Hormones 01 Flashcards
What are plant hormones?
Signalling molecules produced in small concentrations. Control all aspects of plant growth and development. Each plant cell is capable of producing plant hormones.
Which are the ‘classical’ hormones?
Auxins
Cytokinins
Gibberellins
Ethylene
Abscisic Acid
Which are the ‘new’ hormones (in terms of discovery, not biologically speaking)?
Brassinosteroids
Strigolactones
Salicylic Acid
Jasmonic Acid
(Secreted signaling peptides) => as a whole can be considered a class of plant hormones
How is the amount of a hormone controlled
Biosynthesis:
Environmental Input
Programmed development (epigenetics: TFs, regulatory elements)
Compartmentalisation:
e.g. in Vacuole
Degradation (Catabolism)
Inactivation (reversible conjugation)
Efflux
What is a receptor?
A protein that can directly bind a ligand that can induce signaling in order to produce a response
What is the fastest way to turn a signaling pathway/response ON?
Constantly keeping TFs Off by inhibition and turning them On by degarding the inhibitor upon hormone perception
Wich protein mediates degradation of the inhibitory factors?
26S Proteasome
Auxin controls
Growth Processes
Phototropism & Gravitropism
Branching
Embryonic Patterning
Stem Cell maintenance
Organ initiation
and many other processes
“Auxin controls everything”
What is the most bioactive & most abundant Auxin?
IAA = Indole-3-acetic acid
What was discovered by several experiments about Auxin?
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)
Where in a coleoptyle does IAA accumulate?
in shaded sight => side that is being bent
Auxin concentrations
Maximum response at ideal concentration, not the higher the conc the stronger the reaction!
From which biosynthetic pathway do Auxins derive?
Amino-Acid based biosynthesis: Tryptophan
How can IAA be reversibly regulated?
Via Ester conjugation
Can Auxin be stored?
Yes, in a different conjugated form (IAA-Ala or IAA-Leu)
Auxin transport
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)
In what form dies IAA occur in the apoplast vs. cytoplasm?
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
Where are PIN transporters mostly located?
basilateral => explain directional movement from tip of shoot to tip of root
Auxin can act as…
morphogen (gradient => e.g. root elongation) or developmental trigger (localized, actute increase in conc => e.g. lateral root primordium) that trigger a response
Auxin perceprion
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
Auxin Perception at Plasma Membrane
Auxin Binding Protein in apoplast (soluble receptor) binds Auxin and a receptor kinase that leads to fast Auxin responses => signaling event
Cytokinins regulate
shoot division (promotion)
delay leaf senescence
nutrient allocation
root nodule development
environmental signaling and pathogen responses
Auxin action and distribution (counterpart)
and more…
Cytokinin biosynthesis
Adenine-like compounds (adenosine phosphate precursors)
Most active form is trans-zeatin
But also from carotenoids
Cytokinin discovery
Plant cell culture growth
Cytokinin signaling
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)
Where can Cytokinin receptors be found?
different subcellular organization
At plasmamembrane
Different Isoforms e.g. in ER
All converge into activation of of regulator of transcription
Cell culture: plant cell redifferentiation
Auxin/Cytokinin ratio determines form of cell culture
+ auxin => roots
+ CK => shoot
+ both => callus (mass of non- differentiated cells)
Giberellins regulate
seed germination (counterpart of ABA)
growth
flowering, fruit growth (+)
sex determination in some species
Which is the most active form of Gibberellic acids?
Depends on species
In A. thaliana GA4
GA Discovery
Accidental
Rice fungus (giberella fujikuroi)
Hyperelongated stems (Gibberellins were underlying cause?)
Gibberellins as plant growth regulators
Promote stem elongation
=> dwarf mutants = gibberellic acid biosynthesis mutants
=> inhibit gibberellins in crops => reduce stems that lay on ground after rain => increase yield
GA biosynthesis
From Terpenoid biosynthesis pathway
Different steps in diff. subcellular compartments (cytosol, plastids, ER)
Seed germination
ABA induces seed dormancy (dessication tolerance)
GA promotes germination after cold period/rain etc
How can biosynthetic and perception mutants be differentiated?
If biosynthesis: promotion of stem elongation if + GA
If perception: no GA signaling, even if + GA
Hypersensitive: always stem elongation
GA perception
very similar to auxin
Which proteins are important in GA signaling pathways?
DELLA
(very complex)
Brassinosteroids
Chemical similarity to animal steroids but diff. function
Lipids
BRs regulate
Mainly cell elongation
Growth
Cell division
Stress tolerance
Reproductive development
Which is the most active BR?
Brassinolide (BL)
How were BRs discovered?
Stem elongation upon treatment with pollen extract
(230 kg of pollen for a few mg of BR)
BR biosynthesis
Terpenoid pathway
But further down than cytokinin and GA
BR perception
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
