IAA conjugates that accumulate in large amounts in cotyledons of mature seeds may be one of the possible sources of IAA required for the growth of bean seedlings (Bialek et al, 1992).

In mutants or transgenic plants that accumulate IAA, the levels of free IAA generally remain normal while conjugated IAA accumulates --- conjugation of IAA may be a control mechanism for IAA levels. In maize kernels IAA esters with myo-inositol, glucose, inositol glycosides and glucans account for essentially all the IAA conjugates present. The first gene of IAA metabolism to be cloned from plants is the maize gene for IAA-Glc synthase (Szerszen et al, 1994). In some species IAA is conjugated with amino acids (Ala, Asp, Leu or Phe). The ilr1 mutant of Arabidopsis is defective in an IAA-conjugate lyase with preference for IAA-Leu and IAA-Phe (Bartel and Fink, 1995). The Iar3 locus also encodes an IAA-conjugate lyase with a preference for IAA-Ala (Davies et al, 1999).

Oetiker and Aeschbacher (1997) have identified a temperature-sensitive variant of henbane (Hysoscyamus muticus) which grows in the absence of auxin at 26 C, but dies rapidly at 33 C unless auxin is added to the medium. The wildtype accumulates IAA-Asp and IAA-Glucose but the variant accumulates mainly IAA-Glucose. The variant rapidly metabolizes IAA-Asp to unidentified compounds that are distinct from IAA; this challenges the popular hypothesis that IAA conjugates are merely slow-release storage forms of IAA [see Kowalczyk and Bandurski (1990) and Bartel (1997) for discussions of the pathways of synthesis and hydrolysis of IAA conjugates which may play a role in homeostatic control of IAA levels].

Chou et al (1996) suggest that IAA-Asp is an intermediate in the oxidative catabolism of IAA in several plant species:

In Arabidopsis, oxindole-3-acetic acid (OxIAA), OxIAA conjugated to a hexose moiety via the carboxyl group, and the conjugates indole-3-acetyl aspartic acid (IAAsp) and indole-3-acetyl glutamate (IAGlu) are the primary products of IAA fed to the plants (Ostin et al, 1998). IAAsp is further oxidized to OxIAAsp and hydroxy-IAAsp (Ostin et al, 1998).

Indole-3-butyric acid (IBA) is a naturally occurring auxin that is found in several species, including maize, peas and Arabidopsis. Like IAA, IBA is found in free and conjugated forms. IBA is synthesized from IAA via a chain elongation reaction similar to fatty acid biosynthesis (Bartel, 1997). The IBA synthase purified from maize uses acetyl CoA and ATP as cofactors (Bartel, 1997).

The tir3 mutant of Arabidopsis displays a variety of morphological defects that are ascribed to changes in IAA distribution (including a reduction in apical dominance), and is defective in polar auxin transport (Ruegger et al, 1997).

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Bartel B, Fink GR 1995 ILR1, an amidohydrolase that releases active indole-3-acetic acid from conjugates. Science 268: 1745-1748.

Bialek K, Michalczuk L, Cohen JD 1992 Auxin biosynthesis during seed germination in Phaseolus vulgaris. Plant Physiol. 100: 509-517.

Chou J-C, Kuleck GA, Cohen JD, Mulbry WW 1996 Partial purification of an inducible indole-3-acetyl-L-aspartic acid hydrolase from Enterobacter agglomerans. Plant Physiol. 112: 1281-1287.

Davies RT, Goetz DH, Lasswell J, Anderson MN, Bartel B 1999 IAR3 encodes an auxin conjugate hydrolase from Arabidopsis. Plant Cell 11: 365-376.

Kowalczyk S, Bandurski RS 1990 Isomerization of 1-O-Indol-3-Ylacetyl-B-D-glucose. Enzymatic hydrolysis of 1-O,4-O, and 6-O-Indol-3-Ylacetyl-B-D-glucose and the enzymatic synthesis of indole-3-acetyl glycerol by a hormone metabolizing complex. Plant Physiol. 94: 4-12.

Ostin A, Kowalyczk M, Bhalerao RP, Sandberg G 1998 Metabolism of indole-3-acetic acid in Arabidopsis. Plant Physiol. 118: 285-296.

Ruegger M, Dewey E, Hobbie L, Brown D, Bernasconi P, Turner J, Muday G, Estelle M 1997 Reduced napthylphthalamic acid binding in the tir3 mutant of Arabidopsis is associated with a reduction in polar auxin transport and diverse morphological defects. Plant Cell 9: 745-757.

Szerszen JB, Szczyglowski K, Bandurski RS 1994 iaglu, a gene from Zea mays involved in conjugation of the growth hormone indole-3-acetic acid (IAA). Science 265: 1699-1701.