Tryptophan-independent pathways of IAA biosynthesis:

Tryptophan-independent pathways of IAA
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
(conjugation formation or hydrolysis)
¤ Physiological functions
transportation: IAA-myo-inositol
Conjugation and degradation of IAA:
Oxindole-3-acetic acid
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
chloroplast 1/3
conjugated IAA
1 exclusively
Auxin transport
– the only plant growth hormone that has been clearly shown to be transported
¤ 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
agar block method
independent of gravity
Adventitious root formation
– basipetal transport, gravity independent
The characteristics of polar
 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
basal ends of conducting cells
(b) P-glycoproteins (PGPs) : ATP-dependent transporters,
to the multidrug resistance/P-glycoprotein (MDR/PGP
are uniformly distributed
can independently and synergistically catalyze auxin
transport with
a BR2 gene encodes a PGP
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