Bio 351 - Exam 3 (hormones, pigments, and seeds)

Where are plant hormones located?

Move through xylem, cells, or air as a gas

Cell signaling stages

Reception, transduction, response

Reception

Env’t signals are detected by receptors (often proteins embedded in the membrane

Transduction

Environmental signals are converted to a form that amplifies the signal to trigger a response

Response

High enzyme activity and gene expression, secondary messengers are produced

etiolation

morphological adaptation for growing in the dark, lack chlorophyll, allocate all resources to stem elongation

phytochrome

A pigment attached to a photoreceptor protein, is converted into active form during reception stage to initiate signaling

Transduction example

Active phytochrome triggers secondary messenger production (cGMP and Ca2+) that activate protein kinases

kinases

enzymes that phosphorylate other molecules using ATP

Role of Ca2+ in plants

A common signaling molecule (secondary messenger) that triggers changes in pH, transcription, and vacuolar pumops

Response example

Activated kinases influence transcription factors that bind DNA to allow transcription of light-response genes and proteins for development

phytohormones

Synthesis occurs in many cell types simultaneously and may act there or leave, not a linear relationship between concentration, degree, and type of activity

Animal hormones

Synthesis in discrete organs and tissues, transport from site of synthesis to site of action, localized, control processes in conc-dependent manner

What must be true for hormones to create a response?

Present in a sufficient quantity, target tissue is sensitive to hormone, there is a receptor to bind that will change shape

primary natural auxin

IAA, indole-3-acetic acid, derived from AA tryptophan

How were auxins discovered?

1. Charles and Francis Darwin covered coleoptile from light and it didn’t bend, proving phototropism

2. Boysen-Jensen proved auxin is a chemical and diffusible through gelatin but not butter

3. Fritz Went isolated IAA

Function of auxin

Links env’t signals with directional growth (apical dominance and tropic responses), cell elongation, alone induces root formation in a callus

gibberelins

Part of terpene group, GA1 (first discovered in plants), GA3 (fungal gibberellic acid), GA4 (most common plant gibberelin)

How were gibberellins discovered?

Fungus Gibberella secreted gibberellins causing rice to grow tall/spindly and topple at maturity

Gibberellin function

Stem/internode elongation, seed germination, flowering, fruit growth

Cytokinin

Adenine derivatives with isopentenyl side chains, first was kinetin derived from degraded DNA

Function of cytokinin

Stimulate cell division, alone induces shoot formation of callus

How were cytokinins discovered?

Early tissue culture experiments using coconut milk or tobacco with modified forms of adenine

How do auxin and cytokinin regulate tissue formation?

Both are required in a correct ratio to maintain cells of an undifferentiated callus

ethylene

Gaseous hormone, C2H4, rapidly produced in response to drought, flooding, injury, infection

ethylene triple response

Allows seedlings to circumvent obstacles. Inhibits hypocotyl, reduces elongation, thickening of hypocotyl, exaggerated curvature

climacteric fruits have…

High respiration and ethylene so they ripen post-harvest

abscisic acid

A terpenoid derived from carotenoids in mature leaves, levels rise under stress

abscisic acid function

Regulates maturation of embryos and seed germination, levels rise at embryogenesis, can prevent vivipary through dormancy

abscisic acid response to water stress

Accumulates in stressed leaves to inhibit stomatal opening before water potential changes, low soil moisture transfers ABA from roots to shoots

brassinosteroids

First discovered in pollen, functions in sex determination and similar to sex hormones of animals

strigolactones

Stimulates seed germination, mycorrhizal associations, apical dominance

Role of sunlight

Supplies energy for photosynthesis, c-fixation, and biomass accumulation

solar tracking (heliotropism)

Mov’t of leaf throughout the day so its surface is always capturing the sun

phototropism

Altering overall plant growth patterns in response to light

photomorphogenesis

Influence of light on developmental responses (ex. seedlings)

photoperiods

flowering in response to day length, describes hours of light given

photoreceptor

Chromoprotein with a light absorbing group (chromopore) attached to apoprotein with catalytic abilities, absorb photons to initiate a response

How are photoreceptors different from photosynthetic pigments?

They absorb different wavelengths, result in conformational change, create developmental response

phytochromes

Two forms: red (660nm) and far-red (730nm)

Role: In all stages of development

Relationship b/t red and far-red light

Far-red is active form that moves into nucleus to interact with transcriptional regulators. A flash of red light converts red photoreceptors to far-red, flash of far-red results in far-red photoreceptors

phytochrome reversibility

Some seed germination is promoted by red light whereas far-red inhibits

Red and far-red light under vegetative canopies

Red light absorbed by chlorophyll, far red enriched and transmitted

cryptochromes

Absorb: Blue light (400-450nm), induces conformational changes exposing signaling domains

Role: Seedling development, reg. circadian rhythms, anthocyanin biosynthesis

What light induces flowering?

Blue and far-red, red light inhibits

phototropin

PM associated photoreceptors that contain LOV domains

Absorb: UV-A (320-400nm)

Role: Phosphorylation of kinase domain, activation of downstream signaling pathways (stomatal opening, chloroplast mov’t, phototropism)

blue light receptors

Cryptochromes, phototropin, uncharacterized UV receptors. Contain flavin molecules as chromophores

uncharacterized UV-receptors

Absorb: UV-B (280-320nm)

Role: Stress response

How does auxin contribute to phototropism?

Auxin is transferred to the shaded side causing cells to elongate creating curvature towards light source

Where are chloroplasts localized under different light intensities?

Low light→ Along abaxial and adaxial surfaces

High light→ Parallel to light to minimize absorbance

Dark→ Settle at the bottom

How does blue light regulate stomata?

