2. Plant development Flashcards
What are the two types of development?
Indeterminate: organs grow post-embryogenesis - plants
Determinate: no new organs post-embryogenesis - animals
What type of growth do plants exhibit?
Indeterminate - plant growth is plastic (adaptable to env) - plants can grow organs after embryogenesis
Meristem-driven growth - apical shoot apex / root apex + lateral root / axillary bud growth
How do shoots grow?
Shoots grow as modular units - easier - same genes reused many times in a loop
What is the structure of shoot apex?
Apical meristem - tip growth
Axillary bud - lateral branch growth
What is the structure of root apex?
Apical vs root meristem
How is lateral root growth initiated?
What do the different patterns of lateral organ formation in shoots / roots reveal about available resources below / above ground?
Below: roots branch laterally - no nutrients found by going down - branch out to maximise absorption area
Above: branches and leaves grow laterally and usually non-overlaping patterns with other branches to maixmise sunlight absorption - leaves evolved flat to have max SA
Why is plant embryogenesis and further vegetative development separated by dormancy period?
In evolution selected for dormancy between embryogenesis and growth because:
- dormancy allows for offspring to survive harsh env conditions (ex. winter) until env is favourable for growth - use guidance cues to know when to end dormancy
- for seed dispersal: need to get further from parental plant - because plants can’t move -> fully dependent on the env to survive - need to spread offspring far away to diff env to maximise survival chances
Describe plant root growth
Plant root growth is a response to env - directionality of growth depends on nutrient availability - blinf searching (foraging) until found -> increased root growth in that area
Give examples of indeterminate and determinate growth
Does cell lineage restrict plant development?
No, lineage doesn’t restrict plant development:
cells adopt the cell fate of surrounding cells - adopt fate according to position, not lineage
Compare animal and plant development
What are two advantages of indeterminate development in evolution?
Adv of indeterminate growth:
- can adapt to env - form new organs in more favourable patterns
- can easily replace damaged organs
What are the advantages of growing organs in modular units?
Adv of growing organs in modular units:
- easier to replace - same genes (GRN) used on a loop
What processes is auxin functionally involved in?
Auxin functionally involved in:
- cell division and cell elongation
- shoot apical dominance
- root growth and development
- vascular development
- phototropism
- gravitropism
What is shoot apical dominance?
Shoot apical dominance - main central stem (apex) grows more vigorously than the lateral shoots -> this dominance ensures that the plant grows upward - beneficial for competing for sunlight - auxin produced in shoot apex supresses growth of lateral buds
Explain auxin molecule
Auxin - indole-3-acetic acid (IAA) - simple amphiphilic molecule derivd from tryptophan
How is auxin amphiphilic?
Auxin is amphiphilic because it can both be hydrophilic and hydrophobic:
- hydrophylic: carboxyl group
- hydrophobic: indole ring
- can lose/gain H+
-> used in polar auxin transport across plant cell wall
Explain polar auxin transport
Polar auxin transport - emergent, self-organising system for directional flow - diffusion based on conc gradient (pH-dep manner) - lowe pH outside -> becomes protonated - moves into cytosol - higher pH - becomes ionic
Auxin transported into plant cells by efflux proteins and exported out by PIN membrane proteins
IAAH enters the cell, IAA- exported out but only through PIN - ionic form can’t diffuse
Why is auxin transport system called self-organising?
Self-organising because it establishes and maintains its own directional flow and patterns of distribution without external instructions:
- PIN proteins redistribute themselves in response to auxin levels
- auxin conc guides where more auxin should accummulate -> influences activity and expression of PIN
=> feedback system - no need external signal
Why is polar auxin transport inhibited by higher auxin concentration?
Polar auxin transport inhibited by higher auxin conc:
PIN proteins are constantly recycled in endosome - when auxin conc rises - inhibits recycling - more PIN stain in cell wall - export auxin
Explain in detail how polar auxin transport works
Auxin moves from high to lower conc between plant cells - at high auxin levels PIN proteins less recycled - stay in membranes to efflux auxin out => creates auxin-PIN regions of high conc
Describe and experiment used to test polar auxin transport in action
Fuse GFP with PIN - observe where localises - photobleach the proteins from GFP with a laser - see how comes back in time - high auxin-PIN regions come back
Explain auxin canalisation?
Auxin canalisation - forming auxin transport paths - directionality
Self-organising, self-reinforcing system: highest -> lowest auxin conc directionality (source + sink) - directional flow becomes more uniform over time as PIN proteins reorganise - increasing auxin cocn PIN increase
Give an example of an auxin canalisation induced developmental process
Plant leaf vein patterning - auxin canalisation creates directional flow -> later directional flow of water through veins
What are the two fundamental properties of auxin?
Fundamental auxin properties:
- amphiphilic + gain/loose H+ -> switch between forms and create cinc gradients outside vs inside of a plant cell
- auxin regulates its own transport by accummulating + moving PIN proteins in cell membrane
How does PIN localisation change in Arabidopsis embryogenesis?
Auxin is involved in patterning of Arapidopsis embryo - involved in apical-basal specification - depending on stage of development re-locates for apical-basal polarity
Waht would happen to auxin localisation if PIN proteins stopped being recycled?
If PIN would not be recycled - would stay everywhere -> auxin everywhere => chaos
If ARA7 function was disrupted - which acts in endosomal transport of PIN recycling -> normal auxin flux pattern disrupted => new local auxin maxima -> faulty patterning: ex. high conc accummulates at cotyledon tips -> converts to root tissue identity
How does auxin localisation change in Arabidopsis embryogenesis?
Why are there multiple different PIN proteins in plants?
Multiple different PIN proteins (PIN1, PIN4, PIN7) allow higher control over auxin mediated processes - usually found in different locations - can turn off one type without disrupting another
Explain gnom mutation
Gnom gene controls apical-basal patterning in Arabidopsis embryogenesis - needed for polar localisation of PIN1 - fixes polar axis
Gnom mutants produce inconsistent embryo shapes
Explain the molecular mechanism how GNOM controls PIN1 localisation
GNOM - auxin response factor - guanine-nulceotide exchange facor (ARF-GEF) - it fixes basal PIN1 polarity by helping to regulate vesicle formation that transport PIN1 from early endosomes back to the plasma membrane -> maintains the polar localization of PIN1 - placed in specific locations on the membrane to direct auxin flow properly
How does auxin initiate response?
Auxin turns specific genes on by forming an SCF-auxin-Aux/IAA complex and degrading Aux/IAA by proteolysis - removes repression by ARF -> gene activation and expression
Example of Aux/IAA - BDL, monopteros (MP)
Give an example where auxin dependent Aux/IAA proteins are needed in plant development
Aux/IAA - ex MP and BDL - are required for PIN1 expression to specify the hypophisis
MP acts to reinforce the commitment to apical-basal axis
Explain the location of PINs and role of auxin in early embryo patterning
What is the role of auxin in early embryo patterning
Auxin indirectly affects cell fate - apical-basal axis formation
Auxin elicits primary response in adjacent cells and forms part of a signal relay mechanism
Is auxin a morphogen?
No, because auxin doesn’t influence directly - has an indirect effect - also no gradient just on/off
But similar to morphogen because auxin patterns just through other players - doesn’t to it itself
What is the main feature of auxin in this lecture?
Auxin at different stages induces different developmental events - auxin does majority of developmental patterning in plants - but not by itself - cooperation with other molecular complexes (effectors)
