Plant Signalling 2 Flashcards
Auxins roles
Auxin = Indole-3-Acetic Acid (IAA)
Roles in:
- tropic growth
- apical dominance
- organogenesis
- pattern formation & polarity
IAA mutants
No mutants that COMPLETELY lack IAA are known
Mutants in auxin perception & response have helped work out the molecular basis of auxin action
Natural & Synthetic IAA
Synthetic = more stable
- used experimentally
Natural = similar to tryptophan
- synthesis begins from tryptophan
- synthesis driven by developmental and environmental signals
IAA modulation/regulation
Controlled by conjugation
- when conjugated as sugar ester forms, IAA is in its inactive storage form
- cleaving ester bond rapidly activates it (useful for quick responses, naste)
IAA transport
Moves in vascular system (fast, non-polar)
Slow cell-cell transport (polar)
Polar IAA transport
Unique to IAA, gives rise to concentration gradients that shape cell responses
Moving cell to cell using chemiosmotic driving force for movement
IAA entering/exiting the cell mechanisms
Entering:
- Diffusion as IAAH (protonated)
- Co-transport of IAA- & H+ through AUX1 influx carrier
- Active transport of IAA via ABC importers
Exiting:
- PIN efflux carrier
- Active transport of IAA via ABC transporter
- Active transport of H+ via H+ ATPase
Movement is based on pH difference across membrane:
- pH 5.5 outside vs pH 7 inside
- IAAH <===> IAA- + H+
Monitoring IAA presence
Done by inserting an auxin responsive promoter (e.g. DR5) with an adjacent downstream reporter like GUS or GFP
IAA’s asymmetric distribution
Achieved by the asymmetric distribution of influx and efflux proteins in the cells:
- Influx on 1 side, efflux on the other
- PIN (efflux) protein localisation change in response to developmental and environmental signals
IAA response to embryo development
1) initial transport into embryo via PIN7
2) following cell division, PIN1 mediates auxin flow between cells inside embryo
3) PIN4 transports IAA to the base of the embryo
IAA regulating tropisms
PIN localisation can change in response to gravity: providing gravistimulation
- Starch granules sense gravity by sedimenting at bottom of root cap cells
- PIN3 relocates and IAA accumulates on lower side of root
- results in asymmetric, downwards root growth
Mechanism for changing PIN protein localisation
PM-endosome cycling
PIN and AUX proteins are synthesised on ER and transported in vesicles to plasma membrane (PM). They are constitutively cycled between the PM and endosomes
- Constantly recycling, the location of fusion with the PM can change
- PIN and AUX can also enter degradation pathway, entering the vacuole
IAA activating genes
Like Jasmonates, involves degrading repressors
- AUX/IAA repressors normally bind to Auxin Responsive Factors (ARFs) on promoter
- Auxins activate TIR1 (F-box) which activates the SCF type E3 ligase (SCFtir1)
- SCFtir1 polyubiquitinates the AUX/IAA repressor leading to proteosomal degradation
