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:

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

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.

Auxin pKa= 4.75

A chemiosmotic model – auxin polar transport

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