Plant development II - Plant hormones coordinate plant development Flashcards
List some plant hormones
- auxin
- cytokinin
- gibberellin
- ethylene
- abscisic acid
- brassinosteroids
- peptides
- salycilates
- strigolactones
- jasmonates
Describe hormones
- molecules devoid of metabolic or catalytic function
- exclusively act as signals
- effective at very low concentrations (nM cellular range)
- endocrine function
Describe endocrine function
- regulate the activity of cells that can be far from those which produce them (signalling cells) through the bloodstream
autocrine
a signalling cell targets itself
paracrine
a signalling cell targets a nearby cell
Describe some processes that hormones regulate
- growth (cell division, expansion, differentiation)
- development
- responses to exogenous stimuli (biotic and abiotic)
Hormones have a …
pleiotropic effect throughout a plant’s lifecycle.
List some abiotic stressors
- heat/cold
- flooding/drought
- salt/nutrient imbalance
Describe some biotic stressors
- pathogens
- insects
- herbivores
Describe hormone synthesis
- precursor
- intermediate
- active hormone
Describe hormone tranport
cell-to-cell or long distance
Describe the life cycle of a hormone
- synthesis
- transport
- perception
- signalling
- response
- inactivation
Describe hormone signalling
relies upon an active and stable effector for amplification
Describe a hormone response
transcription of a transporter
Describe hormone inactivation
conjugation or degradation
Describe plant hormone biosynthesis
- most plant cells are able to produce most hormones
- often enzymes involved in hormone synthesis are regulated by hormone signalling
Describe hormone competition
several hormones have primary and secondary metabolites in common as precursors and therefore compete for their consumption.
Describe auxin synthesis
tryptophan converted to indolacetic acid
Describe ethylene production
from methionine
What can the isoprenoid pathway produce?
- cytokinin
- gibberellin
- strigol
- abscisic acid
- brassinosteroids
Describe jasmonic acid synthesis
from alpha-linolenic acid
Hormone mutants can be rescued by
exogenous provision of the hormone
How is the step inactivated by the mutation deduced?
adding precursors and observing restoration of the wild-type phenotype
Describe synthetic hormones
- synthetic substances with limited structural similarity can bind to the same receptors
- often trigger stronger responses since plants are not equipped for their catabolism and thus are more persistent
Give an example of synthetic plant hormone usage
- Orange Agent sprayed to defoliate forests and fields during the Vietnam war contained 2,4-D
- potent synthetic auxin.
List some native auxins
- IAA
- IBA
- 4-CI-IAA
- PAA
IAA
indole-3-acetic acid
IBA
indole-3-butyric acid
4-CI-IAA
4-chloroindole-3-acetic acid
List some synthetic auxins
- 2,4-D
- alpha-NAA
- dicamba
- tordon or picloram
2,4-D
- 2,4-Dichlorophenoxyacetic acid
- potent synthetic auxin
alpha-NAA
alpha-Naphthalene acetic acid
dicamba
2-Methoxy-3,6-dichlorobenzoic acid
tordon or picloram
4-Amino-3,5,6-trichloropicolinic acid
Describe hormone modification for transport
- can be conjugated to sugars and amino acids as inactive forms to be transported or to be degraded
Describe jasmonates and salicylates
- defense and stress hormones
- methylated to serve as aerial messengers between individuals
Describe methyl salicylate
repels aphids
Describe long distance hormone transport
- via the xylem (root to shoot) and the phloem (source to sink organs)
How was long distance hormone transport revealed?
- complementation of biosynthetic mutants by reciprocal grafting with wild type
Describe cell-to-cell auxin transport
non-mediated transport through the acid trap model.
