Plant Reproduction I (cont.)

Roses are red,

Violets are blue,

Some poems rhyme.

But this one doesn't.

- anonymous

You are here

• Spore formation,

• Gamete formation,

• Pollination,

• Fertilization,

• Germination.

Today

Dormant Seeds• Dormancy: slow metabolic rate and

suspension of growth and development,

– seed coat dormancy (seed coat impermeable to O2 and H2O),

• release by scarification (fire, abrasion, digestive tracts, etc.),

– internal dormancy (physiological),

• the most common internal dormancy is a requirement for cold,

• release by stratification (cold treatment).

Know these terms.

Nelumbo nucifera (Sacred Lotus)

1288 year old viable seeds

Seed Germinationphase changes

• Dormant to metabolically active,

– etiolated growth (dark growth),

• directional response to gravity,

• light,

• temperature,

• touch,

• De-etiolation (dark to light habit),

– vegetative growth program.

Plant Responses to Signals

A weed is a plant that has mastered every survival skill except for learning how to grow in rows.

- anonymous

Home Stretch

• Responses to internal and external signals are studied (to a large extent) by studying the genetics and physiology of...

Germination (phase change)

Etiolated Growth

De-etiolation (phase change)

Signal Transduction

Hormone Responses

Photomorphogenesis

Gravitropism

others...

External Signals• External signals are used by plant cells to alter

their physiology, morphology and development,

– physical environment,

– chemical environment,

– biological environment,

• sometimes other plants,

• Plants receive signals at the cell level, and have no well defined sensory organs,

• Except for gravity, all other signals are constantly variable.

Internal Signals

• Signals can be processed by growing and non-growing cells,

• Signals are transduced into biologically meaningful results through numerous and co-ordinated pathways,

– changes in ion flux,

– regulation of metabolic pathways,

– regulation of gene expression,

– changes in the cytoskeleton.

Signal Transductiongeneral

Fig. 39.2

General Signal Transduction

Signal Transductionmodels

Linear Model

Signal Signal

Response Response

Network Model

Signals

Signal

Signal

Response

Reception

• Cell surface receptors,

– hydrophilic molecules such as peptides and carbohydrates don’t readily cross the membrane,

• are perceived on the cell surface,

• Amphiphilic and hydrophobic molecules may pass through the membrane to receptors,

– steroid hormones for example,

• Light may be perceived at the cell surface, or in the cell

Plant Receptorsi.e. light responses

• Where does one look?

– no clearly differentiated organs (i.e., eyes, ears, etc.),

– sensitive tissues, however, no clearly differentiated cells,

– lots of responses.

• Germination (+/-)

• Stem length (-)

• Leaf expansion (+)

• Flowering (+/-)

• Phototropism (+/-)

• Stomatal opening (+)

• Chloroplast development (+)

• Pigment synthesis (+)

De-Etiolation (italics)

De-etiolationgreening

Etiolated

• Etiolated growth habit,

– long stem,

– unexpanded closed leaves,

• etioplasts vs chloroplasts,

• lack of chlorophyll,

– apical hook,

– short root,

• Photomorphogenesis,

– Inhibited stem growth,

– Expanded leaves,

– Pigmentation.

– Root development.

De-etiolated

Nature Reviews Molecular Cell Biology 3; 85-93

Phytochromeplant photoreceptor

• 1920’s,

– researchers observed chlorophyll deficient mutants (albinos) that underwent de-etiolation when given physiologically active light,

• 1950’s,

– phytochrome discovered,

• Molecular switch, signal transducer.

Phytochrome

• Proteins with tetra-pyrrole chromophores, and transmission kinase domains,

• Phytochrome gene family contains at least five members,

• Gene family members serve different functions.

chromophore kinase domainsN C

?

?

Signalred light

Responsegreening

Phytochrome Signal Transduction

one approach

• aurea (tomato) mutants lack a phytochrome gene and are impaired in greening,

– micro-inject second messengers at physiologically relavent concentrations,

– look for greening,

• cGMP and calcium.

cGMP

Ca 2+

Fig. 39-3

The Experiments

cAMP

cAMP is involved in many signal transduction cascades.

cGMP is also involved in signal transduction (i.e. cGMP transduces

visual signals in rod cells).

Fig. 39.9

…inject cGMP.

cGMP and Transcriptional Regulation

Post Translational Modifications

Fig. 39.9

Ca2+ channel Protein is opened.

Calcium as a Second Messenger

Fig. 39.9

Microinject Ca 2+

cGMP is a second messenger,

Calcium is a second messenger.

Signalred light

Responsegreening

Phytochrome Signal Transduction

one approach

• What do these experiments show?

• What don’t these experiments show?

Phytochrome is a greening receptor,

?

?

Nature Reviews Molecular Cell Biology 3; 85-93

...thousands of genes,

...response.

