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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
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:
Photodestruction/ riboflavin
Turn off light during extraction
Conjugation and degradation of IAA:
Oxindole-3-acetic acid
Oncidium
The distribution of IAA
¤ is regulated by pH
¤ major in cytosol and chloroplast
¤ in tobacco cells
free IAA conjugated IAA
cytosol 2/3 1
chloroplast 1/3 0exclusively
Auxin transport – the only plant growth hormone that has been clearly shown to be transported polarl
y
¤ 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 o
f auxin
Donor-receiver agar block method
independent of gravity
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 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, belongin
g to the multidrug resistance/P-glycoprotein (MDR/PGP protein
s) are uniformly distributed can independently and synergistically catalyze auxin transport
with PINs a BR2 gene encodes a PGP