Describe the acid trap model
- IAAH diffuses from the apoplast through the membrane into the cytosol
- dissociates
- unable to cross phospholipid bilayers again
IAAH
Protonated auxin
Describe influx carriers
facilitate diffusion of IAA-
IAA-
deprotonated auxin
P-glycoprotein ABC transporters couple ATP hydrolysis to
pump IAA- out of the cell
Describe PIN efflux carriers
facilitate efflux from the cell into the apoplast
Describe auxin gradients
PIN1-2-3-4-7
Describe PIN proteins
- polar distribution at specific sides of the PM in a cell-type dependent manner
- maintained by continuous cycling from the ER/Golgi
PIN-mediated polar auxin transport determines the formation of
auxin maxima
Auxin gradients are required for
plant development
Describe the disruption of auxin gradients
- by inhibiting PIN-mediated transport
- abolish QC identity
- alters root development
- can be viewed using fluorescence
Describe morphogen gradients
sufficient to produce patterning
Describe morphogen feedback
cell identity can affect morphogen distribution and perception
Describe hormone perception - the details
- membrane associated receptors bind the hormone ligand
- (in)activated mainly by means of intra- or extra-molecular transfer of phosphate groups
Describe the role of the soluble receptors
- exploit the hormone as a molecular glue for transient protein interactions
- lead to effector activation
Describe proteolytic hormone signalling
- in hormone absence, specific transcriptional regulators (corepressors bound to transcriptional activators, bound to the enhancer) are maintained inactive by interaction with a co-repressor
- when hormone levels increase (binding to corepressor), this is marked for proteasomal degradation by polyubiquination
polyubiquination
ubiquitin ligases bind to hormone
Describe the auxin signalling pathway
- AUX/IAA proteins bind and repress the activity of ARFs via domain III and IV
- TIR1+auxin connect the AUX/IAA repressors with a ubiquitin E3 ligase complex to be polyubiquitinated
- polyubiquitinated AUX/IAA are degraded via the proteasome
- ARFs recruit RNA polymerase II on target genes
AUX/IAA
auxin
ARFs
Auxin Response Factors
Describe phosphorylating hormone signalling
- can be intra- or inter-molecular phosphorylation
- upon hormone binding, receptor transfers one or more phosphate groups to a soluble protein (with possible intermediates)
- protein acts as shuttle to the nucleus where it experiences high turnover
- gene expression is activated or repressed
Describe cytokinin signalling
- phosphorylation-based
- TCS: phosphotransfer between a membrane bound receptor and a soluble shuttle (His to Asp residues)
TCS
Two component system
Give some examples of when hormonal responses are used in metabolism, physiology or development
- activation of proton pumps
- protein relocalisation (PIN)
- cell cycle regulators
- primary and secondary metabolism
- synthesis of other hormones
Describe feedback looped hormonal responses
- inhibition of hormone biosynthesis
- stimulation of hormone degradation
- promotion of hormone sequestration/inactivation
- negative regulators of signal transduction (competitors/co-repressors)
How are enhancers responsible for hormone responsiveness identified
- sequence comparison of hormone-inducible promoter
- progressive deletions and test of reporter gene activity
Describe output reporters
- transcriptional modules able to perceive a specific hormone
- typically consist of several repeats of the same enhancer fused to a minimal promoter, controlling the expression of a reporter gene
Describe soluble receptors
- exploit hormone as a molecular glue for transient protein interactions
- lead to effector activation
Describe ratiometric output
- hormone-sensitive and insensitive modules joined genetically
- independent of the concentration of the reporter
Describe the hormonal control of vegetative development
- SAM maintenance: cytokinin (+)
- production of primordia: auxin (+)
- leaf expansion: gibberellin (+), brassinosteroids
- leaf senescence: cytokinin (-)
- lateral root production: auxin (+)
- axyllary bud dormance: auxin (+), strigolactones (+), cytokinin (-)
- root elongation: gibberellin (+)
- root differentiation: cytokinin (+)
- root meristem maintenance and activity: auxin (+), cytokinin (-)
… and … are the main hormones involved in the regulation of meristem maintenance, identity and activity in both roots and shoots
auxin, cytokinins
Describe the hormonal control of reproductive development
- seed dormancy: abscisic acid (+)
- seed germination: gibberellin (+)
- flowering: gibberellin (+), ethylene (+)
- flower development: auxin, ethylene, gibberellin
- fruit development: gibberellin(+), auxin (+)
- fruit ripening: ethylene, brassinosteroids
- abscision: ethylene, auxin
Reproductive development involves
the coordinated activity of several hormones, which may vary in a species-specific manner.
Describe hormonally-mediated interactions with other organisms
- ethylene, salicylates and jasmonates are rapidly synthesized and signal attack by other organisms and induce defence mechanisms
- strigolactones mediate synbiosis with mycorrhizal fungi
Describe plant defence mechanisms
- production of secondary metabolites
- induction of apoptosis.
Describe the applications of hormones to agriculture
- used for plant propagation
- auxins and cytokinins most used hormones for tissue culture
- auxin main active principle in rooting powders
tissue culture in plants
in vitro regeneration from explants
Describe hormonal control of tissue culture
- shoot induction: cytokinin
- root induction: auxin
- callus induction: auxin + cytokinin
rooting powders
used for clonal propagation of plants.
Describe hormonal applications in ripening fruits
Ethylene synthesis inhibition and ethylene supplementation applied to control fruit ripening
Describe ripening syndrome
- softening: change in cell wall composition
- pigmentation (chlorophyll→carotenoid/anthocyanins) - secondary metabolites accumulate (aroma)
- starch broken down into soluble sugars
Describe the domestic control of tomato ripening
- Low O2 inhibits ethylene production (delays ripening)
- Release ethylene to promote uniform ripening
