Plant Development and Signals

Plants use development to solve ecological problems– Repair, maintain, flourish

– Resist competitors, herbivores, pathogens, abiotic factors

– Seek out resources

Plants respond to environmental and endogenous signals

Signals alter gene reading programs and influence:– Cell division, expansion and differentiation at meristems

– Tissue, organ and whole plant modification

– Fitness

Gene reading programs and flow of information

DNA

m RNA transcript

proteinSignal(s)

promoter

+ -

+ -Protein synthesis

+ -

gene

enzymes

structuralregulatory transporter

cytoskeletal+ -

+ -

Classes of signals

Internal (endogenous); generally chemical– Hormone– Non-hormone– Electrical gradients (but much less so than animals)

External, generally non-chemical– Electrical, light, gravity, temperature, mechanical forces– Some chemical cues (e.g. MeJ)

Internal chemical signals

Hormones (a definition from animal biologists)

– Naturally occurring organic molecules that act in small concentrations

– Sole function is to serve as a developmental signal• Signal is context dependent

– Typically move from source to target

Plant biologist- naturally occurring organic molecule whose sole function is to serve as a development signal

– 5 classes of plant hormones

Auxin• Indoleacetic acid (IAA), IBA• Activity determined by charge separation• Many synthetics• Roles: 0.55 nm+

-

Gibberellins (GA)• Large family of compounds

– Numbered according to discovery

• Three linked rings with side groups

• Roles:

Cytokinins (CK)

• Derivative of adenine

• Different functional (R) groups attached

• Roles:R group

Ethylene (C2H4)• Gas at room temperature

• Roles:

C C

H

H

H

H

Abscisic acid (ABA)

• Named b/c it was discovered in abscised lvs

• Not the direct cause of leaf loss though

• Roles:

Internal non-hormone chemical signals

Phenolics (Phe)– Coumarin– Anthocyanin– Salicylic acid– Ferulic acid

O

R

Methyl jasmonate (MeJ)

• Related to stress response(s)

• Root initiation, tendril coiling, pollen germination

• Volatile interspecies ‘messages’

Oligosaccarides (Oligos)

• Polymers of carbohydrates

• Related to defense response– Phytoalexins, chitinase, and protease

inhibitors

External chemical signals• Nitrate (NO3

-) – Controls nitrogen assimilation

• Hydroquinones– Can signal parasitic plants to start

‘parasite gene reading program’

Principles of plant hormone action

1. Each hormone causes many responses

IAA

Abscission-

Root initiation+

Stem elongation

+

-+

Bud expansion

Vascular repair

Principles of plant hormone action

2. A hormone’s effect can differ between organs and species, and with age.

Wheat seedling elongation

Young Old

IAA Yes No

GA No Yes

10-6 IAA

+

-

Root elongation

Stem elongation

Principles of plant hormone action

3. Many hormones can affect the same response

Cell division

CK IAA GA

+ + +

Hormone sensorsSince signals vary, the meaning of a signal depends on receptors and sensitivity (kinds of sensors a cell contains)

Cells must have sensors/receptors; when hormone binds w/ following (blue) molecules response set off

• Membrane channels or carriers receive and transport signal (A)

• Signal activates enzyme (CD)

• Receptor is regulatory protein (B)

Gene reading

C D

+ -

+ + -+ -

A

A

B

Setting off a responseResponses may occur when the cell has a threshold number of receptors bound to hormones

Equilibrium conditions predicts that response can occur when:

– Hormone concentration increased

– Receptor concentration increased (= sensitivity)

Receptor

Hormone

HR

Response

+

Response depends on concentration• Induction – on/off

response once threshold hormone level is exceeded

• Modulation – on-going log linear dose response

[Hormone]

[Hormone]

Response

Response

0%

100%

Physiologic range

10-610-8

Hormone concentration control

• Synthesis

• Transport

• Destruction

• Sequestration

• Conjugation

• Example - IAA

Concentration control

• Self-inhibition

• Interactions between IAA and C2H4

• Interactions between CK and GA

• De-stabilizing concentrations

Measuring hormone concentrations

• Bioassays – Grow specimens in a range of known

concentrations– Generate dose response curve– Find value for unknown

• Chromatography

• Mass spectrometry

• Antibody

Control of phenolic synthesis

Phenolics may act as 2o messengers

Phenylalanine

Cinnamic acid

Cinnamyl CoA

Chalcone

Phenolics and flavonoids, including some phytoalexins

Lignin

PAL

Chalcone synthase

aaInjury

Pathogens

+

+

gene

gene

+

+

+

+

Ferulic acid