[Show abstract][Hide abstract] ABSTRACT: INTRODUCTION Indole-3-acetic acid (IAA) is involved in virtually all aspects of plant growth and development (Davies, 1995). Plants produce active IAA by de novo synthesis and by hydrolyzing IAA conjugates (reviewed in Normanly et al., 1995; Bartel, 1997; Normanly, 1997). IAA conjugation activity is widely distributed in the plant kingdom from mosses to angiosperms (Sztein et al., 1995), and most IAA in plant tissues is conjugated via its carboxyl group to sugars, high molecular weight glycans, amino acids, or peptides (Cohen and Bandurski, 1982; Bandurski et al., 1995). Different conjugates may perform different functions in the plant. For example, IAA--Asp is an intermediate in IAA destruction (Tsurumi and Wada, 1986; Monteiro et al., 1988; Tuominen et al., 1994; stin et al., 1998). Other conjugates may serve as reservoirs of inactive IAA that can be hydrolyzed to supply the plant with active hormone, as in maize germination when conjugate hydrolysis provides free IAA to th
[Show abstract][Hide abstract] ABSTRACT: Amide-linked conjugates of indole-3-acetic acid (IAA) are putative storage or inactivation forms of the growth hormone auxin. Here, we describe the Arabidopsis iar3 mutant that displays reduced sensitivity to IAA-Ala. IAR3 is a member of a family of Arabidopsis genes related to the previously isolated ILR1 gene, which encodes an IAA-amino acid hydrolase selective for IAA-Leu and IAA-Phe. IAR3 and the very similar ILL5 gene are closely linked on chromosome 1 and comprise a subfamily of the six Arabidopsis IAA-conjugate hydrolases. The purified IAR3 enzyme hydrolyzes IAA-Ala in vitro. iar 3 ilr1 double mutants are more resistant than either single mutant to IAA-amino acid conjugates, and plants overexpressing IAR3 or ILR1 are more sensitive than is the wild type to certain IAA-amino acid conjugates, reflecting the overlapping substrate specificities of the corresponding enzymes. The IAR3 gene is expressed most strongly in roots, stems, and flowers, suggesting roles for IAA-conjugate hydrolysis in those tissues.