Phytohormones And Their Functions 3s1h29

Chapter 11 – Plant responses to hormones & environmental stimuli  Responses

include

– Developmental transitions Dormancy Germination Flowering

– Growth

Hormones & environmental signals involve signal transduction pathways

Internal and external signals

Hormones influence gene expression  Gene

expression regulated by – microRNAs – transcription factors

Plant hormones & growth

(abscisic acid)

Hormones interact to promote/inhibit development

Auxins

tryptophan

Responses involving auxin  Phototropism  Gravitropism  Cellular

elongation  Initiation of leaf primordia  Apical dominance  Root development  Fruit development

Tropisms  Permanent,

directional growth in response to an external stimulus – Positive tropisms – Negative tropisms

Phototropism  Stems

are positively phototropic

 How

can plants grow towards light?

Auxin and cell elongation

Acidified cell walls have increased elasticity

Phototropism research Phototropin (NPH1) and phototropism -initiates a signal transduction pathway -nph1 mutants non-phototropic

Gravitropism

Gravitropism

How can it be demonstrated that auxin is involved in gravitropism?

Gravitropism and root cap amyloplasts



Gravity regulated auxin transport, Ottenschlaumlger, Iris et al. (2003) Proc. Natl. Acad. Sci. USA 100, 2987-2991

Auxin and initiation of leaf primordia



Pin mutant link

Responses involving auxin  Apical

dominance

Responses involving auxin Formation of adventitious roots

Auxin produced by seeds promotes ovary tissue growth

Plant hormones 1. 2. 3. 4. 5.

Are proteins encoded for by genes Act individually to bring about changes in plant development Function as receptors for environmental signals Both 1 and 3 None of these

Auxin 1. 2.

3.

4. 5.

Prevents apical dominance Is produced in shoot apical meristems Promotes seed development inside fruit All of these None of these

Phototropin 1.

2. 3.

4. 5.

Is a type of auxin Promotes apical dominance Is involved in stem growth towards light Is produced by seeds All of these

Cytokinins – cell division and differentiation

Cytokinin & tissue culture

From callus to somatic embryos

Gibberellins

Promotes: • Germination • Stem elongation • Flowering • Fruit development

Gibberellins  Breaking

dormancy

– Seed germination

Gibberellins  Promotes

cell

division & elongation

Gibberellins  Promotes

bolting in biennials

Gibberellins

Promotes: • Germination • Stem elongation • Flowering • Fruit development

Gibberellin is part of a complex signal transduction pathway

(see supplemental reading, for related information)



Della proteins restrain growth – GA and GID2 degrade Della proteins

Gibberellins and germination

Gibberellin promotes vegetative growth

Abscisic acid  Inhibits

growth  Promotes dormancy  Closes stomata

Abscisic acid – inhibits germination – Promotes dormancy – Leached by imbibition

ABA and stomatal closure

ABA delays flowering  FCA

– an RNA binding protein  FY – an mRNA processing factor  Flowering Locus C – a flowering repressor

Ethylene (CH2=CH2)    



Fruit ripening (promotes) Flowering (inhibits) Abscission (promotes) Sex expression in monoecious species (ratio of ♀ to ♂) Thigmomorphogenesis (reduced stem elongation in some environments)

Thigmomorphogenesis

Brassinosteroids (BRs)

60 types, brassinolide most common  Stimulates cell elongation, leaf expansion  BR mutants – extreme dwarfs, small crinkled leaves – Dark grown BR mutants – de-etiolated 

Plant Genes on Steroids

Science, Vol 307, Issue 5715, 1569-1570 , 11 March 2005

BIN2 catalyzes breakdown of BES1 & BZR2 proteins (phosphorylation) BR regulates activity of key growth transcription factors -BES1(activator) -BZR1(repressor)

Figure 13.12 (p.290)

Figure 13.12 (p.290)

Plant Genes on Steroids

Science, Vol 307, Issue 5715, 1569-1570 , 11 March 2005

BIN2 catalyzes breakdown of BES1 & BZR2 proteins (phosphorylation) BR regulates activity of key growth transcription factors -BES1(activator) -BZR1(repressor)

Responses to environmental stimuli: light  Phototropism  Stomata

opening  Stem elongation  Photodormancy (photoblastism)  Photoperiodism

Phytochrome  Phytochromes

are proteins with a light absorbing pigment attached (chromophore) – Mediates stem elongation, seed germination, timing of flowering

Phytochrome structure

Two forms of phytochrome

Phytochrome & stem growth •Etiolation occurs in low light or dark …why? •Does Pfr inhibit or promote stem elongation? Phytochrome and hormonal control of stem elongation

Phytochrome and seed germination  Photodormancy

seeds

& photoblastic

– Germination activated by light Some

plants, by red light Some plants, by far-red light Negative

photoblastism (tomato), Pfr inhibits germination Positive photoblastism (lettuce), Pfr promotes germination

Lettuce is positively photoblastic

30-60% lettuce seed germinate in dark  85-95% lettuce seed germinate in light 

Phytochrome, photoperiodism & flowering

Manipulation of photoperiod  Poinsettia

industry  Chrysanthemums

 Why

won’t my Christmas cactus bloom?

Brassinosteroids 1.

2. 3. 4. 5.

Promote seed germination in response to light Promotes flowering in response to day length Are proteins with an attached light absorbing chromophore Regulate transcription factors involved in growth All of these

Which of the following is true of phytochrome? 1.

2.

3. 4. 5.

Pfr absorbs red light and Pr absorbs far red light Pr is the active form of phytochrome and Pfr is the inactive form of phytochrome Pfr promotes germination in seeds requiring light All of these None of these

Photoperiodism 1.

2. 3. 4. 5.

Determines seed dormancy/germination in response to light/dark Determines flowering in response to day length Is a protein with an attached light absorbing chromophore Controls stem growth in response to light/dark All of these

Circadian rhythms – sleep movements (nyctinasty)

Nyctinasty

Solar tracking (heliotropism)

Response to mechanical stimuli (seismonasty)

Seismonasty – Mimosa pudica

Seismonasty

Seismonasty  Venus

flytrap

Response to environmental stimuli: Induced resistance  Herbivore

attack, systemin (18aa polypeptide hormone) & jasmonic acid (1-alpha, 2-beta-3-oxo-2-(cis2-pentenyl)-cyclopentane acetic acid)

Figure 1 Model for the activation of defense genes in tomato in response to wounding and insect attack. After wounding, systemin is released from its precursor prosystemin by proteolytic processing. Systemin subsequently binds a membranebound receptor to initiate an intracellular signaling cascade, including the activities of a MAP kinase, a phospholipase, a calcium dependent protein kinase, an extracellular alkalinization, and the release of linlenic acid from membranes. Linolenic acid is converted to jasmonic acid, a messenger for early defense gene activation. Catalytic activity of polygalaturonase, an early gene, leads to generation of hydrogen peroxide acting as a second messenger for late gene activation. R, receptor; MAPK, MAP kinase; Ca2+PK; calcium dependent protein kinase; PLA2, phospholipase A2; LA, linolenic acid; JA, jasmonic acid; pm, plasma membrane.



H2O2 prevents cell wall digestion by fungal pectinases

Response to environmental stimuli: Induced resistance  Pathogens

& the hypersensitive response (HR)

HR response & systemic acquired resistance (SAR)

SAR responses  Lignification

of cell walls  Antimicrobial molecules – PR-proteins (pathogen related proteins) – Chitinases – Phytoalexins (inhibit protein synthesis

SAR model