Plant Responses to Signals

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

- anonymous

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 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)

Seed Germinationphase changes

• Dormant to metabolically active,

– etiolated growth (dark growth),

• respond to gravity,• light,• temperature,• touch,

• De-etiolation (dark to light habit),

– vegetative growth program.

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

or

cGMP is a second messenger,

Calcium is a second messenger.

Signalred light

Responsegreening

Phytochrome Signal Transduction

Phytochrome is a greening receptor,

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.

Nature Reviews Molecular Cell Biology 3; 85-93

...thousands of genes,

...response.

…hundreds of transcription factors,

phytochromes

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 a 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 diffusible 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 apex to base).

Synthesized in the SAM

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).

Auxins …promote gene expression,

Genes

Products

5 - 50 minutes

Concept Map

Auxin

DiscoveryReceptor

TransportFunction(s)