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. Are proteins encoded for by genes

2. Act individually to bring about changes in plant development

3. Function as receptors for environmental signals

4. Both 1 and 35. None of these

Auxin

1. Prevents apical dominance2. Is produced in shoot apical

meristems3. Promotes seed development

inside fruit4. All of these5. None of these

Phototropin

1. Is a type of auxin2. Promotes apical dominance3. Is involved in stem growth

towards light4. Is produced by seeds5. 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 & photoblastic seeds– Germination activated by light

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

Negative photoblastism (tomato), Pfr inhibits germinationPositive 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. Promote seed germination in response to light

2. Promotes flowering in response to day length

3. Are proteins with an attached light absorbing chromophore

4. Regulate transcription factors involved in growth

5. All of these

Which of the following is true of phytochrome?

1. Pfr absorbs red light and Pr absorbs far red light

2. Pr is the active form of phytochrome and Pfr is the inactive form of phytochrome

3. Pfr promotes germination in seeds requiring light

4. All of these 5. None of these

Photoperiodism1. Determines seed

dormancy/germination in response to light/dark

2. Determines flowering in response to day length

3. Is a protein with an attached light absorbing chromophore

4. Controls stem growth in response to light/dark

5. 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-(cis-2-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 membrane-bound 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