmentally limited rosette plant (eg. Brassica sp. and Spinacea oleraceae) generally do not flower in the rosette form  Before flowering, plants undergo extensively internode elongation; known as bolting  Bolting, due to environmental signal eg. photoperiod or low temperature  Bolting in rosette plants can be induced by exogenous application of GA

2. Seed germination  Cereal grain seeds like Hordeum vulgare consists of embryo and non embryonic regions  Embryo region synthesized GA and induced release of α-amylase to hydrolyze endosperm (starch)  Non embryonic region if treated with GA will stimulate to produce α-amylase at high concentration

3. Stimulate mobilization of nutrient reserve during germination  Occurs of cereal grain  During imbibition water is absorbed by seed to ensure GA secretion  GA moves from the embryo to the aleurone where stimulated α -amylase secretion (α -amylase synthesized) and synthesis protease enzyme  Aleurone ⇨ a layer of cell surrounding the endosperm in seed  Protease converts an inactive β -amylase to the active form  α -amylase and β -amylase together digest starch to glucose ⇨ which mobilized to meet the metabolic demands of the growing embryo

β -amylase (inactive)

aleurone endosperm

β -amylase (active) starch α -amylase

protease

glucose

scutellum coleoptile

GA

plumule radical

embryo

4. Flowering Flowering is induced by gibberellins Eg. Pharbitis nil, Chenopodium rubrum will flower immediately  Many perennial plants capable of flowering after pass through a juvenile phase  Gibberellins overcome juvenile phase in many conifers and stimulate precocious flowering  Gibberellins promote maleness in unisexual flowers while auxin promote femaleness  

5. Inhibition of gibberellins biosynthesis  Growth of stem can be inhibited or reduced by synthetic chemical that block gibberellin biosynthesis i.e growth retardant or anti gibberellins (AMO-1618, cycocel, Phosphan-D, ancymidol)  These compound has commercial application in production of ornamental plants  Effects: ⇨ To reduce stem elongation ⇨ Results in shorter and compact plants with darker green foliage  Spraying the plant (Wheat) with antigibberellin produce a shorter, stiffer stem and preventing lodging

CYTOKININS  Cytokinin- adenine derivatives  Kinetin- first compound found with cytokinin activity  Synthetic cytokinin prepared by heating DNA  Zeatin- first natural cytokinin discovered and most widespread  Isopentenyl adenine (iP)  Dihydrozeatin (diHZ) : leass active than zeatin  Benzyl adenine (BAP)

1.Cytokinins regulate cell division  Cytokinin is major factor in regulating cell division in the presence of auxin  Have capacity to initiate division in plant cells and in quiescent or non dividing cells (tissue culture)  Initiate cell division by controlling cell cycle at two points  1. Catalyze transition from G2 phase to mitosis  2. Control G1 to S phase transition

2. Stimulate cell proliferation  In the case of neoplastic (tumorous growth)  Bacterium Agrobacterium tumefaciens pathogens that causes tumorous growth on stems known: crown gall  Crown gall tissue can be excised and maintained on simple medium without hormone  Have capacity to synthesis auxin and cytokinin  When bacteria invade host tissue, it transfer these genes  Genes replicated in host cell  Produced elevated level of auxin and cytokinin  Stimulate neoplastic growth

Crown gall

2. Organogenesis  Cytokinin and auxin stimulate organogenesis: organ formation  Development of shoot and root  Cell culture growth required cytokinin and auxin  High Cytokinin/auxin ration stimulate root formation  Low cytokinin/auxin ration stimulate shoot formation

3. Senescence Mature leaves and fruits express senescence Senescence: breakdown of protein, nucleic acids, other macromolecules, loss of chlorophyll, accumulation of amino acids  Cytokinins will delay senescence while ethylene promote senescence  Cytokinins direct nutrient mobilization and retention by stimulating metabolism in the area of application  Creates a new sink: area that attract metabolites from region of application  

senescence

Experiment : role of cytokinins in nutrient mobilization Control

Treatment with cytokinins kinetin

kinetin

radioactive

C A Radioactive spreads into vascular tissue for export through petiole

B Radioactive accumulates in the area treated with kinetin

Radioactive retain near poin of application

-Radioactive labeled nutrient are fed to the plant -In control (A) radioactive spreads into vascular tissue for export through petiole -In treatment (B) one part/half of leaf is treated with cytokinin -Radioactive accumulates in the area treated with kinetin -In other treatment (C) cytokinin applied on part of leaf (right) -Radioactive retain near point of application -Cytokinins direct nutrient mobilization and retention by stimulating metabolism in the area of application -creates a new sink area that attract metabolites from region of application

Other effects of cytokinins

 Stimulate cell enlargement  Regulate vascular differentiaiton  Promote axillary bud and release apical dominance

Abscisic Acid       

Abscisic acid is a single compound Occur in mature, green leaves Synthesized in cytoplasm of leaf mesophyll cell and accumulated in chloroplast Able to move quickly out of leaves sink tissues Actions: induce storage protein synthesis during seed development Regulating stomatal closure during water stress Also involved in regulating abscission and bud dormancy

ABA physiological effects 1. Stomatal closure  In drought, leaves will synthesized high level of ABA  Allowed stomata closure  Water will be stored during drought/water stress

2. Bud/seed dormancy  Woody plants in temperate zone  ABA concentration maximum in early winter and low end of winter  ABA prevents bud development and seed germination

 ABA actions antagonistic with other hormone:  Inhibits amilase which produced by seed treated with giberellins  Promote chlorosis which have been inhibited by cytokinins  Inhibits cell wall elasticity and cell enlargement by IAA

ETHYLENE Simple hydrocarbon gaseous : H2C=CH2 Not required for normal vegetative growth Synthesized primarily in response to stress Produced in large amounts by tissues undergoing senescence or ripening  Occurs in all plant organs    

Physiological effects of ethylene Vegetative development  Stimulate elongation of stems, petioles, roots and floral structure of aquatic and semiaquatic plants  Ethylene promotes gibberellin synthesis in rice to promote root and shoot elongation  While in peas, root and shoot elongation inhibited by ethylene  Stimulate abnormal growth response such as swelling of stem tissues and downward curvature of leaves (epinasty)

Fruit Ripening  Stimulate fruit ripening: banana, apple avocado etc..  Ethylene is autocatalytic  release of ethylene gas by ripening fruits  will in turn stimulate premature climacteric  and ethylene production by other fruits stored near  Number of qualitative metabolic changes are initiated in fruit

Changes in fruit ripening:  During ripening promote production of sugars, which increase sweetness (breakdown of starch and acid) and odor  Induced rupture of cell membranes and water loss from tissues: increase cell wall softening by the action of enzymes  Involved breakdown of chlorophyll and synthesis of pigments  Synthesis of flavor

Ethylene has important commercial uses

 Storage facilities developed to inhibit ethylene production and promote preservation of fruits have a controlled atmosphere of low O2 concentration and low temperature that inhibits biosynthesis  High concentration of CO2 (3-5%) prevents ethylene action

 Low pressure to remove ethylene and oxygen from storage chamber  Use of inhibitors of ethylene action  Such as CO2 and Ag+ (silver)  Will delay or prevent ripening  Ethylene has high diffusion rate  Difficult to apply in a gas form  Spray ethylene releasing compound such as ethepon  When taken up by plant tissue, ethepon is converted to ethylene

Brassinosteroids  Steroid hormones  Chemical structure similar to animal steroid hormones  Brassinolides most biologically actve  Functions: brassinosteroids promote stem elongation in mutant plants, shoot elongation and ethylene production  Inhibits root growth and development