Auxin biosynthesis pathways in plants

Indole-3-acetic acid (IAA) is known to be synthesized de novo using tryptophan (Trp) as a precursor or using a Trp-independent pathway.
It has been clearly demonstrated that Trp-dependent auxin biosynthesis is essential for
embryogenesis,
seedling growth,
flower development,
vascular pattern formation, and
other developmental processes
In contrast, the molecular components and the physiological functions of the proposed Trp-independent pathway are not known.
Auxin is made in actively growing tissue which includes young leaves, root tips, fruits, and especially the shoot apex.
Two major pathways for IAA biosynthesis have been proposed:
the tryptophan (Trp)-independent pathway and
Trp-dependent pathways.
Trp is synthesized from chorismate via indole-3-glycerol phosphate in the chloroplast.
The ASA1 and ASA2 genes of Arabidopsis thaliana encode the a-subunit of anthranilate synthase, the enzyme catalysing the first reaction of the tryptophan biosynthetic pathway.
The IGS gene of A. thaliana encodes indole-3-glycerol phosphate synthase, which catalyses the conversion of 1-(O-carboxyphenylamino)-1-deoxyribulose-5-phosphate to indole-3-glycerol phosphate.
Trp is used to produce many indole containing substances in plants, such as IAA, indole glucosinolates, phytoalexins, and TAM derivatives.
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