Auxin - an essential growth hormone

Auxin:
- pattern formation
- elongation
- required for viability like cytokinins
- synthesized at the tips of young leaves
- promotes apical (height) dominance
- prevents leaf abscission (loss)
- promotes fruit development
- generation when cut is made
- promote seedless fruits
- herbicides in dicot

Biosynthesis
- use Trp as precursor
- controlled by conjugation (add methyls to convert it to storage forms)

All auxins: Aromatic ring + carboxyl group (0.5nm separation)
eg. IAA - Indole-3-acetic acid

* phototropism (coleoptile curvature test) - differential growth

Auxing Transport
- Basipetal (shoot to root) shift in auxin production
- polar transport from shoot tip (apex) to root tip (base)
- experiment is the donor block of IAA, no IAA obtained in donor when shoot was reversed
- inside of cell is pH 7.2 (more basic than outside pH 5)
- ~ 50% IAA is protonated & can diffuse down a conc. gradient
- ~ 50% is charged (IAA-) and enters via a 2H+/IAA- symporter (influx carrier) (encoded by AUX1 gene)
- IAA- exits cell via efflux carriers (encoded by PIN1 gene) (only found at the basal end of the cells) -> polar transport
- IAA- helps with H+ extrusion: 1. helps in H+-ATPase gene expression 2. membrane trafficking 3. stabilize H+-ATPase stabilization