Hormones 01

Question 1

What are plant hormones?

Answer 1

Signalling molecules produced in small concentrations. Control all aspects of plant growth and development. Each plant cell is capable of producing plant hormones.

Question 2

Which are the ‘classical’ hormones?

Answer 2

Auxins
Cytokinins
Gibberellins
Ethylene
Abscisic Acid

Question 3

Which are the ‘new’ hormones (in terms of discovery, not biologically speaking)?

Answer 3

Brassinosteroids
Strigolactones
Salicylic Acid
Jasmonic Acid
(Secreted signaling peptides) => as a whole can be considered a class of plant hormones

Question 4

How is the amount of a hormone controlled

Answer 4

Biosynthesis:
Environmental Input
Programmed development (epigenetics: TFs, regulatory elements)

Compartmentalisation:
e.g. in Vacuole

Degradation (Catabolism)

Inactivation (reversible conjugation)

Efflux

Question 5

What is a receptor?

Answer 5

A protein that can directly bind a ligand that can induce signaling in order to produce a response

Question 6

What is the fastest way to turn a signaling pathway/response ON?

Answer 6

Constantly keeping TFs Off by inhibition and turning them On by degarding the inhibitor upon hormone perception

Question 7

Wich protein mediates degradation of the inhibitory factors?

Answer 7

26S Proteasome

Question 8

Auxin controls

Answer 8

Growth Processes
Phototropism & Gravitropism
Branching
Embryonic Patterning
Stem Cell maintenance
Organ initiation
and many other processes

“Auxin controls everything”

Question 9

What is the most bioactive & most abundant Auxin?

Answer 9

IAA = Indole-3-acetic acid

Question 10

What was discovered by several experiments about Auxin?

Answer 10

There has to be a molecule that must diffuse from the bottom to the top on the side that is being bent, on the shaded side of a coleoptyle that is necessary and sufficient for inducing a reaction, even in the absence of light (naturally response triggered by light)

Question 11

Where in a coleoptyle does IAA accumulate?

Answer 11

in shaded sight => side that is being bent

Question 12

Auxin concentrations

Answer 12

Maximum response at ideal concentration, not the higher the conc the stronger the reaction!

Question 13

From which biosynthetic pathway do Auxins derive?

Answer 13

Amino-Acid based biosynthesis: Tryptophan

Question 14

How can IAA be reversibly regulated?

Answer 14

Via Ester conjugation

Question 15

Can Auxin be stored?

Answer 15

Yes, in a different conjugated form (IAA-Ala or IAA-Leu)

Question 16

Auxin transport

Answer 16

Polar => directional
Moves long distances through phloem or via Auxin transport proteins (AUX/LAX for Influx, PIN for efflux and ABCB for both directions)

=> usually movement from tip of shoot towards tip of root, but moves up at root tip (basipetally)

Question 17

In what form dies IAA occur in the apoplast vs. cytoplasm?

Answer 17

Apoplast/cell wall (pH 5.5): uncharched IAAH

cytoplasm (pH 7): charged Anion IAA-

uncharged can cross plasma membrane into cell, charged needs specific transporters to exit

Question 18

Where are PIN transporters mostly located?

Answer 18

basilateral => explain directional movement from tip of shoot to tip of root

Question 19

Auxin can act as…

Answer 19

morphogen (gradient => e.g. root elongation) or developmental trigger (localized, actute increase in conc => e.g. lateral root primordium) that trigger a response

Question 20

Auxin perceprion

Answer 20

Receptor (TIR1/AFB) dircetly binds Auxin and is an F-Box protein => associated with E3 ubiquitin ligase (linked directly to proteasome)

Auxin perception activates protein degradation of repressor of TF by increasing the receptors affinity for the repressor protein

Question 21

Auxin Perception at Plasma Membrane

Answer 21

Auxin Binding Protein in apoplast (soluble receptor) binds Auxin and a receptor kinase that leads to fast Auxin responses => signaling event

Question 22

Cytokinins regulate

Answer 22

shoot division (promotion)
delay leaf senescence
nutrient allocation
root nodule development
environmental signaling and pathogen responses
Auxin action and distribution (counterpart)
and more…

Question 23

Cytokinin biosynthesis

Answer 23

Adenine-like compounds (adenosine phosphate precursors)
Most active form is trans-zeatin

But also from carotenoids

Question 24

Cytokinin discovery

Answer 24

Plant cell culture growth

Question 25

Cytokinin signaling

Answer 25

Phosphorelay system

CK binds to membrane-bound Receptor Histidine Kinase => leads to autophosphorylation at histidin
RHK phosphorylates Histidine Phosphotransfer Proteins (HPTs)
HPT transfer phosphoryl group to response regulators (transcriptional regulators)

Question 26

Where can Cytokinin receptors be found?

Answer 26

different subcellular organization
At plasmamembrane
Different Isoforms e.g. in ER
All converge into activation of of regulator of transcription

Question 27

Cell culture: plant cell redifferentiation

Answer 27

Auxin/Cytokinin ratio determines form of cell culture

+ auxin => roots
+ CK => shoot
+ both => callus (mass of non- differentiated cells)

Question 28

Giberellins regulate

Answer 28

seed germination (counterpart of ABA)
growth
flowering, fruit growth (+)
sex determination in some species

Question 29

Which is the most active form of Gibberellic acids?

Answer 29

Depends on species
In A. thaliana GA4

Question 30

GA Discovery

Answer 30

Accidental
Rice fungus (giberella fujikuroi)
Hyperelongated stems (Gibberellins were underlying cause?)

Question 31

Gibberellins as plant growth regulators

Answer 31

Promote stem elongation
=> dwarf mutants = gibberellic acid biosynthesis mutants
=> inhibit gibberellins in crops => reduce stems that lay on ground after rain => increase yield

Question 32

GA biosynthesis

Answer 32

From Terpenoid biosynthesis pathway
Different steps in diff. subcellular compartments (cytosol, plastids, ER)

Question 33

Seed germination

Answer 33

ABA induces seed dormancy (dessication tolerance)
GA promotes germination after cold period/rain etc

Question 34

How can biosynthetic and perception mutants be differentiated?

Answer 34

If biosynthesis: promotion of stem elongation if + GA
If perception: no GA signaling, even if + GA
Hypersensitive: always stem elongation

Question 35

GA perception

Answer 35

very similar to auxin

Question 36

Which proteins are important in GA signaling pathways?

Answer 36

DELLA
(very complex)

Question 37

Brassinosteroids

Answer 37

Chemical similarity to animal steroids but diff. function

Lipids

Question 38

BRs regulate

Answer 38

Mainly cell elongation
Growth
Cell division
Stress tolerance
Reproductive development

Question 39

Which is the most active BR?

Answer 39

Brassinolide (BL)

Question 40

How were BRs discovered?

Answer 40

Stem elongation upon treatment with pollen extract
(230 kg of pollen for a few mg of BR)

Question 41

BR biosynthesis

Answer 41

Terpenoid pathway
But further down than cytokinin and GA

Question 42

BR perception

Answer 42

Plasma membrane receptors (2 receptor kinases)
• extracellular domain
• single path transmembrane domain
• cytosolic kinase domain

=> one of receptor kinases binds BR with LLR (leucin rich repeat)
=> recruits co-receptor (diff. type of receptor kinase)
=> upon formation of complex: kinase domain is actvated and starts signaling pathway

!Ligand acts as molecular glue!
Conformational change, that enhances affinity

Downstream negative regulation of negative regulation