…hundreds of transcription factors,

phytochromes

Phytochrome Signal Transduction

for real

• Active phytochrome (Pfr) is transported to the nucleus,

… binds and inactivates transcriptional repressor (PIF3),

phytochrome (Pfr)

…enhances the expression of MYB,

red light activates phytocrome, active phytochrome (Pfr) is transported to the nucleus,

CCA1 expression, in turn, enhances greening proteins.

…MYB transcription factor is expressed, in turn activates CCA1 transcription,

… CCA1 enhances the expression of CAB (chlorophyll A/B) proteins.

Plant Responses to Signals IIPhytohormones

No one can look at the plants growing on a bank or on the borders of a thick wood, and doubt that the young stems and leaves place themselves so that the leaves may be well illuminated...they are extremely heliotropic; and this probably serves...as a guide (for) the buried seeds through fissures in the ground or through overlying masses of vegetation, into the light and air.

- Charles Darwin

“The Power of Movement in Plants” (1880)

Charles Darwin was Plant Physiologist,

Phototropism,

Introduction to the plant hormone Auxin.

Phytohormones

…a plant product that is able to stimulate physiological responses at very low concentrations,

– either in the tissue in which it is synthesised,

– or in other regions of the plant to which it is transported,

...do not operate in isolation from one another, but often act in co-ordination to produce subtle responses,

…affect gene expression, enzyme activity and membrane function.

Charles Darwin and his son Francis localized the location of perception for blue light phototropism.

Tropism: a growth response in plants that results in curvature toward, or away from a stimulus.

Phototropism

Peter Boysen-Jensen demonstrated that a diffusable substance was involved .

Positive Phototropism: growth toward a light stimulus

Peter Went demonstrated that the difusable substance resulted in cell expansion. Went isolated the active compound.

Natural auxins...

IAA

Bioassays

• Bioassay: identification (or quantitation) of a biologically active substance by measuring the effect the substance has on living material.

Oat Coleoptile (2 cm)

auxin

Oat Coleoptile (> 2 cm)

Auxin Transportpolar

Auxin moves basipetally (from apical ends to basal ends).

Synthesized in the SAM

Polar Transpotrequires energy

Note pH

• IAA is in anionic form at cytosolic pH levels,

– the plasma membrane is relatively impermeable to IAA- ,

– IAA- builds up in the cell,

• IAA- carrier proteins are differentially expressed at the basal ends of cells,

• IAA- exits via efflux carriers once the concentration builds,

• IAA- is protonated to become a neutral compound IAAH.

Polar Transpotrequires energy

• IAAH enters the next cell passively, through the plasma membrane,

– IAAH is also imported by secondary active transport (not shown in the book’s diagram),

H+

IAAH

• Once in the next cell, IAAH returns to IAA- (at the higher cytosolic pH),

• H+-ATPases pump protons out of the cell,

– driving down the pH outside,

– and driving in IAAH in via secondary active transport (down the proton gradient,

• IAA- builds up in the cell.

Polar Transpotrequires energy

H+-ATPases set up a membrane potential, (positive on the outside),

IAA- efflux is facilitated.

H+

IAAH

Auxin…induces apical dominance,

basipetal movement of

auxin inhibits axillary buds,

SAM intact

axillary buds are released.

SAM removed

“Leader”

Apical Dominance

basipetal movement of

auxin inhibits axillary buds,

SAM intact

axillary buds are released,

SAM removed

Auxins …induce vascular differentiation

…xylem differentiation occurs around the wound,

...following the path of auxin diffusion.

…xylem differentiates between hydathodes and leaf vasculature

...following the path of basipetal auxin transport.

hydathode

Auxins …induces lateral and adventitious root formation,

Rooting compounds. [ auxin ]

Honeysuckle cut stems

Auxins …promotes fruit development,

Strawberry

seeds removed +

auxin

seed

seeds removed

Normal

…embryo produces auxin that stimulates fruit development.

Auxin…promotes cell expansion,

Auxin activates the H+-ATPase.

Acidification of the cell wall activates expansin (cell wall proteins).

Fig. 39.7

Acid Growth Hypothesis

Time course of action. Mode of action(hypothesis)

AUXIN BINDING PROTEIN

ABP + Auxin

…activates the pump.

Auxin Binding Proteinauxin receptor?

over-express ABP1 gene

add auxin

• Auxin receptor has not conclusively been identified,

– a protein that is found to bind auxin, (Auxin Binding Protein, ABP) has been proposed as a candidate receptor,

• A common test for receptors, is to over-express (express more than the normal amount of protein), and test to see if the signal is amplified,

• ABP passes this test.

Auxin Binding Proteinauxin receptor?

wild type abp mutant

ABP controls cell division and elongation in embryogenesis.

dead

Auxins …promote gene expression,

Genes

Products

5 - 50 minutes

Concept Map

Auxin

DiscoveryReceptor

TransportFunction(s)

Note: Ch 39, pp 817 - 824…quiz style questions.