Closes stomata to optimize CO2

1. Phototropins in guard cells initiate signaling cascades

2. Activates H+-ATPase pumps

3. H+ exported out causing (-) charge, K+ accumulates inside

4. High turgor opens stomata

development

Changes in form from embryo to seedling, not random and can be reversible

embryogenesis

Formation and development of the embryo inside a seed

growth

Irreversible quantitative change in cell number, size, and/or volume

cell growth = increase in size

tissue and organ growth = increase in cell number and size

callus

A mass of cells resulting from those isolated from plants that can de-differentiate

totipotency

Ability of cells too revert to embryonic state without passing through a reproductive stage, most plants

animal development

Establish body plan during embryogenesis, evolved mobility

• lineage dependent mechanisms involving transcription factors

• homeotic (Hox) genes for proper placement or structures

plant development

Adaptive with more post-embryonic development, evolved flexibility to environment

• cell position determines fate

• no migration

• continuous meristems

2-cell stage (embryogenesis begins)

Zygote forms (1st diploid cell after fertilization), asymmetry and polarity determined. Apical (becomes embryo) and basal (becomes suspensor) cells

suspensor

anchors embryo to maternal tissue to facilitate nutrient/hormone transport

globular stage

Apical cell undergoes series of divisions, primary tissues differentiate into protoderm (establishes radial symmetry), meristem, procambium (early vascular tissues)

polarity

directionality, structural or chemical differences at opposite ends

heart stage

Cotyledon primordia establishes bilateral symmetry, shoot system organization

Torpedo stage

Elongation along the apical-basal axis

Bent-cotyledon stage

After elongation in torpedo stage, cotyledons bend around the hypocotyl and embryo is anatomically complete

Two patterning systems in embryogenesis

Apical-basal axis→ shoots vs roots, longitudinal growth

Radial axis→ tissue organization (dermal, ground, vascular)

Periclinal cell divisions

Produces cells parallel to radial axis for tissue formation, adds cell layers, dermal tissue produced first

Anticlinal divisions

Produces cells perpendicular to radial axis, increase number of cells within a layer to maintain radial integrity

Auxin role in embryo development

Determines position of SAM, cotyledons, RAM

2-cell stage→ auxin asymmetrically distributed along apical-basal axis

globular stage→ auxin redirected to basal region of embryo

heart stage→ complex and directional transport

How is auxin transported?

Protonated IAAH (uncharged, H+ cotransport) form can enter cells, loses H+ in neutral pH of cytoplasm to become charged and incapable of diffusion across PM

chemiosmotic mechanism of auxin transport

pH gradients facilitate entry/exit because cell wall is acidic (H+-ATPase maintains acidity), cytoplasm is basic and IAA- accumulates,

PIN proteins

Mediate exit/entry of IAA/IAAH through the PM, can reorient

ABCB transporters

Actively export auxin using ATP, work with PIN proteins to increase transport efficiency

gurke

lack apical development, polarity fail from disrupted auxin transport

fackel

defective in central (hypocotyl) region from disrupted auxin transport

monopteros

lack proper basal structures (ex. root) from disrupted auxin transport

gnom

lack both apical and basal structures from disrupted auxin transport

Three features of all seeds

embryo, food storage (often 3n endosperm from double fert), testa (seed coat)

endospermic seeds

Endosperm (3n, major storage tissue) is retained at maturity, embryo small, cotyledons thin and specialized for absorption, reserves mobilized to embryo during germination

scutellum (monocots)

Single cotyledon modified to facilitate absorption from endosperm

coleoptile (monocots)

Protective sheath that covers primary leaves of grass seedling

coleorhiza (monocots)

Protective sheath around the radicle

mesocotyl (monocots)

“stem” similar to the hypocotyl

non-endospermic seeds

Endosperm is used during embryogenesis, cotyledons serve as primary food storage tissue, embryo large, typical of dicots, reserves mobilized to cotyledons during germination

exogenous dormancy

Seed coat or other seed structures inhibit germination (water impermeability, gas exchange interference, mechanical constraint, chemical inhibitors)

endogenous dormancy

Embryo dormancy (abscisic acid or embryo size)

photoblasty

Light requirement for seed germination, often small seeds, phytochrome responses

Phase 1 of seed germination

Imbibition, rapid water uptake driven by water potential fuels cell expansion (matric potential low creating a gradient, water potential becomes less negative as water binds to surfaces)

Phase 2 of seed germination

Lag/metabolic activation with little growth, seed volume increases and coat ruptures, transcription/translation occuring

Phase 3 of seed germination

Post germination, radicle emerges, water uptake increases, cells expanding and dividing as starches, lipids, and oils are mobilized

Hormone balance theory

ratio of ABA:GA determines seed dormancy

When is dormancy promoted?

ABA is synthesized and GA degraded, ABA inhibits embryo growth and reserve mobilization

When is germination promoted?

GA synthesis and ABA degradation, GA stimulated embryo growth and reserve mobilization

vivipary

Germination of seeds on the mother plant, allow quick establishment in complex env’ts (ex. tides, salility)

Preharvest sprouting

Seeds germinate on parent plant, dormancy lost before harvest, triggered by wet or humid env’ts

Precocious germination

Seeds germinate during development before entering dormancy due to mutant that disrupts ABA biosynthesis

stratification

Cold treatment (5 degrees C) for a period of time that releases seeds from dormancy, germination increases with duration

How can gibberellins mobilize endosperm reserves?

1. GA is produced by scutellum

2. Stimulates production of amylase near aleurone layer to stimulate transcription/translation

3. Amylose hydrolyzes starch to simple sugars which gets transferred to embryo (ABA can inhibit amylase)