HORT640 - Metabolic Plant Physiology
Secondary products derived from aromatic amino acids
Alternative IAA synthesis pathways
IAN (indole-3-acetonitrile) is a candidate IAA precursor in the tryptophan-independent pathway of auxin biosynthesis (dashed line of figure above) (Normanly et al, 1993). IAN accumulates 6- to 11-fold in the Arabidopsis trp3 and trp2 mutants that overaccumulate IAA conjugates (trp3 mutants are defective in tryptophan synthase [EC 220.127.116.11] A subunit; trp2 mutants are defective in tryptophan synthase B subunit) (Bartel, 1997). Genes encoding nitrilase [EC 18.104.22.168] enzymes have been cloned from Arabidopsis and several other higher plants (Bartling et al, 1994; Normanly et al, 1997).
IAN has several auxin-like effects in Arabidopsis, and is metabolized to IAA. An Arabidopsis mutant defective in the nitrilase encoded by the NIT1 gene has been identified, but the half-life of IAN and the levels of IAA and IAN were unaffected in the nit1 mutant. This may be because Arabidopsis has other functional nitrilases (e.g. 3 nitrilase genes, NIT1, NIT2 and NIT3 are clustered on chromosome 3). However, overexpression of NIT2 enhanced IAN turnover and increased sensitivity to IAN (Normanly et al, 1997).
Bartel B 1997 Auxin biosynthesis. Annu. Rev. Plant Physiol. Plant Mol. Biol. 48: 51-66.
Bartling D, Seedorf M, Schmidt RC, Weiler EW 1994 Molecular characterization of two cloned nitrilases from Arabidopsis thaliana: key enzymes in biosynthesis of the plant hormone indole-3-acetic acid. PNAS 91: 6021-6025.
Normanly J, Cohen JD, Fink GR 1993 Arabidopsis thaliana auxotrophs reveal a tryptophan-independent biosynthetic pathway for indole-3-acetic acid. Proc. Natl. Acad. Sci. U.S.A. 90: 10355-10359.
Normanly J, Grisafi P, Fink GR, Bartel B 1997 Arabidopsis mutants resistant to the auxin effects of indole-3-acetonitrile are defective in the nitrilase encoded by the NIT1 gene. Plant Cell 9: 1781-1790.
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