Tryptophan-independent pathways of IAA
biosynthesis:
Orange pericarp (orp) mutant of maize
inactive tryptophan synthase
a true tryptophan auxotroph
IAA level is 50-fold higher than those of wild-type plant
[15N] anthranilate fed  labeled IAA, not in tryptophan
Glycosides of anthranilic acid and indole are accumulated in pericarp
orange color is due to excess indole
Trp-dep. or Trp-indep.
varied with tissues and
developmental stages.
Conjugated (or bound) auxins
– inactive form
¤ types: low MW compounds conjugated
Glc, myo-inositol, amide
high MW compounds
glucan (7-50 Glc per IAA), protein
depend on the specific conjugating enzymes,
environmental factors such as light and
gravity
(conjugation formation or hydrolysis)
¤ Physiological functions
transportation: IAA-myo-inositol
storage:
protection:
Conjugation and degradation of IAA:
Oxindole-3-acetic acid
Oncidium
Photodestruction/ riboflavin
Turn off light during extraction
The distribution of IAA
¤ is regulated by pH
¤ major in cytosol and chloroplast
¤ in tobacco cells
free IAA
cytosol
2/3
chloroplast 1/3
conjugated IAA
1 exclusively
0
Auxin transport
– the only plant growth hormone that has been clearly shown to be transported
polarly
¤ basipetal transport: from the apical to the basal end
via the vascular parenchyma cells of stems and leaves
¤ acropetal transport: toward the tip (in the root)
via phloem transport (major), xylem parenchyma of the stele
more than one mechanism (pathway) is responsible for the distribution
of auxin
Donor-receiver
agar block method
independent of gravity
Adventitious root formation
– basipetal transport, gravity independent
The characteristics of polar
transport
 in a cell-to-cell fashion, rather than via the symplast
 require metabolic energy
O2 deprivation and metabolic inhibitor
 the velocity is 5-20 cm/h
diffusion rate: 7.2 cm/h;
phloem translocation rate: 0.3-1.5 m/h;
xylem translocation rate: 4 mm/s (14.4 m/h).
 the specific protein carriers of plasma membrane involve
only for active auxins, both natural and synthetic.
A chemiosmotic model – auxin polar transport
Auxin pKa= 4.75
A chemiosmotic model – auxin polar transport
 Auxin influx (or uptake) (pH + E) (p. 477R)
(a) passive diffusion: protonated IAAH
(b) secondary active transporter: 2H+-IAA- symporter
efficiency is greater than simple diffusion
e.g., AUX1, a permease-type auxin uptake carrier
aux1 mutant: agravitropic growth for root
 Auxin efflux
(a) auxin anion efflux carrier: (E)
PIN proteins, are integral membrane proteins, localized at
the
basal ends of conducting cells
(b) P-glycoproteins (PGPs) : ATP-dependent transporters,
belonging
to the multidrug resistance/P-glycoprotein (MDR/PGP
proteins)
are uniformly distributed
can independently and synergistically catalyze auxin
transport with
PINs
a BR2 gene encodes a PGP